Long Term Morphological Modelling of Venice Lagoon

Transcription

1 Prepared for: Long Term Morphological Modelling of Venice Lagoon Report October, 2004 Z 2839

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3 Prepared for: Long Term Morphological Modelling of Venice Lagoon D.G.J. Maas Report October, 2004

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5 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, 2004 Preface This report describes the study done within the framework of my Master of Science thesis and forms the final part of my study Civil engineering at the Delft University of Technology, Faculty of Civil Engineering and Geosciences, Division Hydraulic and Offshore Engineering. The study has been carried out in cooperation with in the Netherlands and the University of Padua, Department of Hydraulic Engineering, Italy. This report describes the adaptation of the long term morphological model ASMITA to the area of Venice Lagoon and supplies insight into the present problem of sediment loss. The deterioration of the salt marshes and the influence of the ebb delta and adjacent coast received special attention. The new model ASMITA Venice is able to simulate and predict the morphological evolution of the lagoon and the aspects that influence this evolution. I would like to thank the members of my graduation committee Prof. Dr. Ir. H.J. de Vriend, Prof. Dr. Ir. M.J.F. Stive, Dr. Ir. Z.B. Wang, Ir. A. Crosato, Prof. Dr. Ir. H.H.G. Savenije and Prof. Dr. Ir. N van de Giesen for their comments during the accomplishment of my thesis. For the months I spent in Padua I would like to thank Prof. Ing. G. Di. Silvio and Ing. L. Dal Monte sharing their time and their knowledge of the Lagoon of Venice. For their financial support I would like to thank the Consortium for Coordination of Research Activities concerning the Venice Lagoon System (CORILA). Furthermore I would like to thank my family and friends for their interest and support during this study. Delft, October 2004 Davor Maas. Preface

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7 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, 2004 Contents List of Figures List of Tables 1 Introduction General Problem analysis Objective Structure of this study Inter Tidal Areas Outside area Tidal gorge Tidal basin Hydrodynamic classification of tidal inlets Venice Lagoon Introduction Geographical characteristics The Adriatic Sea The lagoon coastline Venice Lagoon Forcing factors Waves Wind Tides Relative Sea level rise i

8 October, 2004 Z 2839 Long Term Morphological Modelling of Venice Lagoon 3.4 General characteristics Tidal divides in Venice Lagoon Sediment Vegetation History of human intervention Present Morphological situation ASMITA Introduction Schematisation of the area Morphological equilibrium Equilibrium sediment concentration Morphological changes Input parameters Comparing two systems Introduction Comparing simulation areas Comparing morphological models Problems in applying ASMITA to Venice Lagoon Introduction Restructure the basin area Schematisation basin area Definition basin area Restructure sea side area Current situation Schematisation of the sea side area ii

9 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, Definition of the sea side element Final model adjustment Long term morphological predictions Introduction Input of ASMITA Venice Predictions Morphological evolution Lido inlet Evaluation ASMITA Venice run Conclusions and Recommendations Conclusions Part 1, Analysis Part 2, Part 3, Implementation of ASMITA Venice Recommendations A The Di Silvio model...a 1 A.1 Introduction...A 1 A.2 Conservation equations...a 2 A.3 Constitutive equations...a 4 A.4 Equilibrium configuration...a 5 B C D E Sediment transport formulas...b 1 Lagoon wave climate...c 1 Offshore wave climate...d 1 Offshore wind climate...e 1 F Sediment balance... F 1 iii

10 October, 2004 Z 2839 Long Term Morphological Modelling of Venice Lagoon List of Figures Figure 1 1: Average sediment balance in Venice Lagoon during one year, values in m 3 /year Figure 1 2: Schematisation of the current research Figure 2 1: (a) overview of a tidal inlet system [van Goor, 2001], (b) schematisation of a tidal inlet system Figure 2 2: Schematic diagram of the outer delta residual sediment transport Figure 2 3: Characteristic areas inside a basin Figure 2 4: Hydrodynamic classification of tidal inlets Figure 3 1: (a) Satellite image Venice Lagoon, (b) Geographical location Italy and Venice Lagoon Figure 3 2: (a) Current at the Adriatic Sea, (b) Current in front of the Venice coast Figure 3 3: Coastline of Venice Lagoon. (a) location coastal strips, (b) groynes at the Pellestrina coastline Figure 3 4: Location of the three inlets Figure 3 5: (a) View Chioggia inlet, (b) View Malamocco inlet, (c) View Lido inlet Figure 3 6: Sediment transport forces of waves travelling towards the coast, breaking waves (a), long shore current (b), wave setup (c) Figure 3 7: Currents in the lagoon, induced by wind Figure 3 8: Tidal inlet system adapting to accelerated relative sea level rise and regaining a new constant water depth. [Van Goor, 2001] Figure 3 9: Tidal inlet system unable to adapt to accelerated sea level rise. [Van Goor, 2001] Figure 3 10: Lagoon of Venice separated into three basins by tidal divides Figure 3 11: Evolution of the salt marshes Figure 3 12: Venice Lagoon at the beginning of the 14 th century. [ 14 Figure 3 13: Venice Lagoon at the end of the 15 th century. [ 14 Figure 3 14: Venice Lagoon at the end of the 16 th century. [ 15 Figure 3 15: Venice Lagoon at the end of the 17 th century. [ 15 Figure 3 16: Venice Lagoon at the end of the 18 th century. [ 16 Figure 3 17: Venice Lagoon at the end of the 19 th century. [ 16 Figure 3 18: Venice Lagoon at the end of the 20 th century. [ 17 Figure 3 19: Levelling of the lagoon bottom Figure 3 20: Flow pattern at the Lido and Malamocco inlet due to construction of iv

11 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, 2004 the jetties Figure 4 1: Computational procedure ASMITA Figure 4 2: Schematisation of a tidal inlet in ASMITA Figure 4 3: Cross section of delta, (a) coast without an inlet used as reference coast, (b) volume ebb tidal delta Figure 4 4: Cross section tidal basin, representing the flat and channel volume Figure 4 5: sediment balance between the elements Figure 5 1: Structure of chapter Figure 5 2: Sediment exchange between elements, (a) ASMITA three element model, (b) Di Silvio model Figure 6 1: Schematisation of the current ASMITA model, (a) cross section, (b) top view.6 2 Figure 6 2: Hypsometry of Venice Lagoon Figure 6 3: Schematisation of ASMITA variation one, (a) cross section, (b) top view Figure 6 4: Schematisation of ASMITA variation two, (a) cross section, (b) top view Figure 6 5: Sediment fluxes between the elements, (a) two elements in the basin, (b) three elements in the basin Figure 6 6: Schematisation of the ebb delta before construction of the jetties, (a) location of the sand banks, (b) cross section over ebb delta area Figure 6 7: Schematisation of the ebb delta after construction of the jetties, (a) location of the shallow area, (b) cross section over ebb delta area Figure 6 8: Sedimentation near a coastal construction as a result of blocking the longshore current Figure 6 9: Schematisation into a fixed element Delta, alternative one Figure 6 10: Schematisation into a variable element Delta, alternative two Figure 6 11: Schematisation into the elements Delta and Beach, alternative three Figure 6 12: Sediment transport mechanisms and element schematisation of the Lido sea side area Figure 6 13: Sediment transport mechanisms and element schematisation of the Malamocco sea side area Figure 6 14: Sediment transport mechanisms and element schematisation of the Chioggia sea side area Figure 6 15: Sediment exchange between the element Delta and its surrounding Figure 6 16: Sediment exchange between the element Beach and its surrounding Figure 6 17: Cross section A B over the element Beach v

12 October, 2004 Z 2839 Long Term Morphological Modelling of Venice Lagoon Figure 6 18: Final schematisation ASMITA Venice Figure 6 19: Cross Section of the lagoon, schematisation of the volumes Shallow area and volume Channel Figure 7 1: Schematisation ASMITA Venice for the Lido inlet Figure 7 2: Evolution element Beach, year 1930 until Figure 7 3: Evolution element Delta, year 1930 until Figure 7 4: Evolution element Channel, year 1930 until Figure 7 5: Evolution element Shallow area, year 1930 until Figure 7 6: Evolution element Marshes, year 1930 until Figure 7 7: Sediment transport between elements generated by tide and wave activity Figure 7 8: Sediment transport between the elements generated by long shore transport Figure 7 9: Sediment balance year 2000, values in m 3 /day vi

13 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, 2004 List of Tables Table 3 1: Tidal levels with respect to MSL [Admiralty Tide Tables, 1987] Table 3 2: Coordinates tidal divides in Venice Lagoon Table 4 1: Empirical coefficient delta element. [Bijsterbosch 2003] Table 4 2: Equilibrium values for the Dutch Wadden Sea, Eysink 1990 respectively Stive Table 5 1: Comparison between the Dutch Wadden Sea and Venice Lagoon Table 5 2: Values used in the morphological models Table 7 1: Morphological evolution of the elements at the Lido inlet in the period Table 7 2: Input parameters ASMITA Venice for Lido vii

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15 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, 2004 Summary Introduction The Lagoon of Venice is an important element in the ecological economical balance of the region. Unfortunately the lagoon is undergoing morphological changes that are considered harmful for the preservation of its present function. Human intervention, like river diversion, channel dredging and jetty construction together with natural processes, like land subsidence and the rising of the sea level, have accelerated the evolutionary tendency of the lagoon towards the gradual erosion of its most typical elements, the ecologically important salt marshes and sedimentation of its channels. In managing the lagoon, tools are necessary to predict these processes. The success of the numerical tool, ASMITA, in the management of the Dutch Wadden Sea, has provided the basis for the present study. Study objectives The objective of this study is to develop ASMITA Venice, a version of ASMITA for use in Venice Lagoon, and to investigate the applicability of ASMITA Venice to Venice Lagoon. For that purpose the study integrates results and experience of the Padua group of Prof. Di Silvio. It will focus on the modelling of the sediment balance of the Venice Lagoon, more in particular on the transfer of sediment from the shallower areas to the channel system and on the loss of sediment to the Adriatic Sea. Specific adjustments to ASMITA are developed to be able to reproduce past morphological evolution and predict the future evolution in the Venice Lagoon. These adjustments can be made on three different levels, viz. the schematisation into elements, the equations used to describe the sediment transport and the parameters in the model. Study content Preliminary investigations in Delft and Venice showed that the adjustments to the schematisation would involve both modifications of the present elements and adding new elements to the model. Also re evaluation of the predominant force factors in ASMITA on the basis of the Padua experience proved necessary. Furthermore special attention was needed on the availability of data for the calibration of the modified model. This has led to the following phased approach of the study: Analysis phase In this phase a comparison was made between ASMITA used in the Dutch Wadden Sea and the Di Silvio s model developed for the Venice lagoon. From this comparison the most important adjustments were identified. Development phase In the development phase adjustments were elaborated and subsequently implemented into ASMITA Venice, the adjusted model. Summary

16 October, 2004 Z 2839 Long Term Morphological Modelling of Venice Lagoon Implementation phase In the implementation phase the available data were used to calibrate the model and an analysis was made of the limitations to its usefulness in the light of scarce data and necessary assumptions Results For this study the most important findings were the necessary adjustments in the schematisation and the associated equations. In particular the following elements were added or modified to ASMITA: Marshes This element represents the evolution of the salt marsh areas. In view of its ecological function it is expressed as a surface. Interaction with the shallow areas is based on concentration differences and cliff erosion. Shallow The influence of wind over tide for this element in the Venice setting is expressed in modified equations for this element. Beach The element Beach represents the evolution of the coast due to the construction of a jetty. Its expression uses the length of the jetties as a variable. The effect of the long shore current on the sediment transport is expressed in the model. The calibration of the model proved difficult due to lack of data. The resulting model shows good agreement with the available data. However, the model is based on a number of assumptions, which need to be verified by e.g. field observations. Evaluation The modified model ASMITA Venice integrates knowledge and experience gained in the Netherlands with ASMITA and the insights of the University of Padua. In this way it can potentially provide a basis for further cooperation. The cooperation will be necessary to tune the model ASMITA Venice and even more important, to provide support for some of the assumptions made in the adjustments and the calibration Recommendations include the continued cooperation with Padua University and more in depth studies on the sediment transport mechanisms in the Venice lagoon and similar area s. Summary

17 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, Introduction 1.1 General A large percentage of the population of the world is situated in coastal areas. These regions are constantly changing due to human intervention and natural forcing. This study contributes to the existing knowledge on coastal behaviour. It focuses on a particular part of the coastal area, the tidal inlets. Using a mathematical model, insight is obtained in the morphological evolution of tidal inlet systems. The mathematical model used in this study is ASMITA (Aggregated Scale Morphological Interaction between a Tidal inlet system and the Adjacent coast, Stive and Wang 1996). This semi empirical model is based on sediment transport formulae as well as on empirical relations. By schematisation of the coastal area in several elements, the model calculates the sediment fluxes between these units and the evolution of their sediment content. The number of elements depends on the modelled area, usually three to five. Coefficients in the model are calibrated against historical bathymetric data. The model ASMITA has been optimised for the Dutch tidal inlets, of which it is capable of computing the long term morphological evolution with satisfying accuracy. In this study the model has been adapted to simulate the morphological behaviour of a tidal inlet having different characteristics, the lagoon of Venice. Differences can be found in the extension of salt marshes and in the presence of jetties at the inlet. Therefore adjustments are made on the schematisation and constituting of equations in ASMITA. The adjusted ASMITA model can give an insight into the morphological evolution of Venice Lagoon and contribute to the analysis of the problems encountered in this area. 1.2 Problem analysis The Lagoon of Venice, located in the North East of Italy is connected to the Adriatic Sea by three inlets. The lagoon is undergoing morphological changes that are considered harmful to the preservation of the present functions of the lagoon. Various aspects are involved in dealing with the Lagoon of Venice. The city of Venice, inside the Lagoon, is confronted with floods and problems of water quality. The basin gives access to the Venice harbour, where large ships and oil tankers berth. Additionally the lagoon has an important function as fishing water and recreation area. Next to the human activities, the lagoon is an ecologically important area. All these functions will benefit from good management of the Lagoon on the short and long term. 1 1

18 October, 2004 Z 2839 Long Term Morphological Modelling of Venice Lagoon The characteristics of Venice Lagoon have drastically changed over the centuries. Human intervention, such as river diversion and channel dredging, together with natural processes, such as land subsidence and the rising of the sea level, have turned the evolutionary tendency of the lagoon from gradual infilling towards gradual erosion. The changing characteristics have led to four major effects: Floods: The lagoon islands, including the historic city of Venice, are affected by sea level rise and land subsidence. The relative sea level rise is causing ever more frequent floods in the city centre and threatens the invaluable historical treasures there. The loss of solid material to the Adriatic Sea: The inflow and outflow of sediment are out of balance. According to the Consorzio Venezia Nuova, an average more than one million cubic meters a year of solid material is transported from the lagoon to the Adriatic Sea. A first order sediment balance of the lagoon is given in Figure 1 1: On average m 3 of sediment is brought into suspension from the lagoon bed and settle again on the lagoon bottom. Some m 3 of sediment enters the lagoon through the rivers and m 3 of sediment erodes from the salt marshes m 3 of sediment is lost to the Adriatic Sea via tidal currents and m 3 is dredged from the lagoon channels. The current policy is that all dredged material is dumped in the lagoon to reconstruct eroded salt marshes or mud flats. Marshes Lagoon River Adriatic Sea Figure 1 1: Average sediment balance in Venice Lagoon during one year, values in m 3 /year. Ecological deterioration: The lagoon is in a situation of ecological deterioration due to human interventions, i.e. increase of industrial activity and the discharging of the untreated sewage into the lagoon. Salt marshes, heavily vegetated areas that are submerged only during the highest tides, are important areas for growth of plants as well as feeding and breeding places for birds. These areas, which are indispensable to the ecosystem in Venice Lagoon, are eroding rapidly. Sedimentation in the channels: The internal transport of sediment within the lagoon causes erosion of the shallow areas and sedimentation of the channel system. The latter leads to an increase of dredging activities. 1 2

19 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, 2004 The eroding characteristics of the lagoon are recognised as a process which harms the present function of Venice Lagoon as an ecological, recreational and economical area. The continuing export of sediment is a result of the changes made to the lagoon over the last centuries. Important at this stage is to get insight into the evolutionary tendency of the present lagoon. In this way, predictions of the future evolution can be made and one can decide which precautionary measures need to be taken to create a sustainable situation in safeguarding the Venice Lagoon with its present characteristics. 1.3 Objective This study will focus on the sediment balance of Venice Lagoon, referring to the transfer of sediment from the shallower areas to the channel system and the loss of sediment to the Adriatic Sea. By studying the past morphological evolution of the lagoon, predictions can be made of the future evolution and the influence of human interventions on it. The contribution includes the application of the long term morphological model, ASMITA, to Venice Lagoon. This model has been used for the Dutch tidal inlet systems with satisfying results. As the lagoon has different characteristics and different boundary conditions compared to the Dutch coast, adjustments to the model are necessary. Differences can be characterised by internal factors, such as sediment characteristics and bed structure, and external factors, such as the wave activity in the region and the tidal range. Adjustments can be expected in the schematisation of ASMITA into elements and the equations used to describe the sediment transport. Furthermore, the parameters in the model need to be calibrated against the past morphological evolution. The objective of the presented study can therefore be summarised as follows: The objective of this study is to develop ASMITA Venice, a version of ASMITA for use in Venice Lagoon, and to investigate the applicability of ASMITA Venice to Venice Lagoon. For that purpose the study integrates results and experience of the Padua group of Prof Di Silvio. It will focus on the deterioration of the salt marshes and the influence of the characteristic sea side area on the evolution of the lagoon. For future management the model takes into account the influences of human intervention, i.e. diversion of rivers, construction of jetties and artificial protection of the salt marshes. 1 3

20 October, 2004 Z 2839 Long Term Morphological Modelling of Venice Lagoon 1.4 Structure of this study In obtaining these objectives this study follows the structure presented in Figure 1 2: Information tidal inlet systems Chapter 2 Information Venice Lagoon Chapter 3 Information model Chapter 4 Analysis Dutch Wadden Sea ASMITA A B Chapter 5 Venice Lagoon Di Silvio model Development Extending and Adjusting ASMITA Chapter 6 Implementation Simulation Venice Lagoon Chapter 7 Figure 1 2: Schematisation of the current research. First, the report gives some general information. Chapter 2 gives information about tidal inlet systems in general. To get familiar with the area to be modelled, chapter 3 summarises the geographical and general characteristics of Venice Lagoon, the natural and human activity and the present morphological situation. To model the morphological evolution of Venice Lagoon, chapter 4 introduces ASMITA in its present formulation used for the Dutch Wadden Sea. After this general information the study focuses on adjusting and applying ASMITA to Venice Lagoon. Starting point of this part of the study is the existence of two models, namely the application of ASMITA to the Dutch Wadden Sea (Stive et al 1996, van Goor 2001 and Kragtwijk 2001) and Di Silvio s model on Venice Lagoon (Di Silvio 1991 and Di Silvio 1999) (Figure 1 2). Using these two existing systems, knowledge on how to apply 1 4

21 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, 2004 ASMITA to Venice Lagoon is gained by comparing the two areas and by comparing the two mathematical models. In chapter 5, relations A and B are discussed (Figure 1 2). The first compares the Dutch Wadden Sea with Venice Lagoon. The second makes a comparison between the Di Silvio model and ASMITA. Based on these relations section 5.4 sums up the points which need to be studied before applying ASMITA to Venice Lagoon. In chapter 6, adjustments to the mathematical model are presented. The result is the model ASMITA Venice optimised to simulate the evolution of Venice Lagoon. In chapter 7, the adjusted model is applied to Venice Lagoon. The model input is described as well as the results of the calibration. Chapter 8 presents the conclusions of this research and some recommendations for future research on this field. 1 5

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23 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, Inter Tidal Areas This chapter describes the tidal inlet systems in general. Tidal inlets are water systems connected to the sea through a relatively small inlet, compared to the total basin area. A rough schematisation of a tidal inlet on macro scale can be given by considering three different areas: The outside area, region on the sea side of the inlet and the adjacent coast, The gorge, the passage between sea and basin, The tidal basin, the inside area consisting of channels and shallow areas. Outside area Gorge Tidal Basin (a) (b) Figure 2 1: (a) overview of a tidal inlet system [van Goor, 2001], (b) schematisation of a tidal inlet system These different areas in a tidal inlet are described in the following sections. Section 2.1 describes the outside area, section 2.2 discusses the tidal gorge and section 2.3 presents information on the tidal basin area. Finally section 2.4 gives a general classification of tidal inlet systems. 2 1

24 October, 2004 Z 2839 Long Term Morphological Modelling of Venice Lagoon 2.1 Outside area The outside area of a tidal inlet consists of the coastline near the inlet and the ebb tidal delta. With the ebb tidal delta, the system of shallow areas at the sea side of the tidal gorge is defined. The existence of an ebb tidal delta and the evolution of the adjacent coast depend on the balance of sediment transport in the area. This balance is primarily influenced by the following three aspects: wave action, tidal currents and outflow of rivers. Wave patterns are complex due to the variable occurrence of the waves and processes near the coast, as refraction, diffraction and reflection. Averaging the wave field, two effects are important for the sediment balance: wave generated long shore current and sediment suspension due to waves. The long shore current transports sediment towards or away from the outside area. When shallow areas are present in front of the gorge the current will follow the outside boundary of the banks (Figure 2 2). Besides generating a long shore current, waves have an effect on the sediment concentration in the water. As waves increase the suspension of bottom material, the sediment concentration in the water increases. Tidal currents are strongest through the gorge, where the cross section is relatively small. During flood they bring sediment into the basin and during ebb they transport sediment towards the sea. When the sediment settles in front of the gorge, this contributes to the forming of the ebb tidal delta. Wave field outside area Barrier island Barrier island Inlet throat Wave induced Tide induced Figure 2 2: Schematic diagram of the outer delta residual sediment transport. Besides wave and tidal action a third aspect, the sediment import by rivers, is important for the forming of the outside area. Rivers flowing out into the nearby sea increase the concentration of sediment in the seawater. This sediment can be brought to the system by wave and current action. Rivers that flow out into the basin bring sediment into the basin area. The river water will flow through the tidal gorge into the outer area transporting sediment in this direction. This sediment can contribute to the forming of the outer delta area. 2 2

25 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, Tidal gorge A tidal basin is described as a water body only connected with the sea through a relatively small passage. This passage is called gorge or inlet. It is the passage for tidal currents generated by sea level changes and water coming from possible rivers discharging into the basin. These water flows transport sediment, which can lead to sedimentation or erosion of the tidal basin. Sediment transported towards the sea settles in front of the inlet and form the ebb tidal delta. Sediment transported into the basin travels through the channel system and settles when the current velocity is too low to transport the sediment particles. These imported particles may form shallow areas inside the basin. The dimensions of the gorge, width and depth, depend on the balance between the forces which induce sediment suspension and the resistance of the bed against these forces. The resistance of the bed depends on the soil structure and particle characteristics. The forces on the sediment are generated by the tidal current through the gorge and to a small degree by waves. These factors are described by empirical relations, resulting in a relation between the cross sectional area of the gorge and the tidal prism [Eysink, 1991]. 2.3 Tidal basin The tidal basin is the water body connected to the sea through a gorge, it consists of the areas constantly under water and the regions under influence of tidal action. Considering the tidal basin, distinction is made on the characteristics of the areas, looking at morphological and hydrological features. As an indication the following characteristic areas for a tidal basin are summarised: Channels, the deeper area of the basin defined on their function of transporting water and sediment through the tidal basin. The channel system is a branching network of gullies, starting at the inlet as a wide deep channel and splitting to form many small trenches. Shoals, areas shallower than the channels. They have less transport capabilities than the channel because the local currents are lower. They consist of banks most of the time below water level. Flats, defined as the area between mean low water and mean high water. They consist of different banks, under water with high tides and dry with low tides. Marshes, characterized as areas covered by thick vegetation. Most of the time they are above the water level, except during the highest tides. 2 3

26 October, 2004 Z 2839 Long Term Morphological Modelling of Venice Lagoon Marshes Shoals Flats Bottom tidal basin Channels Tidal range MHW MLW Figure 2 3: Characteristic areas inside a basin. More distinction is possible, but for this study, only the above mentioned terms are of interest. As stated above, the tidal basin exchanges sediment with the sea. The sediment balance of the tidal basin can be either negative or positive, resulting in different evolutions of the tidal basin. Roughly stated, the evolution of a tidal basin can lead to three possible results: If the sediment balance is positive a tidal basin tends to silt up and become part of the surrounding land. If export and import of solid materials into the basin are in balance, the basin is considered in equilibrium. If the sediment balance is negative the tidal basin transforms into a part of the sea, a bay. 2.4 Hydrodynamic classification of tidal inlets There are many forms and sizes of tidal inlets in the world. Effort is made to characterise these areas. Characterisation is helpful when comparing different areas, as is done in this study by comparing the Dutch Wadden Sea with Venice Lagoon. Studying characteristics of a tidal inlet, classifications can be made on basis of the hydrological systems, which play a role in the morphological evolution of the area, specifically the tidal action and the wave intensity. Following a classification made by Hayes (1979), the following groups are formulated: Concerning the average tidal range: Micro tidal: tidal range < 1.0 meter Low meso tidal: tidal range meter High meso tidal: tidal range meter Low macro tidal: tidal range meter High macro tidal: tidal range > 5.5 meter Concerning the mean significant wave height at deep water, H s : Low wave energy H s < 0.6 meter Medium wave energy 0.6 meter <= H s <= 1.5 meter High wave energy H s > 1.5 meter 2 4

27 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, 2004 Combining these criteria s, Hayes uses five different classes to characterize a tidal inlet. 1. Wave dominated inlets 2. Mixed energy wave dominant 3. Mixed energy tide dominant 4. Tide dominated, low 5. Tide dominated, high Friesche Zeegat Venice Lagoon Figure 2 4: Hydrodynamic classification of tidal inlets Figure 2 4 indicates the five classes, depending on the wave action and tidal intensity. For this study, Venice Lagoon and the Wadden Sea are of interest. In Venice, the mean tidal range equals 0.6 meter and the average wave height in front of the inlets equals about 0.5 meter. Following the classification by Hayes, the Lagoon of Venice is characterized by a mixed energy, wave dominant tidal inlet. The Dutch Wadden Sea inlets fall in the mixed energy, tide dominant category. As an example, the Friesche Zeegat is mentioned, which has an average wave height of 0.7 meter and a mean tidal range of 2.3 meter. 2 5

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29 Long-Term Morphological Modelling of Venice Lagoon Developing ASMITA-Venice Z Venice Lagoon 3.1 Introduction October, 2004 In this chapter an insight is given into the region that is analysed. The area under attention is the Lagoon of Venice. By studying the history of the lagoon, we obtain insight into the particular characteristics of the region. This increases the predictability of future developments and helps simulating the morphological evolution with ASMITA. The geographical characteristics of the region of Venice Lagoon will be discussed in section 3.2, the description of the Adriatic Sea in section 3.2.1, the coastline in section and the lagoon area in section The external forcing factors of the Venice region are treated in section 3.3, including waves, wind, tidal wave and relative sea level rise. Some general characteristics of the lagoon are described in section 3.4. Section 3.5 looks at the historical evolution of the lagoon, in which human interventions played a crucial role. The present morphological situation of Venice Lagoon is discussed in section 3.6. Italy Adriatic Sea N N (a) (b) Figure 3-1: (a) Satellite image Venice Lagoon, (b) Geographical location Italy and Venice Lagoon. 3-1

30 October, 2004 Z 2839 Long Term Morphological Modelling of Venice Lagoon 3.2 Geographical characteristics The Adriatic Sea Venice Lagoon is situated at the northern Adriatic coast of Italy. The Adriatic Sea is about 1000 km long and on average 200 km wide. The northern part of the Adriatic Sea is quite shallow with water depths generally less than 50 meters with large areas of depths less than 30 meters. The Adriatic Sea has a particular counter clockwise current circulation. Warmer saltier waters enter through the passage of Otranto, coming from the Mediterranean Sea and go north along the west coast of the Adriatic Sea. Then, the current turns south along the Italian coast, where it joins the great input of fresh water from the river Po. A broad current system is thus formed in the Adriatic Sea, as illustrated in Figure 3 2a. At the coast near Venice Lagoon the circulation of water is illustrated in Figure 3 2b, the actual current system is more complex due to the existence of local eddies. Besides these local variations, the current system is influenced by the tides, the Bora wind (from the northeast) and the Scirocco wind (from the south, south east). N N Italy Adriatic Sea (a) Figure 3 2: (a) Current at the Adriatic Sea, (b) Current in front of the Venice coast. (b) The lagoon coastline The coastline is made up of strips of land that separate Venice Lagoon from the Adriatic Sea. With a length of 60 km, it includes from north to south, the coastal strips of Jesolo, Cavallino, Lido, Pellestrina, Sottomarina and Isola Verde, as illustrated in Figure 3 3a. 3 2

31 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, 2004 (a) (b) Figure 3 3: Coastline of Venice Lagoon. (a) location coastal strips, (b) groynes at the Pellestrina coastline. The coastline has changed over the years under the influence of currents, waves and human intervention. As a result of these influences, the Venetian coast originally looked different than it does today. Before man occupied the coastal territory there was a system of dunes with vegetation and extensive beach areas. The morphology of the coast was a result of erosive forces from the Adriatic Sea and the supply of sediment by the rivers. With the constructions of dams across the rivers and the building of the northern jetties at the intakes of the lagoon, the supply of sand to the Venice coastline reduced. This has increased the erosion of the barrier islands. To counteract the erosion of the coastal barrier, different projects were executed which form the identity of the coast today. A coastline formed by a sea defence wall, Murazzi, and different groynes, perpendicular to the coast which trap the sediment transported along the coastline. Figure 3 3b shows these groynes along the Pellestrina coastline Venice Lagoon The Lagoon of Venice is situated at the northeast coast of Italy. The total surface of the lagoon area is 540 square kilometres. Inside the brackish lagoon, the city of Venice is situated. Venice Lido inlet Malamocco inlet Chioggia inlet Figure 3 4: Location of the three inlets. 3 3

32 October, 2004 Z 2839 Long Term Morphological Modelling of Venice Lagoon The lagoon is connected to the Adriatic Sea by three different inlets (Figure 3 4). From south to north: The Chioggia inlet: this is geographically fixed by jetties at both sides of the inlet and has a width of 400 meters (Figure 3 5a). The Malamocco inlet: this has a width of 400 meters and the construction of jetties at both sides of the inlet allows modern ships access to the port (Figure 3 5b). The Lido inlet: this is geographically fixed by jetties at both sides of the inlet and has a width of 800 meters (Figure 3 5c). (b) (c) (d) Figure 3 5: (a) View Chioggia inlet, (b) View Malamocco inlet, (c) View Lido inlet. The major elements in which the lagoon can be subdivided are the islands, fish farms, and the tidal area. Descriptions of these areas are given below: Islands The islands inside the lagoon cover an area of 44 square km, being 8 percent of the total surface area. The origin of the island is either natural or artificial. Natural islands are remains of dunes or areas created by the depositing and accumulating of solid materials transported by the rivers, which emptied or still flow into the area. The artificial islands, mostly originate from the 19 th century, when man created habitation and industrial areas. Fish farms Fish farms are closed wet areas created by man for the purpose of fish breeding and sometimes hunting. The most farms are located in the North and West part of Venice Lagoon and take up a total surface of 92 square kilometres. Fish farms are separated from the lagoon by banks that prevents water exchange with the lagoon. Cut off from the tidal action, structures are present which regulate the water circulation inside the farms. Tidal area The tidal areas of the lagoon, with a total surface of 420 square kilometres, include channels, shoals, mud flats and salt marshes. Channels are the deepest parts of the Lagoon. They can be men made or naturally formed. Natural channels form a branched system that starts at the inlets and spreads over the water area into smaller creeks. The channel system transports water and sediment through the basin. Men made channels are constructed to accommodate large seagoing vessels. Shoals are areas that are mostly under water. It is the area of lagoon between the particular channel system and the shallower mud flats. 3 4

33 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, 2004 Mud flats are areas that are normally under water, emerging only during particular tidal conditions as is the case with low tides during spring tide cycles. They have no vegetation and are soft muddy areas. Erosion or sedimentation occurs by tidal currents. These currents transport sediment, brought in motion by wave or current friction at the bed surface. Salt marshes are vegetated areas, which are usually above water and are only submerged during the highest tides. They are largely located in the northern and western part of the lagoon and are important because of their role in regulating the lagoon hydrodynamics. Salt marshes hinder water exchange and lessen the action of wave motion. Ecologically they are important as they give home to a wide variety of vegetation and bird live. Salt marshes are covered by a thick growth of plants, mostly halophytic species. These plants give a great support to the soil structure. 3.3 Forcing factors Waves Waves play a role in two regions of the tidal inlet, namely the outside area and the basin area. Both wave activities are discussed below, starting with the wave on the coast and ebb tidal delta. Waves are generated by wind blowing over the water surface. Due to different windintensity and wind direction, the wave pattern at the coast is not uniform. When looking at sediment transport, three aspects of waves travelling towards the coast are important. Described are the aspects: breaking waves, long shore current and wave setup. Waves travelling towards the coast will break at the point where the depth is approximately two times the wave height. The zone between the coast and the point of wave breaking is called the breaker zone. Breaking waves induce turbulence and thus friction at the bottom, leading to movement of sediment. The sediment concentration inside the breaker zone is relatively high due to the wave action. When the waves approach the coast at an angle, a long shore current will be generated inside the breaker zone. Waves that approach the coast at an angle, generate a shear stress in the direction parallel to the coast, this friction force induces a water flow along the shoreline. Along with this water current, sediment can be transported. Wave setup occurs inside the breaker zone where breaking waves cause a net shoreward mass transport. The gradient in the waterline, leads to the creation of an undertow along the bottom towards the sea. The undertow can carry sediment in seaward direction. 3 5

34 October, 2004 Z 2839 Long Term Morphological Modelling of Venice Lagoon Breaking wave Wave Breaker line (a) Sea bottom Turbulence (b) Long shore current Coast Wave setup Breaker line Coast Undertow Sea bottom Sea (c) Figure 3 6: Sediment transport forces of waves travelling towards the coast, breaking waves (a), long shore current (b), wave setup (c). Waves inside the basin are considered to play an important role in the eroding of the shallow areas of Venice Lagoon. Analysing this wave action two different groups can be recognised, waves generated by ship activities and waves generated by wind. Ship activity on the lagoon concerns the access to the harbour of Venice and the Marghera petrolchemical centre as well as ship activity for fishery and recreation purposes. The waves that have the largest influence on shallow areas originate from the smaller ships, as they travel closer to the shore and generate higher waves. Wind waves are generated by the shear stress on the water surface due to wind action. The intensity of these waves depends on the wind speed and the water length on which the wind is active, the fetch. The Lagoon of Venice deals primarily with strong winds coming from the northeast (Bora) and blowing over the total length of the lagoon and wind from the southeast (Scirocco) with a smaller fetch. As mentioned, the waves inside the basin area are considered to play an important role in the morphological evolution of Venice Lagoon. The waves are generated in the deeper parts of the lagoon and travel towards the shallow marshes and flats. When a depth of approximately twice the wave height is reached, the wave breaks due to friction with the bottom. Breaking waves cause turbulent activities at the bottom, evolving in sediment movement at the bottom. With the stirring up of solid material, the waves account for higher sediment concentrations in the water. When horizontal water flows from the shallow to the deeper parts, this sediment movement will result in erosion of the shallower areas. An indication of the height of wind waves on the lagoon is given using the formulas of Bretschneider (appendices C). The wave intensity in front of Venice Lagoon coastline is based on wind data measured between 1988 and Appendices D give the offshore wave climate. 3 6

35 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, Wind Wind above the lagoon area generates shear stress at the water surface, which causes wind waves and drift currents. The effect of wind waves is described in chapter Drift current is a water flow in the upper layer of the water volume, enforcing a return current opposite to the wind direction. Drift currents, which flow towards an obstacle, like the coast, cause a slope in the water surface, wind set up. This slope generates a horizontal pressure difference in the water. This leads to a second flow near the bottom, opposite to the drift current. Finally, there is a current near the water surface flowing in the wind direction and a return flow near the bottom opposing the wind direction. Wind set up Drift current Return current Wind Figure 3 7: Currents in the lagoon, induced by wind. In a tidal basin a complex geometry with channels and shallow banks are present. As the return current will choose the way of low resistance, it will concentrate in the relatively deep areas. The drift currents will be active on the shallow areas and in this way contribute to the sediment transport in these areas. The wind intensity in front of Venice Lagoon coastline is derived from wind data measured between 1988 and Appendices E give the offshore wind climate Tides The tidal wave generated by gravitational forces, induces two different water movements. One movement in vertical direction, vertical tide, and one in horizontal direction, horizontal tide. Vertical tides refer to the variation of the water level when a tidal wave is passing. Besides the astronomical factors, which influence the vertical tides, meteorological factors play a role in the Adriatic Sea. The pressure and the wind influence the tides at the northeast coast of Italy. The situation of low pressure and the Scirocco and Bora winds accentuate the high tides, causing the northern Adriatic Sea to swell up. Conversely, high air pressure and winds from the north west can cause the water in the Northern Adriatic to drop. Tidal waves are no constant phenomenon. When studying the tidal data an average could be constructed. As estimation the tidal wave at the Adriatic Sea in front of Venice Lagoon is semi diurnal. Vertical variation of the water level at the coastline of Venice Lagoon is derived from the Admiralty Tide Tables (Table 3 1). The tidal range is expressed in meters as the difference between average high and average low tide. The Consorzio Venezia Nuova presents the value of 0.6 metres as yearly average tidal range at the three inlets. 3 7

36 October, 2004 Z 2839 Long Term Morphological Modelling of Venice Lagoon Place Height with respect to MSL, in meters. High Water Low Water Mean Springs Mean Neaps Mean Springs Mean Neaps Chioggia Malamocco Venezia Table 3 1: Tidal levels with respect to MSL [Admiralty Tide Tables, 1987] Horizontal tide is the current resulting from the vertical variation of the water level. These currents depend on the driving force generated by the vertical tide and the local conditions, friction and storage capacity. The horizontal tides are an important factor for the sediment transport in the tidal inlet. The water difference between the Adriatic Sea and the tidal basin generates a current inside the gorge and the channels. This flow can transport sediment in suspension and due to the friction on the bottom, it can bring bed materials in movement. Horizontal tide is also active in the coastal area. The tidal wave propagates in the Adriatic Sea in anticlockwise circular direction. This generates a current in front of the lagoon coastal strip from north to south. This current is one of the factors that accounts for the sediment transport along the coast. Through flow measurements, values could be given for the longitudinal current generated by the tides. In the current study, no value is taken for this flow as the phenomenon is incorporated into the model formulas. Important for the modelling of a tidal inlet is the amount of water exchange between the sea and the basin. This water transports sediment and thus influences the morphological evolution of the area. As stated above, the vertical tidal variation induces a current through the gorge. The volume of water that is transported during one tidal period is called the tidal prism. For smaller basins, the spatial variation of water level over the basin is assumed to be zero, in this way the tidal prism equals the total water volume between mean low water and mean high water Relative Sea level rise Relative sea level rise is the sum of two different effects: subsidence of the soil and sea level rise. In the last century, the city Venice dropped 23 centimetres compared to the Adriatic Sea level. The relative rise of water level is considered as one of the main problems of Venice Lagoon. Subsidence can be split into two factors; one has a natural origin and one results of human interference in the lagoon. The average natural rate of lowering of the lagoon bottom and its hinterland is 0.4 mm per year. This is due to consolidation of the soil. The man induced factor is made up of exploitation of underground liquid resources for industrial uses, starting in the 1930 s. The tapping of the underground water supply caused a reduction in pressure in the subsoil, which resulted in a contraction of the ground, 6 mm per 3 8

37 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, 2004 year. After 1970, the exploitation of water resources in Venice Lagoon area was prohibited and the result was a rapid increase of groundwater pressure, which lead to a small decrease of subsidence. Eustatism is the variation in sea level, induced by the changes in world s climate. During the last century, the eustastic rise for Venice Lagoon, independent of its subsidence, averaged 13 centimetres. Studying the effect of relative sea level rise on tidal inlets, it can be stated that the bathymetry of the tidal basin follows the water level variation. This can be explained by the slight depth increase of the system, which causes a slight deceleration of the current in the tidal basin. This reduced flood stream has a smaller transport capacity for sediment and thus results in the settlement of sediment inside the basin. The reduction of ebb tidal currents lowers the capacity to bring sediment into suspension and the settled sediment will not be transported out of the system. This process is called the sediment retention mechanism of a deepened basin. Considering the tidal inlet in a dynamic equilibrium, it compensates for the slight relative sea level rise by increasing the bottom level. Accelerated rising leads to an increase of sediment needed to maintain the same water depth. Changing of the system to this new constant water depth will require some time in which the average depth of the tidal basin increases, see Figure 3 8. Figure 3 8: Tidal inlet system adapting to accelerated relative sea level rise and regaining a new constant water depth. [Van Goor, 2001] If the supply of sediment from outside the system cannot compensate the rising of sea level, the system s morphological state will deviate increasingly from its old configuration and evolves to a new situation in which the depth of the system is highly increased. A boundary is reached, referred to as SLR limit, above which the system is incapable of compensating the changing sea level (van Goor, 2001). The tidal basin will gradually and persistently lag behind the rise of sea level and constantly increase depth. 3 9

38 October, 2004 Z 2839 Long Term Morphological Modelling of Venice Lagoon Figure 3 9: Tidal inlet system unable to adapt to accelerated sea level rise. [Van Goor, 2001] To assess the future evolution of subsidence and eustatism, in Venice Lagoon area, the following scenario by the Consorzio Venezia Nuova as the most realistic is further used. It state that the tendencies recorded in the last century are transferred to the next century, without the subsidence due to industrial groundwater tapping. This results in a relative sea level rise of sixteen to twenty centimetres over the next century. 3.4 General characteristics Tidal divides in Venice Lagoon The tidal wave enters Venice Lagoon through the three inlets, Lido, Malamocco and Chioggia thus three waves propagate through the lagoon area. Where two waves meet each other, a tidal divide can be recognized. Here the current velocities are very low and ridges arise. Tidal divides are not totally fixed and due to the fluctuation of tidal currents it is hard to predict their exact location. The Lagoon of Venice has three tidal inlets, all resulting in a tidal wave propagate into the basin area. This introduces two tidal divides separating the three basins, Lido, Malamocco and Chioggia. Based on a model by Carzon, 2003 the result is given in Figure Although there is more research published on this subject, in this report a straight line will represent the tidal divide draw in Figure The locations of these lines are expressed by the coordinates of the start and endings point. The coordinate system used is originating from a well known system used in Venice Lagoon. 3 10

39 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, 2004 Tidal divide starting point ending point North South axis East West axis North South axis East West axis Lido Malamocco Malamocco Chioggia Table 3 2: Coordinates tidal divides in Venice Lagoon. Lido basin ( , ) Malamocco basin ( , ) ( , ) Chioggia basin ( , ) Figure 3 10: Lagoon of Venice separated into three basins by tidal divides Sediment Besides the forcing factors described in section 3.3 the characteristics of the sediment is an important aspect for the morphological evolution of the lagoon. This section will give a general description of the sediment characteristics. Before the diversion of rivers emptying in the lagoon, the discharge of river water was accompanied by the import of sediment into the basin. This sediment originated from the inland and consisted of a mixture of sand, silt and clay particles. As the flow velocity dropped when entering the lagoon, most sediment settled inside the lagoon. The present shallow areas inside the lagoon are formed in this period consisting of a high percentage of clay. Today, the import of sediment from the rivers is decreased to a level that can be neglected when considering the volumes of sediment moving inside the lagoon. The exchange with the sea is the dominant factor in sediment import and export. This exchange occurs through the inlets that existed between the jetties build in the 19 th century. Result of this construction is 3 11

40 October, 2004 Z 2839 Long Term Morphological Modelling of Venice Lagoon the decrease of sediment import from the sea. Especially the larger sand particles are trapped before entering the lagoon. The small amount of sediment that enters the lagoon consists of fine sand, silt and clay particles. The dominant part of the bed material inside the lagoon is silt or silty clay. As only the larger particles settle at high flow velocities, they generally deposit in the regions close to the inlets. The smaller particles, silt and especially clay settle further inwards of the lagoon, settling on the shallower areas. This process gives the general sorting of the bed material in which sand and silt is situated in the channels close to the inlets and silt and clay particles form the bed surface of shallow and inner areas Vegetation The erosion of the lagoon bottom is effected by the growth of plants. Vegetation protects the soil from eroding by strengthening the soil with the root structure and reducing the local erosive forces on the bottom. Besides, high contents of organic material, related to the presence of vegetation, increase the soil cohesiveness. A short description of the location and species of vegetation is given in this section. The salt marshes, areas that are only submerged during the highest tides, are covered by a thick growth of plants. The salty soil is inhospitable for most plant types but ideal for halophytic species. In the central area of the lagoon some deeper areas until a water depth of about one meter are covered by four different species of eelgrass. In areas with larger water depths, the eelgrass as well as larger algae has disappeared due to the shadowing effect by increasing densities of micro algae caused by eutrophication. The eutrophication, caused by human intervention as pollution and river diversion, has its effect on all the flora inside the lagoon. At the deeper parts, eelgrass has disappeared and in the shallower parts green algae have replaced the former eelgrass communities. The result is a much more unstable situation on the bottom surface, increasing the erodability of these areas. The evolution of the salt marshes from the year 1901 on till 1987 is described in Figure

41 Long-Term Morphological Modelling of Venice Lagoon Developing ASMITA-Venice Z 2839 October, Salt marshes Figure 3-11: Evolution of the salt marshes. 3.5 History of human intervention The human interventions in the Lagoon area during the last 800 years are mentioned as an important factor for the changing characteristics of Venice Lagoon. Result of these activities is a tidal lagoon, which is losing a great deal of sediment to the Adriatic Sea. The coastal islands, which separates the lagoon from the sea, are also under influence of erosive currents. A description of the human activities of the last 700 years is given below: 14th century: At the beginning of the 14th century, little intervention in the natural evolution of Venice Lagoon had occurred. The human activity around the lagoon was high as the city of Venice was build and the lagoon was an important source for fishery. At that time the lagoon was connected to the Adriatic Sea with five unstable gorges. Numerous rivers emptied in the lagoon, including the Brenta, Bacchiglione, Sile, and Piave. 3-13

42 October, 2004 Z 2839 Long Term Morphological Modelling of Venice Lagoon During this century, it was clear that the lagoon was silting up with sediment. The material transported into the lagoon by the rivers, resulted in two problems. First, the growth of swamp area and the low salinity in the lagoon endangered the public health due to the malaria mosquitoes in these areas. Secondly, the sedimentation of the lagoon endangered the safety of the city of Venice as the city relied on the protection of the surrounding water. Work began to divert the river Brenta to discharge into the sea instead of in the lagoon. This project finished at the beginning of the 16 th century. Brenta Sile N Bacchiglione Piave Venice Figure 3 12: Venice Lagoon at the beginning of the 14 th century. [ 15 th century: During the 15 th century the digging of the Canale Maggiore started, this moved the mouth of the Brenta River further south towards the port of Malamocco. Brenta N Figure 3 13: Venice Lagoon at the end of the 15 th century. [ 3 14

43 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, th century: In this century, the project started to divert the river Piave. From the second half of the 16 th century until the year 1726, the Santo Spirito Channel was constructed, to allow ship access from the Malamocco inlet to the city of Venice. The mouths of the Brenta (via Brenta Nova) and Bacchiglione rivers were diverted to the Adriatic Sea, to prevent them from depositing sediment into the lagoon area. Brenta N Brenta Nova Bacchiglione Piave S. Spirito Figure 3 14: Venice Lagoon at the end of the 16 th century. [ 17 th century: In this period, the Novissimo Cut was constructed. This channel connected the Muson with the port of Brondolo. At the end of the 17th century, the Sile Cut was made, redirecting the river Sile to the old riverbed of the Piave. In the meantime, the diversion of the Piave to the port of Santa Margherita was dredged. Diverting the river towards the port involved the forming of a big lake and the opening of a natural passage, which emptied out, into the sea around the area of Cortellazzo. Muson Sile N Novissimo Piave Sile Brondolo Figure 3 15: Venice Lagoon at the end of the 17 th century. [ 3 15

44 October, 2004 Z 2839 Long Term Morphological Modelling of Venice Lagoon 18 th century: In the 18 th century the building of the Murazzi walls started. The walls were constructed along the coastal barrier to prevent the coast from eroding. The preservation of the barrier islands is important for the protection of the lagoon and the city Venice. To insure the navigability of the lagoon inlets, more channels were constructed including the Rocchetta channel that leads to the port of Malamocco. N Port Malamocco Figure 3 16: Venice Lagoon at the end of the 18 th century. [ 19 th century: Different projects were initiated in this period, to enhance the port and commercial capacity of Venice. The railway bridge and a new commercial port including infrastructure was built. To insure the stability of the inlets, jetties at the Malamocco inlet were constructed. N bridge Malamocco Lido Figure 3 17: Venice Lagoon at the end of the 19 th century. [ 3 16

45 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, th century: In the first half of this century, the construction of the outer jetties at the Lido inlet was completed. This included the combining of the northern inlets into one inlet with jetties at both sides. The stability of the Chioggia inlet was secured by the construction of two jetties. The first industrial area in Marghera was built and a canal was dug connecting it directly to the Lido inlet. Parallel to the railway bridge a road bridge was build, this allowed Venice to be accessed by car. Fish farms were created in the north and south of the lagoon, this meant the closure of the lagoon for a total surface of 8600 hectare. From 1930 until 1970, the industrial zone near the lagoon exploited ground water. In the second half of the 20 th century, 500 hectare of the lagoon was reclaimed to accommodate the second industrial area. The construction of a third industrial area, 1000 hectare, has never been finished. A channel for Oil tankers was dug between Marghera and the Malamocco inlet. The building of the Venice airport was done by reclaiming the Tessera salt marsh area. N Marghera Malamocco Lido Chioggia Figure 3 18: Venice Lagoon at the end of the 20 th century. [ 3 17

46 October, 2004 Z 2839 Long Term Morphological Modelling of Venice Lagoon 3.6 Present Morphological situation At present, Venice Lagoon loses sediment to the Adriatic Sea. The loss of solid materials together with a reduction of the sediment input from rivers, results in an erosion of the shallow areas in the tidal basin. The material lost on the shallow areas flows towards the deeper parts of the lagoon, which causes sedimentation of the channel system. Another effect of the deepening of the shallow areas is the increase of wave force on the marshes. As the waves do not lose energy due to friction with the shallow bottom area the erosive forces by the wave s increases on the marshes. Because of these processes, a situation is present in which the salt marshes are reducing (Figure 3 11), the shoals are becoming deeper and the channels are in sedimentation (Figure 3 19). Besides the levelling of the depth in the lagoon basin, the coastal strip is under influence of erosive processes. The reduction of sediment deposited in the coastal area and the blocking effect of the jetties are the cause of the disappearance of beaches and the deepening of the sea area in front of the Venice coastline. mainland lagoon sea PAST PRESENT Figure 3 19: Levelling of the lagoon bottom. The erosion of shallow areas in the lagoon, the sedimentation of the channels and the erosion of the coast are caused by a combination of the following factors: The diversion of the rivers has let to a reduction of the sediment input into the lagoon basin. These rivers deposited considerable quantities of sand and silt in the basin channels, which was partly transported to sea and partly deposited on the shallow areas in the lagoon. The diversion of these rivers directly to the sea lead to a reduction of the sediment input in the lagoon, but due to canalization and the construction of hydroelectric basins, also the sediment transport of the rivers decreased. In this way, less material is deposited in front of the coast, which causes the beaches to erode. The dredging of artificial canals and the maintenance dredging in the lagoon basin caused an imbalance in the hydraulic system. The extra volume of channel area reduces the velocity in natural channels and increases the velocity in shallow areas next to the artificial canals. 3 18

47 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, 2004 The construction of artificial islands and the closure of the fishing valleys reduced the water volume of the basin. This lead to the reduction of tidal volume and thereby the loss of velocity in the channels. The building of the jetties achieved its aim of deepening the lagoon inlets but it also influenced the currents in front of the coast. It altered the direction of the long shore current, which transported sediment to the beaches. As they reached into the deeper parts of the sea, they blocked the current close to the coast. By blocking the currents the transport of sediment to the coast is also blocked, which increased the erosion along the barrier islands, and accumulation of sand in the lee areas of the jetties, north side of the Lido Inlet and the south side of the Chioggia Inlet. Besides the influence on the existing long shore current, the jetties generated a complex new flow pattern. This lead to an increase in current velocity in front of the jetties, see Figure The effects of the jetties on the sediment balance in the lagoon area are the disappearance of the shallow area in front of the inlets, the tidal delta, an increase of the current at the adjacent coast and the blocking of sediment delivery to the beaches. Figure 3 20: Flow pattern at the Lido and Malamocco inlet due to construction of the jetties. Pollution in the lagoon influences the biological system. For sediment transport, this system is important because plants are a protection of the bottom. Plants reduce the current velocity at the bottom and strengthen the soil structure. The loss of plant material increased the erosion of shallower areas in the lagoon and islands, which used to be protected by sea grass. The increase of ship activities in the lagoon has its influence on the morphological evolution of the lagoon. Due to the increase of waves generated by ships, the forcing on the salt marshes and shallow areas increases. This leads to the erosion of shallow areas and erosion of the edges of the salt marshes. The effects of relative sea level rise are extensively described earlier in this report (section 3.3.4). 3 19

48

49 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, ASMITA 4.1 Introduction This chapter looks at the zero dimensional model ASMITA, used in this study as a basis for predicting the long term morphological evolution of Venice Lagoon. ASMITA (Aggregated Scale Morphological Interaction between a Tidal inlet and the Adjacent coast, Stive et al, 1996) has been developed to calculate the developments of tidal basins and its adjacent coastal environment under the influence of external forcing factors. Beside the application of general sediment transport equations and balance equations, the model is based on the assumption that the equilibrium state of each distinguished element of the system can be defined. Morphological changes are modelled by the non linear dynamic interaction between these elements. ASMITA can be considered as an aggregation and extension of the earlier model ESTMORF (Wang et al, 1996 and Fokkink et al. 1996). Aggregation concerns the fact that the system elements are characterized by only one state variable describing the morphological situation. The extension concerns the incorporation of formulations for the ebb tidal delta and directly adjacent coast as well, without modifying the basic concepts. The structure of this chapter is based on the computational procedures followed in ASMITA, Figure 4 1. The area of interest is separated in different elements characterised by one variable, the volume of that element. A morphological equilibrium volume can be calculated per element based on empirical relations. Based on the difference in the actual volume of an element and its equilibrium volume, an equilibrium sediment concentration is calculated. This equilibrium sediment concentration represents the cubic meters of sediment present in one cubic meter of water, in the equilibrium situation. The exchange of water, with a certain sediment concentration, between two elements, induces morphological change of the elements. These procedures are defined over a specified time step. Schematization area Morphological equilibrium Time step Equilibrium concentration Morphological change Figure 4 1: Computational procedure ASMITA. 4 1

50 October, 2004 Z 2839 Long Term Morphological Modelling of Venice Lagoon The structure of this chapter is as follows: schematization of the area is described in section 4.2. In section 4.3 the morphological equilibrium relations, which are the basis for this model, are discussed. The definition of equilibrium concentrations in ASMITA are handled in section 4.4. The process of morphological changes in the elements, occurring when sediment is transported, is described in section 4.5. Finally, section 4.6 deals with the input parameters necessary to run the model. 4.2 Schematisation of the area Schematization of the model is based on hydrological and morphological characteristics of a tidal inlet system. Based on these criteria, tidal inlets are generally described consisting of the following units: a channel area inside the basin, a shallow area inside the basin and an ebb tidal delta in the sea area. The spatial scale on which these elements are introduced in ASMITA is defined by the interpretation of the ebb tidal delta. The total delta area is interpreted as a single element, this is necessary due to the lack of knowledge in the behaviour of an ebb tidal delta. This leads to a definition of the remaining elements in the same spatial scale. A tidal inlet system in ASMITA consists of the following units (Figure 4 2): Ebb tidal delta: represented by a sand volume which is different then the one that would be present in this area when there is no tidal gorge, Channel : represented by the water volume between MLW and the bed surface, Flat: represented by the sediment inside the lagoon between MLW and MHW. It is possible to introduce a fourth and fifth element, which schematise the adjacent coast: Coast south : represented by the water volume below MSL an above a certain depth line, d cs, Coast north : represented by the water volume below MSL an above a certain depth line, d cn. outside world delta Sea coast coast channels Land flats Lagoon Figure 4 2: Schematisation of a tidal inlet in ASMITA 4 2

51 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, Morphological equilibrium An important aspect of the model ASMITA is the assumption that there exists a morphological equilibrium for each element defined in the system. This assumption is based on empirical relations, which are developed between the morphological state of an element and parameters representing the external forcing. The morphological balance is described by the dry or wet volume of an element. A great deal of research is done on the behaviour of tidal inlets. Reference is made to O Brien (1931, 1967), Jarret (1976) and Walton and Adams (1976), who gave the basics for these relations. Adaptation for the Dutch deltas was made by Gerritsen and De Jong (1985) and Gerritsen (1990). Broad literature surveys of these empirical relations for both Dutch and foreign inlets are given by for instance Biegel (1991), Van Kleef (1991) and Eysink and Biegel (1992). The empirical relations that are used in ASMITA are based on these studies, showing a relation between the element volumes depending on the tidal prism, tidal range and basin area. V e = f( P, H, A ) b (4.1) where: V e = equilibrium volume of the element [m 3 ] P = tidal prism through the gorge [m 3 ] H = tidal range at the gorge [m] A b = surface of the tidal basin [m 2 ] The tidal prism, P, represents the volume of water between the MLW and MHW. Assuming a relative short basin compared to the tidal wave length, the tidal prism is written as the tidal range, H, times the total basin surface, A b, minus the volume of sediment between MLW and MHW, V f : P H Ab Vf = - (4.2) where: P = tidal prism through the gorge [m 3 ] H = tidal range [m] A b = surface tidal basin [m 2 ] V f = sediment volume between MLW and MHW, equal to the flat volume [m 3 ] For the elements in ASMITA, the following empirical relations are derived from literature: Ebb tidal delta Dean and Walton (1975) developed a calculation method for the volume of the ebb tidal delta that appears to be very suitable for comparisons between inlets. The volume of the 4 3

52 October, 2004 Z 2839 Long Term Morphological Modelling of Venice Lagoon delta is described by the solid material, present in the sea area in front of the tidal gorge. This material would not be present if the gorge had not existed and the coast was represented by an extension of the barrier islands. Referring to Figure 4 3, the volume of the ebb tidal delta is equal to the (net) volume of sand above a reference coastline. This coastline equals the coastline, which would have been present without an inlet. Sea Beach Sea Beach Ebb tidal delta Reference coastline Reference coastline (a) Figure 4 3: Cross section of delta, (a) coast without an inlet used as reference coast, (b) volume ebb tidal delta. (b) Walton and Adams (1976) concluded that a strong relation exists between the volume of sand stored in the ebb delta of an inlet and the tidal prism. They further demonstrated that with increasing onshore directed wave energy, the outer delta volume tends to become smaller. The empirical relation between the sand volume in the delta area and the tidal prism is presented as follows: V de = a P d 1.23 (4.3) where: V de = sediment volume of the element Delta in an equilibrium situation [m 3 ] P = tidal prism through the gorge [m 3 ] d = coefficient depending on the local wave conditions, see Table 4 1 [m 1.23 ] H wave Wave Climate d [m 1.23 ] 1 Low energy coast 8.6* Moderate energy coast 6.4* High energy coast 5.3*10 3 Table 4 1: Empirical coefficient delta element. [Bijsterbosch 2003] Channels The channel volume is the amount of water between MLW and the bed surface of the lagoon (Figure 4 4). The volume of the channel in the equilibrium situation is assumed equal to the powered tidal prism times a coefficient. Eysink and Biegel (1992) presented the following empirical relation: V ce = a P c 1.55 (4.4) 4 4

53 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, 2004 where: V ce = water volume of the element Channels in an equilibrium situation [m 3 ] P = tidal prism through the gorge [m 3 ] c = empirical coefficient, c = for the Dutch Wadden Sea [m 1.55 ] Flats The volume of material between MLW and MHW in the lagoon represents the morphological equilibrium state of the element Flat (Figure 4 4). The flat volume is considered to depend on the tidal range and the basin area times a coefficient. The following empirical relation is presented by Eysink (1990): V = a H A (4.5) fe f b where: V fe = sediment volume of the element Flats in an equilibrium situation [m 3 ] f = empirical coefficient [ ] H = tidal range [m] A b = surface of the tidal basin, equal to surface flats + surface channels [m 2 ] This formula is based on research done by Renger and Partensky, who found a relation for the German Bight between the flat surface and the total basin surface: A A f b = A (4.6) b where: A f = surface element Flats [m 2 ] Eysink (1991) derived an empirical relation between the average flat height from MLW and the tidal range. Stating that the coefficient depends on the total basin surface: H f = a H fe (4.7) where: H f = average height of the element Flats [m] fe = A b [ ] The volume of the tidal flats is defined as the product of area and height above MSL: V = a A Ê ˆ HA Ë f f fe Á b Ab (4.8) 4 5

54 October, 2004 Z 2839 Long Term Morphological Modelling of Venice Lagoon In ASMITA, a constant average surface is taken for the tidal basin and the flat area. This means that the relation (4.6) is not taken into account and that the equilibrium relation is used as stated above, in equation (4.5), with: a - ( ) ( ) A A 0.5 = - - f b b (water) (water) (sediment) Figure 4 4: Cross section tidal basin, representing the flat and channel volume. 4.4 Equilibrium sediment concentration Per element a sediment concentration is defined which can represent the equilibrium of this unit at each time interval. The sediment concentration is the cubic meters of sediment present in one cubic meter of water. Two different sediment concentrations are defined: Local equilibrium sediment concentration, c e : the concentration of sediment in one element when the element is in equilibrium. Global sediment concentration, c E : the concentration of sediment at the outside boundary of the model, thus the concentration in the outside world (Figure 4 2). The local equilibrium sediment concentration is defined in a way that it equals the global sediment concentration when the system is in morphological equilibrium (c e = c E ). If the local equilibrium concentration is smaller than the global concentration, a tendency exist to accrete (c e < c E ). A tendency to erosion exists when the local equilibrium concentration is larger than the global concentration (c e > c E ). The value of the local equilibrium sediment concentration depends on the difference in volume between the actual element volume and the equilibrium volume of this element, as stated in the empirical relations. The relation between these volumes should represent the morphological behaviour described above. To represent this, a power relation is used when calculating the local equilibrium sediment concentration for each element. For an element X the following formula is given: 4 6

55 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, 2004 c xe c ÊV ˆ xe = EÁ Vx Ë N (4.9) where: c xe = local equilibrium sediment concentration of the element [ ] c E = global sediment concentration [ ] V x = the actual sediment volume of the element [m 3 ] V xe = sediment volume of the element in an equilibrium situation [m 3 ] N x = coefficient [ ] In the areas in which ASMITA is used the coefficient N is generally set on the value two till four. This power corresponds with a third till fifth power for the sediment transport as a nonlinear function of the mean flow velocity: Reference is made to appendices B in which an introduction to transport formulas is given. In this section follows the formulas for the local equilibrium sediment concentration for each element: Ebb tidal delta The morphological situation of a tidal delta is described by the volume of sand present in this area. A difference between the local equilibrium volume (V de ) and the current volume (V d ) of the element represents a tendency towards erosion when V de < V c and towards sedimentation when V de > V c. The local equilibrium concentration for the tidal delta is written as follows: c de N d ÊV ˆ d = ceá Vde Ë (4.10) where: c de = local equilibrium sediment concentration of the element Delta [ ] c E = global sediment concentration [ ] V d = the actual sediment volume of the element Delta [m 3 ] V de = sediment volume of the element Delta in an equilibrium situation [m 3 ] N d = coefficient [ ] Channels The volume of the channel element, representing the morphological situation, is defined as a wet volume. This means that a tendency exists towards erosion when the local equilibrium volume is larger than the current volume (V ce > V c ) and a tendency towards sedimentation when the equilibrium volume is smaller than the current volume of the element (V ce < V c ). The equilibrium relation is now written as follows: c ce c ÊV ce = EÁ Vc Ë ˆ N c (4.11) 4 7

56 October, 2004 Z 2839 Long Term Morphological Modelling of Venice Lagoon where: c ce = local equilibrium sediment concentration of the channel [ ] c E = global equilibrium concentration [ ] V c = the actual water volume of the element Channels [m 3 ] V ce = water volume of the element Channels in an equilibrium situation [m 3 ] N c = coefficient [ ] Flat The volumes reflecting the flat element are considered as sediment volumes. The relation between the local equilibrium volume and the current volume follows the same behaviour as the tidal delta. The local equilibrium concentration of the flat element is written as follows: c fe c ÊV ˆ f = E Á V fe Ë N f (4.12) where: c fe = local equilibrium sediment concentration of the element Flats [ ] c E = overall equilibrium sediment concentration [ ] V f = the actual sediment volume of the element Flats [m 3 ] V fe = sediment volume of the element Flats in an equilibrium situation [m 3 ] N f = coefficient [ ] 4.5 Morphological changes Morphological changes in the element are represented by two processes. In the first place, the exchange between two elements or the exchange between an element and the outside world, in the second place, by the average settlement of sediment inside the element called the vertical exchange. Both processes are shortly described. The sediment exchange between the elements and the outside world is based on the difference in local concentrations and the value of residual transport capacity between the elements. In a formula this is written as follows: ( ) S = d c - c (4.13) where: S 23 = sediment exchange between element 2 and element 3 [m 3 /s ] 23 = diffusion coefficient between element 2 and element 3 [m 3 /s]. c 2 = local sediment concentration element 2 [ ] c 3 = local sediment concentration element 3 [ ] 4 8

57 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, 2004 The settlement of sediment inside the element is based on the difference between the actual concentration and the local equilibrium concentration. When the present concentration in the element is larger than the concentration would be in an equilibrium situation (c 2 > c 2e ), sedimentation in the element will occur. When the present concentration is smaller, erosion will occur (c 2 < c 2e ). The speed in which this difference contributes to the volume change depends on the total area where the exchange with the bottom takes place (A 2 ) and a coefficient representing the amount of vertical interaction (w s2 ). In formula the settlement of sediment is written as follows: ( ) S = w A c - c (4.14) 2 s2 2 2e 2 where: S 2 = sediment exchange between the wet and dry area of element 2 [m 3 /s] w s2 = vertical exchange coefficient of element 2 [m/s] A 2 = surface of element 2 [m 2 ] c 2 = local sediment concentration element 2 [ ] c 2e = local equilibrium sediment concentration element 2 [ ] These sediment exchange formulas can be computed for each element in ASMITA. As seen in Figure 4 5, the mass balance for the elements delta, channel and flat can be computed. The balances are written down in such a way that a positive value represents an increase of volume, both water volumes for channels as well as sand volume for delta and flat: fc (c c c f ) dc (c c c d ) do (c E c d ) w sf A f (c f c fe ) V c w sc A c (c c c ce ) w sd A d (c d c de ) V f V d Flats Channel Delta Sea Dry volume Wet volume Figure 4 5: sediment balance between the elements Delta The mass balance for the delta element is formulated as follows: ( ) d ( ) d ( ) w A c - c = c - c + c - c (4.15) sd d d de dc c d do E d where: w sd = vertical exchange coefficient element Delta [m/s] 4 9

58 October, 2004 Z 2839 Long Term Morphological Modelling of Venice Lagoon A d = surface element Delta [m 2 ] c ce = local equilibrium sediment concentration element Delta [ ] c d = sediment concentration element Delta [ ] c c = sediment concentration element Channel [ ] c E = overall equilibrium sediment concentration [ ] c d = sediment concentration element Delta [ ] dc = diffusion coefficient between element Delta and element Channel [m 3 /s] = diffusion coefficient between element Delta and outside world [m 3 /s] do Channel The mass balance for the channel element is formulated as follows: ( ) d ( ) d ( ) w A c - c = c - c + c - c (4.16) sc c ce c fc c f dc c d where: w sc = vertical exchange coefficient of element Channel [m/s] A c = surface element Channel [m 2 ] c ce = local equilibrium sediment concentration element Channel [ ] c c = sediment concentration element Channel [ ] c f = sediment concentration element Flat [ ] c d = sediment concentration element Delta [ ] fc = diffusion coefficient between element Flat and element Channel [m 3 /s] = diffusion coefficient between element Flat and element Channel [m 3 /s] dc Flat The mass balance for the flat element is formulated as follows: ( ) d ( ) w A c - c = c - c (4.17) sf f f fe fc c f where: w sf = vertical exchange coefficient of element Flat [m/s] A f = surface element Flat [m 2 ] c fe = local equilibrium sediment concentration element Flat [ ] c f = sediment concentration element Flat [ ] c c = sediment concentration element Channel [ ] fc = Diffusion coefficient between element Flat and element Channel [m 3 /s] Morphological changes of the elements can now be calculated by the use of the above formulas. These calculations are done per time interval, defining the actual concentration and calculating the difference between the local equilibrium concentration and the actual concentration. By applying the vertical exchange coefficient and the total surface of the element, a volume variation can be calculated for this time step. In a formula the volume change per time step is given as follows: 4 10

59 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, 2004 dv dt 2 ( ) =± w A c - c (4.18) s2 2 2e 2 where: V 2 = volume of element 2 [m 3 ] w s2 = vertical exchange coefficient of element 2 [m/s] A 2 = surface element 2 [m 2 ] c 2e = local equilibrium sediment concentration element 2 [ ] c 2 = sediment concentration element 2 [ ] ± = negative for sand volume and positive for wet volume Dredging, dumping and sea level rise It is possible to take dredging activities, dumping activities and the relative sea level rise into account in ASMITA. The dredging activities are represented by a removal of sand volume [ I ], positive for water volume variation and negative for sand volume variation. dv dt 2 ( ) =± w A c - c ± (4.19) s2 2 2e 2 I where: I = dredging volume [m 3 /s] ± = negative for sand volume and positive for wet volume Note that dumping (adding of sediment in the element) is the opposite of dredging (removal of sediment in the element). Dumping can be taken into account by introducing a negative dredging value (I). Relative sea level rise is represented by a volume change equal to the depth increase times the total surface. This rise is considered positive for water volumes as stated in the channel element and negative for sand volumes as stated in the delta and flat elements. dv dt 2 dz =± ws2a2( c2e - c2) ± I ± A2 (4.20) dt where: = relative sea level rise [m] ± = negative for sand volume and positive for wet volume For each element in ASMITA, formula (4.20) can be written as follows: Delta dv dt d dz =-wsd Ad ( cde -cd) - Id -Ad (4.21) dt 4 11

60 October, 2004 Z 2839 Long Term Morphological Modelling of Venice Lagoon Channels dv dt c dz =+ wscac( cce -cc) + I c + Ac (4.22) dt Flats dv dt f dz =-wsf Af ( cfe -cf ) - I f -Af (4.23) dt 4 12

61 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, Input parameters To run the model, initial parameter values have to be inserted. These parameters either are derived from field data and earlier studies or should be calibrated in the model. The values ascribed to these variables are given in section 7.2, current chapter will give a description of the parameters. Geometric parameters The model needs the initial surface, volume and concentration for each element. The surface and initial volume are generally generated from field data. The surface is kept constant in the ASMITA model and thus the initial value will be used during the total simulation. The model is not sensitive for the initial concentration, an indication of this value is satisfying. Global sediment concentration An important parameter in the model is the global sediment concentration. This variable represents the long term averaged availability of sediment to the tidal inlet system. The sediment concentration is considered constant. In this way, the value of this parameter presents the concentration in the total system when the global equilibrium situation is present. The sediment concentration is scaled in a way that it gives information about cubic metres bottom per cubic metres water instead of kilograms sediment per cubic metres water: c E c sed = (4.24) - r ( 1 e p ) where: c E = overall equilibrium concentration [ ] c sed = concentration sediment in water [kg/m 3 ] = density of sediment [kg/m 3 ] p = porosity of bottom layer [ ] Defining the value of the global sediment concentration, we should look at the sediment concentration in the sea area of the tidal inlet. This concentration is generated by the forces acting in the coastal area. Analyzing the influence of these forces, an indication can be given of the concentration in this area. For the Dutch Wadden Sea, this study resulted in a global sediment concentration of m 2 /m 2 to m 2 /m 2, assuming that the concentration was generated by wave induced transport and currents only [van Goor, 2001]. 4 13

62 October, 2004 Z 2839 Long Term Morphological Modelling of Venice Lagoon Tidal range An average tidal range is introduced into the model. This value is constant during the simulation and represents the average vertical tidal variation in front of the gorge. The tidal range is given in meters, which is the difference between mean low water and mean high water. Tidal wave is variable per tidal cycle, an average value for this wave action is thereby based on historical observations. It represents not only the astronomical tidal variation but includes all the aspects that influence the vertical variation of water at the tidal gorge (for example wind setup). Equilibrium relations The equilibrium relations for the different elements are presented in chapter 4.3. With these relations, coefficients are introduced into the model. For each element, one coefficient is introduced: for the delta element d, for the channel element c and for the flat element f. The values of these coefficients come from empirical studies of the tidal inlets system under examination. When no studies are present, the coefficients should be derived on historical data of the simulation area. The values used in the Dutch Wadden Sea are given in Table 4 2. Location c [m 1.55 ] d [m 1.23 ] f [ ] Dutch Wadden Sea Table 4 2: Equilibrium values for the Dutch Wadden Sea, Eysink 1990 respectively Stive N parameter This parameter appears in the formulas for calculating the local equilibrium sediment concentration (see equations: (4.10), (4.11) and (4.12)) The N parameter is based on the power commonly used in sediment transport equations. Based on the velocity powered by 3 to 5, in the non linear relation between sediment transport and mean water velocity, the N parameter used in ASMITA is set on 2 to 4. Diffusion coefficient The diffusion coefficient represents the residual exchange capacity between the elements mutually and the outside world. This exchange is a result of a non zero residual transport. At the scale of aggregation considered in ASMITA this exchange is described as a diffusive phenomenon. Considering the three elements, the model incorporates three diffusion coefficients. Namely the transport between the outside world and the delta ( od ), the transport between the delta and the channel ( dc ) and the transport between the channel and the flats ( cf ). 4 14

63 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, 2004 Vertical exchange coefficient The vertical exchange coefficient is defined per element and represents the net vertical exchange per unit per second between the dry and wet area of the element. The coefficient covers all kinds of processes involving erosion and sedimentation, and acts as the long term residual change of sediment amount in the element. Present in the model are three vertical exchange coefficients: w sd for the delta element, w sc for the channel element and w sf for the flat element. As the value is influenced by the forces on the bottom, the wave action will stir up sediment. In the delta area where wave action is relatively high, this results in a lower exchange coefficient for the flat area where wave action is calmer. At the other end, the sediment particles in the flat area are finer which results in a lower coefficient in the flat area. For the Dutch Wadden Sea the values are ranging from m/s to m/s [Buijsman, 1997 and van Goor, 2001]. Calculation period and time step The period over which the simulation needs to be run is an input parameter into the model, as well as the duration of the time step taken to calculate the volume variation. Dredging and Sea level rise Introducing dredging activities and sea level rise into the model, the following parameters are needed in the model: dredging volumes (I) per time step and relative sea level rise ( ) per time step (equation (4.20)). 4 15

64

65 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, Comparing two systems 5.1 Introduction The objective of this chapter is to obtain insight in the problems occurring when ASMITA is used on the Lagoon of Venice. To achieve the objective, it is necessary to gain knowledge of the area and the functioning of the model. By studying earlier reports in modelling Venice Lagoon and the application of ASMITA on different tidal inlet systems this knowledge is obtained. Two existing systems are used to gain knowledge of applying ASMITA and to gain insight in modelling Venice Lagoon (Figure 5 1): Earlier application of the model ASMITA on the Dutch Wadden Sea, Earlier application of the zero dimensional model of Di Silvio on Venice Lagoon. These two systems are used because they are both successful in simulating long term morphological behaviour of the different areas by applying two models that are comparable. Both models are zero dimensional and semi empirical. They split the tidal inlet system in different boxes defining a sediment concentration per box. Based on the difference of sediment concentration between the boxes a diffusive sediment transport is calculated. The exchange of sediment finally defines the morphological changes of the area. Reference is made to studies using the ASMITA concept in analysing the morphological behaviour of the Wadden Sea, Stive et al (1996), van Goor (2001) and Kragtwijk (2001). The construction and application of the Di Silvio model is described in Di Silvio (1991) and Di Silvio (1999). With the knowledge that the model ASMITA is suitable for the Dutch Wadden Sea and the model of Di Silvio is suitable for Venice Lagoon, two comparisons are made: A comparison between the tidal inlet systems of the Dutch Wadden Sea and the tidal inlet systems of Venice Lagoon (Figure 5 1, relation A), A comparison between the morphological models ASMITA and the Di Silvio model (Figure 5 1, relation B). 5 1

66 October, 2004 Z 2839 Long Term Morphological Modelling of Venice Lagoon Dutch Wadden Sea A Venice Lagoon ASMITA B Di Silvio model Extending and Adjusting ASMITA Figure 5 1: Structure of chapter 5. Analysing these relations contributes to the objective of this study. It will provide insight in the different characteristics of a tidal area similar to the Dutch Wadden Sea and a tidal area similar to the Lagoon of Venice. Subsequently analysing the Di Silvio model in relation to ASMITA will give insight in the differences between a model designed to simulating the Lagoon of Venice and ASMITA, suited for the Dutch Wadden Sea. These differences help to adjust and extent ASMITA into a model which can predict the morphological evolution of the Venice Lagoon. Section 5.2 compares the two simulation areas, the Dutch Wadden Sea and the Venice Lagoon. Section 5.3 compares the two models, ASMITA and the Di Silvio model. A conclusion of these comparisons regarding the points that need attention to adjust and extent ASMITA are given in section Comparing simulation areas In this section, the similarities and differences between the Dutch Wadden Sea and Venice Lagoon are analysed (Table 5 1). Only those points that are considered important in modelling the tidal inlets are mentioned. Similarities between the two areas: The inflow of river water is small compared to the tidal volume. Both areas exchange water with the surrounding by tidal action through the gorge and fresh water discharge from the rivers. When comparing the amount of water transported into the basin, in both situations the input of river water can be neglected. Both tidal inlet systems exchange water with the adjacent sea. The exchange occurs through tidal inlets connecting the basin with the sea. When more then one inlet is present the water 5 2

67 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, 2004 exchange during ebb and the exchange during flood in not necessary the same. The assumption made in this case is that the tidal basin can be separated in different areas, each connected with the sea by only one tidal inlet. Both the areas Dutch Wadden Sea and Venice Lagoon have more than one tidal inlet, and in both areas the above assumption is made. In both the tidal inlet systems a channel system is present. Formed by the ebb and flood currents a branched channel system covers the basin area. It starts at the tidal inlet and covers the total basin area. Through these channels the main transport of water and sediment is situated. Both areas have a protected coastline. The Dutch and Italian coast are protected against erosion by the construction of groynes. These groynes block the erosive forces, current and waves. The presence of these constructions influences the sediment transport in the tidal inlet system. Differences between the two areas: The intensity of the tidal action. The tidal range in Venice has an average of 0.6 meters where the Dutch Wadden Sea has an average tidal range of approximately 2 meters. The result is a lower tidal volume and velocity for the Lagoon of Venice, leading to a decrease of sediment transport capacity of the flow. The effect of the tide on the morphology of Venice Lagoon is thus smaller. This leads to a relatively stronger influence of other forcing factors as wind, waves, and human interference. The presence of areas characterized as marshes. These regions are covered by thick vegetation and are flooded only during high tides. In Venice Lagoon these areas are considered more important as the total surface of marshes is much larger compared to the Dutch Wadden Sea. In the past the Dutch Wadden Sea was covered with large vegetated areas. In the thirties, the vegetation disappeared probably due to diseases. Today only small areas are present by the island Terschelling and at places near the coast of Groningen. In Venice Lagoon, about ten percent of the area is covered by salt marshes (year 2000). This percentage is decreasing rapidly, generating concern for the ecological value of the lagoon. There is a difference of the sea side area of the tidal inlets. At the sea side of the Dutch Wadden Sea inlets, a characteristic delta area is present. This is represented by the shallow banks in the North sea in front of the inlets. The general idea of the delta is that it acts as a sediment buffer between the basin and the sea. At the sea side of Venice Lagoon inlets relatively small sediment banks are present. They are located at the southern end of the inlets, and can be considered as very small deltas. The inlets at Venice Lagoon are strongly fixed due to solid structures. The inlets at the Dutch Wadden area are free. When analysing the bottom material inside the two areas, conclusion is a difference in particle size on the bed surface. The particles at the Dutch bottom consist mostly of sand 5 3

68 October, 2004 Z 2839 Long Term Morphological Modelling of Venice Lagoon with a diameter of 125 to 500 m. The bottom of Venice Lagoon consist mostly of silt mixed with sand or clay, silt has a diameter of 4 to 63 m. The adjacent coast of both the tidal inlet systems accommodate groynes to protect the beaches, difference is the existence of large jetties at the coast of Venice Lagoon. Result is a difference in suspended sediment between the areas of the Dutch Wadden Sea coast and Venice Lagoon coast. In the Adriatic sea the concentration of suspended solids near Venice Lagoon coast is considered about ten times smaller than the concentration in front of the Dutch Wadden Sea inlets. The particles of this sediment are smaller in Italy, consisting of fine sand whereas the sediment at the Dutch Wadden Sea has a high percentage of coarse sand. Finally Table 5 1 sums up some values for both the Dutch Wadden Sea, represented by the two inlets Zoutkamperlaag and Marsdiep, and for Venice Lagoon represented by the three inlets Lido, Malamocco and Chioggia: Comparison tidal basins Forcing Wadden Sea, Zoutkamper laag Wadden Sea, Marsdiep Venice Lagoon Lido [data 1992] Venice Lagoon Malamocco [data 1992] Venice Lagoon Chioggia [data 1992] Sea level rise and subsidence [m/century] Average wave intensity sea side [m] Tidal range [m] Tidal period about 12 hours about 12 hours Tidal Prism [m 3 ] 2* * * * *10 8 Geological aspects Total basin surface, wet area [km 2 ] Average depth [m] Water volume below 1.5* * * * *10 8 MLW [m 3 ] Volume delta [m 3 ] 1.2* *10 8 Very small Surface delta [m 3 ] 0.8* *10 8 Very small Surface between MHW and MLW [m 2 ] Surface below MLW [m 2 ] Fresh water input [m 3 /s] Other aspects 78* * * * * * * * * *

69 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, 2004 Bottom material mostly sand silt with some sand and clay Plant growth negligible see grass at 1 m below MSL, high growth starting at 0.15 to 0.3 m+msl gorge natural inlet natural inlet location fixed by jetties location fixed by jetties location fixed by jetties Classification by Hayes Mixed energy tide dominant Mixed energy wave dominant Table 5 1: Comparison between the Dutch Wadden Sea and Venice Lagoon. Conclusion From comparing the areas, Dutch Wadden Sea and Venice Lagoon some conclusions can be drawn regarding the extension and adjustment to ASMITA: The characteristic marshes area in Venice Lagoon should be introduced into the ASMITA model as they play an important role in the total sediment displacement in the tidal inlet system. Thereby the erosion of the marshes is an important subject for the ecological value of the Lagoon. Attention should be given to the decrease of the dominance of tidal action in the inlet system. Different sediment transport systems become more important, mentioned is the effect of waves inside the lagoon area. The presence of jetties at the sea side area of the inlet leads to a new schematisation and definition of the elements in this region. 5.3 Comparing morphological models This section will give a comparison between the two mathematical models, ASMITA (chapter 4) and the Di Silvio model (appendices A). Both models are semi empirical and zero dimensional, designed to simulate long term morphological evolution of a tidal inlet. Similarities between the two models A tidal inlet can be schematized into elements, based on their hydrological and morphological characteristics. For each element, an equilibrium situation can be formulated, represented by one variable characteristic. In both models the tidal inlet will be in a state of morphological equilibrium when the global sediment concentration is present in all the elements. During one simulation, the global sediment concentration is considered constant and equal to the sediment concentration at the sea side boundary of the model. 5 5

70 October, 2004 Z 2839 Long Term Morphological Modelling of Venice Lagoon Sediment exchange between the elements is present based on the difference between the sediment concentration in each element. The capacity of sediment exchange depends on diffusive transport between the elements. The tidal basin is considered a closed water system, only connected to the sea through a relatively small inlet. This means that the input of rivers or precipitation is neglected. Both models consider tidal divides to separate the water volumes, in case there are more connections to the sea. An average tide is formulated, which represents the vertical fluctuation of the sea level. Differences between the two models The element definition in the model is interpreted differently. As the Di Silvio model is based on Venice Lagoon, the considered elements are matching the characteristics of this area. This leads to defining three different areas inside the basin and an outside boundary condition directly at the sea side of the gorge. ASMITA is considering only two elements inside the basin area and is developed to take the ebb tidal delta and adjacent coast into account. The boundary condition is set at the sea side where the banks of the ebb tidal delta ends. The elements defined are as follows: Di Silvio: Marshes = above 0.15+MSL to 0.3+MSL meters, Shallow area = between marshes and 2 meters below MSL, Channel = between 2 meters below MSL and the bottom. ASMITA: Flat = between MLW and MHW, Channel = between MLW and bottom, Delta = above a fictive sea bottom, Coast = between MSL and a certain depth. In the ASMITA model, the exchange between the elements is considered to be in series connection. Assuming sediment fluxes between the elements Flat and Channel, Channel and Delta and between Delta and the outside world. The Di Silvio model considers a parallelconnected system. This system defines an exchange between marshes and shallow areas, shallow areas and channel, marshes and channel and between channel and the outside world. 5 6

71 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, 2004 Channel Flats Channel Shallow area Marshes Delta Coast Sea Sea (a) (b) Figure 5 2: Sediment exchange between elements, (a) ASMITA three element model, (b) Di Silvio model Difference in the definition of the element equilibrium situation. Both models define a local equilibrium sediment concentration, this is the concentration of suspended solid material in each element. The two models describe this variable through different formulas and assumptions. These formulas are defined as follows: Di Silvio model Channel Venice: c, = f ( flow, h ) (5.1) channel e channel Shallow areas Venice: c shallow e, = f ( wind, h ) (5.2) shallow Marshes Venice: c marshes, e = 0 (5.3) ASMITA Channel: c, = f( c, V, P) (5.4) channel e boundary Flat: c, = f( c, V, H, A ) (5.5) flat e boundary flat basin Delta: c, = f( c, V, P) (5.6) delta e boundary delta 5 7

72 October, 2004 Z 2839 Long Term Morphological Modelling of Venice Lagoon Comparing sediment concentration in shallow area: For the sediment concentration in the shallow areas of the tidal basin, a comparison can be made between the equilibrium concentration formula used in the Di Silvio model and the derived sediment transport formula for the ASMITA model: Di Silvio, Venice Lagoon (Appendices A): c s : f h wind s (5.7) where: c s = local equilibrium sediment concentration of the element Shallow area [ ] f wind = coefficient [m] = average depth of the element Shallow area [m] h s ASMITA, Dutch Wadden Sea (combine equation (4.12), (4.5) and (4.2)): c : b Q 2 f f tide (5.8) where: c f = local equilibrium sediment concentration of the element Flats [ ] f = coefficient [s 6 /m 6 ] Q tide = water volume through the inlet per tidal cycles [m 3 /s] We see that in the ASMITA formulation, the base for calculating the sediment concentration is the flow activity in the shallow areas (element Flat). In Venice, the wind is considered the important factor when constructing a formula for the concentration in this element. Therefore, the basin in the Di Silvio model for concentration calculation is set by a coefficient, which is dependent on wind activity. Comparing sediment concentration in deeper area: For the concentration in the deeper parts of the tidal basin (element Channel) the following comparison is made: Di Silvio, Venice Lagoon (Appendices A): c c : f current 5 hc (5.9) where: c c = local equilibrium sediment concentration of the element Channel [ ] f current = coefficient [m 5 ] h c = average depth of the element Channels [m] ASMITA, Dutch Wadden Sea (combine equation (4.11) and (4.4)): 5 8

73 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, 2004 c c Q : bc V 3.10 tide 2 c (5.10) where: c c = local equilibrium sediment concentration of the element Channel [ ] c = coefficient [m 3,3 /s 3,1 ] Q tide = water volume through the inlet per tidal cycles [m 3 /s] V c = sediment volume of the element Channel [m 3 ] Both formulas deal with the current as a driving factor for the value of sediment concentration, a difference is found in the extent in which the current effects the sediment concentration. Definition of the local sediment concentration is different in the two models: The Di Silvio model formulates sediment concentrations based on formula (5.1) to (5.3). This sediment concentration is considered present in the element at that particular time step. ASMITA gives an equilibrium sediment concentration based on formula (5.4) to (5.6). The difference between this sediment concentration and the local sediment concentration represents the sediment demand of the element at that particular time step. The actual local sediment concentration is generated from the mass balance equation. Some values used in the models are given in Table 5 2. The values of ASMITA are used in a study on the Zoutkamperlaag, a basin in the Dutch Wadden Sea. The values of the Di Silvio model are used in a study at Venice Lagoon: ASMITA [Zoutkamperlaag] Di Silvio [Venice Lagoon] dimension outside sediment concentration before the jetties with the jetties tidal range 2,25 0,6 m horizontal exchange coefficient 10, outside delta 8, delta channel 6, channel flat Table 5 2: Values used in the morphological models. About equal to the water exchange of the element during one day. m 3 /day Conclusion From comparing the models, ASMITA and the Di Silvio model some conclusions can be drawn regarding the extension and adjustment to ASMITA: At the moment ASMITA lacks the ability to simulate areas which are characterised as marshes. The possibility of introducing these areas with thick plant growth will be handled in this study. Difference between the models is found in the equations used to define the areas. The reason is a difference in the characteristics of the areas and the forcing 5 9

74 October, 2004 Z 2839 Long Term Morphological Modelling of Venice Lagoon factors which play a dominant role in the sediment transport. Adjustments to the ASMITA equations in defining the equilibrium situation are necessary. Probably not only the value of the parameters should be calibrated but the structure of the equations should be changed. 5.4 Problems in applying ASMITA to Venice Lagoon The objective of this study is to apply the ASMITA model to Venice Lagoon. In the previous sections, a comparison is made between two tidal areas and the morphological models used to predict their evolution. Based on these results, the following points need attention when using ASMITA to predict the morphological evolution of Venice: The schematisation of the tidal basin used in ASMITA is not representative and has to be adjusted to the Lagoon of Venice. The difference is found in the boundaries between the characteristic elements and the way they interact. To account for the adjusted schematisation and the local characteristics the definition of the equations used to represent the elements has to be redefined. Equations under attention are the definition of the sediment concentrations and the formulation of the equilibrium situation. De sea side boundary conditions should be redefined to account for the different characteristics of Venice Lagoon. The accent lies on the presence of human build constructions. 5 10

75 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, Introduction This chapter deals with the problem of making ASMITA applicable to Venice Lagoon. As it is clear from chapter 5, there are differences between the characteristics of Venice Lagoon and the basis of ASMITA. To produce reliable predictions of the morphological evolution of the lagoon, adjustments to and extension of the present ASMITA (Stive et al, 1996) are necessary. After the comparison between the two areas Dutch Wadden Sea and Venice Lagoon (section 5.2), and the comparison between ASMITA and the model of Di Silvio (section 5.3), the issues of adapting ASMITA to Venice Lagoon can be reduced to three problem areas: the elements defined in the schematisation of the tidal inlet system, the definition of the equations for the different elements, The schematisation and definition of the sea side area. ASMITA makes a distinction between four different elements of the tidal inlet system. Inside the basin, it defines the element Flat, for the shallower parts, and the element Channel for the deeper parts of the basin. At the sea side of the tidal inlet ASMITA defines the element ebb tidal delta representing the volume of sand stored in the ebb tidal delta area, and the elements coast, representing the adjacent coast: Flat: the volume of sediment in the basin between MLW and MHW, Channel: the volume of water in the basin below MLW, Delta: the volume of sediment as far as it differs from that in an undisturbed, continuous coastline. Coast: the volume of water below MSL and above a certain depth boundary. Conform the problem areas mentioned above it is important to reconsider the element schematisation and definition made in ASMITA. There are two regions to be studied, the elements representing the sea side area of the inlet and the elements representing the characteristics of the basin itself. For both regions the schematisation into elements and the definition of equations is studied. Section 6.2 studies the basin area, where section focuses on the schematisation into elements and section focuses on the definition of the equilibrium concentrations. In section 6.3 the sea side area is discussed. Section describes the current situation of the sea side area, the schematisation is constructed in section and the definition of the elements is presented in section Finally, section 6.4 summarises ASMITA Venice. 6 1

76 October, 2004 Z 2839 Long Term Morphological Modelling of Venice Lagoon 6.2 Restructure the basin area Schematisation basin area In Venice, the basin is normally modelled with at least three elements. This is necessary because of the different characteristics inside the basin. To make a comparison with the Dutch Wadden Sea, the current element definition of ASMITA (Figure 6 1) is applied to the bathymetric data of Venice Lagoon. Channel Flats Outside area Tidal Volume Volume channel (water) (b) Volume Flat (sediment) (a) Figure 6 1: Schematisation of the current ASMITA model, (a) cross section, (b) top view. Applying these definitions, a surface percentage relation is given for the two elements: Flats and Channel: In Venice Lagoon the ratio of channel surface versus flats surface is 80/20, In the Dutch Wadden Sea this ratio is 30/70. This difference originates from a lesser tidal range in Venice Lagoon and a difference in the bottom level. In ASMITA the area below MLW (Mean Low Water), the channel system, is characterised as a system of gullies in which the transport of water is concentrated. In Venice Lagoon the water area below MLW, is relatively large, based on the percentage mentioned above. Studying this lagoon area it is clear that the characteristics used for an element Channel are no longer valid for this larger region. To comply with the Channel characteristics, a lower depth boundary for the element Channel is necessary. Therefore, the area under MLW is split in ASMITA when applied to the Lagoon of Venice. Two variations of the present ASMITA schematisation are discussed: The first schematisation is based on studies by Rinaldo (1999). A reasonable interpretation of the boundary depth between the channel element and the shallow areas element is the point where the hypsometric curve has the largest curvature. The following boundaries are derived from the hypsometric curves in (Figure 6 2): 6 2

77 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, 2004 Element Channel total basin: area under 2 MSL, Element Channel Lido basin: area under 1.5 MSL, Element Channel Malamocco basin: area under 2 MSL Element Channel Chioggia basin: area under 2 MSL Figure 6 2: Hypsometry of Venice Lagoon. The results are the following depth boundaries for the elements (Figure 6 3): Element Flat: area between MHW and MLW, Element Shallow areas: area between MLW and 2.0 MSL, Element Channel: area under 2.0 MSL. A flats A basin A shallow area A channels Channel Shallow area Flats 2 meter Outside area Volume Flats (sediment) (b) Volume Shallow area (sediment) Volume Channels (water) (a) Figure 6 3: Schematisation of ASMITA variation one, (a) cross section, (b) top view. 6 3

78 October, 2004 Z 2839 Long Term Morphological Modelling of Venice Lagoon The advantage of this subdivision is in maintaining the definition of the element Flat. In this way the volume of the flat area can still be used to calculate the tidal prism (equation (4.2)). The second schematisation is based on general interpretation of Venice Lagoon area used in numerous studies. This definition is based on three characteristic areas of Venice Lagoon basin: Marshes: areas with thick plant growth, under water only during high water spring and extreme high tides, Shallow area: areas which are dry during low water spring and extreme low tides, Channels: areas below the water line that is important for the water circulation. By using these definitions, we can take advantage of the knowledge and experience existing in Venice Lagoon. This is due to the fact that the separation of elements used in this schematisation is common in studies of Venice Lagoon. Moreover, the areas of interest in the lagoon, viz. the marshes, are then included in the model. The partition of these areas is still subject of research and differ between different studies. Separation in this study is based on the characteristics of the area and the research of the hypsometry described in this section (Figure 6 4): Marshes: area with thick plant growth, Shallow areas: area above 2 meters below MSL, excluding the marshes, Channels: area between 2 meters below MSL and the lagoon bottom. A marshes A basin A shallow area A channels Channel Shallow area Marshes 2 meter Outside area Volume Shallow area (sediment) (b) Volume Channels (water) (a) Figure 6 4: Schematisation of ASMITA variation two, (a) cross section, (b) top view. Note that the definition of elements by depth boundaries only, is tentative. For example, there are areas with sufficient depth to be considered as a channel volume, but which have a different function. These areas can be seen as underwater lakes and do not contribute to the circulation of water and sediment (for instance the area of Fondi dei Settemorti in the Malamocco basin). These areas are considered part of the element Shallow area rather than part of the element Channels based on their characteristics. As the elements are defined, the aspect of sediment exchange between these compartments is discussed. In ASMITA the elements are put in series, which results in an exchange 6 4

79 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, 2004 between flats and channel and between channels and the outside area. This follows from the geographical layout of the simulation area. In a tidal basin where only two elements can be identified, it is clear that exchange takes place between these two compartments and through the gorge. Representation of these sediment flows are schematised by introducing fluxes between the three elements (Figure 6 5 a). When adjusting ASMITA to Venice Lagoon, introducing a third element inside the basin area is suggested. With the introduction of this extra element, a horizontal exchange is introduced between the element Shallow area and the element Marshes. It is also possible to modulate an extra horizontal sediment exchange between the element Channel and the element Marshes. The Di Silvio model introduces such an exchange in the Venice Lagoon environment. This leads to the schematisation presented in Figure 6 5 b, where a non serial horizontal exchange system is introduced by the interaction between the elements Channel and Marshes. Channel Flats Channel Shallow area Marshes Sea side area Sea side area (a) (b) Figure 6 5: Sediment fluxes between the elements, (a) two elements in the basin, (b) three elements in the basin. This extra horizontal exchange introduced in Venice Lagoon, refers to the steep cliffs visible at the boundary of the marshes. These cliffs are almost vertical and about half a meter to one meter in height, they are caused by the wave action against the salt marsh area where the soil is cohesive. The definition of the elements Channel as an area with a certain depth boundary, prevent the introduction of a horizontal exchange between the elements Channel and Marshes when their height differ more than one meter in height. With the Channel defined as the area under two meters below mean sea level, the extra horizontal exchange is not introduced in the adjusted ASMITA formulation. Result of this all is a new schematisation of the ASMITA model referred to as ASMITA Venice. This schematisation fits best to the distinctive characteristic areas in Venice Lagoon. In this way the exchange of knowledge and results between different studies of the lagoon is possible. Disadvantage of this schematisation is the uncertainty in applying the empirical relations for defining the equilibrium situations. This aspect will be discussed in section

80 October, 2004 Z 2839 Long Term Morphological Modelling of Venice Lagoon Definition basin area Every element in ASMITA is represented by a local sediment concentration. Referring to section 6.2.1, the following elements are introduced in ASMITA: Marshes: area with thick plant growth, Shallow areas: area above 2 meters below MSL, excluding the marshes, Channels: area between 2 meters below MSL and the lagoon bottom. For each element, a local equilibrium concentration should be introduced. This section will discuss the formulation of the equilibrium concentration in the ASMITA Venice model. Local equilibrium sediment concentration ASMITA The equilibrium relations used in the ASMITA model are formulated earlier in the report (section 4.3). Empirical studies resulted in these relations, which formulate an equilibrium volume for the different elements Subsequently an equilibrium sediment concentration is defined based on the difference between the equilibrium volume and the actual volume. The principle of the ASMITA relations are as follows: Define an equilibrium volume for the element based on empirical relations (equations (4.3), (4.4), (4.5)). Define an equilibrium sediment concentration for the element, based on the relation between the equilibrium volume and the actual volume (equations (4.10), (4.11), (4.12)). Studying the possibilities in adjusting these formulas, we look at the variation of the coefficients in these relations. The equilibrium volume relations are variable in changing the value of the coefficients. The value of these coefficients can be found in analyzing the simulation area in a quasi equilibrium situation or use the value of earlier research in regions similar to the current simulation area. The equilibrium sediment concentration relations can change by variation of the N parameter. This value represents the transport capacity of a water flow. A higher value means a higher sediment transport capacity of the water flow. For application on the Dutch Wadden Sea, the power N is commonly taken as two. Sediment transport in Venice Lagoon is generally calculated with a transport formula that ascribes a higher power to the velocity. 6 6

81 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, 2004 Local equilibrium sediment concentration Di Silvio Di Silvio (2003) describes equilibrium relations for the Lagoon of Venice. These relations give a sediment concentration per element depending on the morphology of the element and the local forces that influence the sediment suspension. Formulas are defined in which the sediment concentration in the element Channel is a function of its depth and the tidal action. The sediment concentration in the element Shallow area is a function of its depth and local wind above the lagoon. The sediment concentration in the element Marshes is considered zero due to the protection of the bottom by plants growth, which prevents erosion. c c c c s m = = = 0 ( c, ) (, lokal wind) f h P f h s The equilibrium sediment concentrations for Venice Lagoon can be computed with the following formulas (Di Silvio, 2003): Channel: Representing the area between 2 MSL and the bottom: c ce = f current 5 hc (6.1) where: c ce = equilibrium sediment concentration element Channel [ ] f current = current coefficient [m 5 ] h c = average depth element Channel [m] Shallow areas: Representing the area above 2 MSL (excluding the Marshes): c se = f h wind s (6.2) where: c se = equilibrium sediment concentration element Shallow areas [ ] f wind = wind coefficient [m] = average depth element Shallow areas [m] h s Marshes: Representing area with thick plant growth: c me = 0 (6.3) where: c me = equilibrium sediment concentration element Marshes [ ] By applying the relations suggested by Di Silvio (2003), there will be a relation for each element between the equilibrium sediment concentration and the physical factors considered important inside this element. 6 7

82 October, 2004 Z 2839 Long Term Morphological Modelling of Venice Lagoon The relations are based on the same element schematisation as suggested in the ASMITA Venice schematisation. Therefore, there is no problem in defining these relations for the simulation area. Difficulties are found in implementing the relations in the ASMITA concept. If a comparison is made between the model ASMITA and the long term sediment transport model by Di Silvio, difference is found in the formulation of the local sediment concentration of the elements: In ASMITA, the local sediment concentration is formulated based on the sediment exchange between the elements and the interaction with the bottom. Sediment exchange between the elements depends on the sediment concentration difference between the elements concerned. Interaction with the bottom depends on the difference between the sediment concentration in the element and the equilibrium sediment concentration calculated with the equilibrium relations. In the model of Di Silvio, the local equilibrium sediment concentration is equal to the local sediment concentration and is formulated based on the sediment exchange between the elements. This exchange depends on the difference in sediment concentration between the elements concerned. The exchange with the bottom is equal to the sediment exchange with the elements. By using the relations of Di Silvio in ASMITA Venice, attention should be given to the meaning of the equilibrium sediment concentration in the ASMITA concept. If the relations of Di Silvio are used, there must still be a difference between the local sediment concentration and the local equilibrium sediment concentration. This difference is used in ASMITA to calculate the sediment settling in the element. To overcome this problem, the Di Silvio relations represent the equilibrium sediment concentration for each element when applied in ASMITA Venice. By doing this, the meaning of the Di Silvio relations changes. They no longer represent the sediment concentration in the element, but give a sediment concentration of the element when it is in equilibrium. In this way, the difference between the local sediment concentration and the local equilibrium sediment concentration in ASMITA Venice can still be used to define the vertical exchange inside each element. Final definition basin area This section formulates the final definition of the equilibrium situation for each element. Criteria s that influence the forming of the relations are as follows: The best representation of the actual average concentration in the area schematised by this element, The formulation that matches the best with the current ASMITA concept. Referring to section 6.2, the ASMITA Venice model includes the following elements: Channel: area between 2 meters below MSL and the lagoon bottom, Shallow areas: area above 2 meters below MSL, excluding the marshes, Marshes: area with thick plant growth, 6 8

83 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, 2004 Each of these elements should be represented by a local equilibrium sediment concentration. The current chapter defines an equilibrium sediment concentration for the different elements: Channel The characteristics of the new channel element differ not much from the area defined between MLW and the bottom in the current ASMITA schematisation. It is still the area that is responsible for the transport of water and sediment inside the lagoon. Based on this similarity the relation of the equilibrium volume in ASMITA is considered representative for the water volume between 2 MSL and the bottom. The equilibrium volume is than written as follows: V ce = a P c 1.55 (6.4) where: V ce = water volume of the element Channels in an equilibrium situation [m 3 ] P = tidal prism through the gorge [m 3 ] c = coefficient [m 1.55 ] In the relation between the concentration and the volume, the N parameter is introduced. The currents are the main force that induces sediment transportation through the channel element. This N parameter corresponds with the power for the sediment transport as a nonlinear function of the mean flow velocity. Based on assumptions that the sediment transport in Venice Lagoon is equivalent to the velocity powered by four to six, the value of three to five is used for the N parameter. The local equilibrium sediment concentration for the element Channel is then written as follows: c ce N c ÊV ˆ ce = ceá Vc Ë (6.5) where: c ce = local equilibrium sediment concentration of the element Channels [ ] c E = overall equilibrium concentration [ ] V c = the actual sediment volume of the element Channels [m 3 ] V ce = sediment volume of the element Channels in an equilibrium situation [m 3 ] N c = coefficient (3 to 5) [ ] Shallow areas We aim for a formulation in the form of the current ASMITA definitions, defining an equilibrium volume relation and a relation for the equilibrium concentration. First, the relation for the equilibrium volume is studied. Considering the existing empirical relations for the current ASMITA element Flat, a dependence of the volumes on the tidal currents is found: V f a P = a H Ab = (6.6) 1 - a 6 9

84 October, 2004 Z 2839 Long Term Morphological Modelling of Venice Lagoon The equilibrium situation in Venice Lagoon is considered related to the wind activity on the shallow areas, conform formula (6.2). The current generated by tidal activity, is assumed less important. Under this assumption, the equilibrium volume in the ASMITA model should be related to the wind induced forces in these areas. A coefficient shallow is introduced, representing the water depth above the shallow area surface, in a situation where equilibrium exists between the bottom morphology and the active forces. This coefficient is considered a function of the wind velocity, fetch length and resistance of the bottom to erosion: a = shallow f ( u, Fe,resistance bottom) wind (6.7) The equilibrium volume of the shallow areas is written as a function of the coefficient shallow. As the volume refers to a sediment volume and the shallow represents a water depth above the shallow area, the maximum depth of the shoals is introduced, h max,s. The height of the sediment in the shallow area is than written as the difference between the maximum height and the shallow. The formula for the equilibrium volume is written as follows: V = ( h -a ) A (6.8) se max, s shallow shallow where: V se h max,s shallow A shallow = sand volume of the element Shallow area, = maximum water depth of the shallow area, = coefficient depending on the local wind and bottom resistance, = surface of the element Shallow areas. The N s parameter, representing the power over the volume difference in the concentration relation, should be set considering the Di Silvio formulations. Waves are considered as the driving force behind the sediment movement from the bottom, water currents are less important. To express this effect the N s parameter is set on one. The local equilibrium sediment concentration, for the element Shallow areas is then written as follows: c se c ÊV s = EÁ Vse Ë ˆ 1 (6.9) where: c se = local equilibrium sediment concentration of the element Shallow area [ ] c E = overall equilibrium concentration [ ] V s = the actual sediment volume of the element Shallow area [m 3 ] V se = sediment volume of the element Shallow area in equilibrium [m 3 ] Marshes The element Marshes is unknown in the current ASMITA concept. This results in the formulation of equilibrium relations based on research on Venice Lagoon without backup from the current ASMITA model. These relations should represent the sedimentation process and erosion process of the element. These formulas are used in ASMITA Venice to calculate the sand volume change of the element per time step. 6 10

85 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, 2004 The marshes area consists mostly of clay covered by thick vegetation. Their boundaries are characterised by cliffs up to one meter in height. The function of marshes is generally considered as an element where sediment is trapped. When the area floods, sediment is transported into this region. When the water flows back, the plants prevent high velocity at the bottom and prevent sediment movement. This process results in the sedimentation of the marshes area. The loss of sediment from marshes occurs when a part of the area collapses into the neighbouring element. This takes place when current and especially waves attack the boundaries of the marshes. To introduce the element Marshes in ASMITA, a formulation of an equilibrium sediment concentration relation is considered. The presence of plants highly effects the sediment concentration in the element. As the root system strengthens the soil and the stalks protect against erosive forces, the plants prevent suspension of the sediment. Based on this knowledge, the sediment concentration in this element is considered zero. The local equilibrium sediment concentration is written as follows: c em = c = 0 (6.10) m where: c em = local equilibrium sediment concentration of the element Marshes [ ] The element Marshes exchange sediment with neighbouring elements based on difference in sediment concentration (section 4.5). When the sediment concentration in the element Marshes is considered zero, there will only be an import of sediment into the element. Assuming the vertical exchange infinite, results in a settlement on the bottom of all the imported sediment, representing the trapping effect of the marshes. The export of sediment can be related to the depth difference between the marshes and the neighbouring element. The larger this difference, the higher the wave force on the marshes, and the lower the stability of the edges. When the depth difference, ms is too high, the element Marshes will lose a certain amount of sediment to the neighbouring element, ms. This increases the depth of the neighbouring element and decreases the depth of the element Marshes. This can be written in formulas as follows: hs + hm D ms bms = constant h + h <D b = 0 s m ms ms (6.11) where: h s = average depth shallow areas below MSL [m] h m = average height marshes above MSL [m] ms = maximum height boundary marshes shallow areas [m] ms = volume of sediment [m 3 ] 6 11

86 October, 2004 Z 2839 Long Term Morphological Modelling of Venice Lagoon 6.3 Restructure sea side area ASMITA is developed to simulate the behaviour of a tidal inlet system including the adjacent coast and the ebb tidal delta. This section discusses the sea side area of Venice Lagoon, being the area of the Adriatic sea near the tidal inlet. Purpose is to introduce this area into ASMITA Venice in this way being able to simulate the long term morphological evolution of the sea side area and predict its influence on the evolution of the tidal basin as a whole. The ebb tidal delta is considered to play a dominant role in sediment exchange between the sea and the tidal basin. The development of an ebb tidal delta and the influence on the tidal inlet system is subject of other studies. Reference is made to De Vriend et al., 2002 and Coastal Engineering Manual, The presence of an ebb tidal delta is a balance between the supply of sediment and the erosive forces in this area. Natural this balance is made by three dominant factors: the tidal current through the inlet, the outflow of river sediment into the basin, and the wave activity at the sea. In Venice Lagoon the balance of sediment in the ebb tidal delta is strongly influenced by human activity. This section discusses the current situation of the sea side area of the Venice Lagoon inlets (section 6.3.1). Based on this analysis a general schematisation is made for a tidal inlet bound by jetties (section 6.3.2). Followed by a specification for the three inlets of Venice Lagoon. In section the new elements are defined based on the ASMITA concept Current situation The present situation at the sea side of the inlets is strongly influenced by human intervention in the last hundred years. This area changed from a natural ebb tidal delta to a region dominated by human build structures. The natural ebb delta can be described as an area with numeral sandbanks in front of the inlets (Figure 6 6). These banks consisted of sediment from the lagoon transported by tidal currents and sediment from the adjacent coast delivered by long shore current. 6 12

87 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, 2004 B Sea Ebb Delta A B waterline Ebb Delta A (a) Lagoon bed level Figure 6 6: Schematisation of the ebb delta before construction of the jetties, (a) location of the sand banks, (b) cross section over ebb delta area. (b) The present situation is dominated by the presence of jetties. The numerous sandbanks disappeared and the region in front of the inlet deepened (Figure 6 7). A small deposit of sediment is located obliquely in front of the inlet, originating from lagoon and coastsediment. Against the jetties a deposit of sediment is located, increasing the beach in seaward direction. B Sea Ebb Delta A waterline Ebb Delta B A (a) Lagoon bed level Figure 6 7: Schematisation of the ebb delta after construction of the jetties, (a) location of the shallow area, (b) cross section over ebb delta area. (b) The following human interventions in Venice Lagoon are mentioned as they strongly affect the evolution of the ebb tidal delta: River diversion: the rivers Brenta, Bacchiglione Sile, Piave and branches of the Po are diverted from or off the lagoon. This resulted in a reduction of the sediment input, into the lagoon and the adjacent coasts which both lead to a shortening of sediment supply to the ebb tidal delta. Construction of jetties: this influenced the water flow by blocking the long shore current and guiding the tidal current through the gorge. 6 13

88 October, 2004 Z 2839 Long Term Morphological Modelling of Venice Lagoon Construction of groynes: groynes along the coastline interrupt the existing longshore current. Result of these interventions on the dominant sediment transport factors are discussed. The diversion of rivers resulted in a reduction of the sediment input into the lagoon and the adjacent coast. As the sediment input in the lagoon decreased the sediment output directed towards the ebb tidal delta decreased. As the sediment concentration at the adjacent coast reduced the supply of sediment towards the ebb tidal delta decreased. The diversion of the rivers thus lead to a shortening of sediment supply towards the ebb tidal delta. The construction of groynes and jetties influenced the long shore current at the Adriatic coast. This long shore current is generated by wind waves and located relatively close to the coastline. The presence of coastal structures, blocks the long shore current. As the longshore current velocity drops the sediment transport capacity of the water flow decreases. Result is sedimentation in these areas (Figure 6 8) and a reduction of the sediment concentration in front of the inlets. Sea Jetty Jetty coast sedimentation Inlet coast Tidal ebb current Long shore current Lagoon Figure 6 8: Sedimentation near a coastal construction as a result of blocking the long shore current. Another effect of the jetties is the guidance of tidal flow through the inlet. The ebb current, directed towards the delta is guided by the jetties resulting in a higher velocity when flowing into the Adriatic sea. This jet current transports sediment further from the tidal inlet. These particles will settle further from the coast and not contribute to the forming of the ebb tidal delta and adjacent coast Schematisation of the sea side area Currently ASMITA schematises the sea side area into an element Delta, representing the sand banks in front of the gorge, and two elements coast representing the adjacent coastline at both sides of the inlet (section 4.2). Section 4.3 to 4.5 discusses the formulas used to calculate the morphological evolution of these elements. This section discusses the schematisation of the sea side area of the three Venice Lagoon inlets. Purpose is to simulate the morphological evolution of the sea side area and thereby predict the future evolution and the influence on the evolution of the whole tidal basin. 6 14

89 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, 2004 Principle is to analyse the current formation of the sea side area. From this analysis the dominant forcing factors for the present morphology can be found. Subsequently alternative schematisations are suggested. Forcing factors At first we analyse the current formation of the sea side area. Three processes can be distinguished, the accretion of the coastline near the jetties, the deepening of the area in front of the gorge and the presence of small banks obliquely of the inlet. The accretion of the coast is generated by the residual long shore current, this current deposit sediment when interrupted by the jetties. The small banks are formed by sediment from the adjacent coast and by sediment from the lagoon, as a result of the long shore current and the tidal flow through the gorge. The sediment on the banks is again brought in suspension by wave action at the sea. The presence of jetties result in a guidance of the tidal flow through the gorge. The flow velocity increases and causes a deepening of the area in front of the gorge. Alternatives schematisations Considering the sediment transport processes mentioned above different schematisations of the sea side area are considered. The area under attention is limited by the end of the jetties and includes the total surface influenced by the presence of the tidal inlet. The region between the jetties is considered part of the channel element. Alternative one (Figure 6 9): Conform the current ASMITA definition a geographically fixed delta surface is set. The volume of the delta is represented by the sediment difference between the actual coastline and a fictive coastline without a tidal inlet. sea delta channel Figure 6 9: Schematisation into a fixed element Delta, alternative one. Alternative two (Figure 6 10): The surface of the element Delta is variable, changing with the erosion and accretion of the coast. The surface boundaries are set by the mean sea level at the coast and fixed boundaries at the sea. The volume of this element is defined based on the difference in cross section of the actual coastline and a fictive coastline without a tidal inlet. 6 15

90 October, 2004 Z 2839 Long Term Morphological Modelling of Venice Lagoon delta sea Figure 6 10: Schematisation into a variable element Delta, alternative two. channel Alternative three (Figure 6 11): To define the element a fictive line is drawn, from the top of the jetties longitudinal to the coast. The surface of the element Delta is fixed between this line and a sea side boundary. The sediment volume of this element is defined based on the difference in cross section of the actual coastline and a fictive coastline not interrupted by a tidal inlet. The area between the delta element and the coast is schematised by a separate element, Beach. This element represents the sediment build up by long shore transport. delta beach north channel beach south Figure 6 11: Schematisation into the elements Delta and Beach, alternative three. Definition of the element Delta is that it represents the sediment that participates in the process of morphological changes of the tidal system. The accretion of the coast results in an increasing beach area. Sediment located at the beach has a relatively fixed location and therefore not involved in the morphological evolution of the tidal inlet system. Including the new beach area in the delta element as is done with a fixed delta surface in alternative one is therefore not desirable. Alternative two takes into account this problem by varying the delta surface. This varying surface gives a new problem in defining the volume in the system. As the surface changes the sediment volume in the delta element changes, in this way sediment disappears from the system without a sediment transport mechanism to simulate this loss. In alternative three the sediment deposit at the beach is kept in the system. A separate element, Beach is introduced to simulate the evolution of the coast. Alternative three is considered the best schematisation, as it fits the different morphological characteristics of the area and separates the regions based on different sediment transport mechanisms. The delta area simulates the processes of deepening in front of the gorge and 6 16

91 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, 2004 evolution of sandbanks forced by tidal and wave action. The element Beach represents the beach accretion as a result of the blocking of the long shore current. As the sediment transport forces and characteristics of each of the three inlets of Venice Lagoon are different, the schematisation of each inlet is discussed separately. In this schematisation the elements Beach differ between the different inlets. Schematisation sea side area Lido Dominant in the Lido area is the long shore current from the north. This flow is generated by the strong wind from the north east, the Bora. This has led to accretion of the coast northern of the Lido inlet. To take this morphological process into account an element representing the north coast is included. The effect of the wind on the south part is negligible and therefore this area is not included in the model. Beach Delta Tidal current Long shore current Figure 6 12: Sediment transport mechanisms and element schematisation of the Lido sea side area. Schematisation sea side area Malamocco Long shore currents in the region of the Malamocco inlet are small as the coast on both sides are interrupted by groynes and the jetties of the Lido and Chioggia inlets. Never the less a small accretion of the northern beach is present. An increase in this accretion is expected as the sediment passing the Lido inlet is increasing due to the sedimentation of the north coast at Lido which reaches the end of the jetty. The evolution of the northern coast is therefore taken into account in the schematisation. 6 17

92 October, 2004 Z 2839 Long Term Morphological Modelling of Venice Lagoon Beach Delta Tidal current Long shore current Figure 6 13: Sediment transport mechanisms and element schematisation of the Malamocco sea side area. Schematisation sea side area Chioggia The schematisation of the sea side area at Chioggia is represented by the element Delta and an element Beach located at the south of the inlet. The dominant sediment flow is coming from the south as the south east wind, Sirocco is dominant in this area. The dominance of the Sirocco wind is found in the position of the coastline south of the inlet. The coast is located in a south west direction, resulting in a long shore current in southern direction. Delta Beach Tidal current Long shore current Figure 6 14: Sediment transport mechanisms and element schematisation of the Chioggia sea side area. 6 18

93 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, Definition of the sea side element In ASMITA Venice a schematisation is made of the sea side area into three different elements; the Delta element, the northern Beach, and the southern Beach. Conform the ASMITA concept (Chapter 4) each element should be defined by one variable indicating the morphological situation of the element. An equilibrium situation is defined based on empirical relations between the forces generating sediment transport and the condition of the element. The morphological evolution is influenced by sediment transport between neighbouring elements based on difference in sediment concentration. Distribution of sediment inside the element is based on the difference between the current sediment concentration and the sediment concentration in the equilibrium situation. Definition delta element The morphological situation of the element Delta is represented by the sand volume in this area. Considered is the volume of soil, which wouldn t be present if the coastline wasn t interrupted (section 4.3). Morphological changes of the element occurs through sediment exchange with the surroundings (Figure 6 15). Sediment exchange between the delta and the channel is represented by a diffusion coefficient times the difference between the local sediment concentrations. The sediment exchange between the delta and the outside world is represented by a diffusion coefficient times the difference between the local concentration and the global concentration. The global concentration is the concentration present in the sea. This value is used in ASMITA as a constant boundary condition. The sediment exchange between the coast and delta is controlled by two forces, the tidal flow and the long shore current. The tidal action is simulated as a diffusive process, calculated with a diffusion coefficient and the sediment concentration differences between the elements. The sediment transport of long shore current is simulated as a sediment flow from the element Beach to the element Delta. bd (c b c d ) do (c E c d ) Beach q bd (c b ) Delta q do (c d ) Sea dc (c c c d ) Channel Figure 6 15: Sediment exchange between the element Delta and its surrounding. In the delta area two sediment transport forces are active, the tidal current and the wind activity. Assuming that both these forces effect the sediment transport in that amount that 6 19

94 October, 2004 Z 2839 Long Term Morphological Modelling of Venice Lagoon none is negligible, the equilibrium situation is a function of the wind activity and the tidal current. In ASMITA the sand volume in the delta element is considered. Translating these independences to a sand volume the following relation is given: Vde = fwind + fcurrent (6.12) where: V de = sand volume in the element Delta [m 3 ] f wind = expressing the effect of wind on the sediment transport f current = expressing the effect of current on the sediment transport The parameter f wind is considered a function of the water depth in the delta area, the wave height and the delta surface. f current considered is a function of the tidal volume in the delta region. The sediment concentration is calculated as follows: c de c ÊV ˆ d = EÁ Vde Ë N d (6.13) where: c de = local equilibrium sediment concentration of the element Delta [ ] c E = global sediment concentration [ ] V d = the actual sediment volume of the element Delta [m 3 ] V de = sediment volume of the element Delta in an equilibrium situation [m 3 ] N d = coefficient [ ] In the current situation the delta element has a large depth relatively to the wave height. Including the wind waves as a dominant sediment transport mechanism seems unrealistic. With an average depth higher than ten meters and an average wave height of half a meter, the wind influence is not taken into account in the Venice Lagoon delta area. The sand volume is thus depending on the tidal action in the region. Based on Walton and Adams (1976) the following relation is considered: V de = a P d 1.23 (6.14) where: V de = sediment volume of the element Delta in an equilibrium situation [m 3 ] P = tidal prism through the gorge [m 3 ] d = coefficient [m 1.23 ] This relation is still to be proven for the Venice Lagoon region. The coefficient is calibrated for the Lido area, see section

95 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, 2004 Definition element Beach The element Beach has a sediment balance depending on the long shore current. The amount of sediment settling in this region depends on the velocity of the long shore current and the sediment concentration in this flow. The velocity is varying according the bathymetry in the region. The more the coast grows towards the end of the jetty the more sediment passes the element Beach. The sediment exchange with the surrounding is considered as a diffusive exchange with the outside world and the element Delta. Besides this transport the dominant long shore current is represented by a sediment flow from the outside world into the element Beach and a sediment flow from the element Beach into the element Delta. sb (c E c b ) bd (c b c d ) Sea q sb (c E ) Beach q bd (c b ) Delta Figure 6 16: Sediment exchange between the element Beach and its surrounding. Characteristic for this area is the growth of the coast in seaward direction (Figure 6 17). This accretion continues until the coastline reaches the end of the jetty, in this situation the sediment input, from long shore current equals the sediment output. A B Past B A B Beach Lequilibrium A A beach B Present A A beach Equilibrium L equilibrium Figure 6 17: Cross section A B over the element Beach. Important for the behaviour of this element is thus the accretion of the beach against the jetty. If this accretion is small a large percentage of the sediment is trapped in this element, if this sedimentation length equals the jetty length the trapping of sediment is negligible. 6 21

96 October, 2004 Z 2839 Long Term Morphological Modelling of Venice Lagoon The sediment concentration in the element is thus depending on the length of the beach against the jetty. Conform the ASMITA definitions the concentration is calculated with the following formula: c be c Ê L ˆ b = EÁ Lbe Ë N b (6.15) where: c be = local equilibrium sediment concentration of the element Coast [ ] c E = global sediment concentration [ ] L b = the actual sedimentation length of the element Coast [m] L be = sedimentation length of the element Coast in an equilibrium situation [m] N b = coefficient [ ] The equilibrium situation, L be is present when the beach accretion reaches the end of the jetty. The actual length, L b of the coastline is the distance in cross shore direction between the coast before the construction of the jetty until the actual coast. The value of the actual length is calculated according to the sediment balance in the element Coast using the following empirical relation between the volume and the length of the growing beach: L = a V (6.16) N b beach b where: L b = the actual sedimentation length of the element Beach [m] beach = coefficient [m 1.5 ] N = coefficient [ ] V b = the actual sand volume of the element Beach [m 3 ] The coefficient, beach and N, follows from empirical study using data between the year 1930 until the year For the northern sedimentation at the Lido inlet of Venice Lagoon the values are set on beach = and N = 0,

97 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, Final model adjustment Chapter 6, focuses on adjusting ASMITA to Venice Lagoon. This adjustment is based on conclusions made in chapter 5 about the differences between morphological simulation of Dutch Wadden Sea and morphological simulation of Venice Lagoon. The points analysed in this study are as follows: The schematisation of the tidal basin into characteristic elements, The definition of equations describing the morphological evolution of the elements, The schematisation of elements and definition of equations of an area located at the sea side of the inlet and dominated by human build structures. Result from this study is an adjustment and extension of ASMITA. The new version, named ASMITA Venice is designed to simulate the morphological evolution of the three tidal inlets at Venice Lagoon and has the ability to simulate regions with similar characteristics as Venice Lagoon. The final structure of ASMITA Venice is presented in this section. To gain insight into the principles used, reference is made to chapter 4 in which ASMITA is described. The model consists of four basic elements with the possibility to introduce two extra elements, Beach north and Beach south. The elements discussed are the following (Figure 6 18): Marshes Shallow area Channel Delta Beach north and Beach south Marshes Channel Shallow area Beach Beach Delta Sea Figure 6 18: Final schematisation ASMITA Venice. 6 23

98 October, 2004 Z 2839 Long Term Morphological Modelling of Venice Lagoon Marshes The marshes are the area covered with thick plant growth. Due to this thick vegetation the sediment concentration in the element itself is considered zero. Changes are taking place at the boundaries of the marshes. The element is characterised by its total surface. The surface of the marshes changes as a result of sediment exchange with the neighbouring element, Shallow area (Figure 6 18). An exchange takes place depending on the sediment concentration differences between the elements. As the sediment concentration is zero in the element Marshes itself the sediment transport is directed towards the element Marshes. A second sediment exchange mechanism is based on the average depth difference between the elements. If the depth difference is too high, due to cliff forming, sediment transport is taking place in the direction of the Shallow area. In formula these processes are described as follows (see equations (6.10) and (6.11)): Equilibrium sediment concentration: c em = c = 0 m Sediment exchange on height difference: hs + hm D ms bms = constant h + h <D b = 0 s m ms ms Balance equations: dv dt m dz = dsm( cs -cm) -bms -Am - Im (6.17) dt da dt m = dv dt h m m (6.18) where: c em = local equilibrium sediment concentration of the element Marshes [ ] h s = average depth shallow areas below MSL [m] h m = average height marshes above MSL [m] ms = maximum height boundary marshes shallow areas [m] ms = volume of sediment [m 3 ] V m = actual sand volume of the element Marshes sm = diffusion coefficient between elements Shallow area and Marshes [m 3 /s] c s = sediment concentration element Shallow area [ ] c m = sediment concentration element Marshes [ ] A m = actual surface element Marshes [m 2 ] = relative sea level rise [m] = dredging volume [m 3 /s] I m 6 24

99 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, 2004 Shallow area The Shallow area is the area between the marshes and the channels. The Shallow area is defined by the sand volume above two meters minus mean sea level (Figure 6 19). An equilibrium situation for the element is expressed depending on the wave activity in the area. The waves stir up sediment from the bottom, induce erosion and preventing sedimentation. The sediment height by which the Shallow area is in equilibrium is expressed as the maximum depth minus a coefficient. This coefficient is obtained from historical information of Venice Lagoon or similar tidal areas. Sediment exchange is present with the element Marshes and with the element Channel. The sediment transport with the marshes is described above as two processes. One process based on sediment concentration differences between the elements and the second based on the difference in height. Sediment transport between the Shallow area and the Channels is based on the sediment concentration differences between these two elements. In formula these processes are described as follows (see equations (6.8) and (6.9)): Equilibrium condition: V = ( h -a ) A se max, s shallow s Equilibrium sediment concentration: c se c ÊV ˆ s = EÁ Vse Ë 1 Sediment exchange: ( ) d ( ) T = d c - c + c - c (6.19) sc s c sm s m Balance equation: dv dt s dz = wssas( cs - cse) + bms -As - Is (6.20) dt where: V se = sediment volume of the element Shallow area in equilibrium [m 3 ] h max,s = maximum water depth of the element Shallow area [m] shallow = coefficient depending on the local wind and bed resistance [m] A s = surface of the element Shallow areas [m 2 ] c se = local equilibrium sediment concentration of the element Shallow area [ ] c E = overall equilibrium concentration [ ] V s = the actual sediment volume of the element Shallow area [m 3 ] T = sediment exchange with the surrounding [m 3 /s] sc = diffusion coefficient between elements Shallow area and Channel [m 3 /s] c s = sediment concentration element Shallow area [ ] c c = sediment concentration element Channel [ ] sm = diffusion coefficient between elements Shallow area and Marshes [m 3 /s] 6 25

100 October, 2004 Z 2839 Long Term Morphological Modelling of Venice Lagoon c m = sediment concentration element Marshes [ ] w ss = vertical exchange coefficient of element Shallow area [m/s] = relative sea level rise [m] = dredging volume [m 3 /s] I s A basin A marshes A shallow A channel 2 meter below MSL Volume Shallow area (sediment) Volume Channel (water) Figure 6 19: Cross Section of the lagoon, schematisation of the volumes Shallow area and volume Channel. Channel The element Channel consists of the channel system located in the tidal basin and the area between the jetties. The element is defined as the wet volume below two meters minus mean sea level (Figure 6 19). The equilibrium situation depends on the tidal action through the channel system. Sediment exchange is present with the neighbouring elements, Shallow area and Delta. The amount of transported sediment is calculated with a diffusion coefficient times the difference in sediment concentration between the elements. The following equations are used to describe the evolution of the element Channel (see equations (6.4) and (6.5)): Equilibrium situation: V ce = a P c 1.55 Equilibrium sediment concentration: c ce N c ÊV ˆ ce = ceá Vc Ë Sediment exchange: ( ) d ( ) T = d c - c + c - c (6.21) sc c s dc c d 6 26

101 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, 2004 Balance equation: dv dt c dz = wscac( cce - cc) + Ac + Ic (6.22) dt where: V ce = water volume of the element Channels in an equilibrium situation [m 3 ] c = empirical coefficient [m 1.55 ] P = tidal prism through the gorge [m 3 ] c ce = local equilibrium sediment concentration of the channel [ ] c E = global equilibrium concentration [ ] V c = the actual water volume of the element Channels [m 3 ] N c = coefficient [ ] T = sediment exchange with the surrounding [m 3 /s] sc = diffusion coefficient between elements Shallow area and Channel [m 3 /s] c s = sediment concentration element Shallow area [ ] c c = sediment concentration element Channel [ ] dc = diffusion coefficient between elements Delta and Channel [m 3 /s] c d = sediment concentration element Delta [ ] A c = surface of the element Channel [m 2 ] w sc = vertical exchange coefficient of element Channel [m/s] = relative sea level rise [m] = dredging volume [m 3 /s] I c Delta: The Delta area is the area located at the sea side of the jetties. The area covers the surface influenced by the interaction between the lagoon and the sea. The element is defined by the sand volume present in the area different from the sand volume that would be present if the coast continued in a straight line without an inlet. An equilibrium situation for this element is based on the tidal action Sediment exchange is present with the element Channel, based on the sediment concentration difference of the two elements. Transport of sediment between the element Delta and the element Beach is based on two processes. The first process is diffusive depending on the sediment concentration differences between the elements. The second process is forced by the dominant long shore current, it is expressed as a sediment transport in the direction of the long shore current depending on the sediment concentration in this current. Exchange of sediment between the delta element and the outside world is also based on these two processes considering there is only one element Beach. In formula this is expressed as follows (see equations (6.13) and (6.14)): Equilibrium situation V de = a P d 1.23 Equilibrium sediment concentration: 6 27

102 October, 2004 Z 2839 Long Term Morphological Modelling of Venice Lagoon c de c ÊV ˆ d = EÁ Vde Ë N d Sediment exchange: ( ) ( ) ( ) ( ) T = d c - c + d c - c + d c - c + q c - c (6.23) sd d s cd d c bd d b bd b d Balance equation: dv dt d dz = wsd Ad ( cde - cd ) + Ad + Id (6.24) dt where: V de = sediment volume of the element Delta in an equilibrium situation [m 3 ] P = tidal prism through the gorge [m 3 ] d = coefficient [m 1.23 ] c de = local equilibrium sediment concentration of the element Delta [ ] c E = global sediment concentration [ ] V d = the actual sediment volume of the element Delta [m 3 ] N d = coefficient [ ] T = sediment exchange with the surrounding [m 3 /s] sd = diffusion coefficient between elements Shallow area and Delta [m 3 /s] c d = sediment concentration element Delta [ ] c s = sediment concentration element Shallow area [ ] cd = diffusion coefficient between elements Channel and Delta [m 3 /s] c c = sediment concentration element Channel [ ] bd = diffusion coefficient between elements Beach and Delta [m 3 /s] c b = sediment concentration element Beach [ ] q bd = flow coefficient between elements Beach and Delta [m 3 /s] A d = surface of the element Delta [m 2 ] w sd = vertical exchange coefficient of element Delta [m/s] = relative sea level rise [m] = dredging volume [m 3 /s] I d Beach north and Beach south The element Beach represents the area located against the jetty and between the coast and element Delta. An element Beach can be introduced at both sides of the inlet. It simulates the enlargement of the beach in seaward direction as a result of sediment build up at the downstream side of the jetty. The evolution of the element Beach is expressed by the cross shore length of the emerging beach. This is assumed equal to zero when the cross shore length of the emerging beach is equal to that of the undisturbed beach (without jetty). The equilibrium situation is reached when the cross shore length of the beach equals the length of the jetty. Sediment transport is present between the element Beach and its surrounding, the element Delta and the outside world. In both situations two processes are present. The first process is 6 28

103 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, 2004 diffusive depending on the sediment concentration differences between the elements. The second process is forced by the dominant long shore current, it is expressed as a sediment transport in the direction of the long shore current depending on the sediment concentration in this current. The element Beach is represented by the following equations (see equation (6.15)): Equilibrium sediment equation: c be c Ê L ˆ b = EÁ Lbe Ë N b Sediment exchange: ( ) d ( ) ( ) T = d c - c + c - c + q c - c (6.25) sb b E db b d sb E b Balance equations: dv dt b dz = wsbab( cbe - cb) + Ab + Ib (6.26) dt dl dt b 0.5 dvb a Ê ˆ b = Á Ë dt (6.27) where: c be = local equilibrium sediment concentration of the element Coast [ ] c E = global sediment concentration [ ] L b = the actual sedimentation length of the element Coast [m] L be = sedimentation length of the element Coast in an equilibrium situation [m] N b = coefficient [ ] T = sediment exchange with the surrounding [m 3 /s] sb = diffusion coefficient between Sea and element Beach [m 3 /s] c b = sediment concentration element Beach [ ] bd = diffusion coefficient between elements Beach and Delta [m 3 /s] c d = sediment concentration element Delta [ ] q sb = flow coefficient between Sea and element Beach [m 3 /s] V b = the actual sediment volume of the element Beach [m 3 ] A b = surface of the element Beach [m 2 ] w sb = vertical exchange coefficient of element Beach [m/s] = relative sea level rise [m] = dredging volume [m 3 /s] I b 6 29

104

105 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, Long term morphological predictions 7.1 Introduction This part of the Study focuses on the application of the ASMITA Venice model. This model, presented in section 6.4 is designed to predict the morphological evolution of the tidal basins at Venice Lagoon. It is an adjustment and extension of the ASMITA model. To test the capability of ASMITA Venice this part of the study will run the model for the northern inlet at Venice Lagoon. The purpose of this study part is to show that the model is suited to predict the evolution of a tidal inlet with the characteristics of the Lagoon of Venice. Before predictions are made on the morphologic evolution, different input parameters are necessary. These parameters simulate the behaviour of the elements regarding the sediment transport forces and the intensity of sediment exchange. To set the value of these parameters knowledge is necessary of the past morphological evolution in the area. This information results from historical measurements in the field and satellite images of the lagoon. Of the period 1930 until the year 2000 data are available for this study. This period is suitable as it starts after the large human interventions in the region, diversion of rivers and construction of the jetties. It should be mentioned here that the available data for this study are an indication of the past evolution and are to be treated with care because of the uncertainty about the reliability. Nevertheless these data are useful and fit for the purpose of this study. This chapter will have a run of ASMITA Venice for the Lido inlet at Venice Lagoon. Section 7.2 presents the past morphological evolution in the area and the input parameters after the calibration of the model. Section 7.3 gives the morphological evolution of the different elements for the period 1930 until the year An evaluation of the simulation is presented in section

106 October, 2004 Z 2839 Long Term Morphological Modelling of Venice Lagoon 7.2 Input of ASMITA Venice Five elements are included in the simulation (Figure 7 1) referring to the basin schematisation in section 6.2, the schematisation of the sea side of the Lido inlet in section 6.3. Definition of these elements are given in section 6.4. Marshes Channel Shallow area Beach Delta Sea Figure 7 1: Schematisation ASMITA Venice for the Lido inlet. The model is calibrated on bathymetric measurements and satellite images of Venice Lagoon in the period 1930 until the year The values available for this study are presented in the Table 7 1: Basin Year L beach [m] V delta [*10 6 m 3 ] V channel [*10 6 m 3 ] V shallow [*10 6 m 3 ] A marshes [*10 6 m 2 ] Lido Table 7 1: Morphological evolution of the elements at the Lido inlet in the period

107 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, 2004 The input parameters in ASMITA Venice for the Lido inlet are presented in Table 7 2. These parameters are defined according to knowledge of the area gained during this study and the calibration of the model over the period 1930 until the year Coast north Lido Type Beach Area 3,36E+07 m 2 Global equilibrium concentration 8,0E 05 Local equilibrium concentration 8,0E 05 Vertical exchange 8.64 m/day Coefficient n 3 Initial Volume 1,4E+03 m 3 Initial concentration 8,0E 05 Delta Lido Type Delta Area 2,3E+07 m 2 Global equilibrium concentration 8,0E 05 Local equilibrium concentration 8,0E 05 Vertical exchange 8.64 m/day Coefficient n 5 Initial Volume 1,0E+00 m 3 Initial concentration 8,0E 05 Channel Lido Type Channel Area 2,1E+07 m 2 Global equilibrium concentration 8,0E 05 Local equilibrium concentration 6,6E 05 Vertical exchange 8.64 m/day Coefficient n 3 Initial Volume 7,3E+07 m 3 Initial concentration 8,0E 05 Shallow area Lido Type Shallow Area 1,24E+08 m 2 Global equilibrium concentration 8,0E 05 Local equilibrium concentration 8,7E 05 Vertical exchange 8.64 m/day Coefficient n 1 Initial Volume 1,84E+08 m 3 Initial concentration 8,0E

108 October, 2004 Z 2839 Long Term Morphological Modelling of Venice Lagoon Marshes Lido Type Marshes Area 2,5E+07 m 2 Global equilibrium concentration 8,0E 05 Local equilibrium concentration 0 Initial Volume 1,0E+07 m 3 Initial concentration 8,0E 05 Average depth 4,0E 01 m Diffusion coefficient beach out 8,0E+07 m 3 /day beach delta 8,0E+07 m 3 /day delta out 1,6E+08 m 3 /day channel delta 1,0E+08 m 3 /day shallow channel 1,5E+07 m 3 /day marches shallows 1,0E+04 m 3 /day Flow coefficient q beach out 2,0E+08 m 3 /day q beach delta 2,0E+08 m 3 /day q delta out 2,0E+08 m 3 /day Alfa coefficient beach 3,6E 01 m 0,66 delta 1,3E 02 m 0,81 channel 4,5E 05 m 0,65 shallow 5,0E 01 m Extern coefficients Tidal range 6,0E+01 m Dredging 1,0E+05 m3/year Sea level rise 0.23 m/century Tidal volume 8,5E+07 m 3 Length jetty 2,4E+03 m cliff height marshes 0,9 m deterioration marshes 200 m 3 Table 7 2: Input parameters ASMITA Venice for Lido. 7.3 Predictions Morphological evolution Lido inlet In the following figures the predictions of the morphological evolution of the elements in the Lido inlet is presented using ASMITA Venice. The period is expressed in days, 7 4

109 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, 2004 representing the year 1930 until the year Based on the values in Table 7 1 the past morphological situation of the element is plotted by the white dots. The element Beach is represented by the cross shore length of the emerging part of the beach measured near the jetty. Point zero on the length scale represents the situation on the beach when no jetty was present yet (year 1930) (Figure 7 2). Figure 7 2: Evolution element Beach, year 1930 until The simulation of the element Beach represents the growth of the beach area. A stable situation is present at the moment that the beach enlargement reaches the end of the jetty (2400 meters seaward). Conform the present situation the increasing coast has about reached this situation. The element Delta is represented by a sediment volume calculated according to a reference level. This reference level is present in a situation where the coast is not interrupted by the presence of an inlet (Figure 7 3). Sediment volume [m 3 ] Length [m] I Year 1930 Time [years] I Year 2100 I Year 1930 Time [years] I Year 2100 Figure 7 3: Evolution element Delta, year 1930 until

110 October, 2004 Z 2839 Long Term Morphological Modelling of Venice Lagoon The evolution of the Delta area represents an increase in sediment volume based on the assumption that the bathymetry of this area in the year 1930 (completion of the jetties) is at its reference level. Based on the measurements in the year 1970, 1990 and 2000 the sediment volume represents a relative stable situation. This constant situation is predicted by ASMITA Venice for the coming years. The behaviour of the delta element supports the general idea that the area in front of the inlet is currently in a morphological stable situation. This points out that the effect of the delta area on sediment transport on the long term is relatively small. To simulate the evolution of the lagoon a boundary condition between the jetties should be sufficient. This conclusion is limited to the situation in which no drastic changes occur in the delta region. The element Channel is represented by the water volume inside the lagoon below two meters minus mean sea level (Figure 7 4). Wet volume [m 3 ] I Year 1930 Time [years] I Year 2100 Figure 7 4: Evolution element Channel, year 1930 until The predictions by ASMITA Venice show the increase of Channel volume between the years 1930 and 1970, as a result of channels being dredged and the erosive tendency of the lagoon. As a prediction the channel system reaches a balance between loss of sediment by maintenance dredging plus exchange with the Adriatic Sea and the input of sediment from the shallower areas. The element Shallow area is represented by the sediment volume above two meters minus mean sea level in the area of the lagoon that is not characterised as Salt Marsh area (Figure 7 5). 7 6

111 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, 2004 Figure 7 5: Evolution element Shallow area, year 1930 until The evolution of the element Shallow area simulated by ASMITA Venice represents the current evolution of the shallower areas in the lagoon. In both cases a deepening of the area (and consequentially loss of sediment) is shown, this sediment is assumed to flow in the direction of the channel system. This is a result of the artificial construction of channels and the maintenance dredging. The erosive tendency of the Shallow area continues in the future according to the predictions of the ASMITA Venice model. The prediction seems reliable as the channel system is presently kept on depth to allow ships access, supporting the transport of sediment towards the deeper channel area. The element Marshes is represented by the surface covered with thick vegetation inside the lagoon (Figure 7 6). Surface [m 2 ] Sediment volume [m 3 ] I Year 1930 Time [years] I Year 2100 I Year 1930 I Year 1986 Time [years] I Year 2100 Figure 7 6: Evolution element Marshes, year 1930 until Presented in the predictions of the Marshes is a continuing decrease of the salt marshes. This decrease shows the erosion of the cliffs as a result of the present instability of the cliffs 7 7

112 October, 2004 Z 2839 Long Term Morphological Modelling of Venice Lagoon (ecological deterioration) and the increase of wave attack due to an increase in depth of the neighbouring area, in this study characterised as Shallow area. The decrease of erosion, shown in Figure 7 6 around year 1986 is the result of artificial sediment input into the Salt Marsh area. In the simulation of the model it is assumed that this artificial restoration works will be continued. The sediment transports are given in the following figures. The lines represents the transport of particles between the different elements as schematised in Figure 7 1. A distinction is made between the sediment transports generated by tide and wave activity (Figure 7 7) and the sediment flow generated by the dominant long shore current (Figure 7 8). A From Marshes towards Shallow Sediment flux [m 3 /day] C D E B A B From Shallow towards Channel C From Channel towards Delta D From Sea towards Beach E From Beach towards Delta F From Delta towards Sea F I Year 1930 I Year 2000 Time [years] I Year 2100 Figure 7 7: Sediment transport between elements generated by tide and wave activity. Sediment flux [m 3 /day] D F E D From Sea towards Beach E From Beach towards Delta F From Delta towards Sea I Year 1930 I Year 2000 Figure 7 8: Sediment transport between the elements generated by long shore transport. Time [years] I Year

113 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, 2004 Extracting the values of the year 2000 from Figure 7 7 and Figure 7 8 result in the following sediment balance: Marshes 80 Channel Shallow Beach Delta +200 Sea 1180 Figure 7 9: Sediment balance year 2000, values in m 3 /day Figure 7 9 presents the sediment balance per element as a result of natural sediment transport between the areas. Excluded from this balance is the effect of sea level rise and the dredging inside the areas. The evolutionary tendency of the areas expressed in figure 7 2 to figure 7 6 is a balance of the above factors. For the separate influence of these factors reference is made to appendix F. 7 9

114 October, 2004 Z 2839 Long Term Morphological Modelling of Venice Lagoon 7.4 Evaluation ASMITA Venice run The model ASMITA Venice is able to follow the past morphological evolution of the Lido inlet, referring to the figures in section 7.3 which follow the historical information presented by the white dots. Based on this result predictions of the future evolution of the tidal inlet system are given. Because of this ability it is possible to predict with the help of the model ASMITA Venice the future evolution of the tidal inlet. Therefore the conclusion is justified that developed model ASMITA Venice is fit for Venice Lagoon situation and can be a useful tool for the studies and works to be carried out in the future in Venice Lagoon. However some notes on this positive conclusion should be given and taken into account. The equilibrium relations of the element are dominant for the final situation the element is represented by. The parameters used in the equilibrium relations are calibrated on the period 1930 until the year In this period, for most of the elements, information is available for this study in the years 1930, 1970 and This results in a calibration of the equilibrium situation depending on three measurements, which already have an uncertainty of themselves. To increase the reliability of the model the present definition of the equilibrium situation should be supported by more information of the evolutionary tendency of Venice Lagoon. As the model is developed now and proved to fit in the present situation of the area it is recommended to follow up the field surveys and collection of data and carry out more calibrations of the ASMITA Venice model with the additional field data. The tidal volume is assumed constant during the simulation period. In ASMITA this value is a relation of the area between mean low water and mean high water. The evolution of this region is not calculated in ASMITA Venice. The influence of a constant tidal volume on the evolution of the tidal inlet system is therefore not considered in the simulation. Assumed is that during the simulation period of the run no large human interventions took place. This assumption is necessary as ASMITA Venice predicts the future evolution based on empirical information. If interventions in the area take place which influence the behaviour of the sediment transport in or between the different elements, the model is unable to account for these changes. Adjustments of the model will then be necessary taking into account these manmade changes and will lead to different parameters to be used in the ASMITA Venice model, then are used in this study. Mentioned here are three possible interferences by man. The artificial protection of the salt marshes, this affects the sediment transport from the element Marshes towards the element Shallow area by decreasing the parameter deterioration marshes. The possibility to increase the sediment input in the future by redirecting the rivers to flow into the lagoon, this introduces a sediment flow from the outside world into the element Channel. The possibility to reconstruct the jetties, this varies the jetty length which is included in the model by the length of the beach accretion. 7 10

115 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, Conclusions and Recommendations 8.1 Conclusions The objective of this study is to develop ASMITA Venice, a version of ASMITA for use in Venice Lagoon, and to investigate the applicability of ASMITA Venice to Venice Lagoon. In the introduction of this study, chapter 1, the separate parts were introduced. They consist of an Analysis phase, the Development phase and the Implementation phase. Here we summarize the conclusions of the chapters involved and review the end result Part 1, Analysis The first part of this study (chapter 2,3,4) describes the theory concerning tidal inlets, Venice Lagoon area and the morphological model. Chapter 5 combines the knowledge from studies on the Venice Lagoon and the Dutch Wadden sea. From these chapters it becomes clear that the current ASMITA model is not suited for predicting the long term evolution of Venice Lagoon. The following points are found: The current schematisation of ASMITA is not suitable to simulate an important area of the Venice Lagoon, the Marshes, Comparing the external forcing factors of Venice Lagoon and the Dutch Wadden Sea indicates a reduced average tidal action, leading to an increasing importance of the wind and wave forcing, The sea side boundary conditions should be redefined due to the presence of a strongly reduced ebb tidal delta and the presence of the jetties in Venice Lagoon area, The effect of wind on the sediment transport within shallow areas of the lagoon needs to be incorporated in relation to tidal effects Part 2, Subsequently the following adjustments are made for ASMITA Venice: Schematisation of the area, New elements were introduced based on characteristics of the region and function: Marshes: areas with thick plant growth, with an ecological impact in relation to their surface, Shallow: areas consisting of shallow banks without thick plant growth, above 2 MSL meter, 8 1

116 October, 2004 Z 2839 Long Term Morphological Modelling of Venice Lagoon Channel: area with relative high water and sediment transport capacity, bound between 2 MSL meter and the maximum depth of the lagoon, Delta: area with shallows at the sea side of the gorge, bound between a fictive coastline without a tidal inlet and the coastline with a tidal inlet, Beach: area of beach accretion due to the construction of jetties, bound between a coastline without a jetty and the coastline with a jetty. Influence of the forcing factors A different influence of forcing factors led to the adjustment of ASMITA in defining the equilibrium situation and sediment transport processes for some elements. For these elements, the dominant forcing factors and the resulting formulas are defined as follows: Marshes The controlling factor on the marshes is the wave attack and depth of the surrounding shallows. These waves are generated by wind and ships. They cause progressive retreat of the marsh boundary, accelerating with increasing wave activity and deepening of the shallow area. Sedimentation of the marshes occur based on concentration difference with the Shallow element. The concentration in the element Marshes is considered zero due to the thick plant growth, in this way the salt marshes are simulated as sediment traps. Shallow In the element Shallow the wind activity is considered most important. The equilibrium situation is defined based on the wind waves active in this region. Delta In the element Delta both tidal currents and wave action are considered. Due to the large depth in this region the tidal current is considered dominant. The equilibrium situation is defined based on the tidal volume. The element Delta exchanges sediment with the surrounding elements based on concentration differences. Besides this transport process a sediment flow in the direction of the dominant long shore current is considered. This flow is directed from the element Beach through the element Delta towards the Outside world. Beach The element Beach is defined as a function of the jetty length. The equilibrium situation is reached when the cross shore length of the beach is equal to the length of the adjacent jetty. This is not only a good representation of the beach accretion but in this way it is possible to predict the influence of the jetty length on the sediment balance of the tidal inlet system. Sediment transport is present between the element Beach and its surrounding, the element Delta and the Outside world. In both situations two processes are present. The first process is diffusive depending on the sediment concentration differences between the elements. The second process is forced by the dominant long shore current, it is expressed as a sediment 8 2

117 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, 2004 transport in the direction of the long shore current depending on the sediment concentration in this region Part 3, Implementation of ASMITA Venice The available data for calibration of the modified model, representing past evolution, consisted of 3 sets of data covering the period of The number of tuning or calibration factors in the model equations are high so that substantial assumptions had to be made. Combined with the fact that there are relatively few data points for the calibration, the assumptions are difficult to verify. The output and value of the model is therefore qualitative rather then quantitative and presently gives an indication of tendencies only. The results presented in chapter 7 assume that the available data correctly represent past evolution and that no drastic changes occur during the simulation years apart from the dredging activities. Within these conditions the results of the Lido inlet provide insight in the effects of human interference in the area, such as dredging, salt march restoration and protection, and the influence of jetties. The results of the delta element supports the general idea that the area in front of the inlet is currently in a morphological stable situation. This points out that the effect of the delta area on the long term sediment balance is relatively small. To simulate the evolution of the lagoon a boundary condition between the jetties, as done in the model of Di Silvio, should be sufficient. This conclusion is limited to the situation in which no drastic changes occur in the delta region. Overall conclusion The modified model ASMITA Venice integrates knowledge and experience gained in the Netherlands with ASMITA, and the insights of the University of Padua. In this way it can potentially provide a basis for further cooperation. The cooperation will be necessary to tune the model ASMITA Venice and even more important, to provide support for some of the assumptions made in the adjustments and the calibration 8.2 Recommendations This study was limited in its scope due to the lack of reliable data. It is recommended to continue the cooperation with the Padua University, starting with the collection of more data and improving the adjusted model with this extra information. In this way the value of the parameters can be defined more accurate. During the present study assumptions are made which were based on the available information. By giving special attention to the following parameters the ASMITA Venice 8 3

118 October, 2004 Z 2839 Long Term Morphological Modelling of Venice Lagoon model should be able to give a more accurate prediction of the evolution of the Venice lagoon: Surface and Volumes of the different elements, Calculation of the outside sediment concentration, The value of the tidal volume and the variation over the simulation period, The influence of waves on the shallow areas expressed in the parameter shallow, The value of the long shore sediment transport, The value of cliff erosion and the stability of these cliffs, The dredged volume in the elements. The ASMITA Venice model should be applied on more areas with characteristics similar to Venice Lagoon. This will give insight in the possibilities and reliability of the ASMITA Venice model. 8 4

119 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, 2004 References Biegel E.J., 1991; Equilibrium relations in the ebb tidal delta, inlet and back barrier area of the Frisian Inlet system. Rapport GEOPRO /GWAO Rijksuniversiteit Utrecht, Vakgroep Fysische Geografie. Bijsterbosch L., 2003; Influence of Relative Sea level Rise on Tidal inlets. Note Z2958, Delft Hydraulics, Delft, march Brown, 1928; Inlets on sandy coasts, Proc. ASCE, vol. 54, part II, February. Buijsman M.C., 1997; The impact of gas extraction and sea level rise on the morphology of the Wadden Sea, Delft Hydraulics, Report H Dean R.G., Walton T.L., 1975; Sediment transport processes in the vicinity of inlets with special reference to sand trapping. In Estuarine Research, L.E. Cronin, Academic press, 2, pp Di Silvio G. 1989; Modelling the morphological evolution of tidal lagoons and their equilibrium configurations. 23 rd IAHR Congress, Ottawa, Canada, pp Di Silvio G., 1991; Averaging operations in sediment transport modelling: short step versus long step morphological simulations., International Symposium on the Transport of Suspended Sediments and its Mathematical Modelling, Florence, September 2 5. Di Silvio G., 1999; Interaction between marshes channels and shoals in a tidal lagoon, IAHR Symposium on River, Coastal and Estuarine Morphodynamics, University of Genoa, Italy. September 6 th 10 th. Di Silvio G., Barusolo G., and Sutto L., 2001; Competing driving factors in estuarine landscape, 2nd IAHR Symposium on River, Coastal and Estuarine Morphodynamics, Obhiro Japan, Sept., Di Silvio G., 2003; Sediment balance, morphodynamics and landscape restoration., International Discussion Meeting, Churchill College, Cambridge, England, 14 th 17 th September. Di Silvio G., Dal Monte L., 2003; Ratio between channel cross section and tidal prism in short lagoons: validity and limits of the Law of Jarret, 3 rd IAHR Symposium on River, Coastal and Estiarine Morphodynamics, pp , Barcelona. Eysink W.D., 1990; Morphological response of tidal basins to change. Proc. 22 nd Coast. Eng. Conf., ASCE, Delft, July 2 6, Vol. 2, The Dutch Coast, Paper no. 8, 1990, pp Eysink W.D., 1991; Impact of sea level rise on the morphology of the Wadden Sea within the scope of its ecological function, Delft Hydraulics, Report H1300. Eysink W.D. Biegel E.J., 1992; ISOS*2 Project, phase 2. Impact of sea level rise on the morphology of the Wadden Sea in the scope of its ecological function. Investigations on morphological relations. Delft Hydraulics. Report H1300. Fokkink R.J., Karssen B., Wang Z.B., Kerckhoven J. van, Langerak A., 1996; Morphological modelling of the Western Scheldt estuary. 8 th International Biennial Conference on Physics of Estuaries and Coastal Seas, The Hague, September 1996 References 1

120 October, 2004 Z 2839 Long Term Morphological Modelling of Venice Lagoon Gerritsen F., Jong H. de, 1985; Stability of flow profiles in the Wadden Sea. Rijkswaterstaat, Adviesdienst Vlissingen, Nota WWKZ 84, Vol. 6. Gerritsen F., 1990; Morphological stability of inlets and channels of the western Wadden Sea. Ministerie van Verkeer en Waterstaat, dienst getijdewateren. Nota GWAO Goor M.A. van, 2001; Influence of Relative Sea Level Rise on Coastal Inlets and Tidal basins. Delft Hydraulics, Report Z2822. Goor M.A. van., Stive M.J.F., Wang Z.B., Zitman T.J., 2003; Impact of sea level rise on the morphological equilibrium state of tidal inlets. Marine Geology, November , pp Goor M.A. van., Zitman T.J., Wang Z.B., Stive M.J.F., 2003; Impact of sea level rise on the morphological equilibrium state of tidal inlets. Marine Geology Hayes M.O., 1979; Barrier island morphology as a function of tidal and wave regime. In Barrier Islands, S.P. Leatherman (editor), Academic Press, New York, pp Jarret J.T., 1976; Tidal Prism Inlet Area Relationships. GITI Report 3, US Army Coastal Engineering Research Center, Ft. Belvoir, VA. Kragtwijk N.G., 2001; Aggregated Scale Modelling of Tidal Inlets of the Wadden Sea; Morphological Response to the Closure of the Zuiderzee. Delft Hydraulics, Report Z2822. Kragtwijk N.G., Stive M.J.F., Wang Z.B., Zitman T.J., 2004; Morphological response of tidal basins to human interventions. Coastal Engineering, may 2004, pp O Brien M.P., 1931; Estuary tidal prisms related to entrance areas. Civil Engineering. ASCE, Vol. 1 No. 8, pp O Brien M.P., 1969; Equilibrium flow areas of inlets on sandy coast. J. of the Waterways and Harbors Div., Proc. Am. Soc. Civil Engineering. Renger E., Partenscky H.W., 1974; Stability criteria for tidal basins, Proc. 14 th Coast. Eng. Conf., ASCE, 2, pp Rinaldo A., Fagherazzi S., Lanzoni S., Marani M., Dietrich W.E. 1999; Watershed delineation and comparative network morphology, Water resources, Vol. 35 no 12, pages , December Stive J.F., Capobianco M., Wang Z.B., Ruol P., Buijsman C., 1996; Morphodynamics of a tidal lagoon and the adjacent coast. 8 th International Biennial Conference on Physics of Estuaries and Coastal Seas. The Hague, 9 th 11 th September. Stive M.J.F., Wang Z.B., Van Dongeren A., De Vriend H.J., Dronkers J., 2000; Coastal Inlets and Tidal Basins. College handleiding CT5303, Faculteit der Civiele Techniek en Geowetenschappen, Technische Universiteit Delft. Van Kleef A.W., 1991; Empirical relationships for tidal inlets, basins and deltas. Report GEOPRO Rijksuniversiteit Utrecht, Faculteit der Ruimtelijke Wetenschappen, Vakgroep Fysische Geografie. References 2

121 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, 2004 Walton T.L., Adams W.D., 1976; Capacity of Inlet outer bars to store sand, Proceedings of 15th Coastal Engineering Conference, ASCE, Honolulu, Hawao, pp Wang Z.B., Karssen B., Fokkink R.J., and Langerak A., 1996; A dynamic/empirical model for estuarine morphology. 8 th International Biennial Conference on Physics of Estuaries and Coastal Seas, The Hague, September Internet addresses References 3

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123 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, 2004 A The Di Silvio model A.1 Introduction A conceptual zero dimensional morphological model developed for predicting the long term evolution of a tidal lagoon subject to possible changes of the driving factors. The model is able to describe the long term evolution of marshes, shallow areas and channels when the lagoon is subject to any change of the external driving factors. Even more important, it determines the final equilibrium configuration of each compartment of the lagoon when the driving factors remain stationary for a sufficiently long period. The model is built up out of a three element model, marshes, shallow areas and channels. When the driving factors are considered constant the values of, the surfaces, the channel cross section at the inlet, the average depth of the shallow areas, and the elevation of the marshes, will evolve towards their equilibrium value. The total surface of the basin is defined by the surface of the three elements. In formula written as: Stotaal = Smarshes + Sshoals + Schannel (A.1) where: S total = surface of the total basin [m 2 ] S marshes = surface of the marshes [m 2 ] S shoals = surface of the shallow area [m 2 ] S channel = surface of the channels [m 2 ] The total water volume flowing into the tidal basin is written as: Vtotaal = Vmarshes + Vshoals + Vchannel + Vriver (A.2) where: V total = annual water volume through the inlet [m 3 ] V marshes = annual tidal prism of the marshes [m 3 ] V shoals = annual tidal prism of the shoals [m 3 ] V channel = annual tidal prism of the channel [m 3 ] V river = fresh water run off from the rivers [m 3 ] A 1

124 October, 2004 Z 2839 Long Term Morphological Modelling of Venice Lagoon Figure A 1: Distribution of surface versus elevation in zero dimensional schematization of a tidal lagoon. [Di Silvio et al, 2001] A.2 Conservation equations The model is based on the conservation equations of water and sediment in the three elements. Referring to figure A 2 the equations can be expressed as follows: Balance between vertical and horizontal annual fluxes of sediment for the channel area: D = T + T - T + I (A.3) c s, c m, c c, out r, c where: D c = net erosion over the channel [m 3 ] T s,c = sediment flux between shoals and channel [m 3 ] T m,c = sediment flux between marshes and channel [m 3 ] T c,out = sediment flux between channel and the outside world [m 3 ] I r,c = external input into the channel [m 3 ] Balance between vertical and horizontal annual fluxes of sediment for the shoals: D = T - T + I (A.4) s ms, sc, rs, where: D s = net erosion over the shoals [m 3 ] T m,s = sediment flux between marshes and shoals [m 3 ] T s,c = sediment flux between shoals and channel [m 3 ] I r,s = external input into the shoals [m 3 ] Balance between vertical and horizontal annual fluxes of sediment for the marshes: D =-T - T (A.5) m mc, ms, where: D m = net erosion over the marshes [m 3 ] T m,c = sediment flux between marshes and channel [m 3 ] A 2

125 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, 2004 T m,s = sediment flux between marshes and shoals [m 3 ] Figure A 2: Sediment fluxes between element in the zero dimensional schematization of a tidal lagoon [Di Silvio at al, 2001] The net horizontal fluxes to and from the channels are expressed as the difference between the outward flux (during the ebb tide) and the inward flux (during the flow tide). As the tidal basin is considered to be closed, the inward and the outward water volumes are perforce the same. By contrast, the outward and inward sediment volumes are different because the concentration in the donor compartment is different. The net sediment fluxes are expressed as the difference between outward and inward fluxes, namely: T, = ( c -c ) V (A.6) c out c out total T, = ( c -c ) V (A.7) s c s c shoals Tm, c = ( cm -cs ) Vmarshes (A.8) where: c c = sediment concentration in the channel [m 3 ] c out = sediment concentration in the outside world [m 3 ] c s = sediment concentration in the shoals [m 3 ] c m = sediment concentration in the marshes [m 3 ] The sediment flux between the marshes and shoals depends not on the water exchange between the two sub compartments. In fact the flooding and drying of the salt marshes solely occur through the network of the marsh creeks which belongs to the channel compartment. Thus the sediment flux T m,s represents a transformation of marshes into shoals, controlled by the wave action. As soon as the depth of the shoals increases beyond a certain critical value, the cliff of the marshes collapses to compensate the shoal deepening. On the contrary, if the depth of the shoals is too small, sediment accumulate against the cliff above the mean sea level increasing the marshes. This sediment flux is then expressed as: A 3

126 October, 2004 Z 2839 Long Term Morphological Modelling of Venice Lagoon T m, s ds dt marshes =-hm (A.9) where: h m = mean elevation of the marshes above MSL [m]. A.3 Constitutive equations Constitutive equations are given in the model to express the representative sediment concentration in the three elements. Considering that sediment transport is respectively controlled by the tidal flow in the channels and by the wave height in the shallow areas, the following equations can be written for respectively sediment concentration in the channel element and sediment concentration in the shallow areas: c V n channel c = fx n Ac (A.10) c s f = h y m s (A.11) where: A c = channel cross section at the inlet [m 2 ] f x = constant for the sediment concentration in the channel h s = average depth of the shoals [m 2 ] f y = coefficient for the sediment concentration in the shoals The parameter f x can be assumed as a constant for a certain geographical location as it depends on the sole erodibility of the channel bottom. The parameter f y depends not only on the erodibility of the shoal bottom, but also on the local wave climate, As the wave climate depends in its turn on the intensity of the local wind and on the fetch, it seems more appropriate considering f y as a function of the ratio S shoals /S total. Formula (A.11) is than written as: c s f ÊS ˆ m shoals = m Á hs Stotal Ë (A.12) where: f m = local constant f m is a local constant increasing with the wind intensity and decreasing with the strength of the bottom. x These equations are a simplified form of the monomial transport equation, S = a U in which the power x varies between five to six (e.g. Brown 1928, Kalinske, Engelund and Hansen) and U is a representative flow velocity near the bottom. The flow velocity is A 4

127 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, 2004 proportional to the tidal prism in the channels, while depends on the wave height and the local depth in the shoals. On this ground the exponent s n and m in equation (A.10) and (A.11) can be respectively assumed equal to about 5 and 1 (Di Silvio and Dal Monte 2003). When the bottom is covered by thick vegetation, as on the salt marshes, the value of f m tends to zero. The equivalent concentration over vegetated salt marshes is in fact zero as they act as perfect traps for the sediments conveyed by the tidal channels over their surface. Elevation and surface of the salt marshes, in fact, depend on the collapse of their steep edge under the action of gravity when it becomes too high. The edge stability condition postulates a relation between the elevation of the marshes above the lower vegetation limit and the depth of the shallow areas below it that should depend on the ration (S marshes /S shoals ) of their respective surfaces. From the data collected in the Lagoon of Venice, a preliminary simple form of this relation appears to be: Dk -em - d ÊS ˆ = marshes 2hs Á kd Ë Sshoals (A.13) where: = boundary depth marshes shallow areas k = maximal admissible height between marshes and shallow areas e m = height marshes above boundary marshes shallow areas k = shear resistance of the soil (including vegetation) to collapse, practically constant A.4 Equilibrium configuration Combining the conservation and constitutive equations of the model in stationary conditions, and assuming f ÊS shoals y = fmá Stotal Ë ˆ, leads to the following equation: 3 2 Ê c ˆ Ê c ˆ ( 1 2W 4E 2RW) c c Á + Á Ëcout Ëcout Ê c ˆÊ c ˆ E E W WE WR E c 2 c 2 Á Á = 0 Ëcout Ë cout (A.14) E is the ratio between the rate of relative sea level rise,, and the product of the annual tidal range in the shallow areas, r s, and the sea turbidity. a E = r c s out (A.15) W is the ratio between the wind action on the shallow areas and the sea turbidity multiplied by a geotechnical factor (in square brackets). A 5

128 October, 2004 Z 2839 Long Term Morphological Modelling of Venice Lagoon W f È m = Í21- cout Î ( kb ) k D D k (A.16) If a river conveys water and sediments in the basin, two more non dimensional parameters appear in equation (A.14). R is the ratio between the annual run off volume of the river and the annual volume exchanged between channels and shoals, expressed as the product of shoals surface and average tidal range on the shoals: R = S V r shoals r s (A.17) I is the ratio between the annual sediment input from the river and the annual sediment input from the sea, should the shoals act as perfect traps: I = I x + I y S r c total s out (A.18) The following equations, combined with, (A.14) provide the non dimensional quantities that define the equilibrium configuration of a lagoon, namely the cross section A c of the channel at the inlet, the surface S m of the marshes, the depth h s of the shoals and the elevation e m of the marshes. S S m Ê cc ˆ Á - E c out = Á 2W Á Ë (A.19) È Ê c ˆ c Í2W -Á -E h c s out = Í Ë D k Í Ê c ˆ c E 2k Í Á - D ÍÎ Ëcout (A.20) em a 2 max = 1-4E cc (A.21) c out where: a max = the amplitude of the maximum annual spring tide. A 6

129 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, 2004 B Sediment transport formulas Sediment transport plays an important role in coastal engineering problems. The export of sediment from an area results in erosion of this area, the import of sediment concludes in sedimentation. Transport of sediment is defined as the product of sediment concentration and velocity. S = cu (B.1) Integrated over the water depth h Ú 0 ( ) ( ) S = c z U z dz (B.2) In coastal areas, distinction can be made between two physical conditions, namely the movement of water due to waves and water movement due to current. The wave induced friction mainly loosens material on the bottom and stirs it up, currents mainly transport material in vertical direction. Besides the distinction in physical condition, published sediment transport formulas take account for two different transport mechanisms, both bottom transport and suspended transport. Bottom transport or bed load, is the movement of particles in a layer near the bottom. Suspended transport or suspended load, is the transported material in the layer some height above the bottom. Many researches have been done to predict the movement of sediment in different physical conditions, resulting in sediment transport formulas. Many of these formulas are complex and none provides an ideal answer to the system of practical movement. The basic structure for most formulas is the product of a coefficient and a powered velocity. S = mu n (B.3) S = sediment transport in m 3 s 1 m 1 m = coefficient U = (average) current velocity in ms 1 n = power varying from 3 to 6 The value of m and n varies very strongly between different sediment transport formulas. Using different transport formulas in calculating the bottom and suspended transport, the results can change by a factor more then 10. B 1

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131 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, 2004 C Lagoon wave climate The wave climate in the lagoon is an important factor for transport and erosion of bed sediments. The actual wind wave condition is a function of the wind speed, the fetch, the wind duration and the water depth. The wave conditions are calculated according to Bretschneider (1952). This routine assumes sufficient duration of the wind for the full development of the waves. It is found that for the relevant conditions the duration is not a significant factor, as the wind is only required to blow for approximately one hour to obtain steady conditions. The fetch is typically in the range from one to ten kilometres. Considering the transport of sediment an average between the wave heights and periods predicted for a fetch of one and ten kilometre is taken. These average parameters are not far from the predictions made for a fetch of three kilometres. The average wave heights and wave periods generated by wind, are presented in the following figures: Wind wave height wave height [m] 0,7 0,6 0,5 0,4 0,3 0,2 0,1 depth 2m depth 1m depth 0,5m wind speed [m/s] Figure C 1: Wind wave height at the lagoon. Wind wave period wave period [s] 3,5 3 2,5 2 1,5 1 0,5 depth 2m depth 1m depth 0,5m wind speed [m/s] Figure C 2: Wind wave period at the lagoon. C 1

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133 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, 2004 D Offshore wave climate The deep water wave climate condition at the location D (figure D 2) is shown in figure D 1. The data is obtained by transformation of the wind climate at the CNR tower (Appendices E). 330 N 0 N 8% 7% 6% 30 N 5% 300 N 4% 3% 2% 1% 60 N 4 m 3.5 m 3 m 2.5 m 270 N 0% 90 N 2 m 1.5 m 1 m 0.5 m 240 N 120 N 0 m 210 N 150 N 180 N Figure D 1: Deep water wave climate at location D. Venice Lagoon location D Adriatic Sea Figure D 2: Location where the deep water wave climate is calculated. D 1

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135 Long Term Morphological Modelling of Venice Lagoon Z 2839 October, 2004 E Offshore wind climate The wind climate at the CNR tower (figure E 2) is shown in figure E 1. The data is obtained from measurements at this location between 1988 and Figure E 1: Wind climate at the CNR tower. Venice Lagoon CNR tower Adriatic Sea Figure E 2: Location CNR tower. E 1