Pilot study Gdynia Numerical modelling

Transcription

1 Pilot study Gdynia Numerical modelling "Application of ecosystem principles for the location and management of offshore dumping sites in SE Baltic Region (ECODUMP) Tomasz Marcinkowski, Tomasz Olszewski Department of Aquatic Ecology Maritime Institute in Gdańsk Gdańsk, December 2014

2 TABLE OF CONTENTS 1. Introduction... Error! Bookmark not defined. 2. Location and overall characteristics of Gdynia dumping site... Error! Bookmark not defined. 3. Disposal of dredged material and its resuspension... Error! Bookmark not defined. 4. Environmental conditions... Error! Bookmark not defined. 5. Numerical modeling... Error! Bookmark not defined MIKE21 general information... Error! Bookmark not defined Mesh generation, input data... Error! Bookmark not defined Initial conditions... Error! Bookmark not defined Environmental forcings... Error! Bookmark not defined Scenarios... Error! Bookmark not defined. 6. Verification of data sources for boundary conditions... Error! Bookmark not defined Sea level variations... Error! Bookmark not defined Wave parameters... Error! Bookmark not defined. 7. Numerical model validation... Error! Bookmark not defined. 8. Calculation results... Error! Bookmark not defined Scenario Dredged material deposition case study "A".. Error! Bookmark not defined Scenario dredged material deposition case study "B".. Error! Bookmark not defined Scenario resuspension process case study "C"... Error! Bookmark not defined. 9. Summary... Error! Bookmark not defined. References... Error! Bookmark not defined. 2

3 LIST OF FIGURES FIG LOCATION OF GDYNIA DUMPING SITE IN THE GULF OF GDAŃSK... 6 FIG BOTTOM TOPOGRAPHY OF GDYNIA DUMPING SITE... 7 FIG DUMP SCOW LOADING IN A PORT (LEFT). DREDGED MATERIAL LOADED ON A DUMP SCOW AND READY TO BE TRANSPORTED (RIGHT)... 9 FIG DISCRETIZATION OF THE COMPUTATIONAL AREA WITH A GRID OF TRIANGULAR ELEMENTS (FLEXIBLE MESH TYPE), CONCENTRATED IN THE REGION OF GDYNIA DUMPING SITE FIG BOUNDARY CONDITION IN THE MIKE MODEL AS A TIME SERIES OF RELATIVE CHANGES IN SEA WATER LEVEL [M], SIGNIFICANT WAVE HEIGHT [M], WAVE PERIOD [S] AND DIRECTION OF WAVE PROPAGATION [0] (ARROWS)FIG FIG BOUNDARY CONDITION IN THE MIKE MODEL AS A TIME SERIES OF RELATIVE CHANGES IN SEA WATER LEVEL [M], SIGNIFICANT WAVE HEIGHT [M], WAVE PERIOD [S] AND DIRECTION OF WAVE PROPAGATION [0] (ARROWS)FIG FIG REAL FORCING CONDITION IN THE MIKE MODEL AS A TIME SERIES OF CHANGES IN WIND SPEED [M/S] AND WIND DIRECTION FIG REAL FORCING CONDITION IN THE MIKE MODEL AS A TIME SERIES OF CHANGES IN WIND SPEED [M/S] AND WIND DIRECTION FIG FORCING CONDITION AS A SCENARIO IN THE MIKE MODEL A TIME SERIES OF CHANGES IN WIND SPEED [M/S] AND WIND DIRECTION FIG CHANGES IN SEA WATER LEVEL [CM], FROM TO , AT 5 COASTAL WATER GAUGE STATIONS (VISTULA RIVER MOUTH, NORTHERN PORT OF GDAŃSK, GDYNIA, PUCK, HEL) FIG COMPARISON OF CHANGES IN THE WATER LEVEL [CM] AT THE WATER GAUGE STATIONS GDYNIA (TOP) AND HEL (BOTTOM) WITH DATA OBTAINED FROM THE HIROMB MODEL FIG LOCATION OF AN AWAC INSTRUMENT (Λ= , Φ= ) FIG COMPARISON OF SIGNIFICANT WAVE HEIGHT HS [M] TIME CHANGES MEASURED WITH THE AWAC AND OBTAINED FROM THE REGIONAL WAM MODEL FIG COMPARISON OF WAVE PERIOD T P [S] TIME CHANGES MEASURED BY AWAC AND OBTAINED FROM THE REGIONAL WAM MODEL FIG COMPARISON OF SEA LEVEL CHANGES IN TIME ( ) BETWEEN THE RESULTS FROM THE MIKE AND HIROMB MODELS AND DATA MEASURED BY AWAC IN THE CHOSEN POINT WITHIN THE DUMPING SITE FIG COMPARISON OF SIGNIFICANT WAVE HEIGHT CHANGES IN TIME ( ) BETWEEN THE RESULTS FROM THE MIKE MODELS AND DATA MEASURED BY AWAC IN THE CHOSEN POINT WITHIN THE DUMPING SITE FIG COMPARISON OF CURRENT VELOCITY CHANGES IN TIME ( ) BETWEEN THE RESULTS FROM THE MIKE MODELS AND DATA MEASURED BY AWAC IN THE CHOSEN POINT WITHIN THE DUMPING SITE FIG RESULTS OF THE SIMULATION OF DREDGED MATERIAL DISPOSAL UNDER REAL HYDRODYNAMIC CONDITIONS, FOR THE CASE STUDY A, AT T1 TIME STEP (WHITE SPOT POINT OF DISPOSAL WITHIN THE DUMPING SITE) FIG RESULTS OF THE SIMULATION OF DREDGED MATERIAL DISPOSAL UNDER REAL HYDRODYNAMIC CONDITIONS, FOR THE CASE STUDY A, AT T2 TIME STEP (WHITE SPOT POINT OF DISPOSAL WITHIN THE DUMPING SITE) FIG RESULTS OF THE SIMULATION OF DREDGED MATERIAL DISPOSAL UNDER REAL HYDRODYNAMIC CONDITIONS, FOR THE CASE STUDY A, AT T3 TIME STEP (WHITE SPOT POINT OF DISPOSAL WITHIN THE DUMPING SITE) FIG RESULTS OF THE SIMULATION OF DREDGED MATERIAL DISPOSAL UNDER REAL HYDRODYNAMIC CONDITIONS, FOR THE CASE STUDY A, AT T4 TIME STEP (WHITE SPOT POINT OF DISPOSAL WITHIN THE DUMPING SITE) FIG RESULTS OF THE SIMULATION OF DREDGED MATERIAL DISPOSAL UNDER REAL HYDRODYNAMIC CONDITIONS, FOR THE CASE STUDY A, AT T1 TIME STEP FIG RESULTS OF THE SIMULATION OF DREDGED MATERIAL DISPOSAL UNDER REAL HYDRODYNAMIC CONDITIONS, FOR THE CASE STUDY B, AT T1 TIME STEP (WHITE SPOT POINT OF DISPOSAL WITHIN THE DUMPING SITE) FIG RESULTS OF THE SIMULATION OF DREDGED MATERIAL DISPOSAL UNDER REAL HYDRODYNAMIC CONDITIONS, FOR THE CASE STUDY B, AT T2 TIME STEP (WHITE SPOT POINT OF DISPOSAL WITHIN THE DUMPING SITE)

4 FIG RESULTS OF THE SIMULATION OF DREDGED MATERIAL DISPOSAL UNDER REAL HYDRODYNAMIC CONDITIONS, FOR THE CASE STUDY B, AT T3 TIME STEP FIG RESULTS OF THE SIMULATION OF DREDGED MATERIAL DISPOSAL AND THE EFFECT OF RESUSPENSION UNDER REAL HYDRODYNAMIC CONDITIONS, FOR THE CASE STUDY C, AT T1 TIME STEP FIG RESULTS OF THE SIMULATION OF DREDGED MATERIAL DISPOSAL AND THE EFFECT OF RESUSPENSION UNDER REAL HYDRODYNAMIC CONDITIONS, FOR THE CASE STUDY C, AT T2 TIME STEP FIG RESULTS OF THE SIMULATION OF DREDGED MATERIAL DISPOSAL AND THE EFFECT OF RESUSPENSION UNDER REAL HYDRODYNAMIC CONDITIONS, FOR THE CASE STUDY C, AT T3 TIME STEP

5 1. Introduction This report constitutes a part of the project "Application of ecosystem principles for the location and management of offshore dumping sites in SE Baltic Region (ECODUMP). The aim of this study was to investigate the usefulness of numerical models for the analysis of hydrodynamic processes occurring during marine operations including disposal of dredged material at sea (offshore dumping sites). Dredged material from deepening harbour basins, characterized by a large number of fine fractions: silts and clays, is deposited in dumping sites at sea. In Poland there are 9 such dumping sites and their names are most often directly linked to the location. Therefore, they are called as follows: Gdynia, Gdańsk, DCT, Łeba, Ustka, Darłowo, Kołobrzeg, Szczecin Świnoujście Port, Szczecin Świnoujście Maritime Office. Gdynia dumping site has been chosen as a pilot research site in the area of Polish territorial waters. The main investigated issue was the dispersion process of fine grained sediments suspended in the water column. In the case of dredged material deposition, it was taken into account that the generation of suspended sediments can take place in two phases: initial during dredged material disposal, and secondary as a result of sediment resuspension caused by currents and waves. Numerical modeling can be helpful when analyzing different phenomena both in qualitative and quantitative terms. In modeling, much attention is paid to the most reliable definition of boundary and initial conditions. 5

6 2. Location and overall characteristics of Gdynia dumping site Each of the ECODUMP project partners selected an existing dumping site in the territorial waters for the purpose of numerical model construction. In the case of Poland, the choice fell on the constantly used area in the vicinity of Gdynia. Gdynia dumping site is located in the Gulf of Gdańsk near the approach channel to the port of Gdynia and approx. 8 km from the port. It has the shape of a quadrangle with the vertices located at the following points: ,6 N ,85 E ,7 N ,85 E ,7 N ,85 E ,9 N ,85 E This area covers approx. 6.4 km 2 Fig Fig Location of Gdynia dumping site in the Gulf of Gdańsk Dredged material has been deposited at Gdynia dumping site since About 4.6 million m 3 of the sediments had been disposed there up to The last disposal before starting the research took place in the period from to , amounted to 1.7 million m 3 and the material came from the port of Gdynia. It consisted mostly of fine fractions, from fine sand to sandy clayey silt and clay. Macroscopic analysis of core samples collected at the dumping site (detailed information on the research can be found in the report Dembska G. et al. (2014)) showed that the surface sediment layer consists mostly of fine sands, silty sands and silts. Fine fractions observed in sediments from the area adjacent to the dumping site indicate the dispersion of dredged material during operations of its disposal. The uppermost layer of sediments surface layer of the dumping site, can also be subject 6

7 to resuspension during extreme storm events. On the basis of the research, it has been clearly proved that the surface layer of sediments at the dumping site is of anthropogenic origin. Bottom topography of the dumping site and the directly adjacent area was described in the report Dembska G. et al. (2014). However, in this study the image of bottom topography is presented (Fig. 2.2.). Accurate bathymetric measurements conducted in the framework of the ECODUMP project were used in numerical modeling. Fig Bottom topography of Gdynia dumping site 3. Disposal of dredged material and its resuspension 3.1 Dredged material deposition Dredging works in the Polish coastal zone are usually carried out using trailing suction hopper dredgers (TSHD), bucket ladder dredgers and grab dredgers. They are equipped with own hoppers or operate together with self propelled and non self propelled dump scows. Dredged material obtained as a result of works aimed at maintenance of sufficient depth of waterways in port aquatories and harbor basins is most often deposited at offshore dumping sites. Dredged material can be transported to a disposal site with a pipeline, in dredger hoppers or in dump scows. Disposal of dredged sediments and physical phenomena occurring when it is discharged into the sea at the dumping site, depending on the technology used, has been shown schematically in Fig

8 Transport by pipeline Deposition from a hopper dredger Deposition from a dump scow sea current Fig Physical phenomena during deposition of dredged material with the use of different technologies (EPA/USACE, 2004) The slurry (mixture of sediment and water) is transported through pipelines. All sediment fractions are largely mixed. Consequently, the use of pipelines gives the greatest spatial dispersion of sediments on the bottom, and the largest amount of dredged material goes into suspension. In the case of discharge of the material from a TSHD or from dump scows (this technology is used for the investigated Gdynia dumping site), the process of deposition is different. The consistency of the material in hoppers is not uniform i.e., cohesive sediments are present as compacted, plastic lumps and in a liquid state. Non cohesive sediments undergo initial segregation as a result of transport. When emptying the hopper, non cohesive sediments and lumps of cohesive sediments fall onto the bottom quickly and basically do not disperse, however, they cause additional disturbance of deposits on the bottom and put them in suspension. The finest fractions in part of the material that remains in a liquid state while dropped in the water column, form a plume which is subject to transport processes. A self propelled dump scow which is transporting dredged material has been shown in Fig Such vessels are most commonly used during works connected with dredging within the areas of Polish ports. The carrying capacity of such scows may exceed 1000 m 3, their maximum speed during transport is 5 7 knots, whereas during discharge the speed is reduced to 1 2 knots. Dump scows can be unloaded either through hatches in their bottom or by hydraulic opening of the hull (split hull dump scow). 8

9 Fig Dump scow loading in a port (left). Dredged material loaded on a dump scow and ready to be transported (right) Various, in terms of grain size composition, sediments can be deposited in different ways. Sands and gravels almost immediately fall onto the bottom and stay there. Single fine particles of diameter of mm (silts) and <0.002 mm (clays) behave differently. In contrast to the coarse fraction, i.e. sands, gravels and boulders, those fine particles remain in the water quite long before they finally settle on the bottom. Their discharge may cause long term and long range turbidity, which can have a significant impact on marine ecosystem functioning. Turbidity plumes composed of suspended silt and clay particles can move over distances of more than ten kilometers before they fully settle on the bottom. Key elements to be determined in terms of environmental impact assessment of the project are as follows: duration of the suspension, its concentration and range. 3.2 Resuspension Sediments on the seabed, particularly those deposited relatively recently, can be characterized by a loose structure. Thus, any enforcement in the form of increased waves or stronger currents, for which the critical value of the shear stress in the surface layer of sediments is exceeded, will cause dislodging and dispersion of sediment particles. In the model, the finest material which could go into suspension is represented by the fraction for which the settling velocity was assumed to be 0.5 mm/s (settling velocity for medium silts). For such material, the critical shear stress required to start erosion is 0.1 Pa. The critical shear stress below which the sedimentation process begins was set at 0.07 Pa. The process may involve both single particles in the initial phase and intensive mass transport under considerable exciting force. This phenomenon is called resuspension and occurs almost every time when the seabed is disturbed. Remobilization of suspension can also be caused by mechanical factors, such as dragging a net, trawl or anchor across the bottom and, to a lesser extent, by underwater works performed by divers. Moreover, sediment resuspension can be induced by deposition of dredged material or occur when pipelines are embedded in bottom sediments. Every issue concerning the modeling of suspended sediment dispersion, in particular its concentration, range and duration, requires the occurrence of resuspension to be considered. The amount of sediments dislodged from the bottom can be significant under favorable hydrological conditions. This results in the occurrence of sediment suspension plumes of high concentration (Fig. 3.3) which can travel long distances before they are dispersed or undergo sedimentation. 9

10 In this study, the process of resuspension is analyzed for the period after deposition of dredged material at the dumping site and while meteorological and hydrodynamic conditions correspond to storm conditions. Fig Underwater plume of suspended sediments generated in laboratory conditions 4. Environmental conditions Knowledge of environmental conditions controlling a marine water body and acting within this body of water is necessary for the construction of a numerical model to simulate hydrodynamic processes occurring during deposition of dredged material on a dumping site and to analyse the behaviour of deposited sediments under the influence of environmental forcings. The best results in numerical models are achieved when empirical data (based on measurements) are used as boundaries of these models. However, such situation happens extremely rarely. Therefore, data from regional models or hypothetical scenarios are most frequently used in practical application. In each case, the model in the construction phase is verified by measurements at specific points, for example, marine stations, measurement buoys or measurement sensors. The sources of information which were used in determination of boundary, initial and forcing conditions in the numerical model constructed for Gdynia dumping site have been presented below. 4.1 Meteorological conditions Fundamental forcing conditions, largely responsible for generation of currents, wind waves, and contributing to the variability in water levels, are wind conditions. In particular, the parameters which should be determined are: direction and speed of the wind. In the modeling, wind forcing was applied as "real" or hypothetical (e.g. modeling of extreme events) distribution of two dimensional variable (wind speed and direction). In the case of the application of real wind forcings, data from one of the following regional scale atmospheric models: COAMPS, UMPL or HIRLAM, were used for specific points. 10

11 For the area of the Gulf of Gdańsk, measurements of wind parameters are carried out by the Institute of Meteorology and Water Management at the first order stations in Hel and Gdańsk (Northern Port). Previously conducted studies showed that there are differences between the results of measurements of offshore conditions and those at shore based stations. 4.2 Hydrological conditions For the Baltic Sea, such parameters as sea water level variations and the current field are included in HIROMB model with a grid resolution of 1 n.m. As boundary conditions, it is essential to determine variations in sea water level. This condition can be combined with the characteristics of spatiotemporal flows at offshore boundaries of the model (components of velocity and directions). There is also the possibility to include a separated condition in the form of sea water level variation, components of flow velocity or discharge at the adopted boundaries. Sea water level observations in the area of the Gulf of Gdańsk are conducted at 5 coastal water gauge stations in: Hel, Northern Port of Gdańsk, Gdynia, Puck and at the Vistula River Mouth. 4.3 Wave conditions Surface waves, described in a satisfactory way, are included in the spectral WAM model (Gűnter, 2002). This model has been repeatedly verified and shows its practical usefulness (Cieślikiewicz and Swerpel 2005, Paplińska 1999). In the calculations, on which sea waves have significant impact, it is necessary to determine characteristics of this motion in the spectral form for appropriate boundaries. In this context, wave motion can be described by: significant wave height H S, wave peak period T P and wave direction θ together with the spreading factor n. Moreover, wave motion at the boundary of the model can be imposed as wind sea waves with or without taking into account low frequency swell. In the area of the Gulf of Gdańsk, continuous measurements of waves are not carried out. However, within the ECODUMP project, periodical measurements of waves, currents and variability in sea water level were carried out at the point located within Gdynia dumping site. 5. Numerical modeling 5.1 MIKE21 general information Numerical computations were performed with the use of the licensed, Danish software package MIKE 21 Coupled Model FM. It is a computation package, which has been developed for years in the Danish Hydraulic Institute (DHI), intended for calculating water flows, wave motion and sediment transport in the coastal zone of the sea and in the open sea. The Coupled version enables the simulation of hydraulic issues in a dynamic manner and with a mutual interaction of all the applied modules. This program is widely used in the world (inter alia Burcharth et al., 2007). For the purpose of performing computations related to the transport of suspended sediments, which occurs during the disposal of dredged material, the following modules were used: Hydrodynamic module (HD), Spectral Wave module (SW), 11

12 Mud Transport module (MT). The hydrodynamic (HD) module allows to simulate variations of current field and water level in response to a variety of forcing functions in the open sea and coastal areas. In the module, the following hydraulic effects and facilities are included in computations (DHI, 2013a): Bottom shear stress and wind shear stress, Barometric pressure gradients, Coriolis force, Sources and sinks, Rainfalls and evaporation, Flooding and drying of water areas, Wave radiation stresses. The spectral wave module (SW) enables to calculate the parameters of the wave field (height, period and direction of waves) generated by the wind with direction and speed varying in time. In the performed computations the following was considered (DHI, 2008): Nonlinear wave wave interaction, Dissipation due to bottom friction, white capping and depth induced wave breaking, Refraction and shoaling due to depth variations, Nonlinear interaction of waves with currents, Storm surges variable in time, Coexistance of wind waves and swell. The mud transport module (MT) describes the erosion, transport and sedimentation of the finest fractions, caused by the impact of sea currents and wave motion. The MT module can be used both for silt and clay sediments, as well as for the mixture of these sediments with sand, in which, however, fine fractions predominate and a significant feature of such mixture is its cohesion (DHI, 2013b). The MIKE 21 software package reaches the maximum of its effectiveness in the case of the simulation of medium period hydrodynamic phenomena, lasting from a couple of days to a couple of months, in a limited basin. The methodological construction of the numerical model involves the limitation of the computation area by boundaries and generation of a numerical mesh corresponding with the surveyed issue. Physical conditions must be defined in the boundaries of the model for each of the considered issues, which are called boundary conditions. These conditions can originate from other models, actual surveys or can be formulated in an artificial way, so as to analyze extreme events or the accurately determined forcings. In the case of entering the conditions obtained from computations in the boundaries of the model, it is suggested to verify these values with survey data if such exist. It is also crucial to determine the state of the model in the moment of its commencement by the so called initial conditions. The results of initial in situ surveys carried out in the area for which the numerical model is created, enable a more precise recreation of the environment s characteristics. 12

13 The next methodological step in model construction is its validation and, in result, if it is necessary, its calibration. For the issues included in this work, the calibration of the results was carried out based on surveys concerning variability in the water level, sea currents and data characterizing the wave field in the selected places. 5.2 Mesh generation, input data The hydrodynamic model was constructed based on a numerical grid of Flexible MESH type (diverse triangular and rectangular elements). For the issues related to the disposal of dredged material and resuspension of sediments, a grid of elements and nodes, with a maximum area of a single element not greater than m 2 was applied for the model. Bathymetric data obtained from multibeam echosounder measurements were assigned to each mesh. Resolution of bathymetric data for the area of Gdynia dumping site was 5x5 m, for the western part of the Gulf of Gdańsk 25x25 m and for the rest of the basin 1000x1000 m (Error! Reference source not found.). Fig Resolution of bathymetric points 13

14 Fig Discretization of the computational area with a grid of triangular elements (Flexible MESH type), concentrated in the region of Gdynia dumping site In order to obtain the most accurate results of the simulation, in the area of Gdynia dumping site and the adjacent area, the numerical grid was gradually refined so that the surface of the smallest grid elements did not exceed m 2 (Fig. 5.1). To achieve the highest accuracy of numerical calculations, it is necessary to choose boundary conditions which are the most similar to real conditions. Their determination, as previously mentioned, requires a detailed analysis of measurement data or the use of data from regional numerical models. In the absence of real measurement data, which is common situation for marine areas, in engineering practice, the results of operational numerical models are used. Variability in the water level, spatio temporal characteristics of the flows at offshore boundaries of the model (velocity or fluctuation and direction) and basic parameters of surface wave motion presented in the spectral form were defined as boundary conditions. For the issues presented in this study, several different sets of boundary conditions were applied: A. for the real situation characterized by benign environmental conditions, which occurred in the period from to in the southern Baltic Sea; B. for the real situation characterized by moderate environmental conditions, which occurred in the period from to ; C. for the real situation lasted from to , including extreme storm conditions (storm Xaver). It should be emphasized that the boundary conditions corresponding to the first two situations ( A and B ) represent calm and moderate sea states, not exceeding level 3 of sea, which allow to carry out such works as deposition of dredged material from self unloading barges. On the other hand, the 14

15 third situation ( C ), concerning much more dynamic environmental conditions, was selected to evaluate the intensity of resuspension of previously deposited sediments. For example, for the A situation, boundary condition related to variability in the water level and parameters characterizing the wave spectrum at the northern boundary (offshore) of the model has been shown in Fig Boundary condition in the MIKE model as a time series of relative changes in sea water level [m], significant wave height [m], wave period [s] and direction of wave propagation [0] (arrows)fig Fig Boundary condition in the MIKE model as a time series of relative changes in sea water level [m], significant wave height [m], wave period [s] and direction of wave propagation [0] (arrows)fig Similarly, for the B situation, boundary condition related to variability in the water level and parameters characterizing the wave spectrum at the northern boundary of the model has been shown in Fig Boundary condition in the MIKE model as a time series of relative changes in sea water level [m], significant wave height [m], wave period [s] and direction of wave propagation [0] (arrows)fig

16 Fig Boundary condition in the MIKE model as a time series of relative changes in sea water level [m], significant wave height [m], wave period [s] and direction of wave propagation [0] (arrows)fig Boundary condition concerning a time series of water level variability and parameters characterizing the wave spectrum at the northern boundary of the MIKE model for the most dynamic C situation has been presented in Fig Boundary condition in the MIKE model as a time series of relative changes in sea water level [m], significant wave height [m], wave period [s] and direction of wave propagation [0] (arrows) Fig Boundary condition in the MIKE model as a time series of relative changes in sea water level [m], significant wave height [m], wave period [s] and direction of wave propagation [0] (arrows) 16

17 5.3 Initial conditions An initial condition determines the physical state of a mathematical model at the moment (t 0 ) of starting numerical simulation. For the issues described in this study, the initial conditions are as follows: in the hydrodynamic module (HD): spatial distribution of sea water levels, distribution of velocities for current field, in the spectral wave module (SW): spatial distribution of waves, in the mud transport module (MT): concentration of different fractions of suspended sediments, thickness of all the mobile sediment layers. In the implemented simulations, water level value at the time t 0 for the entire water body corresponds to the water level value of the boundary condition at the analogous point in time, and surface waves are not taken into account. Additionally, concentration of different fractions of suspended sediments, as an unknown value in the investigated water body, was set at zero level (implementation of the excess type issue). 5.4 Environmental forcings The environmental forcings in the conducted numerical simulations are: sea waves, sea currents, variability in the water level, as well as the effect of wind over the water body, which, as a result of the sheer stress, transfers the energy affecting wave motion and current variability. While the first three factors, mentioned above, are imposed in the boundary conditions of specific model configurations, the last condition is defined as variable in time and standardized for the entire study area. Depending on the adopted computational scenarios, it is assumed to be a "real" distribution of two dimensional variable (wind speed and direction) the result of the atmospheric regional scale model HIRLAM. Wind forcings applied in the presented calculations have been shown in Fig Real forcing condition in the MIKE model as a time series of changes in wind speed [m/s] and wind direction, Fig. 5.5, Fig. 5.6Error! Reference source not found. which correspond to the situations described above: A, B and C. 17

18 Fig Real forcing condition in the MIKE model as a time series of changes in wind speed [m/s] and wind direction Fig Real forcing condition in the MIKE model as a time series of changes in wind speed [m/s] and wind direction 18

19 Fig Forcing condition as a scenario in the MIKE model a time series of changes in wind speed [m/s] and wind direction It should be noted that works connected with dredged material disposal from self unloading dump scows and with embedding power cables or pipelines are conducted when a sea state do not restrict the vessel s ability to operate. Thus, the application of environmental forcings at low or moderate levels is most correct. 5.5 Scenarios Dredged material deposition Dredging works are carried out under favorable environmental conditions: weak waves and calm or moderate wind conditions. The issue of dredged material deposition was analyzed assuming A and B boundary conditions presented in section 5.2, and under appropriate wind forcing characterized in section 5.4. In the study periods, different direction of winds occurred and it was included in the analysis of suspended sediment dispersion. The adopted scenarios were based on mathematical modeling of processes that are close to those occurring during actually performed dredging works. Multi bucket or single bucket dredgers, similarly as in the case of the port in Gdynia, deepen harbor basins and load dredged material onto dump scows. In the scenario, self propelled and split hull dump scows were used, in order to allow for self unloading. The carrying capacity of SM660 dump scow is 660 m 3. The analysis of scow operating time enabled to determine the period of time between subsequent discharges of dredged material on the dumping site. In the model scenario, it was assumed that dredged material is unloaded four times a day for the following 25 days and the time of unloading is 3 min. Each time, dredged material discharged from a dump scow consisted of: lumps of cohesive sediment, fine grained sand, mixture of silts, clays and water. For the purpose of numerical modeling, it was assumed that one half of the load was fine sand and the other half was mostly silty material. Only about 3% of sand fraction goes into suspension, while in case of silt fraction with high water content, the amount which can be suspended is much higher, reaching up to 35%. The information concerning dredged material and dredging machinery/equipment was obtained from the Port of Gdynia. The most important, from the point of view of numerical 19

20 modeling, are the particles of diameters smaller than mm, which go into suspension and remain as suspended particles for a period from several to several tens hours. Non cohesive sediments and lumps of cohesive compacted solids reach the bottom after a short time and remain there. Time, place and load of every single discharge are assumed within each computational scenario. Dredged material (divided into fractions) is defined by parameters characterizing its behavior in the water column, such as: fall velocity, limiting concentrations for the flocculation process or initiation of hindered settling, dispersion and critical shear stress, below which deposition occurs. Furthermore, the parameters such as: bottom roughness, density of different sediment layers, critical shear stress for resuspension process in different bottom layers, are defined. Gdynia dumping site can be characterized by relatively large water depths in the range of m. For modeling dredged material discharges of small volume, morphological changes, having a negligible effect on flow hydrodynamics, were not taken into account in the scenarios. Resuspension process Resuspension of sediments can occur only in the environmental conditions described as storm conditions. These conditions exceed acceptable limits for using dredging equipment. As a result, in order to consider the process of resuspension, the following scenario has been proposed: environmental forcings described in the situation B are appropriate to conduct disposal operations, and extreme forcings, described in the situation C occur immediately after the disposal and can potentially cause resuspension process. The boundary conditions for this scenario have been presented in the description of C situation in section 5.2. It has been assumed that immediately after the deposition of dredged material, the surface layer of the seabed within the area of the dumping site is partially covered with new sediments and the conditions for resuspension of these particles have been characterized in section 3. Forcing conditions in the scenario prepared to analyze the process of resuspension are the conditions adopted from the real storm Xaver, which probability of occurring is lower than once in 50 years. 6. Verification of data sources for boundary conditions Due to the lack of measurement data, for the local MIKE model, it was decided to use boundary conditions and environmental forcings taken from the regional scale models (e.g. HIROMB, WAM). In order to verify this approach, the results of the regional models were compared with measurements conducted at hydrological coastal stations and with the results from the measuring instrument installed in the area used for model computations. 6.1 Sea level variations Temporal variability of sea water levels [cm] in the period from to was analyzed at 5 coastal water gauge stations (Vistula River Mouth, Northern Port of Gdańsk, Gdynia, Puck, Hel). The results from observations at these stations comply with one another very well (Fig Changes in sea water level [cm], from to , at 5 coastal water gauge stations (Vistula River Mouth, Northern Port of Gdańsk, Gdynia, Puck, Hel) ). It is reflected by high values of linear correlation coefficients (r> 0.9), which are a measure of linear, statistical relationship between two 20

21 processes. For example, for two hydrodynamically extreme coastal stations, Hel and Vistula River Mouth, the linear correlation coefficient is r=0.95. Fig Changes in sea water level [cm], from to , at 5 coastal water gauge stations (Vistula River Mouth, Northern Port of Gdańsk, Gdynia, Puck, Hel) For further analysis, stations (Gdynia and Hel) located close to the modeled area were selected. The results of observations at these stations, for a period of one month ( ), were compared with the results obtained from the regional model HIROMB. In order to be able to compare the results, changes in sea water level obtained from the model (relative) were converted to absolute values, in such a way that their compliance was related to the average values for the investigation period at each station. Results of the comparison revealed that the values were similar for both cases: Gdynia HIROMB and Hel HIROMB (Fig. 6.2), and the coefficients of linear correlation are as follows: r Gdynia HIROMB = 0.96, r Hel HIROMB =

22 Fig Comparison of changes in the water level [cm] at the water gauge stations Gdynia (top) and Hel (bottom) with data obtained from the HIROMB model Due to the fact that the offshore boundary of the local scale numerical model MIKE is considerably far from the shoreline (Fig. 5.2, section Error! Reference source not found.), it is necessary to choose the correct boundary condition for sea water changes. The above analysis, which confirmed high compliance of sea level changes from the HIROMB model and data obtained from the measurements at coastal stations (Gdynia, Hel), justifies the acceptance of data from the regional HIROMB model for further numerical computations in the MIKE model. 6.2 Wave parameters Measurement data from the selected point within Gdynia dumping site were obtained based on the registration of current velocities and wave heights by an acoustic current profiler of AWAC type (Acoustic Wave And Current) set at that point (location of the instrument has been presented in Fig. 6.3). 22

23 Fig Location of an AWAC instrument (λ= , ϕ= ) The results of significant wave heights H s [m] obtained from the measurements by AWAC, for a period of two months ( ), were compared with the results obtained from the regional WAM model (WAM grid point closest to the dumping site). Comparison of time series variation in H s from measurements and from the model has been shown in Fig Fig Comparison of significant wave height Hs [m] time changes measured with the AWAC and obtained from the regional WAM model The graph shows high compatibility of two series of data, and the linear correlation coefficient is r =

24 This analysis also confirms high compliance of the results of significant wave height changes from the WAM model with observational data from AWAC instrument, which justifies the acceptance of data from the regional model WAM for further numerical computations in the MIKE model. Similarly, the results of wave period T p [s] measurements with an AWAC ( ) were compared with the results obtained from the regional WAM model. The comparison of time series variations in T p, from the measurements and from the model, has been presented in Fig Fig Comparison of wave period T p [s] time changes measured by AWAC and obtained from the regional WAM model The graph shows high compatibility of two data series, and the linear correlation coefficient r equals Numerical model validation After numerical simulations conducted with the use of the local scale MIKE model (in which data from regional models HIROMB and WAM were used as boundary conditions and forcing), the results were compared with the real data obtained from measurements at the selected point within the dumping site. The purpose of this comparison was to calibrate the hydrodynamic model, based on measurement data. The following figures show the results of the comparison of data from numerical computations after the model calibration process, and measurement data collected by AWAC instrument. Error! Reference source not found. presents a time series of changes in sea water level, while in Fig. 7.2 we can see the comparison of the variation in time of significant wave height H s. Fig. 7.3 shows the comparison of sea current velocities at the same point. A good agreement of the measurement data with the results of computations was observed for sea water levels, wave heights and current velocities. 24

25 Calibration of the hydrodynamic model included changes in the bottom roughness and characteristics of wind friction. The application of appropriate changes led to the improvement in the agreement between calculated and measured sea currents. Fig Comparison of sea level changes in time ( ) between the results from the MIKE and HIROMB models and data measured by AWAC in the chosen point within the dumping site Fig Comparison of significant wave height changes in time ( ) between the results from the MIKE models and data measured by AWAC in the chosen point within the dumping site Fig Comparison of current velocity changes in time ( ) between the results from the MIKE models and data measured by AWAC in the chosen point within the dumping site 8. Calculation results 8.1 Scenario dredged material deposition case study A 25

26 The results of simulation concerning the dumping of dredged material have been presented for the subsequent time steps (t1, t2, t3, t4, t5) in the figures, Error! Reference source not found., Error! Reference source not found., and Fig. 8.5, in the following way: a) graphs of wind speed and direction variability in time, over the basin (black vertical line represents the moment of simulation shown on the maps of currents, waves and concentrations), b) map of current circulation in the Gulf of Gdańsk (directions and averaged velocities), c) map of suspended sediment dispersion in the zoomed area including Gdynia dumping site (concentration of suspended solids in kg/m 3, which is equal to g/l), and: d) in the case of the last figure (Fig. 8.4) map of changes at the sea bottom within the area of Gdynia dumping site after disposal works, where the subsequent time steps mean: ), 5). t1 moment in time 2 hours after the discharge under the prevailing wind from the south ( t2 moment in time 2 hours after the discharge under the prevailing wind from the west (Error! Reference source not found.), t3 moment in time 2 hours after the discharge under the prevailing wind from the east (Error! Reference source not found.3), t4 moment in time when the spatial impact of suspended sediments is the greatest (Error! Reference source not found.4), t5 moment in time when all the disposal operations have been finished ( Each set of pictures shows the simulation at subsequent time steps of the adopted calculation scenario. At t1 time step, the maximum extent of spreading of the sediment plume is moderate and it reaches a distance of approx. 2.8 km from the point of discharge, with an average concentration decreasing quickly. After 2 hours, the maximum concentration in the center of the plume located close to the point of discharge is 60 mg/l. However, in the second plume, approx. 1.1 km away from the point of discharge, after 8 hours from the time of discharge, the concentration is very low and does not exceed 6 mg/l. During this period, the wind from the south has been blowing over the basin area for a period more than ten hours. Maximum speeds of the current within the dumping site are small ( m/s). The dumping site is in the area of current circulation, and as a result, the suspended sediments disperse in a north westerly direction. 26

27 At t2 time step, the maximum extent of spreading of the sediment plume reaches about 3.2 km from the point of discharge. The concentration of suspended solids decreases rapidly. After 2 hours, the maximum concentration in the center of the plume located close to the point of discharge is 55 mg/l. In the second plume, which is approx. 1.5 km away from the point of discharge, after 8 hours the concentration is low and it equals 7 mg/l. During this period, the wind over the basin has been blowing from the west for over ten hours, at a speed not exceeding 7 m/s. Maximum speeds of the current within the dumping site, for such forcing, are in the range of m/s. The generated current circulation causes that the plume is moving in a westerly direction, i.e. in the direction opposite to the direction of the wind. At t3 time step, the maximum extent of spreading of the sediment plume reaches 1.8 km from the point of discharge. The concentration of suspended solids decreases rapidly. After 2 hours from the time of discharge, the maximum concentration in the center of the plume is 20 mg/l. The plume disappears after 6 hours from the time of discharge. During this period, the wind over the basin changes from a south eastern to eastern direction and its speed decreases from 9 to 5 m/s. Maximum speeds of the current within the dumping site for such forcing reach up to 0.2 m/s, and the direction in which the sediment plume is moving is similar to the direction of the wind. At t4 time step, the spatial area of the plume reaches the maximum size for the entire simulation. The length of the plume is 3.5 km and the maximum width equals 0.6 km. The maximum concentration reaches 67 mg/l. Such situation occured when the southern wind increased. Suspended solids are present at elevated concentrations due to low current velocity. At t5 time step, the state of the bottom after disposal and sedimentation of dredged material has been presented (all the sites of disposal can be easily seen there). A newly formed layer of sediments, locally in the areas of the disposal, has a thickness of up to 0.33 m. The analysis of sediment thickness after finishing the disposal works indicates that sediments formed from the suspension outside the area of the dumping site are negligible and their thickness in the modeling does not exceed 2 mm. 27

28 Fig Results of the simulation of dredged material disposal under real hydrodynamic conditions, for the case study A, at t1 time step (white spot point of disposal within the dumping site) 28

29 Fig Results of the simulation of dredged material disposal under real hydrodynamic conditions, for the case study A, at t2 time step (white spot point of disposal within the dumping site) 29

30 Fig Results of the simulation of dredged material disposal under real hydrodynamic conditions, for the case study A, at t3 time step (white spot point of disposal within the dumping site) 30

31 Fig Results of the simulation of dredged material disposal under real hydrodynamic conditions, for the case study A, at t4 time step (white spot point of disposal within the dumping site) 31

32 Fig Results of the simulation of dredged material disposal under real hydrodynamic conditions, for the case study A, at t1 time step 32

33 8.2 Scenario dredged material deposition case study B The results of simulation concerning the discharge of dredged material have been presented for the subsequent time steps (t1, t2, t3) in the figures Error! Reference source not found., Error! Reference source not found., Error! Reference source not found., in the following way: a) graphs of wind speed and direction variability in time, over the basin (black vertical line represents the moment of simulation shown on the maps of currents, waves and concentrations), b) map of current circulation in the Gulf of Gdańsk (directions and averaged velocities), c) map of suspended sediment dispersion in the zoomed area including Gdynia dumping site (concentration of suspended solids in kg/m 3, which is equal to g/l), and d) in the case of the last figure (Error! Reference source not found.) map of changes at the sea bottom within the area of Gdynia dumping site after disposal works, where the subsequent time steps mean: t1 moment in time 2 hours after the discharge under the prevailing wind from the southwest (Error! Reference source not found.6), t2 moment in time 2 hours after the discharge under the prevailing wind from the west (Error! Reference source not found.7), t3 moment in time when all the disposal operations have been finished (Error! Reference source not found.). Each set of pictures shows the simulation at subsequent time steps of the adopted calculation scenario. At t1 time step, the maximum extent of spreading of the sediment plume is minor and it reaches a distance of approx. 1.4 km from the point of discharge, with an average concentration decreasing quickly. After 2 hours, the maximum concentration in the center of the plume located closer to the point of discharge is 30 mg/l. However, in the second plume, approx. 1.3 km away from the point of discharge, after 8 hours from the time of discharge, the concentration is very low and does not exceed 4 mg/l. During this period, the wind from the south west has been blowing in the basin area for over ten hours. Maximum velocities of the current within the dumping site are small ( m/s). The sediment plume is moving in a north westerly direction i.e., rotated 90 o in relation to the wind direction. At t2 time step, the maximum extent of spreading of the sediment plume reaches about 2.4 km from the point of discharge. The concentration of suspended solids decreases rapidly. After 2 hours, the maximum concentration in the center of the plume located closer to the point of discharge is 47 mg/l. In the second plume, which is approx. 2.2 km away from the point of discharge, after 8 hours the concentration is low and it equals 4 mg/l. During this period, the wind over the basin has been blowing from the west for over ten hours, at a speed not exceeding 9 m/s. Maximum velocities of the current within the dumping site, for such forcing, are in the range of m/s. The generated 33