Modelling and mapping of urban storm water flooding Using simple approaches in a process of Triage.

Transcription

1 Modelling and mapping of urban storm water flooding Using simple approaches in a process of Triage. Blanksby J.* Kluck J.**, Boogaard F.C. *+ ***, Simpson S.****, Shepherd W.* and Doncaster S.* * Pennine Water Group, Department of Civil and Structural Engineering, University of Sheffield, UK ** Tauw bv, Zekeringstraat 43 g, 1014 BV AMSTERDAM, the Netherlands *** Delft university of Technology. Department of Sanitary Engineering, Faculty of Civil Engineering and Geosciences, Delft University of Technology, P.O. Box 5048, NL-2600 GA, Delft, the Netherlands **** City of Bradford Metropolitan District Council, UK corresponding author: floris.boogaard@tauw.nl Abstract Climate change and densification of urban areas are likely to result in more frequent flooding. The long recognised problems of short peak rainfall intensities which exceed the storm water drainage capacity and heavier long duration rainfall causing additional runoff from saturated urban green space are likely to become more common, increasing the probability of flooded streets, buildings and homes. It is becoming accepted that it will not be possible to solve these problems using traditional European drainage methods, and that storm water needs to be managed on and above the ground surface and that this will require the participation of different groups of stakeholders than have been involved in the past. A major problem is how to convince other officials, concerned with non-water related urban disciplines, of the importance of space for water. This problem is being addressed by the EU Interreg IVB projects Skills Integration and New Technologies (SKINT) 1 and Managing Adaptive Responses to changing Flood Risk (MARE) 2 in the North Sea Region and FloodResilienCity (FRC) 3 in the North West Europe Region. All three projects focus on sharing knowledge and best practices of water management issues within urban areas and are developing methods appropriate for application by municipalities which complement national and regional approaches for modelling and mapping urban storm water. 1 Skills Integration and New Technologies (SKINT) 2 Managing Adaptive Responses to changing Flood Risk (MARE) 3 FloodResilienCity (FRC)

2 Introduction From the 1970s there has been a steady progression in the development of modelling techniques for urban storm drainage. At the beginning of this period the Rational Method and its derivative such as Area Time diagrams had been in use since the mid nineteenth century. However, since the 1970s, the development of computers has enabled introducing in increasingly sophisticated hydrodynamic modelling software supported by better understanding of rainfall and surface and sub surface hydrological processes. The UK progression is typical of development in Europe including: In the 1970s the development of the TRRL Hydrograph method (Watkins, 1977) capable of simulating flows and depth up to pipe full capacity In the 1980s the publication of the Wallingford Procedure (DoE 1983) and the complimentary software WASSP capable of simulating surcharged flow in dendritic systems In the 1990s the enhancement of WASSP in WALLRUS and the development of HYDROWORKS using the St Venant equations and Preissman Slot to enable the modelling of heavily bifurcated systems with backflows. All of these developments used one dimensional modelling techniques whereby the effects of depth of flow on the surface could be determined and the modelling of surface pathways as channels could also be achieved, but this was limited in its application. As a result of this the determination of the routing of excess surface flows was carried out in the field. In this period a considerable effort has been put into the development of user interfaces with graphics facilities using digital mapping techniques to build models and display results and in the UK, software such as InfoWorks and Micro Drainage have considerately enhanced to experience of modelling. In the late 1990s and since the Millennium, there has been an increasing use of two dimensional modelling to simulate the passage of flows over the urban surface. Two different approaches have been adopted; grid based (raster) Triangular Irregular Networks (TINs). These approaches have been used independently or linked to 1D approaches for both urban storm drainage and river modelling and given unlimited resources would be used in all circumstances. However resources are limited and this influences the uptake of modelling and because of this, these models are not applicable for all cases yet. A desk top review of 2D surface modelling practice has been carried out by the English Environment Agency (Neelz and Pender 2009) and further information can be found in this report. This paper considers the impacts of the resource limitations and then identifies how the user focussed approaches of SKINT, MARE and FRC are contributing to identification of different components of flooding and hence the stakeholders who should be involved and fund the investigation, design and implementation of urban flood risk management measures and what modelling approaches and data should be used within the investigation process. Digital Elevation Models LIDAR (Light Detection and Ranging) has become the most common method of collecting elevation data. It can be obtained at a range of resolutions using fixed wing and helicopter areal platforms and vehicular land based platforms. Although conventional and GPS surveying techniques can provide more accurate information on local areas, LIDAR is a much more cost effective means of covering large areas.

3 However, there are a number of factors that need to be considered when using LIDAR based digital elevation models for flood risk modelling. Resolution Typically, airborne LIDAR has a vertical resolution ranging between 5 cm and 15 cm with horizontal resolution ranging between 25 cm and 2 metres. The higher resolutions tend to be more expensive, using helicopter platforms flying low level narrow paths whereas the lower resolutions can be obtained using higher level fixed wing platforms flying wider paths, which are more cost effective. Even higher resolution land based surveys can be carried out with 1cm vertical and 5 cm horizontal resolution. This type of survey provides and effective three dimensional image of a street scene, but does not provide information behind properties The issues relating to resolution are as follows: The higher the resolution, the greater the resource required for data capture, storage and processing. The lower the resolution, the less accurate the representation of the urban surface. There is a need to balance the requirements of terrain modelling with the resources required to produce, manage and use the data, but in both the UK and the Netherlands 1m horizontal resolution Lidar data is often viewed as a starting point Storage and processing LIDAR data is generally provided in ASCII format and stored as a raster grid. This means that a 5cm horizontal resolution vehicular based DEM requires 400 hundred times the storage capacity of a 1m resolution fixed wing DEM, and that excludes the storage of data on the faces of buildings. The impact on data processing is even greater, but this will be discussed on the section on modelling. Representation of the urban surface It goes without saying that urban flow pathways are affected by relatively minor surface features such as narrow walls, earth mounds under hedges, and road kerbs. Although some of these may be picked up by medium resolution LIDAR, it is only the very high resolution DEMs that will provide a truly accurate representation of the ground surface. The effect is particularly significant on convex surfaces, whereas in concave (valleys) they are less important. Nevertheless, LIDAR users should make themselves fully aware of the potential for lower resolution DEMs to provide misleading results. The difficulty in using vehicle based LIDAR behind buildings should also be considered. Currency of the DEM Any DEM is a snapshot in time and is therefore subject to the effects of urban development. Updating DEMs along major rivers or sea fronts is not a major problem, as in urban areas any activities in these locations will be well documented and new levels and alignments will be readily available. However, away from these areas, in the majority of the urban area where surface water flooding is the main problem, the continuous and widespread process of urban renewal will result in many unrecorded changes such as highway resurfacing which can reduce the capacity for roads to act as pathways. In addition, at times of flooding, people can use temporary flood defences such as

4 sandbags which can have a major impact on flow pathways. This is a reason to treat even the highest resolution DEM with caution when it comes to modelling. Another point is that people tend to forget that a model (or the data for a model) can be imperfect at some points if the results appear more and more realistic. This is certainly the case for a DEM, which gives a very realistic representation of the reality. Also the more data is used in a model, or the larger the area modelled, the more errors it contains and the harder it becomes to find all the relevant errors. Passageways, Bridges and Culverts. Unless an areal based LIDAR survey flies directly over a narrow passage it is possible that the signal will not penetrate to the bottom of the passage and hence the ground elevation within the passage will be incorrectly recorded. In the case of bridges, the recorded ground level will be the bridge deck and not the ground under the bridge. Similarly a LIDAR survey will not identify a culvert passing under and embankment. In all cases, the flow pathways will be wrongly identified and so steps should be taken to provide adequate representation by amending ground levels, removing bridge decks and inserting slits across embankments. However, although in the case of passageways the amendment of ground levels has no side effects, in the case of bridges and culverts, the flow pathways over the top of the bridge or embankment are interrupted by the actions taken. Value of DEMs Despite the above, the value of DEMs is not in dispute. There is no other easy way to identify flow pathways and determine the areas putting critical urban infrastructure at risk of flooding. The lesson to be learned from above is that outputs of modelling using LIDAR based DEMs should not be taken at face value and that where actions are to be taken as a result of modelling, then detailed local checks and enhancements should be made. Nevertheless there are many cases where these problems do not exist, or where their effect is minimal such is in large flat areas as occur in the Dutch polders, or the North European Plain, even the lower resolution DEMs can be fit for purpose. Modelling approaches Over the past thirty years most countries in the North Sea and North West Europe Regions have developed a considerable capacity for hydrodynamic modelling of urban drainage, river and coastal systems. The initial developments in 1D models identified within the introduction have proved to be effective, but the large amount of modelling required by the EU Flood Directive has resulted in the development of 2D modelling typically at low (5m) horizontal resolution for both coastal and river systems and also urban areas. The UK is typical of this for river and coastal flood risk modelling and in addition has produced surface water flood risk maps at 5m resolution taking account of but not modelling drainage system capacity for events with return probabilities of once in thirty and once in two hundred years. The surface water modelling and mapping of the whole of England is a huge task, but although best endeavours have been made to resolve issues the models are only appropriate to preliminary flood risk assessments. However, the nature of the 2D models does not distinguish between the different sources of flooding and so additional approaches are required to do this. Additionally, the modelling

5 approaches described above are highly specialised and expensive to implement. So simplified approaches designed for implementation within municipalities have been designed to: identify the contribution of different types of urban surface to flooding, to identify to those responsible for those surfaces that they should be involved in the management of flood risk, to identify the analytical approach required and the resources needed, and where possible to provide the necessary solutions. For the modelling of flooding in Urban areas a high resolution DEM is needed (at least 1m horizontal resolution) to make sure that possible above ground small water ways likes alleys will be in the DEM, without being blocked by the surrounding houses. For really small alleys this might even be too coarse. However, when using vehicle mounted Lidar with extremely high resolution, including building faces, the amount of data presented can cause confusion and it becomes necessary to leave out some of the data. GIS based modelling Flood component analysis a framework for identifying causes and attributing responsibility for flooding Flood component analysis was developed by David Wilson of Scottish Water as a means of illustrating responsibilities for flooding to the different partners involved in the Glasgow Strategic Drainage Plan. In its initial form the flood component analysis used different modelling techniques to estimate the volume of different sources of flooding, but here GIS based approaches are used as a preliminary assessment. Figure 1 shows the main flood components, with an indication of the impacts of future drivers (climate change and urbanisation) and an example of how the main components may be further sub divided. Possible interactions affecting drainage infrastructure performance Current Surface water and soil Drainage infrastructure Groundwater Streams, rivers and artificial water bodies Coastal water Changing depth of flooding due to climate change and/or urbanisation Depth of flooding + Surface water and soil + Drainage infrastructure + Groundwater + Streams, rivers and artificial water bodies + Coastal water Shorter duration high intensity storm events Impacts of changing rainfall patterns Longer duration, low to medium intensity rainfall events Rural green space Green space at urban fringe Green space within urban area Developed urban surface Sub divisions of surface water and soil

6 Figure 1: Flood component analysis and the impact of future drivers The need for a preliminary flood component analysis There are benefits in carrying out an assessment to identify the potential causes of flooding prior to embarking on detailed diagnostic studies. The results of the assessment will help to identify: The organisations which should be involved Appropriate modelling techniques Data collection needs Resource requirements Costs and their likely distribution. The proposed approach may be used as part of the investigation of a specific flood incident or as a general approach to the assessment of flood risk of high vulnerability infrastructure and buildings. Rationale The approach should be simple and should be capable of automation and should use readily available data Data requirements The analysis requires the following information as a minimum: A digital elevation model Digital maps with layers identifying different types of surfaces Digital flood hazard, probability, risk and extent maps for coastal and river flooding In addition information on the distribution of depth, duration and frequency of local rainfall will provide enhanced outputs, as will basic information of the hydraulic performance of drainage infrastructure. In the absence of the latter, the analysis will enable a preliminary assessment of drainage infrastructure performance to be made where knowledge of the rainfall causing flooding exists. Aim To identify how the following flood components may contribute to flooding Water bodies o Coastal o Rivers o Streams Drainage infrastructure Surface types o Rural green space o Green space at the urban fringe o Urban Green space o Developed urban surfaces Highways Buildings and structures

7 The preliminary flood component analysis process The process has 11 elements as follows: 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) Define surface water management zone boundaries (Figure 2) Identify all pathways and sinks Identify pathways and sinks associated with surface water Identify pathways, sinks and flood extents associated with water bodies (Figure 3a) a) Coastal, rivers and large streams b) Small streams Identify contribution from developed urban surfaces (Figure 3b and 3c) a) Highways b) Buildings, structures and associated ground surfaces Identify contribution from green space (Figure 3c) a) Urban Green space b) Green space at the urban fringe c) Rural green space Assess drainage system capacity Assess decay rates for surface water flooding along developed urban surface (Figure 4) Assess decay rates for flooding from small streams along developed urban surface Assess joint probabilities Finalise flood component analysis The catchment covers the City of Bradford area with a distance of up to 15Km West to East and 8 Km South to North Potential Surface Water Management Zones and example of final selection Figure 2: Example of the sub division of a large city catchment into manageable surface water management zones The process is progressive and may be terminated at various points, depending on the available data. On the completion of Element 4 it will be possible to define those areas subject to flooding from: Coastal water Rivers

8 Large streams Small streams Surface water It will also be possible to identify those areas where there are joint probabilities of flooding and in the case of surface water and small streams identify their relative contribution. Example of potential accumulations from overflowing watercourse Example of potential accumulations from highways Example of potential accumulations from developed areas Example of potential accumulations from green space Figure 3: Potential contributions to flooding form urban surfaces under different ownership Legend Accumulation (s.m.) 2,501 10,000 10,001 50,000 50, , , , ,001 1,000,000 > 1,000,000 On completion of Element 7 it will be possible to identify the potential contributions from small streams and different surface types by comparing the accumulations at any point without any modelling.

9 With a minimum of modelling based on rational approaches, Elements 8 and 9 take account of the capacity within the different urban drainage systems to accept and transport surface water runoff from urban green space and blocked or overloaded stream culverts. Having calculated the runoff, this approach, calculates a rate of decay for flooding emanating as it flows along pathways in developed urban areas, and into drainage systems through gullies, providing that the capacity of the drainage systems serving those areas are not exceeded. This provides a method of determining the extent of the impact of runoff from green space saturated by long duration, heavy, (but not intense) cyclonic rainfall, which is the cause of the majority of flooding incidents in and around urban areas in the UK. Runoff from green space enters the highway at points 1 4. Gulleys located at points A - T Graph showing decay in surface water discharge and depth as it flows past gulleys with spare capacity Figure 4: Simple modelling of flood decay as flows from green space enter drainage system Finally, Elements 10 and 11 draw together the results of the preceding elements to provide a preliminary quantification of the different flood components and to inform the different players of their involvement Modelling and communication using simple approaches in a process of triage For a good assessment of the situation and the possible solutions it is important to choose the model which takes into account the right processes. However, sometimes it is better to start using models which do not take into account all processes, but which give a quick and dirty first insight on what might be situation or the effect of possible solutions. We promote the use of an analysis of the ground elevation such as that described above. This provides quick and understandable insight in where water might collect in depressions, how deep these depressions would be and what the catchment area is of the water in a depression. The results are directly presented in maps and should be combined with other maps of the flooding such as recorded flood events and areas vulnerable to flooding, In a workshop these maps are best discussed with the possible stakeholders who should be involved in solving problems of urban flooding: such as local citizens supported by water engineers, urban

10 planners, decision makers, architects etc. This helps to promote the interaction based on local data and can be done using a touch screen on which several GIS-maps have been prepared. Similar methods developed independently in the Netherlands and the UK provide insight in the above ground discharge of storm water. They assume that the storm event is so heavy that the sewer system is completely filled and used. In the Netherlands it is assumed that from typically 60 mm of rainfall (the maximum expected rainfall in one hour for a return period of 100 year), 20 mm is stored in and discharged by the underground drainage system. The other 40 mm is stored on or flows over the surface. Based on the DEM and the type of surface the locations of depressions and the flow path are estimated and presented. There is no flow simulation used, only a evaluation of what is higher and what is lower. This general insight is easily carried out by GIS specialists rather than water engineers and the results can be communicated with urban planners, decision makers and other stake holders who should be involved in solving problems of urban flooding. The principles of such a model are easily understood and it is our experience in the Netherlands that because of this the deficiencies in the model are easily accepted. Used in preliminary assessments such a general approach can also afford not to be correct, because it can quickly be rerun, or people can imagine and point out themselves how the water would flow. This process helps to develop a better understanding of the situation and can result in the synthesis of options. This can stimulate discussion of what measures are acceptable and the prioritisation of the response and what can and can t be solved and what is of no importance. In effect it is a form of triage. The approach can be used to identify where detailed modeling is required and provide a preliminary assessment of options prior to the selection of specific options for more detailed modeling. If in the case of detailed models, an important omission in the data is found after complex simulations have been made, the validity of the whole model can be thrown into question. A complex model not only requires more data and more computational time, it is also far more difficult to check that all essential data is in it correctly. For that reason we recommend to start exploring the situation and possible solutions using simple models in the first case prior to the use of more complex models in critical locations and for critical options. References DoE 1893, Department of the Environment/National Water Council Standing Technical Committee. The Wallingford Procedure, HR Wallingford Kluck J., et al. Modelling and mapping of urban storm water flooding, Novatech Lyon Neelz S., and Pender G., Desktop review of 2D hydraulic modelling packages, Science report, SC080035, Environment Agency, July 2009, ISBN ,

11 Watkins L.H. The TRRL hydrograph method of urban sewer design adapted for tropical conditions, Proceedings of the Institution of Civil Engineers, Part 2, 1977, 63, June,