1 in 30 year 1 in 75 year 1 in 100 year 1 in 100 year plus climate change (+30%) 1 in 200 year

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1 Appendix C1 Surface Water Modelling 1 Overview 1.1 The Drain London modelling was designed to analyse the impact of heavy rainfall events across each London borough by assessing flow paths, velocities and catchment response. The Drain London Data and Modelling Framework specified that the direct rainfall method should be used in the modelling approach. This method incorporates conservative allowances for the drainage network and infiltration. The following key assumptions were made to generate the model input: Initial Loss None Infiltration Loss None Allowance for Drainage System A constant value of 6.5mm/hr was applied No aerial reduction factor applied Summer profile was used 1.2 To comply with the Drain London framework requirements rainfall inputs were generated at a standard 10km grid square resolution. As specified in the framework guidance hyetographs for the following rainfall events were generated: 1 in 30 year 1 in 75 year 1 in 100 year 1 in 100 year plus climate change (+30%) 1 in 200 year 1.3 Total rainfall depths at each 10km grid centroid for all required return periods were extracted from the FEH CD-ROM (v3) Depth Duration Frequency (DDF) model. A comparison between the peak rainfall depths in adjacent 10km grid squares was completed Version 27/05/11 C1-1

2 to confirm the suitability of the 10km grid resolution for modelling purposes. The difference in total rainfall depths between the grid centroids was less than 7% which suggests that the data is suitable for use in the study. 1.4 Hyetographs corresponding to the total rainfall depths were created by importing the parameters for each 10km grid centroid into an isis FEH unit. The rainfall depth from the FEH CD-ROM was entered into the isis FEH unit as observed depth in the event rainfall data tab. A summer profile was applied, with a duration of 3 hours and time step 0.2 hours as agreed for the Drain London modelling (see below). 1.5 Runoff coefficients for varying surfaces were standardised and are specified in the Drain London Data and Modelling Framework. These coefficients were applied to the rainfall profiles in order to simulate an appropriate level of infiltration for each land use type. 1.6 Critical duration is a complex issue when modelling large areas for surface water flood risk. The critical duration can change rapidly even within a small area, due to the topography, land use, size of the upstream catchment and nature of the drainage systems. The ideal approach would be to model a wide range of durations. However, this is not always practical or economic when modelling large areas using 2D models which have long simulation times such as within the Drain London study. 1.7 A high level investigation was undertaken to understand the effect of rainfall event duration on the Drain London Study area using a rapid modelling technique. The intention of the investigation was to show variation in critical duration across the study area and thus identify whether it was possible to identify single critical durations for each sub-model. The study used the 1 in 100year hyetographs for 1, 3, 6 and 12 hour durations along with a simplified terrain model to route overland flow. The key result was that critical duration is highly variable across surface water catchments but the influence was not sufficiently significant to justify considering multiple event durations within the Drain London Study. Therefore, a single duration of 3hrs was selected Version 27/05/11 C1-2

3 for all model runs to ensure result consistency and comparability across the Greater London area. 1.8 This technical note deals with the modelling carried out for Group 3 and Group 6 of the Drain London project. 1.9 Group 3 includes the London Boroughs of: City of Westminster; City of London; Camden; The Royal Borough of Kensington and Chelsea; Hammersmith and Fulham; and Islington 1.10 Group 6 includes the London Boroughs of: Lewisham Bromley Bexley Greenwich 2 The model 2.1 The model used for group 3 and group 6 is a 1d-2d TUFLOW model utilising the direct rainfall method. The version number of TUFLOW is AA-iDP-w The following return periods were modelled: (a) (b) (c) (d) (e) 30 year 75 year 100 year 200 year 100 year plus climate change allowance 2.3 Modelling was carried out, as required, at a 5m grid resolution for a 6 hour duration (twice the critical storm duration). Version 27/05/11 C1-3

4 3 Hydraulic model extents 3.1 The model extents were primarily determined by the boundaries of the London boroughs being investigated. However, in some cases, there was a large area outside the boroughs contributing to flow within them. Alternatively, the group 3 and group 6 boroughs contributed flow to other boroughs outside of the group. 3.2 As a result, it was decided to construct hydraulic models based upon the predominant hydrologic catchment and then exchange results for other boroughs with the relevant consultant. Figure 1 and Figure 2 illustrate these additional areas for group 6 and group 3. Version 27/05/11 C1-4

5 Figure 1- Group 6 boundaries Version 27/05/11 C1-5

6 Figure 2- Group 3 boundaries Version 27/05/11 C1-6

7 3.3 Due to the required grid cell size for the models, the large area covered by the boroughs and the high number of wet cells, several models were required per group. As a result the groups were broken down into sub-catchments. 3.4 Group 3 was split into 3 hydrological sub-catchments between which there was negligible flow. Figure 3- Group 3 sub-catchment model boundaries 3.5 Due to its size it was not possible to split group 6 into small distinct hydrological catchments. The fluvial nature of many group 6 catchments meant that most flow gathered in river valleys. Therefore, sub-catchments were divided so that the only flows anticipated to cross their boundaries were along river corridors. In total there were 21 sub-catchments. Version 27/05/11 C1-7

8 3.6 Using these divisions of the catchment, a large model was run at 30m resolution and hydraulic data at the sub-catchment boundaries were recorded. These flows and levels were then used as boundary conditions for the smaller sub-catchment models. Figure 4- Group 6 sub-catchment model boundaries Version 27/05/11 C1-8

9 4 Boundary conditions 4.1 Several different types of boundary conditions were used for each model and these are summarised in this chapter. Rainfall 4.2 The most important boundary condition for the model was the rainfall boundary condition. This was applied as direct rainfall to every cell of the model using a TUFLOW 2d_rf boundary. Rainfall was applied as gross rainfall from FEH (see chapter 1). Losses due to different land uses and an allowance for the drainage network were applied within the TUFLOW model. 4.3 Varying losses were applied to the rainfall depending on the land use represented by each TUFLOW cell. The following runoff coefficents were applied using the TUFLOW boundary condition database file based upon the feature code defined in OS Mastermap (Table 4-1). To carry this out the rainfall boundary condition GIS layer was attributed with the feature code values from OS Mastermap data. 4.4 A secondary, continuous loss of 6.5 mm/hr was applied for some land uses (Table 4-1) to represent the capacity of the drainage network. This was homogeneous across the whole drain London project and does not take into account local variations in network capacity. Version 27/05/11 C1-9

10 Feature Code Descriptive Group Comment Runoff Coefficients Drainage - Continuous Loss (mm/hr) Mannings Roughness Building General Surface Residential yards General Surface Step General Surface Grass, parkland Building Glasshouse Land; Heritage And Antiquities Water Inland Natural Environment (Coniferous/NonConiferous Trees) Heavy woodland and forest Roads Tracks And Paths manmade Roads Tracks And Paths tarmac or dirt tracks Rail Roads Tracks And Paths Tarmac Roads Tracks And Paths (roadside) Pavement Structures Roadside structure Structures Generally on top of buildings Water foreshore Water tidal water Land (unclassified) Industrial Yards, Car parks Table 4-1- Model assumptions for OS Mastermap land use types Tidal 4.5 A boundary condition was applied along the Thames to allow surface water flows to exit the model into the Thames. This assumed the free-overflow of water into the Thames through a lack of defences or the correct functioning of any flap valves in the Thames defences. External 4.6 One boundary condition was included in Group 6 that was external to the overall model extent. The area can be seen in Figure 1 and was applied due to the lack of availability of LIDAR. Flow locations and catchment area were estimated using both ordnance survey maps and FEH. FEH was then used to calculate the flows from these small catchments. These flows were then applied as point inflows at the bottom of the valleys where it enters the model domain. 4.7 Upstream and downstream conditions were also provided for each sub-catchment in group 6 where fluvial flows were recorded entering or exiting the sub-catchment in the 30m grid model. These flows and stages were applied in TUFLOW as QT, HT and HQ boundary conditions. Version 27/05/11 C1-10

11 4.8 In some cases the sub-catchment boundary conditions related to point flows through 1d model elements such as culverts and bridges at the model boundary. 5 Roughness 5.1 Different roughness values were applied for each cell in the model depending on the land use indicated by OS Mastermap at that point. The values used can be seen in Table Ground levels 6.1 Ground levels for the model were taken from filtered LiDAR provided as part of the Drain London project at 1 m resolution. In some cases the LiDAR needed editing to provide a better representation of ground levels and flow routes. 6.2 Buildings were edited to be raised above the LiDAR ground level. This was carried out using the technique suggested by the tier 1 consultant. This involved setting the building elevation to the average level indicated by LiDAR across the building polygon. A further 0.1 m was added to this level to further delineate flow routes. 6.3 Ground levels were also edited in the TUFLOW model using zlines and zpolygon layers to delineate features such as underpasses, culverts and railway cuttings. 6.4 As part of the Drain London specification it was also required to model rivers as full. To achieve this, a zpolygon layer was used in TUFLOW to set ground levels within main river channels to the level of the river banks. The bank levels were determined by taking the average level of 20m long transects at 10m intervals along the river. 7 Model limitations 7.1 The model has made several assumptions about the data available and the behaviour of surface water for this study. These assumptions and limitations are summarised below. Version 27/05/11 C1-11

12 (a) (b) (c) (d) (e) (f) (g) Sewers and surface water drainage are not modelled and are only included as part of the 6.5 mm/hr drainage allowance which does not represent local variations in capacity. The model relies heavily on the accuracy of LiDAR data which is assumed to be filtered correctly to remove buildings and vegetation. Data cannot be truly validated due to a lack of historical surface water flood data. The model has only been run for one critical storm duration (3 hours). The model has a grid cell size of 5m and therefore does not represent features smaller than 5m. Roughness is assumed not to vary with time. Subterranean flow routes and storage such as the London underground network are not known in detail. 8 Model error 8.1 Model error was specified to be within +/- 5%. Model results show that as a percentage of the total volume of the model the average error for both group 3 and group 6 is within +/- 5%. 9 Results and model outputs 9.1 Model results have been extracted to GIS format using the TUFLOW to GIS utility and post-processed using GRASS GIS. Mapped results can be seen in Appendix D. 9.2 There are two distinct patterns of flooding that can be seen in groups 3 and 6. One pattern shows distinct flow routes while the other shows localised ponding in low points. 9.3 The flow route pattern can be observed in less urban, fluvial catchments, primarily in group 6, where water is free to coalesce in fluvial valleys. In many cases these areas have already been identified as at risk of flooding from fluvial modelling. However, in some cases fluvial valleys have been built in and the watercourse has been culverted or ignored. It is in these places, where the watercourse is not designated as main river, where the surface water modelling has highlighted significant flow routes that may Version 27/05/11 C1-12

13 flood many properties. Due to the absence of a recognised water course in some of these locations, features such as railway lines and highways hold back water and due to a lack of surface flow routes through them. 9.4 In group 3 and more urbanised parts of group 6 a separate pattern can be observed that does not show distinct flow routes. Instead an ice-tray effect can be seen. This is observed in areas where there is a predominance of basements or where buildings are lower than the main road level. In these locations, groups of buildings are often bounded on all sides by raised roads or other features and as a result, modelling highlights water as collecting between the raised features. This is predominant across central London where a majority of buildings are shown as flooded due to the presence of basements and very few flow routes are observed as basements or lowered features collect all the water. As many of these areas also drain to combined sewer systems, this highlights the risk to basements from surface water and their reliance on sewer drainage which is not specifically modelled as part of this study. It also prevents the easy identification of flooding hotspots as almost all of the group 3 boroughs show this diffuse pattern of flooding. Subsequently, borough, or even group wide measures are more appropriate. Version 27/05/11 C1-13