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D. Risk assessment methodology

D.1 Overview

This appendix documents the technical details of the hydraulic and hydrological model build used to undertake the detailed risk assessment. Note that in accordance with the project specification the modelling was undertaken in two stages or tasks:

Task 1 – 2D Surface Water modelling

Task 2 - 1D-2D Surface Water Model

This was undertaken largely to enable results from Task 1 to be utilised at an early stage during the project. The final model (post completion of Task 2) should be used as the base for all further modelling.

The approach taken has been one of integrated modelling of all drainage systems excepting highway drainage and private sewerage (for which no information was available). This relatively detailed approach is justified by the requirement to use the model to test a variety of flood risk management measures to reduce flood risk in Paddock Wood.

D.2 Task 1 – 2D Surface Water Model Build

A Surface Water Flood Risk Model was built and run using InfoWorks ICM version 1.5 modelling software. This was selected for the following reasons:

Variable mesh enables additional detailing where required through Paddock Wood, and coarser modelling in the wider rural catchment. A variable mesh represents the modelled surface using irregular triangles rather than regular grid squares used in some other 2D models. This has the benefit of enabling greater detailing of the 2D surface only where it is required, thus avoiding making the rest of the model excessively detailed (and therefore slow to run).

Compatibility with urban drainage modelling and integrated modelling. In particular, Southern Water sewer model of Paddock Wood, which was constructed in InfoWorks CS.

Ability to model River networks, sewer networks and surface water flow routes in one model.

The Task 1 InfoWorks ICM model contains a 1-D network of nodes, links and sub-catchments representing the Southern Water sewer system, some other 1-D nodes and links representing key culverts under the railway, and an irregular triangular mesh forming a 2-D zone which represents the ground topography.

D.2.1 LiDAR Data

LIDAR (Light Detection And Ranging, is an optical remote sensing technology that can measure the distance to a target by illuminating the target with light, often using pulses from a laser. In this context, LiDAR is captured from a plane to create a digital elevation model (DEM) describing the surface of the land. 1m LIDAR was available for part of the study area. 1m tiles with full coverage used were TQ6841-7, TQ6743-7, TQ6644-7, TQ6546-7, and TQ6447. Partial 1m coverage is available for grid squares TQ6742, TQ6643, TQ6545, and TQ6446.

Some 1m LIDAR tiles were provided, however issues where found with them not lining up with the rest of the 1m DTM so they were not used in this project. The tiles were: TQ6642, TQ6741, TQ6445, and TQ6544. Error! Reference source not found. shows clearly that tile TQ6544 does not line up with the other tiles in the area. The other three tiles had similar issues.

2m LIDAR was available for the entire study area and this was used in those areas not covered by reliable 1m LiDAR.

Figure D-1 Example of misaligned LIDAR

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LIDAR Processing

1m LIDAR was mosaiced in ArcMap using the „‟Mosaic to new Raster‟ tool, the default Mosaic method was used, and Pixel type was change to 32-bit Float.

2m LIDAR was also mosaiced as above and this grid has been re-sampled to 1m, using the „‟Mosaic to new Raster‟ tool.

The 1m and 2m LIDAR mosaics were then mosaic together using the „First‟ mosaic method to stamp the 1m grid onto the 2m grid to form the final combined DTM.

DTM Checking

In areas with mixed coverage, the original 1m and 2m grids have been compared to ensure that the recorded levels are consistent. Within the study area these spot checks show a variability of <0.5m, and in most areas the differences are <0.05. The areas where the difference is greater than 0.05 often coincides with drainage channels and watercourses which are better represented in the 1m LIDAR.

The final DTM has been checked for steps and other issues by checking a series of cross-section. No steps have been identified between 1m and 2m LIDAR.

Once the DTM was been loaded into the InfoWorks software any noticeable anomalies or obstructions to the natural flow paths can be removed using „mesh polygons‟ within the InfoWorks model rather than editing the raw DTM data.

D.2.2 Hydrology

Rainfall

Design rainfall was generated using the Direct Rainfall FEH methodology. FEH DDF parameters were determined for the FEH catchment covering Paddock Wood. The 1 km2 DDF parameters were extracted for the centroid of the catchment. Comparisons were made with neighbouring catchment DDF parameters, and they were all found to be very similar so the same DDF values were used for the entire study area. These DDF parameters were input into the InfoWorks FEH rainfall generator, InfoWorks is then able to produce a hyetograph for a series of return periods and durations.

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

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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. Two durations were modelled the 60 minute and the 600 minute.

For the short duration 60 minute storm summer rainfall profiles were used, a winter profile was used for the longer 600 minute duration storm. The final extent for each Return period will be the „worst case‟ combination of these two durations.

Sensitivity testing was carried out on the Southern Water sewer model to determine the „worst case‟ profile and duration. Generally the 60 minute storm proved to be the „worst-case‟ for most of the nodes in the sewer model. However total flood volume was greater in the 600 minute storm. There was little difference between the summer and winter profiles.

The direct rainfall model has been set so that rainfall falls on the 2D Zone in all areas except the Southern Water Model sub-catchments. Here the rainfall hydrographs are entered into the sub catchments and the water routed into the sewer system according to the relevant runoff factors. Although this means flow paths through these sub catchments are not immediately modelled the direct rainfall flows into the area, and the sewer system surcharges and enters the 2D Zone at points throughout the model, therefore representing likely areas of SW flooding.

Allowance for Drainage System

The Southern Water sewer model representing the foul and surface water systems through Paddock Wood has been included within the model to explicitly model the drainage system to provide the most accurate assessment available of the drainage system capacity. Checks were made to ensure that the entire modelled catchment area was connected to one or the other system, and this was confirmed to be the case. Approximately 13% of the catchment area is connected to the foul system, with the remaining 77% connected to the surface water system. It should be noted that it assumed the system is empty at the start of a model run, with the exception of any domestic / commercial foul flows (foul sewers only) and infiltration (foul and surface water sewers) as included in the Southern Water model..

The Southern Water sewerage model uses the Wallingford Percentage Runoff model. Base flow in the surface water sewerage was modelled using the InfoWorks Infiltration module, with no changes to the set-up supplied by Southern Water. This is also the case for the foul sewerage, where dry weather flow was also modelled as-supplied in the original Southern Water model. .

The modelling approach taken was to use the sewerage model as-supplied, with only minor changes made where necessary to integrate the sewer model with the 1D modelled watercourses and the 2D surface model. Where manholes (storm and foul) within the sewer model show signs of surcharging they have been converted to 2D manhole allowing the surcharged water to enter the 2D zone and flow/flood. Only those Foul manholes which surcharge in the M30 year event have been allowed to connect to the 2D zone.

Infiltration Zones

The rest of the 2D model area – outside the subcatchments- has been split into a series of three infiltration zones.

Urban

Rural

Semi-rural

These areas have been set with a fixed percentage runoff (PR) values. This PR value remained fixed through-out an event.

Urban – 0.7, Semi Rural 0.6, Rural 0.4.

The rural value was checked against the SPR values in the FEH CD_ROM.

Anecdotal evidence was that the area around the Rhoden System was 'soggy'; therefore we looked at the SPR values for this area in detail and increased the runoff value to 0.45.

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D.2.3 Southern Water Sewer Model

The model provided was a calibrated InfoWorks CS model of the Foul and Storm System in the Paddock Wood Area. The model appears to provide a good coverage of the area.

The area around Blackberry Way, Clover Way, Buttercup Close and Green Lane is only modelled as having a foul sewer network – no storm network is included in the model. Consequently, the surface water runoff in this area is modelled using the direct rainfall approach in InfoWorks ICM. Likewise private sewerage within the industrial areas to the north of the railway has not been modelled.

Where there are storm sub catchments no direct rainfall modelling is undertaken it is assumed any rainfall falling on this area passes into the sewerage system, but then is able to surcharge back out onto the 2D mesh where the sewerage system is surcharged.

D.2.4 2D Zone

The 2D zone consists of an irregular triangular mesh, which represents the ground topography. The ground levels of this mesh were based on the most recent LIDAR data available at the start of the project (April 2011).

Mesh Definition

The 2D Mesh is built to represent the local topography and features such as drainage channels and embankments which can have significant impacts on the flow of surface water, and hence on flood risk.

As the area of each triangle in the mesh is variable, the model resolution was varied so that the area of Paddock Wood can be modelled in more detail, whilst the wider catchment which is outside our study area can be modelled at low resolution, as illustrated in Figure . This was done through the use of mesh zones, covering the Paddock Wood area, which generated a mesh with a maximum triangle size of 16m2 while allowing all other areas to have a maximum triangle size of 200m2.

Figure D-2 Illustration of a variable Triangular Mesh using Mesh polygons

D.2.5 Breaklines

Breaklines were used to define the mesh along roads, where kerblines help to direct and contain shallow surface water flows, and railways where embankments or cuttings can transmit, intercept or prevent surface water flows from following the general topography of the ground surface. Breaklines force the mesh to generate triangles along its edge, as in Figure below, which in turn ensures that the areas within or near to near to breaklines have well defined linear features to more accurately model surface water flows.

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Figure D-3 Illustration of a Breakline used in a Triangular Mesh

The breaklines used to model roads and railways were generated using MasterMap data, collecting all polylines with feature codes relating to road edges and railways. A visual inspection was then carried out to remove redundant lines (for example railway sidings have many railway lines, but break lines are only required on the outer lines or where there are significant changes in elevation) to reduce the number of triangles generated between breaklines.

Breaklines have also been use to define the course of the many drainage channels which flow through the 2D, model area. The supplied Digital River Network was used to define these breaklines. For those reaches represented by breaklines the elevation of the watercourse has been determined from the LIDAR, it has been assumed that the LIDAR is representative of the topography along these ordinary watercourses.

D.2.6 Buildings

InfoWorks ICM allows buildings to be explicitly modelled either as permeable walls or as mesh polygons representing the footprint of the building. For this model the buildings were represented using a porous wall with a height of 6m and porosity of 0.1 (10%), this was assumed based on the likely percentage of the building were water could enter (doors, airbricks). The MasterMap polygons associated with building were converted to Porous walls throughout the Paddock Wood area.

Using porous walls with a direct rainfall model can suffer from issues relating to rainfall falling within the building footprint; therefore, buildings were only represented in Paddock Wood where there were storm sub-catchments associated with the Southern Water sewer model that were not being modelled using direct rainfall.

D.2.7 Flow Path through embankments

The only notable flow paths in the 2D model were where the Gravelly Ways and Tudeley Brook flow under the railway. Here 1D networks were used to model the associated culverts with the dimensions taken from the survey data provided.

For the Tudeley Brook crossing a mesh polygon was also required to reinstate the railway bridge which had been filtered out during the LIDAR filtering process. This was achieved by adding a Mesh Zone that was set to raise the ground level to the deck level of the bridge as per survey drawings.

Breakline

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Figure D-4 Step by Step Construction of Task 1 Surface Water Model

Step 1 – terrain

Step 2 - 1D model of Sewer System (nodes and conduits in purple)

Step 3 – watercourses, roads and railways added as breaklines (red lines)

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Step 4 – Buildings as permeable walls (turquoise)

Step 5 – Generate the 2D mesh

D.2.8 Roughness Zones

The roughness is represented using a Manning‟s n value of 0.04. The buildings are being represented as porous walls so not increasing in roughness is necessary.

The use of variable manning's n values in InfoWorks caused an increase in processing and run times, which proved to be unfeasible for surface water modelling. This issue was raised to Innovyze (developers of InfoWorks ICM) but a workable solution was not available during the study. Sensitivity testing in similar studies has identified that this simplification is not normally significant for an assessment of surface water flood risk: however, where the model is used for detailed feasibility studies, it is recommended that sensitivity testing is undertaken to confirm this.

D.3 Task 2 – 1D-2D Surface Water Model Build

The Task 2 InfoWorks ICM model uses the Task 1 model as a base but adds a 1D-2D linked model of the key watercourses in Paddock Wood. These watercourses were Gravelley Ways, Paddock Wood Stream, East Rhoden and West Rhoden. These watercourses were chosen as they are an integral part of the drainage network running through Paddock Wood, with most having a direct connection to the surface water sewer system.

D.3.1 River Reach

InfoWorks ICM represents 1D river channels as River Reaches. A river reach is constructed from a series of cross section lines and river centreline. The cross section lines used topological data from two surveys undertaken in 2005 and 2008 by BW Surveys. The CCTV survey of the Paddock Wood Stream Culvert was used in conjunction with these surveys.

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The cross sections at the downstream limit on the Paddock Wood Stream, East Rhoden and West Rhoden were derived using the LIDAR data and the channel bed lowered according to the general gradient of river reach. This was done to ensure the downstream boundary was downstream of the Paddock Wood area of interest. New survey along these reaches was suggested, but not possible within the time and budget constraints of this project.

Along these 1D reaches structures have been included in detail. The dimensions of such structures have been determined from asset information obtained in the data collection stage; where no specific information was available dimension have been inferred using local knowledge and LIDAR data.

The railway embankment is a significant obstruction to surface water runoff. There are 6 major culverts underneath the railway embankment of varied sizes and capacities, and these have all been explicitly modelled. Following comments from members of the public at the public engagement event, there are understood to be additional culverts beneath the railway to the east and west and between the East and West Rhoden streams, however these are not mapped and their dimensions are unknown and therefore they have not been included in the hydraulic modelling. It is recommended that these are identified and if significant surveyed and added to the model.

The main channels (East Rhoden, Paddock Wood Stream and Gravelley Ways) are assumed to contain a base flow at the beginning of a run. Other minor channels (for example the upper reaches of the Rhoden, Paddock Wood Stream and Gravelley Ways Stream systems) were assumed to be dry at the beginning of a run, with water entering them following surface water runoff. This was considered to be a reasonable compromise because

None of these minor channels have been surveyed and are not identified well by the LiDAR, and consequently the depth of the channel (and therefore its capacity) is under-estimated.

The vast majority of channels modelled in this way are in the upper reaches of the catchment and therefore serve only to feed surface water into the areas of primary consideration.

Where more detailed assessment of the performance of these minor channels is required (for example in an FRA) the channels should be surveyed and explicitly modelled.

As a check the in channel flows from the ICM model were compared to the FEH flows derived as part of the SFRA modelling. The return periods were not explicitly comparable, yet it was possible to assess whether the values fell within a similar range to the fluvial estimates. .It is important to recognise that the FEH Statistical and ReFH estimates do not represent the complex hydraulics of the system, whereas the ICM model does. A good correlation was seen for the Gravelley Ways and West Rhoden. Along the East Rhoden the InfoWorks model is showing less flow than that shown in the hydrology estimates. On the Gravelley Ways Stream peak flows were lower than the FEH and Statistical estimates, possibly reflecting the upstream restriction of the railway culvert. The East and West Rhoden show interaction across their floodplains, which has not been gauged here.

Table D-1 Comparison of FEH flow estimates and modelled flows

Watercourse (all estimates d/s of railway)

Event FEH Statistical (m3/s)

REFH (m3/s) InfoWorks ICM Model

Gravelley Ways Q100 1.3 1.1 0.8

Q100+cc 1.5 1.4 1.0

East Rhoden Q100 2.5 2.6 2.9

Q100+cc 3.0 3.1 2.9

West Rhoden Q100 0.7 0.6 1.6

Q100+cc 0.8 0.7 1.9

Inflow

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The inflow to the river reaches is predominantly being represented using the direct rainfall model and the flow of rainfall from the wider catchment into the area. However, a baseflow was entered into the upstream reach on the East Rhoden, Paddock Wood Stream and Gravelley Ways. No baseflow was added to the West Rhoden as the upstream limit is connected to a storm sewer outfall.

The baseflow for each watercourse was calculated in the ReFH spreadsheet using the relevant FEH catchment descriptors for each watercourse.

Structures

All the structures which were surveyed along the reaches have been included in the model.

The majority have been represented as either orifice units (where the culvert was a short length under a minor bridge) or a culvert with relevant inlet and outlet losses.

Along parts of the Paddock Wood Stream and the East Rhoden, sections from the LIDAR were used to interpolate the river reach to the downstream limit. In these areas structures such as minor bridges have been ignored.

The relief culvert linking the Gravelley Ways and Tudeley Brook was already represented in the sewer model as a conduit. The spill structure from the Gravelley Ways into this culvert was modelled as a weir.

Bank Lines

Bank lines were drawn between cross sections along the length of the river reaches. They were digitized to follow the bank as far as possible using the ground model. The banks were digitized so they could be as far as possible a similar width apart as that of the up and downstream cross sections.

The elevations for the bank lines were determined by querying the LIDAR at 10m intervals. In the absence of any detailed survey this was deemed the best representation possible for the banks topography.

The discharge coefficient was set to 1.0 and the weir modular limit set to 0.9 for all banks.

Roughness

Manning‟s n values were used to determine the roughness values of the river reaches. Where the channels were identified as being the responsibility of the Medway Internal Drainage Board and Manning‟s n value of 0.03 was used. This was thought to be an appropriate value given the maintenance regime employed by the IDB along these watercourses. The Paddock Wood Stream, which falls under a different, Environment Agency, maintenance regime, was given a Manning‟s n value of 0.05. The short open channel section within the Station Car Park, was given a high Manning‟s n value of 0.1 following the site visit and the understanding that channel maintenance here is sporadic at best due to access being restricted by a fence without a gate. This channel was very overgrown, to the extent that the channel bed could not be seen through the fence.

Gravelley Ways – 0.03

East Rhoden – 0.03

West Rhoden – 0.03

Paddock Wood Stream – 0.05

Station Car Park open channel – 0.1

D.3.2 Connection to Sewer System

An existing, calibrated, InfoWorks CS sewer model was provided for this study by Southern Water contained a number of Storm outfalls. On closer inspection, it was clear that these outfalls were connected to the watercourses flowing through and around Paddock Wood. Therefore the surface water sewer network has been connected directly to the 1D river reaches representing the ordinary watercourses where applicable to represent the outfall from the surface water sewer in to the watercourses.

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D.4 Model Runs

The model will be run for the following rainfall return periods:

1 in 30-year,

1 in 75-year,

1 in 100-year,

1 in 100-year plus an allowance for climate change (All values in hyetograph increased by 30%)

1 in 200-year.

10 second timestep with a results multiplier of 30 and default parameters were used for all model runs. The 60 minute simulations were run for 6x the rainfall event duration (six hours).

Mass Balance

The largest event modelled (M200-600) produced a total mass volume error of 637 m3. This

was spread over a number of locations and when investigated was considered reasonable.

D.5 Sensitivity Testing

Sensitivity testing will be undertaken to test the model‟s sensitivity to:

Rainfall event duration

Downstream Water Level

Urban Creep

Roughness

Percentage Runoff

D.5.3 Urban Creep

The sensitivity to urban creep is detailed in Figure 3-5 to Figure 3-8.

D.5.4 Rainfall duration and downstream water level

The sensitivity to rainfall duration and downstream water level was assessed by comparing the results from a 60 minute summer storm with a free-discharge (critical depth) downstream boundary to a 600 minute winter storm with a fixed bank full downstream boundary. It was assumed that a long duration storm was more likely in winter and if this were to occur the levels on the main river Medway and Teise would also be high. The results show there is a marked difference between the two. The longer duration and restricted downstream boundary resulted in an increase in flood extent both downstream and upstream of the railway embankment. The figures below show the difference in results.

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Figure D-5 Comparison of M30-60 and M30-600 model scenarios

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Figure D-6 Comparison of M200-60 and M200-600 model scenarios

D.5.5 Roughness

Sensitivity to roughness was modelled by rerunning the base model with channel roughness and 2D surface roughness both increased to n=0.07. Results (shown in Figure D-7) indicate significant increases in depth on the Tudeley Brook and Gravelley Ways Stream systems. However, there is an unexpected decrease in modelled peak depths in the Paddock Wood Stream and Rhoden systems. The reasons for this have not been investigated, but it does reiterate that the model is sensitive to roughness and that this should be considered carefully when undertaking detailed feasibility studies.

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Figure D-7: Sensitivity test results - roughness

D.5.6 Percentage Runoff

A comparison of the base 1 in 100 year and 1 in 100 year plus 30% increased rainfall was used to test sensitivity to increased runoff. As shown in Figure D-8, the areas to the east and west of Maidstone Rd and South of the railway are most sensitive to this increase in flow, as well as some areas to the north of the railway.

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Figure D-8: Sensitivity to increased rainfall

See also section Overview of flood risk3.3.2 for further discussion of the increases in damages and people at risk due to climate change.

D.6 Model verification

No flow survey data was available for model verification. It is assumed that the sewer model has undergone verification against a short-term flow survey. No details of this verification were available, but it would be reasonable to assume that the sewer model had not been verified against relatively minor rainfall events, and therefore confidence in the runoff modelling decreases with higher return period storms.

For this study, the approach to model verification has involved a comparison of modelled flooding extents with the evidence of historic flooding in the catchment. This is summarised in Table D-2 below for the five major sub catchments modelled. Note that the verification undertaken is subjective in its nature. There was not accurate, detailed information available on the dates, timings, depths and numbers of properties impacted by significant historic events in Paddock Wood, and hence it was not feasible to attempt to simulate specific events.

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Table D-2 Model Verification

Sub-catchment Observed Flooding Modelled Flooding Comparison

Allington Rd / “Station car park” stream

Reports of flooding in the area of:

Allington Road

Ribston Gardens

Laxton Gardens

Bramley Gardens

Old Kent Road

Maidstone road

Railway Station

Good Correlation to Observed. The modelled results compare well with the historic flooding recorded along Ribston Gardens and Allington Road. The extent along Laxton and Bramley Gardens is possibly not as extensive as the anecdotal evidence would have you believe. However the anecdotal evidence is not detailed, it simply highlights areas not specific properties that flooded. The modelled results show some stretches of the Old Kent Road and Maidstone Road to be flooded but not to a great depth (<0.3m). Flooding around the station associated with the open section of channel within the culvert was shown well in the model.

Gravelley Way Stream / Tudeley Brook

Historic flooding reports the Badsell road culvert is exceeded resulting in flooding to bungalows along Ringden Avenue.

Some Correlation with Observed. The modelled flooding is reasonable around here it shows the Badsell Road culvert being exceed with flow overtopping the road and flowing down Ringden Avenue and neighbouring Roads. No extensive property flooding is shown though as depths are shown to be less than 0.1m. But it should be noted the model represents a clean system (i.e. no blockages), and details such as individual property threshold were not available.

Paddock Wood Stream (u/s of railway)

Reports of flooding in the area of:

Waitrose Car Park

Cedars / John Brunt Pub

Maidstone Road

Rowan Close

Putlands Leisure Centre

Good Correlation to Observed. The area around Cedars / John Brunt Pub and the Waitrose car park is shown to be inundated as the evidence indicated. The section of the Maidstone Road which leads to Rowan Close is shown to be inundated, but again only to a depth <0.3m. This discrepancy with the anecdotal evidence could be the result of the modelling the Paddock Wood Stream culvert as 'clean' and dry at the beginning of the storm. The area around Putlands is shown to flood in line with the historic evidence.

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Sub-catchment Observed Flooding Modelled Flooding Comparison

Rhoden system (u/s of railway)

Reports of flooding in the area of:

LeTemple Road and Dimmock Close

Field near Church Farm

Paddock Wood Cemetery

Good Correlation to Observed. There is a good correlation between the modelled and observed flooding around LeTemple Road and Dimmock Close, and near Church Farm. There was very limited flooding shown around Paddock Wood Cemetery. It is thought that the local drainage issues specific to this site could be the cause of this discrepancy as the details were not know and couldn't be included in the modelling. The model also shows flooding around the Green Lane estate then flowing along Warrington Road. There is no anecdotal evidence of flooding here. Therefore the discrepancy is likely to be the result of the sewer system associated with the Green Lane Estate not being explicitly modelled as it was not present in the provided Southern Water Sewer Model.

Paddock Wood Stream / Rhoden system (d/s of railway)

Reports of flooding in the area of:

North of Railway to east of B2160

Lucks Lane

Swatlands Farm

Transfesa road

Hop Pocket lane

Sewage Works

Rhoden Farm

Good Correlation to Observed. There is a good correlation between the modelled and observed flooding.

D.7 Assumptions and Accuracy

D.7.1 Assumptions

The representation of any complex system by a model requires a number of assumptions to be made. In the case of the InfoWorks ICM hydraulic model it has been assumed that:

LIDAR accurately reflects the topography of the area and particularly that the filtered LIDAR has appropriately removed the influence of vegetation.

Design inflows are an accurate representation of a given return period

Model parameters have been determined appropriately

The details of hydraulic structures and the units used to represent them in the model provide an adequate representation of the situation.

Highway drainage and private sewerage have not been modelled as no data was available. Local detailing to include these may be required for feasibility studies.

Section D.2.3 details areas of the catchment where sewerage was not modelled. Again consideration should be given to detailing these areas as required for feasibility studies.

2D breaklines have been used to model some minor watercourses and ditches where no survey data was available, and in these cases minor structures have been ignored.

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There is anecdotal evidence from the public engagement event that other culverts exist beneath the railway to the east and west and between the East and West Rhoden streams, however these are not mapped and their dimensions are unknown and therefore they have not been included in the hydraulic modelling.

Results appear sensible, but further detailing would be required for example to support an FRA.

D.7.2 Accuracy

The accuracy of hydraulic models is heavily dependent on the accuracy of the hydrological and topographic data on which they are based. The largest source of uncertainty in modelled water levels quoted for a given return period is usually the natural uncertainty in the design inflows. Flood frequency estimates are inherently uncertain because they cannot be measured or formally validated against observed data. The equations generally used to model hydraulic systems are approximations of the physical processes involved but after decades of use and of continuous improvement the limitations and implications of the approximations are well understood. Some of the main areas of hydraulic model uncertainty are:

Topographic data errors in survey, LIDAR data and the filtering algorithm used

Estimates of model parameters such as Manning's n value roughness and structure coefficients.

Representation of structures within the floodplain.

While every effort has been made to accurately reflect the situation on the ground and estimate model parameters, these can never be completely certain. Therefore, certain assumptions are made as part of the modelling process. Despite the uncertainties that exist the current model is believed to be suitable for the purposes of flood risk mapping.

D.7.3 Mapping accuracy and limitations

The mapping provided with this report is appropriate for use as an evidence base to support spatial planning so that allowances are made for surface water flooding when allocating land for housing development. The maps can also be used to assist emergency planners in preparing their Multi-Agency response plans. The maps also enable public engagement and allow strategic consideration to be given to levels of investment, maintenance and the requirements of a strategic action plan for the area.

It should be noted that the maps only show the predicted likelihood of surface water flooding for the particular rainfall events defined for the purpose of the study (this includes flooding from sewers, drains, small watercourses and ditches that occurs in heavy rainfall in urban areas). Since actual rainfall events may not be the same as those used in this study there will be frequent occasions when flooding occurs and the observed pattern of flooding does not in reality match the predicted patterns shown on these maps. We have made all reasonable efforts to ensure that the maps reflect all the data available and have applied our expert knowledge to create conclusions that are as reliable as possible. It is essential that anyone using these maps fully understands the complexity of the data utilised in production of the maps, is aware of the limitations imposed by the assumptions made (particularly rainfall) and do not use the maps in isolation without proper understanding of how they are derived.

In order to help improve the accuracy of and confidence in flood estimates and hydraulic models in the future, the following recommendations are made:

That long-term or permanent level and flow monitoring be used to improve understanding of the catchment hydrology and the performance of the drainage networks. Such technology may also offer potential to improve the coverage of the current flood warning service to those at risk from surface water flooding (rather than just those at risk from Main River flooding as is currently the case).

Detailed flood event recording. More detailed recording of flood events which occur will assist future studies to better understand the mechanisms of flooding and confirm the accuracy of models. Surface water flooding by its nature can occur then reside very quickly so a variety of techniques may be required to gather this information, including post-flood surveys and levelling, resident's questionnaires and possibly use of flood wardens or community groups to take photographs etc during flood events.

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D.8 Damages calculation methodology

D.8.4 Summary

Property counts and damage estimates have been calculated for the Paddock Wood SWMP by using FRiSM, JBA‟s in-house flood metrics software.

D.8.5 Flooding Data

The FRiSM calculation was run for the following return periods; 2,10,30,75,100 and 200 year these results were annualised assuming a first flood with a return period of 1 year to obtain average annual damages. The 100 year with climate change was also queried.

All the return periods were queried twice for 2 different flooding thresholds. One run was for depths greater than 0m and one for depths greater than 0.05m. The depth threshold was used to generate a flood outline from the model depth grid. The outline was then used for property counts. Damages were only calculated for properties which were within the flood outline.

D.8.6 Receptor Data

The receptor datasets used for the calculations were the NRD property points layer together with Master Map building polygons. The NRD data for Paddock Wood was filtered to remove properties which did not have a building footprint in Master Map. This removes features such as ponds and post boxes and ensures they are not included in the property count. The NRD data was then further subdivided into residential properties and critical services to define the metrics used for the FRiSM calculation (Table D-3). Table D-4 shows the attribute queries used to create the subdivisions of NRD.

Table D-3 Metric Definition

Metric Name Definition NRD All Count All NRD property points, filtered to remove points without a

corresponding Master Map footprint

NRD Res Count NRD residential properties

Number of people Res properties * 2.34

NRD Critical Count Critical services comprise: schools, hospitals, care homes / nursing homes, police, fire and ambulance stations, prisons, electricity installations and water and sewage installations.

Table D-4 Relationship between individual receptors and the NRD

Receptor Selection by Attributes WHERE Clause Res Properties "mcmcode" = 1

Nursing homes/Care homes/Prisons

"mcmcode" =625

Hospitals "mcmcode" =660

Police Stations "mcmcode" = 651

Fire & Ambulance Stations

"mcmcode" = 650

Sewage & Water Works "mcmcode" = 840

Electricity Installations "mcmcode" = 960

Schools Os_class = 'ADULT EDCUATION' OR "Os_class" = 'EDUCATION' OR "Os_class" = 'FIRST SCHOOL' OR "Os_class" = 'FURTHER EDUCATION' OR "Os_class" = 'FURTHER EDUCATION COLLEGE' OR "Os_class" = 'HIGH SCHOOL' OR "Os_class" = 'HIGHER EDUCATION' OR "Os_class" = 'INFANT SCHOOL' OR "Os_class" = 'JUNIOR SCHOOL' OR "Os_class" = 'MIDDLE SCHOOL' OR "Os_class" = 'NURSERY' OR "Os_class" = 'PRIMARY SCHOOL' OR "Os_class" = 'PRIVATE PRIMARY SCHOOL' OR "Os_class" = 'SCHOOL' OR "Os_class" = 'SCHOOL FOR THE DEAF' OR "Os_class" = 'SECONDARY SCHOOL' OR "Os_class" = 'SPECIAL SCHOOL' OR "Os_class" = 'TECHNICAL COLLEGE' OR "Os_class" = 'UNIVERSITY' OR "Os_class" = 'PRE SCHOOL EDUCATION'

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D.8.7 Property counts

Property counts were undertaken using the detailed counting method. This method utilises the Master Map building footprints in conjunction with the NRD property points. A property point is counted as flooded if its corresponding building footprint is within the flood outline, even if the property point itself may not fall within the flood outline, this is illustrated in Figure D-9.

Figure D-9 Counting method

Detailed Count = 9

Simple Count = 4

NRD points flooded

NRD points

Building Footprints

q100 Outline

Legend

D.8.8 Depths

Each flooded property point is attributed with a min, max and mean depth value these values correspond to the minimum, maximum and mean value of the depth grid within the property footprint. If the property footprint contains less than half a depth grid cell then it will not receive any depth values, although the property will count as flooded.

D.8.9 Damages

Each flooded property point is attributed with a min, max and mean damage value these values correspond to the damage value for the minimum, maximum and mean depth within the property footprint.

The damage value is in pounds and is worked out by obtaining a unit damage value (£/m2) using the depth damage curves from the Multi Coloured Manual 2010 (Flood Hazards Research Centre 2010). The unit damage value depends on the depth at the property and the property type. This damage value is then multiplied by the value in the floorarea field of the NRD to obtain an absolute damage value.

Damages have not been calculated for properties whose floorlevel is „pU‟. These are potential uppers which are generally upper floors in flats, however properties with a floor level of „pU‟ have been included within the property counts. This is because the damage occurred by an upper floor flat is likely to be null however the residents of the property will still be affected by the flooding.

The values of damages to each property have been capped so that they cannot exceed the value of the property. This is in accordance with best practice as set out in the MCM. The property values were calculated as follows:

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Residential properties: Set at the average property value for each housing type, sold in Kent in June 2011, sourced from the Land Registry web-site

10.

Table D-5: Average value by housing type

NRD Housing type Average value (Kent, June 2011)

Notes

Detached £317,000

Semi-detached £185,000

Terraced £143,000

Maisonette / flat £108,000

Unknown £181,000 Based on average for all properties in Kent.

Non-residential properties - An assumed market value was calculated based on 10* the average rateable value for commercial and industrial properties in South East England (10* £75/m2 = £750/m2). This was then multiplied by the building footprint of the property as calculated in the MasterMap data. Source of average rateable values: Department of Communities and Local Government 2009.

Checks were made by comparing capped and un-capped damage costs. Capping had no effect on total damages to the south of the railway, but a very significant impact to the north where the combination of large industrial units and high depths leads to some very high estimates of potential damages.

D.8.10 Reporting Units

Properly counts and damages were summarised on a reporting unit level. The reporting units used for this study were the CDAs shapefile. For each model scenario each reporting units is attributed with a count according to the number of each receptor type flooded within the reporting unit. The max, min and mean depth of individual receptors within the reporting unit is also recorded as well as the max, min and mean damage of individual receptors. Damages are also summed within each reporting unit. There are 3 damages sums for each reporting unit as the minimum, maximum, and mean damage of each individual receptor is summed giving a min, max and mean sum. Table D-4 defines the fieldnames used in the reporting unit feature classes and the excel spreadsheet.

10

http://www.landreg.gov.uk/house-prices

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Table D-6 FRiSM Field definitions

Field Name Prefix

Field Name Suffix Definition

Metric Name Property Count within the reporting unit according to the metric definition

Metric Name Depth Max The maximum depth at an individual property within the reporting unit

Metric Name Depth Min The minimum depth at an individual property within the reporting unit

Metric Name Depth Mean The mean depth at an individual property within the reporting unit

Metric Name Damage Max The maximum damage at an individual property within the reporting unit

Metric Name Damage Min The minimum damage at an individual property within the reporting unit

Metric Name Damage Mean The mean damage at an individual property within the reporting unit

Metric Name Damage Max Sum The sum of the maximum damages at individual properties within the reporting unit

Metric Name Damage Min Sum The sum of the minimum damages at individual properties within the reporting unit

Metric Name Damage Mean Sum The sum of the mean damages at individual properties within the reporting unit

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