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Technical Note Technical Note Project: Wirral SFRA Job No: 60041400 Subject: Functional Floodplain (FZ3b) Prepared by: Mike Williamson Date: 12 th Jan 2009 Checked by: Kevin Keating Date: 12 th Jan 2009 Approved by: Kevin Keating Date: 12 th Jan 2009 Introduction Faber Maunsell, as part of the Wirral SFRA, were required to produce a functional floodplain (FZ3b) for the Wirral SFRA study area. This file note briefly explains the flood mapping methodology applied to create the functional floodplain. There is also a Comments section, supplemented by figures, which explains any decisions of judgment on uncertainties that were applied to the mapping process. Methodology to Produce Fluvial Functional Floodplain The functional floodplain was created using defended 4% AEP (25 year) maximum stage levels (a reasonable approximation of the 1 in 20 year event referred to in PPS25) produced using the HYDRO- 1D hydraulic modelling software for the Morpeth Dock Pumping Station and River Birket Risk Assessment and Flood Study, 2006 (referred to as Morpeth and Birket Study from here on). These model results for the River Birket, River Fender and Greasby Brook were used to produce the functional floodplain. The functional floodplain produced for Arrowe Brook is taken from the Cheshire and Wirral Study, 2004 using undefended 4% AEP maximum stage level data, as this is considered by the Environment Agency to be a more suitable dataset to use than the defended 4% AEP data produced from the Morpeth and Birket Study. The functional floodplain for Dibbinsdale Brook and its tributaries was produced from the most up-to-date Flood Zone 3 (FZ3) outline (dated Jan, 2009). Interpolation of Stage Levels Maximum stage levels for each modelled river section node for the 4% AEP event of the River Birket, the River Fender and Greasby Brook were copied from the Morpeth and Birket Study and exported via .csv files to MapInfo Tables (.tab file), 1 for each watercourse. These .tab files were then joined to a .tab file containing the relevant modelled nodes for each watercourse. The next step was to create left and right banks for each watercourse through which the 4% AEP maximum stage levels could be interpolated over the floodplain. The left and right banks were copies of the river section nodes and were placed where it was considered the functional floodplain would extend to on either bank. There were now 3 separate .tab files containing modelled nodes with 4% AEP maximum stage levels for the Birket the Fender and Greasby Brook that could be processed in MapInfo with Vertical Mapper to produce a grid of flood levels for each watercourse using the following parameters: • Interpolation of modelled river section nodes using a Triangular Irregular Network (TIN) Linear solution with smoothing. • A final triangle side length of 400m for the Birket, 520m for the Fender and 355m for Greasby Brook. Direct Tel: 0151 647 2312 38 Woodside Business Park T +44 (0)151 647 2312 Birkenhead F +44 (0)151 647 4742 Merseyside CH41 1EL E [email protected] Page: 1 of 8 Doc. Revised: www.fabermaunsell.com Technical Note • Average value coincident point aggregation. • Output cell size of 2m. Several versions of the triangular mesh (TIN) of the interpolated levels were reviewed to ensure a robust representation of flood levels and the interaction along the watercourse. These reviews indicated that additional interpolated 4% AEP level nodes were required at certain sections along each watercourse to assist the interpolation process. Interpolate Nodes Interrogation of the TIN and gridded levels data concluded that the triangle side length needed to be improved. To reduce the triangle side length a number of pseudo nodes were added to the nodes Mapinfo .tab files directly to aid interpolation. Levels for the interpolate nodes were derived as averages between the neighbouring upstream and downstream river nodes. The resulting TINs with shorter maximum triangle lengths showed a significant improvement in output. Flood Extents The next step was to create a flood depth grid for each watercourse, using the DTM (made up of unfiltered LIDAR data and SAR data where LIDAR coverage was unavailable) and the flood level grids using the Grid Calculator tool in Vertical Mapper. This flood depth grids were then contoured using the Contour tool in Vertical Mapper. This produced a raw flood extent polygon in Mapinfo .tab format which could then be post-processed to produce the final functional floodplain extent. Flood Extent Post-Processing The first step was to remove any holes that were less than 200m 2 within the functional floodplain extent polygons using the Snap/Thin tool in MapInfo. Step 2 was to digitise and combine, with the extents, any holes covering water bodies such as watercourses and lakes. Any isolated extent polygons not connected to the main extents were deleted from the Mapinfo table of the flood extent. Step 3 was to check the interaction between the functional floodplain and FZ3. Where appropriate, the functional floodplain was trimmed to FZ3 so as to not be exceeding the FZ3 extent. It was also necessary to taper the functional floodplain extents along non-modelled tributaries using the level at the confluence. The final step was to ensure that areas benefiting defences were taken into account. Along defended reaches (raised man-made defences as specified by NFCDD) the extent was trimmed back, where appropriate, to keep the functional floodplain in channel behind the raised defences. This process was implemented at various locations along the Birket and the Fender. Comments There are various comments to consider relating to uncertainties and decisions that were made during the mapping process. Upstream End of River Birket and Newton Brook Fluvial Flood Zone 3 on the Birket begins at the railway line in Meols thus not accounting for the land upstream of here up to Hoylake and also Newton Brook. This is due to the small catchment for the upstream area and Newton Brook being less than 3km 2 and therefore too small to run through JFLOW (the modelling software used to create FZ3). The modelled extent used in the Morpeth and Birket Study begins approximately 400m east of Hoylake. It was decided therefore to create functional floodplain for the low lying area between Hoylake and the aforementioned railway line even though we do not have FZ3 data here (see Figure 1). Direct Tel: 0151 647 2312 38 Woodside Business Park T +44 (0)151 647 2312 Birkenhead F +44 (0)151 647 4742 Merseyside CH41 1EL E [email protected] Page: 2 of 8 Doc. Revised: www.fabermaunsell.com Technical Note Newton Brook has not been modelled as part of the Morpeth and Birket Study. Therefore in order to create functional floodplain for this watercourse, levels were extrapolated over this reach to produce an indicative functional floodplain. The functional floodplain envelope shown on the Municipal Golf Course (circled red on Figure 1) is not shown to flood directly from the River Birket. However this area is very low lying and contains a large drainage network therefore it was judged appropriate to extrapolate the 4% AEP maximum stage level of 4.89m from the most upstream modelled node on the Birket over this area. This also highlights a possible surface water flooding issue in this area. Figure 1 below shows the area described in the paragraphs above where blue is the functional floodplain and red is the fluvial / tidal FZ3. This map is reproduced from Ordnance survey material with the permission of Ordnance survey on the behalf of the Controller of her Majesty's Stationary Office © Crown Copyright. Unauthorised reproduction infringes Crown Copyright and may lead to prosecution to civil proceedings. Licence No. 100019803 Figure 1 – Functional floodplain at upstream reaches of River Birket and Newton Brook Direct Tel: 0151 647 2312 38 Woodside Business Park T +44 (0)151 647 2312 Birkenhead F +44 (0)151 647 4742 Merseyside CH41 1EL E [email protected] Page: 3 of 8 Doc. Revised: www.fabermaunsell.com Technical Note River Birket North of Moreton Figure 2 below shows the functional floodplain in blue and the fluvial / tidal FZ3 in red. There is a gap in the fluvial / tidal FZ3 along the River Birket north of Lingham Farm and Eve-a-lyn Farm. It is likely that this is due to the fact that the fluvial / tidal FZ3 is kept in channel as per the functional floodplain therefore was not considered important to include in the final FZ3. This map is reproduced from Ordnance survey material with the permission of Ordnance survey on the behalf of the Controller of her Majesty's Stationary Office © Crown Copyright. Unauthorised reproduction infringes Crown Copyright and may lead to prosecution to civil proceedings. Licence No. 100019803 Figure 2 – missing Fluvial / Tidal FZ3 along River Birket River Birket / Drain Interaction South of Leasowe Common There is a drain that drains the land south of Pasture Farm (circled in red on Figure 3). It is believed that a flap system operates between the drain and the Birket though this cannot be said with confidence. If a flap system does not operate here then the low lying grassed area to the east of the drain may be susceptible to flooding for a 4% AEP event. Direct Tel: 0151 647 2312 38 Woodside Business Park T +44 (0)151 647 2312 Birkenhead F +44 (0)151 647 4742 Merseyside CH41 1EL E [email protected] Page: 4 of 8 Doc. Revised: www.fabermaunsell.com Technical Note This map is reproduced from Ordnance survey material with the permission of Ordnance survey on the behalf of the Controller of her Majesty's Stationary Office © Crown Copyright. Unauthorised reproduction infringes Crown Copyright and may lead to prosecution to civil proceedings.
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