Flood Risk Assessment
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FLOOD RISK ASSESSMENT on: Land off Holt Road, Horsford for: Carl Palmer Land off Holt Road, Horsford Flood Risk Assessment 206164 Contents Amendment Record This report has been issued and amended as follows: Rev Description Prepared Reviewed Checked Date P1 Preliminary Issue MJWH MEH AM 08.06.2018 Canham Consulting Ltd The Old School School Lane Thorpe St. Andrew Norwich NR7 0EP T: 01603 430650 F: 01603 430672 [email protected] www.canhamconsulting.co.uk Flood Risk Assessment – Land off Holt Road, Horsford June 2018 Issue P1 206164 CONTENTS 1 INTRODUCTION ................................................................................................................. 3 2 DEVELOPMENT DESCRIPTION & LOCATION ................................................................... 4 2.1 TYPE OF DEVELOPMENT WITH LOCATION ........................................................... 4 2.2 VULNERABILITY CLASSIFICATION ......................................................................... 5 2.3 SEQUENTIAL TEST .................................................................................................. 5 3 SOURCES OF FLOODING .................................................................................................. 6 3.1 FLOODING FROM RIVERS ....................................................................................... 6 3.2 FLOODING FROM THE SEA ..................................................................................... 6 3.3 FLOODING FROM THE LAND................................................................................... 7 3.4 FLOODING FROM THE GROUND ............................................................................ 8 3.5 FLOODING FROM ARTIFICIAL SOURCES ............................................................... 8 4 CLIMATE CHANGE ........................................................................................................... 10 5 DETAILED DEVELOPMENT PROPOSAL .......................................................................... 11 6 FLOOD RISK MANAGEMENT MEASURES....................................................................... 12 6.1 LEVEL STRATEGY ................................................................................................. 12 6.2 FOUL STRATEGY ................................................................................................... 12 6.3 SURFACE WATER DRAINAGE (SUDS) .................................................................. 12 6.3.1 OUTFALL AND WATER VOLUME ......................................................................... 12 6.3.2 SUDS OPTIONS .................................................................................................... 14 6.3.3 SUDS DESIGN ...................................................................................................... 16 6.3.4 MAINTENANCE ..................................................................................................... 20 7 RESIDUAL RISKS ............................................................................................................. 21 7.1 LACK OF MAINTENANCE ....................................................................................... 21 8 OFF SITE IMPACTS .......................................................................................................... 22 8.1 IMPACT OF SURFACE WATER DRAINAGE ........................................................... 22 8.2 IMPACT OF DEVELOPMENT ON HYDROLOGICAL MORPHOLOGY ..................... 22 9 SUMMARY ........................................................................................................................ 23 1 Flood Risk Assessment – Land off Holt Road, Horsford June 2018 Issue P1 206164 Appendix List Appendix A Masterplan Appendix B Site Plan with Topographic Information Appendix C Anglian Water Sewer Records Appendix D Exceedance Flow Plan Appendix E Infiltration Test Results Appendix F MicroDrainage Calculations Appendix G Maintenance Strategy Figure List Figure 1.1 Site Location Plan Figure 3.1 Surface Water Flood Map Figure 6.1 Infiltration test locations (indicated by the blue rectangles) Table List Table 2.1 Sequential Test Table 6.1 Site Infiltration Rates Table 6.2 SuDS Component Assessment Table 6.3 Pollution Hazard Indices Table 6.4 Indices Suitability Comparison Table 6.5 Infiltration Basin Sizing Calculations Table 6.6 Infiltration Basing Sizing Calculations [Lower Infiltration Rate] Table 6.7 Infiltration Basin Sizing Calculations [Lower Infiltration Rate & 1-in-30 year storm event] Table 6.8 Attenuation Basin Sizing Calculations Table 6.9 Attenuation Crate Sizing Calculations 2 Flood Risk Assessment – Land off Holt Road, Horsford June 2018 Issue P1 206164 1 INTRODUCTION Canham Consulting Ltd have been commissioned by Carl Palmer to produce a Flood Risk Assessment in support of an outline planning application for a mixed density residential development off Holt Road, Horsford. The proposed development consists of approximately 60-65 residential units. A masterplan of the development can be found in Appendix A. The purpose of this report is to provide information on the flood risks associated with the proposed development, identify mitigation measures for the site, meet the relevant design criteria and to enable the development to proceed. This is to be achieved by ensuring that the development is safe from flooding to the recognised standards and does not increase the risk of flooding to the development or to third parties. This report follows the guidance set out in the National Planning Policy Framework (NPPF), the National Planning Policy Guidance (NPPG), alongside other relevant Local Authority policies and guidance notes from relevant bodies such as the Environment Agency (EA), Anglian Water, CIRIA, The SuDS Manual (C753) and Norfolk County Council’s Guidance on SuDS Design. The following data was collected as part of this assessment: Anglian Water Sewer Records Infiltration Rates 3 Flood Risk Assessment – Land off Holt Road, Horsford June 2018 Issue P1 206164 2 DEVELOPMENT DESCRIPTION & LOCATION 2.1 Type of Development with Location The proposed development consists of low and medium density residential development, open space and associated roads. The location of the site is shown on Figure 1.1. Most of the site is greenfield. There is a private swimming pool and garage within the site. The site location is centred on Grid Reference TG196152. Site Location Figure 1.1: Site Location Plan North of the site there are existing residential dwellings, beyond this is Church Street, further residences and Horsford All Saints Church. East of the site there are several large agricultural fields. Adjacent to the south-east of the site there is a farm. South of the site there is agricultural land, and beyond this is the Northern Distribution Road (NDR). South- west of the site there are agricultural buildings. Adjacent to the west of the site is the B1149 Holt Road and a roundabout which links to the NDR via the realigned Drayton Lane. There is agricultural land to the west of Holt Road. 4 Flood Risk Assessment – Land off Holt Road, Horsford June 2018 Issue P1 206164 The site area is approximately 3.4ha. Most of the site is currently considered as permeable. 2.2 Vulnerability Classification The various types of built development are classified by national planning policy with regard to vulnerability to flood risk. NPPG Table 2 provides a detailed list of which types of development fall into the five flood risk vulnerability classifications. This list is recognised as not being exclusive, but it provides guidance on the various uses and their associated Flood Risk Vulnerability Classification. The proposed development is residential. Residential use falls into the ‘More Vulnerable’ classification. 2.3 Sequential Test The online flood maps indicate that the site is located within Flood Zone 1. Table 3 of the Technical Guidance to the NPPF identifies which types of development are appropriate for the various Flood Zones as shown below in Table 2.1. Flood Risk Essential Water Highly More Less Vulnerability Infrastructure Compatible Vulnerable Vulnerable Vulnerable Classification Zone 1 Exception test Zone 2 required Exception test Exception test Zone 3a required required Exception test Flood Zone Classification Zone 3b required Suitable for Development Not Suitable for Development Table 2.1: Sequential Test As the proposed site is within Flood Zone 1, the ‘More Vulnerable’ usage is considered suitable from the Sequential Test viewpoint. It is considered that the site is in a suitable location from a flood risk perspective. 5 Flood Risk Assessment – Land off Holt Road, Horsford June 2018 Issue P1 206164 3 SOURCES OF FLOODING As the site is shown within Flood Zone 1 on the online flood maps, the most significant source of flooding is deemed to be from surface water. However, for completeness, an analysis of all flood sources has been undertaken to fully consider the flood risk to the development and the surrounding area. 3.1 Flooding from Rivers Fluvial flooding is caused by rivers and occurs when the river channel capacity is exceeded by the flow. Most rivers have a natural floodplain which, in built up areas, is sometimes encroached upon by development. The online flood plain maps indicate that the site is within Flood Zone 1 and therefore at a low risk of flooding from rivers. The nearest watercourse is a stream approximately 500m north of the site. From the information above, the risk of flooding from rivers is