JBA Consulting Report
Total Page:16
File Type:pdf, Size:1020Kb
LP/Ev/9l Cumulative impacts of development on flood risk Final Report December 2020 www.jbaconsulting.com 2016s5363 - Bracknell Cumulative Impacts Assessment (v4.0 Jan 2021) i JBA Project Manager Anna Beasley 8a Castle Street Wallingford OX10 8DL Revision history Revision Ref/Date Amendments Issued to Draft / 16 April 2020 Natalie Hird Final Draft / 24 September Amendments following review Natalie Hird 2020 Final / 18 December 2020 Amendments following review Natalie Hird Final / 13 January 2021 Final amendments Natalie Hird Contract Prepared by ............................... Kirstie Murphy BSc MSc Assistant Analyst Reviewed by ............................... Anna Beasley BSc MSc CEnv MCIWEM C.WEM Anna Beasley Technical Director Purpose This document has been prepared as a Final Report for Bracknell Forest Council. JBA Consulting accepts no responsibility or liability for any use that is made of this document other than by the Client for the purposes for which it was originally commissioned and prepared. JBA Consulting has no liability regarding the use of this report except to Bracknell Forest Council Copyright © Jeremy Benn Associates Limited 2020. Carbon footprint JBA is aiming to reduce its per capita carbon emissions. 2016s5363 - Bracknell Cumulative Impacts Assessment (v4.0 Jan 2021) i Contents 1 Introduction ................................................................................................ 1 1.1 Context ........................................................................................................................... 1 1.2 Overview of flood risk in Bracknell Forest ............................................................................ 3 1.3 Input data ....................................................................................................................... 3 2 Assessment approach .................................................................................. 3 2.1 How catchments were defined ........................................................................................... 4 2.2 How flood risk susceptibility was defined ............................................................................. 4 2.3 How future growth was considered ..................................................................................... 8 2.3.1 Strategic growth in Bracknell Forest ................................................................................................8 2.3.2 Strategic growth in neighbouring Authorities ....................................................................................8 2.4 Results of future growth analysis ......................................................................................11 2.5 Identifying catchments with the most potential for cumulative impact of development on flooding ... 11 3 Assumptions and limitations ....................................................................... 12 3.1 Flood risk .......................................................................................................................12 3.2 Existing properties ...........................................................................................................12 3.3 Proposed development .....................................................................................................12 4 Planning policy considerations ..................................................................... 13 4.1 Higher and medium susceptibility catchments with high development pressure within Bracknell Forest ............................................................................................................................13 4.2 Higher susceptibility catchments with high development pressure draining towards neighbouring authorities ......................................................................................................................14 4.3 Higher susceptibility catchments with less development pressure draining towards neighbouring authorities ......................................................................................................................14 4.4 Lower susceptibility catchments ........................................................................................14 2016s5363 - Bracknell Cumulative Impacts Assessment (v4.0 Jan 2021) ii List of Figures Figure 1-1: Hydrological catchments and local authority boundaries 2 Figure 2-1: The approach to the cumulative impact assessment 4 Figure 2-2: Flood risk susceptibility in Bracknell Forest 7 Figure 2-3: Potential development sites considered within Bracknell Forest and neighbouring districts 10 List of Tables Table 1-1: Spatial data utilised in the cumulative impact assessment 3 Table 2-1: Flood susceptibility scoring 5 Table 2-2: Flood susceptibility by WFD catchment 6 Table 2-3: Distribution of potential development areas by WFD catchment 11 Abbreviations AEP Annual Exceedance Probability NPPF National Planning Policy Framework SFRA Strategic Flood Risk Assessment WFD Water Framework Directive 2016s5363 - Bracknell Cumulative Impacts Assessment (v4.0 Jan 2021) iii 1 Introduction 1.1 Context Under the 2018 update to the National Planning Policy Framework1 (NPPF), strategic policies and their supporting Strategic Flood Risk Assessments (SFRAs), are required to ‘consider cumulative impacts in, or affecting, local areas susceptible to flooding’ (para. 156). At present, the Planning Practice Guidance for Flood Risk and Coastal Change has not been updated to include any guidance on methodology for the assessment of the cumulative impacts of flooding. Cumulative effects arise, for instance, where several developments each have insignificant effects but together have a significant effect; or where several individual effects of the plan (e.g. noise, dust and visual) have a combined effect. This report examines the risk of several developments having a significant effect on flooding. The assessment includes both development within Bracknell Forest and within the neighbouring local authority areas, as hydrological catchments cross these administrative boundaries, draining into, out of and within Bracknell Forest. It assesses the risk of: • Cumulative development in Bracknell Forest adversely impacting flood risk within Bracknell, or within a neighbouring local authority area. • Cumulative development in a neighbouring local authority area adversely impacting flood risk within Bracknell. The assessment focusses on flood risk from rivers, as the greatest impact of cumulative development occurring within a catchment is felt in the river corridor. The assessment involves dividing the Bracknell Forest and neighbouring authority areas into hydrological catchments, assessing the susceptibility of each catchment to current flood risk, and considering the potential impact of the development being considered at this stage in each local authority’s Local Plans. Hydrological catchments cross local authority boundaries, so where catchments are shown as higher risk, it is important to note that the flood risk areas or development pressures are in some cases outside of Bracknell Forest Borough. Figure 1-1 shows Bracknell Forest and its neighbouring authorities, alongside the hydrological catchments which drain into, out of and within Bracknell Forest. Where catchments are identified as sensitive to the cumulative impact of development, the assessment concludes with potential strategic planning policy suggestions to manage the risk. ————————————————————————————————————————————— 1 Ministry of Housing, Communities and Local Government (2018) National Planning Policy Framework. Available at: https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/728643/Revised_NPPF_2018.pdf 2016s5363 - Bracknell Cumulative Impacts Assessment (v4.0 Jan 2021) 1 Figure 1-1: Hydrological catchments and local authority boundaries 2016s5363 - Bracknell Cumulative Impacts Assessment (v4.0 Jan 2021) 2 1.2 Overview of flood risk in Bracknell Forest The Level 1 SFRA (July 2918) considered all sources of flooding including fluvial, surface water, groundwater, sewers and reservoirs within Bracknell Forest. Many of the settlements across Bracknell Forest have experienced flooding in the past, including (but not limited to) Sandhurst, Warfield, Binfield, Bracknell Town, North Ascot, Crowthorne and Winkfield. Fluvial flood risk is generally be confined to the Main River floodplains such as The Cut and the Blackwater River. Overall, fluvial flood risk is in close proximity to watercourses, with a few areas of extensive floodplain. Surface water flooding occurs on small watercourses and through surface water runoff and exceedance of the drainage capacity in urban areas throughout Bracknell Forest, particularly Bracknell itself. In some locations there may be a risk of groundwater emergence during extremely wet winters. The highest groundwater flood risk areas occur across a central band, to the south of Bracknell and into North Ascot. There are also some areas close to the River Blackwater in Sandhurst which may be associated with river gravels. 1.3 Input data This assessment used spatial data to assess the existing risk of flooding and the potential impact of development. The spatial data was either provided by the client or available as open source data from the Ordnance Survey, Environment Agency or neighbouring planning authorities. A list of the data used, and its source has been included in Table 1-1. Table 1-1: Spatial data utilised in the cumulative impact assessment