Level 1 Strategic Flood Risk Assessment
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Broxbourne Borough Council Level 1 Strategic Flood Risk Assessment Final Report May 2016 Broxbourne Borough Council Bishops' College Churchgate Cheshunt WALTHAM CROSS EN8 9XQ This page is intentionally left blank 2016s4048 Broxbourne Borough Council Level 1 SFRA FINAL v1.0 i JBA Project Manager Joanne Chillingworth The Library St Philip’s Courtyard Church End COLESHILL B46 3AD Revision History Revision Ref / Date Issued Amendments Issued to Draft v1.0 / May 2016 Kim Harding, Broxbourne Borough Council Final v1.0 / May 2016 Kim Harding, Broxbourne Borough Council Contract This report describes work commissioned by Broxbourne Borough Council. The Council’s representative for the contract was Kim Harding. Prepared by .................................................. Sophie Dusting BSc Analyst Joanne Chillingworth BSc MSc MCIWEM C.WEM Chartered Senior Analyst Reviewed by ................................................. Claire Gardner BSc MSc MCIWEM C.WEM Chartered Senior Analyst Purpose This document has been prepared as a Final Report for Broxbourne Borough 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 Broxbourne Borough Council. 2016s4048 Broxbourne Borough Council Level 1 SFRA FINAL v1.0 ii Acknowledgements We would like to acknowledge the assistance of: Broxbourne Borough Council; Hertfordshire County Council including Highways; Environment Agency; Thames Water; Lea Valley Regional Park Authority; Canal and River Trust; and Planners at the neighbouring authorities Copyright © Jeremy Benn Associates Limited 2016 Carbon Footprint A printed copy of the main text in this document will result in a carbon footprint of 553g if 100% post- consumer recycled paper is used and 703g if primary-source paper is used. These figures assume the report is printed in black and white on A4 paper and in duplex. JBA is aiming to reduce its per capita carbon emissions. 2016s4048 Broxbourne Borough Council Level 1 SFRA FINAL v1.0 iii Executive Summary Introduction This updated Level 1 Strategic Flood Risk Assessment (SFRA) replaces the Level 1 SFRA originally published by Broxbourne Borough Council in December 2007. The report has been prepared to replace the content included in the previous version of the SFRA and to provide appropriate supporting evidence for the emerging Local Plan. SFRA objectives The Planning Practice Guidance advocates a tiered approach to risk assessment and identifies the following two levels of SFRA: Level One: where flooding is not a major issue in relation to Strategic Housing Land Availability Assessment (SHLAA) sites and where development pressures are low. The assessment should be sufficiently detailed to allow application of the Sequential Test. Level Two: where land outside Flood Zones 2 and 3 cannot appropriately accommodate all the necessary development creating the need to apply the NPPF’s Exception Test. In these circumstances the assessment should consider the detailed nature of the flood characteristics within a Flood Zone and assessment of other sources of flooding. The objectives of this SFRA update are to: 1. To update the Council's existing Level 1 SFRA, taking into account most recent policy and legislation in the National Planning Policy Framework. 2. To update the Council's existing Level 1 SFRA, taking into account the latest available flood risk information and data. 3. To investigate and identify the extent and severity of flood risk from all sources presently and in the future within the local planning authority area of Broxbourne Borough Council. 4. To provide a comprehensive set of maps presenting flood risk from all sources that can be used as evidence base for use in the local plan. The following outputs have been prepared to meet the objectives: Appraisal of all potential sources of flooding, including Main River, Ordinary Watercourse, surface water and groundwater. Review of historical flooding incidents. Mapping of location and extent of functional floodplain. Reporting on the standard of protection provided by existing flood risk management infrastructure. An assessment of the potential increase in flood risk due to climate change. Areas at risk from other sources of flooding, for example surface water, canals or reservoirs. An assessment of the impact of future large-scale developments both within and outside Broxbourne. An assessment of existing flood warning and emergency planning procedures, including assessment of safe access and egress during an extreme event. Recommendations of the criteria that should be used to assess future development proposals and the development of a Sequential Test and sequential approach to flood risk. Summary of Level 1 Assessment The SFRA has considered all sources of flooding including fluvial, surface water, groundwater, sewers, reservoirs and canals within the study area. With regards to assessment methods, fluvial flood risk has been analysed using the results from River Lee 2D Modelling Study (CH2M Hill, 2014), provided by the Environment Agency, as well as Flood Zone 2 and 3 datasets also provided by the Environment Agency. Surface water flood risk has been analysed using the updated Flood Map for Surface Water published online by the Environment Agency and recorded flood incident data supplied by Hertfordshire County Council Highways unit. A number of other data sources have been drawn upon as an evidence base, such as sewer data from Thames Water, canal overtopping data from the Canal and River Trust, National Inundation Reservoir Mapping from the Environment Agency and various geology / groundwater products and datasets from the Environment Agency. The assessment has concluded the following: 2016s4048 Broxbourne Borough Council Level 1 SFRA FINAL v1.0 iv Flood history shows that the borough of Broxbourne has been subject to flooding from several sources of flood risk. Since the 1947 flood, a number of measures have been constructed across the Lower Lee catchment to alleviate the risk of flooding. However, the River Lee Flood Relief Channel almost reached capacity in 1987, 1993 and 2000. The primary fluvial flood risk is located along the River Lee corridor, around the eastern boundary of the borough. In addition, the floodplain associated with the tributaries of the River Lee network are generally narrow until reaching the urban areas and / or towards the confluences with the River Lee network. The borough of Broxbourne has experienced a number of historic surface water / drainage related flooding caused by a number of mechanisms from insufficient storm and combined drainage capacity to poor surface water management. The uFMfSW further shows a number of prominent overland flow routes in the borough; these predominantly follow topographical flow paths of existing watercourses or dry valleys with some isolated ponding located in low lying areas. In addition, a number of these follow local road infrastructure. The Thames Water DG5 register indicates a total of 36 recorded flood incidents in Broxbourne Borough Council administrative area. The more frequently flooded postcodes are EN8 8, with 15 records, followed by EN10 7 with six records. There have been incidents of historic groundwater flooding in the borough of Broxbourne which is thought to primarily be caused by the underlying geology i.e. gravel overlaid on top of London Clay. The study area is known to have a high water table and the bedrock geology of the study area indicates that the borough may be vulnerable to groundwater flooding. This may constrain the suitability of certain types of SuDS techniques such as infiltration. In relation to artificial sources of flooding, there are no records of flooding from reservoirs impacting properties inside the study area. The level and standard of inspection and maintenance required under the Act means that the risk of flooding from reservoirs is relatively low. There are two records of a canal overtopping along the Lee Navigation Channel (Lower) caused by higher water levels associated with debris from vegetation clearance and by water running from a footpath eroding the bank. The extent of the overtopping is unknown; however, it is thought that no properties were flooded. Consideration of the residual risk behind flood defences (including assets and structures) has been undertaken as part of this study, using the Environment Agency AIMS database. A high-level assessment of the potential cumulative impact of development in neighbouring authorities and the Lea Valley Regional Park Authority, to the borough of Broxbourne has also been undertaken. The Sequential approach to development and flood risk has been defined with guidance provided for the application of the Sequential and Exception Tests for both the Local Plan and for detailed, site-specific Flood Risk Assessments. This SFRA provides details of the FRA requirements and guidance for developers. These recommendations include those of the NPPF, Environment Agency standing advice, as well as reference to regional and local policy. In addition, specific recommendations following the findings of this level 1 SFRA have been put forward for development in Flood Zones 1, 2 and 3. Site-specific FRAs should include assessment of mitigation measures required to safely manage flood risk along with the along with promotion of SuDS to create a conceptual