Structural and Civil Engineering Flood Risk Assessment For
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Southsea Hovertravel Site Structural and Civil Engineering Flood Risk Assessment for Proposed Development Prepared for Griffon Hoverworks December 2020 Job No: 20133 Document Ref: 20133-WFB-XX-XX-RO-E-0001 P02 Revision Initial Issue Revision Revision Revision Revision P02 P03 P04 P05 Issue/Status DRAFT Preliminary Report Date December 2020 Dec 20 Prepared by B Morton B Morton Checked by B Morton Contents 1.0 Introduction and Development Description 1.1 Project Introduction and Objectives 1.2 Planning Policy and Guidance 1.3 Site location 1.4 Topography and Existing Land Use 1.5 Geology 1.6 Hydrological Features 1.7 Proposed Development and Flood risk Vulnerability Classification 2.0 Flood Risk Assessment 2.1 Source of Information 2.2 Flood Hazards 2.3 The Sequential Test & Environment Agency Flood Zone Compatibility 2.4 Flood Risk Mitigation Measures 3.0 Summary and Conclusions 4.0 Closure Appendices A: Site Location Plan B: Site Topographic Survey C: Proposed Site Layout Plan 1.0 Introduction and Development Description Development sites of more than 1 hectare (ha) in Flood Zone 1 Development sites less than 1 ha in Flood Zone 1, including a change of use in development type to a more 1.1 Project Introduction and Objectives vulnerable class (for example from commercial to residential), where they could be affected by sources of flooding other than rivers and the sea (for example surface water drains, reservoirs) WFBA Consulting Civil & Structural Engineers, have been commissioned by Griffon Hoverwork, to undertake a Development in an area within Flood Zone 1 which has critical drainage problems as notified by the Environment Flood Risk Assessment (FRA) to support the proposed development of an extension to the existing terminal Agency. building at Southsea Hoverport Terminal, Southsea, Portsmouth. This document represents a site-specific FRA for the proposed development. The Agency’s guidance supports the NPPF and specifies the requirements for FRAs based on the level of flood risk and the vulnerability of the proposed development to flooding. 1.2 Planning Policy and Guidance In addition, Environment Agency consent is required for any proposed works or structures, in, under, over or within This Flood Risk Assessment has been undertaken with due regard to the following national and local planning policy eight metres of the top of the bank of the watercourse, designated a ‘main river’, (or sixteen metres in the case of a and flood risk guidance. tidal watercourse). 1.2.1 National Planning Policy 1.2.3 Local Policy and Guidance The National Planning Policy Framework (NPPF)1 and associated Planning Practice Guidance for Flood Risk2 aim to steer new development to areas with the lowest risk of flooding. The following documents provide local flood risk policy and guidance for development within the city of Portsmouth: New development must also ensure that flood risk is not increased elsewhere, and where appropriate, planning applications should be supported by a site-specific Flood Risk Assessment. The Partnership for Urban South Hampshire’s Strategic Flood Risk Assessment (SFRA)4 and Portsmouth City Council’s Local Flood Risk Management Strategy (LFRMS)5 present details of the regional hydrology, potential The NPPF recommends that development should only be allowed in areas at risk of flooding where the FRA (and sources of flooding across the city and implications for developers. the sequential and exception tests, as applicable) can demonstrate that: Policy PCS12 of Portsmouth City Council’s Core Strategy6 relates to flood risk and includes the following relevant statements: a) within the site, the most vulnerable development is located in areas of lowest flood risk, unless there are overriding reasons to prefer a different location. The city council will reduce flood risk by following the flood risk management hierarchy set out in PPS25. The council will: b) the development is appropriately flood resistant and resilient. Assess the level of flood risk in making allocations and considering planning applications, in particular by c) it incorporates sustainable drainage systems, unless there is clear evidence that this would be inappropriate. reference to strategic and site-specific flood risk assessments. Avoid flood risk by: d) any residual risk can be safely managed; and . Taking a sequential approach to site allocations by prioritising those sites at a lower risk of flooding; . Applying the sequential test to planning applications, with the exception of applications for minor e) safe access and escape routes are included where appropriate, as part of an agreed emergency plan. development and for changes of use, including changes of use to residential. The sequential test will be considered met on: The guidance confirms that major developments should incorporate sustainable drainage systems unless there is . Allocated sites; and clear evidence that this would be inappropriate. Non-allocated brownfield sites in those parts of flood zones 2 and 3 that the Strategic Flood Risk Assessment shows as low or medium hazard areas. 1.2.2 Environment Agency Policy and Guidance . Requiring the exception test to be met in line with PPS25, in particular the requirement that development should be safe over its lifetime. 3 The Environment Agency’s guidance for FRAs confirms that planning applications for the following development . Substitute less vulnerable development types for those incompatible with the degree of flood risk wherever proposals must be supported by a FRA: possible, both in making allocations and on individual development sites. Control flood risk by: Development in Flood Zone 2 or 3 including minor development and change of use 4 1 Ministry of Housing, Communities & Local Government, February 2019, National Planning Policy Framework. Partnership for Urban South Hampshire (PUSH), February 2016, Strategic Flood Risk Assessment, 2016 Update. 5 2 Ministry of Housing, Communities & Local Government, March 2014, Planning Practice Guidance, Flood Risk and Portsmouth City Council, March 2015, Local Flood Risk Management Strategy. Coastal Change. 6 Portsmouth City Council, The Portsmouth Plan, Portsmouth’s Core Strategy, Adopted January 2012. 3 Environment Agency, March 2014 (updated February 2017), Guidance – Flood Risk Assessments if you’re applying for planning permission. Maintaining and improving the city’s flood defences; . Seeking contributions towards flood defences from development; . Where necessary, controlling flood risk on individual sites through on site flood risk management measures; . Seeking opportunities to improve the capacity problems of the existing sewer system, including through the promotion of a link between the east and west interceptor sewers; Portsmouth Application Site . Permitting new developments only where the necessary surface water drainage, foul drainage and sewage Harbour treatment capacity is available, or where additional capacity will be provided in time to serve any individual phase of the development without unacceptably reducing the level of service to existing users, or causing harm to the environment; and . Requiring surface water to be separated within the site and the post development rate of discharge of both surface and foul water to the combined system should be no greater than the existing rate. Mitigate the residual flood risk that remains after the above steps have been considered by: . Ensuring that the design and layout of development minimises the risk to people and damage to property in the event of flooding; and . Having in place effective emergency response plans. 1.3 Site Location Southsea The application site is located off Clarence Esplanade, Southsea, Portsmouth, PO5 3AD, as shown on the Site Location Plan within Appendix 1 and Figure 1 below. The application site is represented by the existing hoverport terminal on the Solent coastline, approximately 1km west of Southsea town centre and 1.5km south-west of Portsmouth city centre. The Environment Agency’s on-line flood map for planning indicates that virtually the entire application site lies within Flood Zone 3 (high probability of flooding) associated with tidal flooding from the sea. Figure 1. Site Location (image extracted from Portsmouth City Council’s LFRMS) The National Planning Policy Framework (NPPF) and associated technical guidance confirms that all applications 1.4 Topography and Existing Land Use for proposed new development within Flood Zone 2 or 3 and development proposals exceeding one hectare within Flood Zone 1 must be accompanied by a site-specific Flood Risk Assessment. The application site is located on the relatively flat, low-lying Solent coastline at Southsea. Ordnance Survey mapping indicates ground levels in the vicinity of the application site of approximately 4mAOD, rising slightly into Southsea to the East. A topographic survey is available for the application site and a copy is included as Appendix 2. The site survey confirms that ground levels are relatively consistent across the application site, and in the range 3.3 to 3.5mAOD. Ground levels within the area of proposed development are approximately 3.3 to 3.4mAOD. The existing land use comprises the Southsea hoverport terminal, with the area of proposed development (extension) comprising shingle beach. The application site is bound to the north-east by Clarence Esplanade, to the north-west by Clarence Pier Pavilion, to the south-west by the Solent and to the south-east by the hovercraft landing stage. 1.5 Geology British Geological Survey mapping indicates that the local geology beneath the application site comprises sand, silt and clay of the Palaeogene Earnley Sand Formation and Marsh Farm Formation, overlain by superficial Storm Beach Deposits (gravel)7. The area of proposed development is located on the shingle beach, where incident rainfall will infiltrate to ground. 1.6 Hydrological Features The application site is located on the South Coast at Southsea, Portsmouth, adjacent to The Solent. There are no watercourses or other surface water features in the vicinity of the application site. The local hydrological site setting is presented as Figure 2. River Wallington & Portsmouth Harbour Application Site The Solent Figure 2.