Flood Study Report - Land North Of Sherfield Road

Parent Category: Planning and Development

Category: Planning Minutes and Reports

Published on Friday, 14 October 2016 14:34

Hits: 2514

Below you will find a copy of the recent flood study report carried out by AAH Planning Consultants on behalf of Bramley Parish Council.  The report is available as a PDF which you can download and view.  A brief summary of the introduction and conclusions has been included on this page but you are advised to refer to the full report.

Introduction

AAH Planning Consultants have been commissioned to undertake a flood study report to assess planning application reference number 15/02708/OUT. The applicant is seeking outline planning consent for a residential development of up to 50 dwellings, including affordable housing with associated access, highway works, drainage works (SUDS), public open space, landscaping and any other associated infrastructure with all matters reserved except for means of access. The site is within the administrative area of Basingstoke and Deane Borough Council, Thames Water, and the Environment Agency each of whom are interested parties in the flood risk and drainage aspects of planning.

The National Planning Policy Framework (NPPF) identifies that flood risk assessments should be conducted for new developments proposed on the floodplains of rivers, sites potentially subject to coastal flooding and for developments of 1 hectare and above. The proposal site lies within an area designated as flood zone 1, 2 and 3 by the Environment Agency (EA).

The NPPF states that residential land uses (class use C3) such as the proposed development are ‘more vulnerable’ to flooding. The existing use of the site for development as open ground is ‘water compatible’. The flood risk vulnerability of the site would increase with respect for the pre and post development site use.

On behalf of Bramley Parish Council AAH Planning Consultants has undertaken quantitative assessment of the floodplain using two dimensional flood modelling software to consider both the Bramley Green Stream and the ordinary watercourse which crosses the site. The instruction is to produce a hydraulic model to meet planning consultation deadlines and where beneficial to expand the study once flow information for the ordinary watercourse becomes available. The model study will be submitted to the EA as a ‘flood map challenge’ to improve the accuracy of their flood map, so safeguarding the position of the Parish Council’s existing constituents, and indeed those of the new development should the Local Planning Authority grant planning permission.

Conclusions

This report summarises the baseline hydrology and flood risk on the watercourses associated with the Bramley Green Stream in Bramley, Hampshire.

The Q100 flood outline yielded by hydraulic modelling (which in annual probability terms is equivalent to fluvial flood zone 3) is similar to the EA flood map flood zone 3 extents, this is to be expected as the input data (LiDAR ground levels and FEH derived flows) are broadly similar. Accuracy of the flood model is improved through incorporation of the developer’s own topographic survey whilst flood levels produced provide comprehensive baseline conditions which are used to identify deficiencies within the developer’s flood resistant housing measures and surface water drainage design.

Sensitivity analysis shows that variations of ± 20% to inflow, channel roughness and gradient produce up to 0.13m increase in flood level. A similar increase in manning’s roughness also has an impact at the west end of the site, increasing flood levels by 0.12m. Changing the downstream gradient has little impact, as the stream is relatively steep and thus there is no backwater effect.

Future flood modelling investigations planned will quantify substantial flooding of the site which it has been shown by indicative flood modelling could occur should an ordinary watercourse (or its associated culvert) located centrally within the site become fully or partially blocked. This flood scenario should be considered as an unknown residual risk to the proposed development which in turn casts uncertainties over the necessary requirements for flood resistant building design.

Attachments:

File	Description	File size	Last modified
Bramley Flood Study Report.pdf		848 kB	Fri, 14 Oct 2016 14:29:55