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  • Strategic Flood Risk Assessment for Greater Manchester

    Methodology for deriving SFRA SuDS Map

    FINAL Report – August 2008

  • Association of Greater Manchester Authorities Strategic Flood Risk Assessment Methodology for deriving SFRA SuDS Map

    Scott Wilson St James's Buildings, Oxford Street, Manchester, M1 6EF, United Kingdom Tel: +44 (0)161 236 8655 Fax: +44 (0)161 228 2581

    www.scottwilson.com

    This document has been prepared in accordance with the scope of Scott Wilson's appointment with its client and is subject to the terms of that appointment. It is addressed to and for the sole and confidential use and reliance of Scott Wilson's client. Scott Wilson accepts no liability for any use of this document other than by its client and only for the purposes for which it was prepared and provided. No person other than the client may copy (in whole or in part) use or rely on the contents of this document, without the prior written permission of the Company Secretary of Scott Wilson Ltd. Any advice, opinions, or recommendations within this document should be read and relied upon only in the context of the document as a whole. The contents of this document do not provide legal or tax advice or opinion. © Scott Wilson Ltd 2008

    Revision Schedule Greater Manchester SFRA – Methodology for deriving SFRA SuDS Map August 2008

    Rev Date Details Prepared by Reviewed by Approved by

    D1 May 2008 Draft Fay Tivey

    Assistant Flood Risk Specialist

    Michael Timmins

    Principal Flood Risk Specialist

    David Dales

    Director

    D2 August 2008 Final Fay Tivey Assistant Flood Risk Specialist

    Michael Timmins Principal Flood Risk Specialist

    David Dales Director

  • Association of Greater Manchester Authorities Strategic Flood Risk Assessment Methodology for deriving SFRA SuDS Map

    FINAL August 2008 i

    Table of Contents

    1 Introduction.......................................................................................2 1.1 Background...................................................................................................................2 1.2 Aim ...............................................................................................................................2

    2 Data Sources....................................................................................4 1.1 Data ..............................................................................................................................4

    3 Methodology and Limitations ............................................................9 3.1 Methodology .................................................................................................................9

    4 How to use the SuDS Map..............................................................12 4.1 Policy & Strategic Planning .........................................................................................12 4.2 Development Control ..................................................................................................12

    5 SuDS Techniques Advice ...............................................................13 5.1 Overview.....................................................................................................................13 5.2 Infiltration Methods......................................................................................................15 5.3 Attenuation Methods ...................................................................................................16 5.4 Combined Methods.....................................................................................................18

  • Association of Greater Manchester Authorities Strategic Flood Risk Assessment Methodology for deriving SFRA SuDS Map

    FINAL August 2008 2

    1 Introduction

    1.1 Background

    As part of the sub-regional SFRA, a series of maps delineating the solid geology, superficial

    geology and groundwater vulnerability of the Greater Manchester area were presented. The

    purpose of the maps was twofold:

    • firstly, to demonstrate where geological conditions may be conducive to groundwater

    flooding and,

    • secondly, to provide information regarding the general suitability of Sustainable Drainage

    Systems (SuDS) across the sub-region.

    To increase the usability of the mapping and the SFRA, a SuDS guidance map was created.

    SuDS are a sequence of management practices and control structures designed to drain surface

    water in a more sustainable fashion than some conventional techniques. Sustainable drainage is a

    concept that makes environmental quality and people a priority in drainage design, construction

    and maintenance.

    The SuDS approach includes measures to reduce surface water runoff at source, prevent pollution

    and provide a range of physical structures designed to receive the runoff. There are many SuDS

    design options that can be tailored to fit all types of development. They can also be designed to

    improve amenity and biodiversity in developed areas.

    1.2 Aim

    The purpose of the SuDS map is to help planners in each of the ten districts in Greater

    Manchester to make decisions regarding the suitability of SuDS techniques into developments at

    an early stage. The maps may also be used by developers to identify which SuDS techniques may

    be suitable for areas and ensure that they are considered during the early stages of the site design

    process and accommodated in site layout and landscaping.

    It should be stressed that this map and advice is to provide a strategic tool to assist policy and

    development control planners in seeking runoff limitations. The map has been created using large

    scale datasets that make several assumptions and therefore has limitations when used in more

    local or site based situations.

    The starting assumption with the map is that all areas within CDC and WODC are suitable for

    SuDS in one form or another. The general permeability of the underlying ground conditions

    (bedrock, superficial deposits and soil) is then queried to determine which SuDS system is

    suitable. Three generic categories of SuDS type have been used:

    • Infiltration based systems,

    • Attenuation based systems,

    • Combined systems.

    Finally, the vulnerability of groundwater resources (aquifers) and source protection zones is then

    queried. For example, those areas that may have suitable geological conditions for infiltration

    based systems may also overly a highly vulnerable aquifer and therefore the pollution risks may be

  • Association of Greater Manchester Authorities Strategic Flood Risk Assessment Methodology for deriving SFRA SuDS Map

    FINAL August 2008 3

    too great to allow widespread infiltration SuDS to be implemented. Consequently, attenuation

    systems would be recommended.

    This user guide highlights the methodologies, limitations and assumptions used to produce the

    SuDS map.

  • Association of Greater Manchester Authorities Strategic Flood Risk Assessment Methodology for deriving SFRA SuDS Map

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    2 Data Sources

    1.1 Data

    The SuDS map has been created based on three main sources of data:

    • British Geological Survey (BGS) Permeability Mapping for Bedrock (solid), Superficial (drift), Mass Movement and Artificial deposits.

    • EA Groundwater Vulnerability Maps.

    • EA Source Protection Zones (SPZ).

    The following descriptions of the datasets used are taken from documents and publications

    provided by the data provider.

    BGS Permeability Data1

    The BGS Permeability dataset provides a qualitative classification of estimated rates of vertical

    movement of water from the ground surface through the unsaturated zone (i.e. above the ‘water

    table’) in geological formations underlying an area. It also provides a useful guide to the relative

    permeabilities of the superficial and bedrock formations present at or near the ground surface.

    The dataset contains three main fields:

    • Predominant Flow Mechanism

    • Maximum Permeability

    • Minimum Permeability

    The Predominant Flow Mechanism indicates how fluid will migrate from the ground surface

    through the unsaturated zone of geological formations and has three classes, intergranular,

    fracture or mixed (intergranular and fracture).

    The Maximum and Minimum Permeability ratings indicate the range of flow rates of water likely to

    be encountered in the unsaturated zone for geological formations at, and immediately below,

    outcrop (rather than at any significant depth). Five qualitative classes have been used for each

    rating:

    • very high

    • high

    • moderate

    • low

    • very low.

    Therefore an area may have a range of permeabilities within the maximum and minimum ratings

    provided. For example, an area may have a maximum permeability of “Very High” and a minimum

    permeability of “Very Low”. The actual permeability for a local site would have to be determined

    using more detailed ground investigation.