Strategic Flood Risk Assessment for Greater Manchester · Groundwater Vulnerability Maps 2’3...

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

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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


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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


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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


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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.


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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.

1 BGS Geosure Data – Information Sheet: Recommendations for using the BGS ‘Permeability Indices’ data in site

assessment or environmental search reports. January 2008.


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The Maximum Permeability represents the fastest potential vertical rate of migration of water

through the unsaturated zone likely to be encountered. The Minimum Permeability represents the

minimum, and in some cases more normal, bulk rate of vertical movement of water likely to be

encountered.

BGS Permeability Dataset Limitations

It is important to understand the assumptions made when creating the permeability dataset. The

main limitations associated with the BGS permeability dataset include:

• It is based on 1:50 000 geological data and hence should only be used for regional planning purposes.

• It can provide a useful guide to the relative permeabilities of the superficial and bedrock formations present at or near the ground surface. Formal evaluation of groundwater vulnerability requires additional information, including the physical and chemical characteristics of the soil, and the thickness of, and depth to water in, the different geological layers present.

• The indices should not be converted to saturated hydraulic conductivity values and used to provide quantitative values for rates of movement of pollutants to the water table.

• Slope and surface topography is not taken into account when assessing the permeability of an area.

Groundwater Vulnerability Maps2’3

Groundwater is an important strategic resource with three-quarters of all the groundwater pumped

from boreholes or taken from springs is used for mains water supply. It directly supplies nearly a

third of the drinking water in England and Wales. In some areas it is the only available drinking

water resource. It also supplies nearly all those who do not have mains water.

Groundwater is not just for private domestic use: many hospitals, bottling and food processing

plants also rely on their own groundwater supplies, as do major manufacturing and other

industries. There are advantages in using groundwater for both public and private supplies:

compared to surface water, it is of relatively high quality and usually requires less treatment prior

to use, even for drinking and other potable purposes.

The widespread presence of groundwater means that any material spilt on or applied to the ground

has the potential to reach the water table. Whether it will or not depends on the material involved

and the ground conditions at that site. Pollutants introduced by people can overwhelm the natural

capacity of the ground to deal with them.

If human activities do pollute groundwater, it is very difficult to return it to its original condition.

Processes that take days or weeks in surface water systems may take decades to centuries in

groundwater. This is because of the relatively slow rates of groundwater flow and the reduced

microbiological activity below the soil zone (due to the general lack of oxygen and nutrients).

Protecting groundwater is therefore essential. The subsurface environment is inaccessible and

complex and groundwater pollution can be very difficult to detect and may not become evident until

a water supply or spring is affected. Pollutants may take months or years to migrate from the

source to a receptor or to a point where they can be detected.

2 Environment Agency Groundwater Protection: Policy and Practice, Part 2 – Technical Framework

3 Environment Agency description and guidance provided with dataset


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The EA provided their Groundwater Vulnerability (GWV) Maps for the Cherwell and West

Oxfordshire District Areas. Published in 1998 and subsequently updated, the groundwater

vulnerability maps were designed to increase general public awareness and aid developers who

were planning new activities, and planners who were assessing new proposals or drawing up

development plans.

The GWV maps are only available in digital GIS layers from the EA and are based on printed

maps originally developed for strategic land use planning. The maps use existing geological and

soil maps and databases held by the British Geological Survey and the National Soil Research

Institute and cover England and Wales at a scale of 1:100,000.

The GWV layers describe the aquifer type, vulnerability and soil leaching potential for an area.

The combination of all three produces the full vulnerability classification.

GWV Aquifer Type and Vulnerability Classification

There are three aquifer types shown on GWV maps:

• Major Aquifer: Highly productive and used for potable supply on a regional scale.

• Minor Aquifer: Variably permeable and potentially important for local water supplies.

• Non – Aquifer: Negligibly permeable, containing insignificant quantities of groundwater.

As well as an aquifer type, a vulnerability classification is assigned to each area on the map.

Vulnerability is classed as High, Intermediate and Low and refers to the contaminant leaching

potential through the soil and into the rock.

Factors that will influence the vulnerability of an aquifer to contamination include whether the

aquifer is classed as confined or unconfined; the depth of the aquifer; whether the Major Aquifer is

overlain by a Minor Aquifer that is in hydraulic continuity with; and the soil vulnerability.

Some strata have a high leaching potential and have very little ability to slow or halt the progress

of contaminants and transmit them readily to the underlying aquifer. Other strata have a low

leaching potential and are thus either impermeable or have a number of natural factors that can

slow or stop the leaching of contaminants

GWV Soil Classification

There are two main types of soil classification used in conjunction with the aquifer and vulnerability

ratings in the GWV maps. These include Soils of High Leaching Potential and Soils of Low

Leaching Potential.

“Soils of high leaching potential” – these soils have little ability to attenuate diffuse source

pollutants and in which non-adsorbed diffuse source pollutants and liquid discharges have the

potential to move rapidly to underlying strata or to shallow groundwater. They are further classified

into the following three sub-classes:

(1) Soils which readily transmit liquid discharges because they are either shallow, or

susceptible to rapid flow directly to rock, gravel or groundwater.

(2) Deep, permeable, coarse textured soils which readily transmit a wide range of

pollutants because of their rapid drainage and low attenuation potential.

(3) Coarse textured or moderately shallow soils which readily transmit non-adsorbed

pollutants and liquid discharges but which have some ability to attenuate adsorbed

pollutants because of their clay or organic matter contents.


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Soil information for urban areas and restored mineral workings is based on fewer observations

than elsewhere. A worst case vulnerability classification (high / H) is therefore assumed for these

areas and for current mineral workings. All are given a designation HU until proved otherwise.

“Soils of low leaching potential” – soils in which pollutants are unlikely to penetrate the soil layer

because either water movement is largely horizontal, or they have the ability to attenuate diffuse

pollutants. Lateral flow from these soils may contribute to groundwater recharge elsewhere in the

catchment. They generally have high clay or organic matter contents.

GWV Dataset Limitations

As with the BGS data, it is important to understand the main limitations associated with the GWV

dataset. These include:

• The maps use existing geological and soil maps and databases held by the British Geological Survey and the National Soil Research Institute and designed to be used at a scale of 1:100,000. They are therefore suitable for strategic planning purposes but not for site-specific assessment.

• More specific vulnerability information can always be determined following more detailed site investigation.

• There are many areas, especially across restored mineral workings and urban areas, where the data is scarce and therefore it has been assumed that a worst case vulnerability exists.

• Slope and surface topography is not taken into account when assessing the vulnerability of near surface deposits.

Source Protection Zone Maps4’5

In addition to the general groundwater vulnerability that is defined across widespread areas using

the GWV maps, there are also specific areas where protection measures are needed for individual

sources, such as springs, boreholes and wells. In these areas, the EA restrict activities that may

pollute the groundwater resource (especially to major public water supplies or other potable water

uses such as brewing) and defines Source Protection Zones (SPZs) to delineate the most critical

areas requiring protection. This is relevant to SuDS as it can potentially restrict the use of

infiltration based techniques such as soakaways.

Generally the closer a polluting activity or release is to a groundwater source the greater the risk of

pollution. According to the EA, of the estimated 100,000 groundwater abstractions in England and

Wales, there are nearly 2,000 major public and food production supply sources with bespoke

SPZs. Most of these are sources producing more than 500,000 litres a day.

Each zone has three subdivisions for each source (EA (1996)). The shape and size of a zone

depends on the condition of the ground, how the groundwater is removed, and other

environmental factors.

SPZ I - Inner Protection Zone:

Any pollution that can travel to the borehole within 50 days from any point within the zone is

classified as being inside this zone. This applies at and below the water table. This zone also has

a minimum 50m protection radius around the borehole. These criteria are designed to protect

against the transmission of toxic chemicals and water-borne disease.

4 Environment Agency Groundwater Protection: Policy and Practice, Part 2 – Technical Framework

5 Environment Agency description and guidance provided with dataset


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SPZ II - Outer Protection Zone:

The outer zone covers pollution that takes up to 400 days to travel to the borehole or 25% of the

total catchment area, whichever area is the biggest. This travel time is the minimum amount of

time that the Environment Agency estimate pollutants need to be diluted, reduced in strength or

delayed by the time they reach the borehole.

SPZ III - Total Catchment:

The total catchment (source catchment) is the total area needed support removal of water from the

borehole and to support any discharge from the borehole.

Prior to any SuDS techniques being developed within SPZ areas, consultation with the EA

groundwater resources team is recommended to determine up-to-date and accurate information

on local conditions.

SPZ Data Limitations

As with the other data highlighted above, it is important to understand the main limitations

associated with the SPZ dataset. These include:

• The maps use existing geological and soil maps and databases held by the British Geological Survey and the National Soil Research Institute and designed to be used at larger scales. They are therefore suitable for strategic planning purposes but not for site-specific assessment. Site specific assessment should be carried out in consultation with the EAs groundwater resources teams.

• As with all spatial geological data, there could be uncertainties in the boundaries and extents of SPZs shown on maps. Therefore detailed consultation should be undertaken with the EA for developments that overlap, abut or are adjacent to SPZs.


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3 Methodology and Limitations

3.1 Methodology

Several steps and assumptions were made in creating the SuDS map. As a starting point, it has

been assumed that all new development and re-development across the sub-region is suitable for

SuDS implementation - infiltration systems, attenuation systems or combined systems (as required

and agreed with the EA). The following steps were then taken:

Step 1: All geological data was merged into one GIS layer. This produces one map with

numerous areas each with metadata relating to permeability, vulnerability and source

protection ratings (Figure 3-1).

Figure 3-1: Geological “Layers” used to create SuDS map

Some of the data on this map is based on data obtained from the British Geological Survey and the Greater Manchester Geological Unit. License No. 2006/1521 BGS Digital Data Licence 2008/151 - IPR/102-154DZ BGS Derived Material Copyright License 2008/151 – DMC – IPR/102-156DZ


FINAL August 2008 10

Step 2: As each area has a minimum and maximum permeability for bedrock, superficial,

artificial and mass movement deposits, it is sometimes difficult to determine which

permeability rating should be used at a strategic scale.

To ensure that a single SuDS map could be created, the descriptive permeability ratings

were converted into numeric ratings so that average permeabilities across all deposits

could be determined for each area. The average rating is based simply on the average

between the minimum and maximum permeability for each deposit and then an average

across all deposits ignoring zero values.

Table 3-1: Numeric Values assigned to permeability ratings

Permeability Rating Numeric Rating

Very High 1

High 2

Medium 3

Low 4

Very Low 5

No Data 0

Step 3: Once average permeability ratings were calculated, they were then converted back into

a descriptive rating for each area.

Step 4: One of the three generic SuDS Systems was applied to each area using the average

permeability ratings. This is a subjective process that is designed to be as simple and

transparent as possible and provides an indication to planners of the potential categories

of SuDS system that could be applied to the area.

It is stressed again that this process is not designed to provide a detailed and

prescriptive result that can be used to condition planning applications or produce site

specific policies in the LDF. Detailed site investigation and consultation with the EA

would be required to determine exactly which type of SuDS system could be applicable

to a particular area.

Table 3-2: Determining SuDS Suitability based on average permeability

Average Rating Descriptive Rating SuDS Suitability

>0<=1 Very High

>1<=2 High Infiltration Systems

>2<=3 Medium

>3<=4 Low Combined Systems

>4 Very Low

0 No Data Attenuation Systems

Step 5: The vulnerability of the underlying geology is then taken into account using the GWV

and SPZ maps to produce a final SuDS recommendation for each area. This is to

ensure that future development proposals not only reduce runoff, but do not increase the

potential for polluting groundwater resources.

For example, where there are major aquifers with a high vulnerability, the SuDS

recommendation is restricted to Attenuation Systems until further information or data is

provided to suggest otherwise. If the SuDS suitability based on permeability alone



suggests infiltration systems are suitable for the area, then the final recommendation is

“Attenuation Systems – Highly Vulnerable Aquifer”.

Table 3-3: Final SuDS Recommendation taking into account GWV

Groundwater Vulnerability

Description SuDS Recommendation

MAJOR_H Attenuation Systems

MINOR_H

High Vulnerability Aquifer Attenuation Systems – Highly Vulnerable Aquifer

MAJOR_I Attenuation Systems – Potentially Vulnerable Aquifer

MINOR_I

Potentially Vulnerable Aquifer Combined Systems

MAJOR_L

MINOR_L

Low Vulnerability Aquifer

Infiltration Systems

SPZ I Inner Zone

SPZ II Outer Zone

SPZ III Total Catchment

Attenuation Systems – SPZ

No Data No Data No Data – Further Investigation Required

The recommendation categories have been thematically mapped to create a final SuDS

recommendation map.



4 How to use the SuDS Map

It is recommended that SuDS are assumed to be applicable across the whole study area in the

first instance. The SuDS map can be used as a guide by strategic planners and also in

development control activities to give an indication as to which SuDS method is thought to be most

suitable.

4.1 Policy & Strategic Planning

The SuDS map for the AGMA sub-region is designed to be used at a strategic scale to give users

of the SFRA an overview of the potential SuDS applicability across the districts. The map is based

on permeability data at the 1:50,000 scale and groundwater vulnerability at the 1:100,000 scale.

The map does not take into account slope, urban areas or more local ground conditions.

In conjunction with the sub-regional SFRA, it is envisaged that policy and strategic planners within

the ten AGMA districts will use the detailed maps to specify SuDS requirements for strategic land

allocations. For example, if a large strategic site is shown to be in a location where only

attenuation SuDS techniques are suitable, the incorporation of a suitable system, such as a

detention basin, should be included in the development brief and proposed layout design from an

early stage in the development process.

As a result, the map is not intended to be used as a definitive and detailed specification on the use

of SuDS across the two districts.

4.2 Development Control

The SuDS map can be used as a reference guide for Development Control to identify which SuDS

techniques would be suitable for a development site and therefore allow the request for specific

information from a site-specific FRA. In addition, the maps can be used by Development Control to

advise developers which SuDS technique is thought to be suitable in an area. This is particularly

prudent in situations where only attenuation techniques are suitable and therefore the subsequent

land-take needs to be considered to incorporate this into the site layout design.

Following receipt of detailed site specific FRAs and consultation with the EA, it may be apparent

that SuDS are not the most suitable way to address surface water runoff from a development. In

these situations, development runoff would need to be discharged in accordance with Approved

Document H: Drainage and Waste Disposal of the Building Regulations 2000, Section 3 or to

British Standard BS EN 752 Drain and Sewer Systems for Outside Buildings, Part 4.



5 SuDS Techniques Advice

5.1 Overview

There are numerous solutions to the implementation of sustainable drainage techniques. A

summary of commonly used SuDS techniques is presented in the proceeding section. In

accordance with the six categories presented in Section 3.1, common SuDS techniques have been

split into three broad categories:

• Infiltration,

• Attenuation,

• Combined.

Any proposed SuDS methods should be planned and incorporated in accordance with relevant

policy and guidance such as PPS25, National SUDS Working Group (2004), BRE 365, CIRIA

report C522 for SUDS, CIRIA 523 (SUDS Best Practice Manual) and CIRIA C697 (the SUDS

Manual). Information presented in the preceding section has been derived from these sources.

Table 5-1 presents a summary of commonly used SuDS techniques with regard to the appropriate

scale of implementation (e.g., regional level / site level) and associated biodiversity benefits.

Association of Greater Manchester Authorities

Strategic Flood Risk Assessment

Methodology for deriving SFRA SuDS Map

FINAL

August 2008

14

Table 5-1: Summary of SuDS techniques and their suitability to m

eet the three goals of biodiversity

Management

Train

Component

Description

Water

Quantity

Water

Quality

Amenity

Biodiversity

Green roofs

Layer of vegetation or gravel on roof areas providing absorption

and storage.

●

●

●

Rainwater harvesting

Capturing and reusing rainwater for domestic or irrigation uses.

○

○

○

Site

Permeable pavements

Infiltration through the surface into underlying layer.

●

●

○

Filter drains

Drain filled with permeable material with a perforated pipe along

the base.

●

●

Infiltration trenches

Similar to filter drains but allows infiltration through sides and

base.

●

●

Soakaways

Underground structure used for store and infiltration.

●

●

Bio-retention areas

Vegetated areas used for treating runoff prior to discharge into

receiving water or infiltration

●

●

●

Area

Swales

Grassed depressions, provides temporary storage, conveyance,

treatment and possibly infiltration.

●

●

○

Sand filters

Provides treatment by filtering runoff through a filter media

consisting of sand.

●

●

Basins

Dry depressions outside of storm periods, provides temporary

attenuation, treatment and possibly infiltration.

●

●

○

Ponds

Designed to accommodate water at all times, provides attenuation,

treatment and enhances site amenity value.

●

●

●

Regional

Catchment

Wetland

Similar to ponds, but are designed to provide continuous flow

through vegetation.

●

●

●

Key: ● – highly suitable, ○ - suitable depending on design



5.2 Infiltration Methods

These type of SuDS rely on discharges to ground, where ground conditions are suitable.

Therefore, infiltration SuDS are reliant on the local ground conditions for their successful operation.

Various infiltration SuDS techniques are available for directing surface water run-off to the ground,

though some of these are covered in Section 5.4, as they also offer some level of attenuation in

addition to their infiltration capacity.

Soakaway

Soakaways are the most commonly used type of infiltration device in the UK. They store rapid

runoff and allow its efficient infiltration into the surrounding soil. Soakaways are sub-surface

infiltrations devices that require no net land take and which are best suited to the infiltration of

storm water runoff from small areas such as roofs of residential housing. When used to drain roads

and car parking areas, pre-treatment may be required to ensure that pollutants are minimised to

acceptable levels before passing flow into the device.

With regard to limitations of their use, soakaways should not be used:

• Within 5 metres of a building or road

• In an area of unstable land without full consideration of the impact of the infiltrating water

• In ground where the water table reaches within 1 metre below the base of the soakaway at

any time of year

• Near any drainage field, drainage mound or other soakaway which would result in the

overall soakage capacity of the ground being exceeded and the effectiveness of any

drainage field impaired

• Where the risk of contamination in the runoff could result in pollution of groundwater.

Permeable Surfaces

Permeable surfaces are designed to allow water to drain through to a sub-base at a rate greater

than the predicted rainfall for a specified event. Permeable surfaces act by directly intercepting the

rain where it falls and control runoff at source. Runoff during low intensity rainfall events is

prevented by permeable surfaces. During intense rainfall events runoff generation may occur from

permeable surfaces. Permeable sub-base can be used to temporarily store infiltrated run-off

underneath the surface and allow the water to percolate into the underlying soils. Alternatively,

stored water within the sub-base may be collected at a low point and discharged from the site at

an agreed rate.

Programmes should be implemented to ensure that permeable surfaces are well maintained to

ensure the performance of these systems is not reduced. The use of grit and salt during winter

months may adversely affect the drainage potential of certain permeable surfaces.

Types of permeable surfaces include:

• Grass/landscaped areas

• Gravel

• Solid Paving with Void Spaces



• Permeable Pavements

5.3 Attenuation Methods

Where ground conditions are not suitable for infiltration techniques, management of surface water

runoff prior to discharge should be undertaken using attenuation techniques. This technique

attenuates discharge from a site to reduce flood risk both within the site and to the surrounding

area. It is important to assess the volume of water required to be stored prior to discharge to

ensure adequate provision is made for storage. The amount of storage required should be

calculated prior to detailed design of the development to ensure that surface water flooding issues

are not created within the site.

The rate of discharge from the site should be agreed with the Local Authority and the Environment

Agency. If surface water cannot be discharged to a local watercourse then liaison with United

Utilities should be undertaken to agree rates of discharge and the adoption of the SuDS system.

Large volumes of water may be required to be stored on site. Storage areas may be constructed

above or below ground. Depending on the attenuation/storage systems implemented, appropriate

maintenance procedures should be implemented to ensure continued performance of the system.

Detention Basins

Basins are areas that have been contoured (or alternatively embanked) to allow for the temporary

storage of surface water run-off. Basins are designed to drain free of water and remain waterless

in dry weather. These may form areas of public open space or recreational areas. Basins also

provide areas for treatment of water by settlement of solids in ponded water and the absorption of

pollutants by aquatic vegetation or biological activity. The construction of basins uses relatively

simple techniques. Local varieties of vegetation should be used wherever possible and should be

fully established before the basins are used. Access to the basin should be provided so that

inspection and maintenance is not restricted. This may include inspections, regular cutting of

grass, annual clearance of aquatic vegetation and silt removal as required.

Retention Ponds and Wetlands

Ponds are designed to hold the additional surface water run-off generated by the site during

rainfall events. The ponds are designed to control discharge rates by storing the collected run-off

and releasing it slowly once the risk of flooding has passed. Ponds can provide wildlife habitats or

water features to enhance the urban landscape and, where water quality and flooding risks are

acceptable, they can be used for recreation. It may be possible to integrate ponds and wetlands

into public areas to create new community ponds.

Ponds and wetlands trap silt that may need to be removed periodically. Ideally, the contaminants

should be removed at source to prevent silt from reaching the pond or wetland in the first place. In

situations where this is not possible, consideration should be given to a small detention basin

placed at the inlet to the pond in order to trap and subsequently remove the silt. Depending on the

setting of a pond, health and safety issues may be important issues that need to be taken into

consideration. The design of the pond can help to minimise any health and safety issues (i.e.

shallower margins to the pond reduce the danger of falling in, fenced margins).

Various types of ponds are available for utilising as SuDS measures. These include:

• Balancing/Attenuating Ponds

• Flood Storage Reservoirs



• Lagoons

• Retention Ponds

• Wetlands

Green Roofs

Within urbanised areas, space is often a large constraint to the use of SuDS measures. However,

there is a large area of roof space that is currently under-utilised which lends itself to the use of

green roofs.

Green roofs are just that, they are a layer of vegetation, placed over a drainage layer that is

designed to intercept and retain rainfall leading to a reduction in the volume of runoff. The use of

green roofs can reduce the size of downstream SuDS and drainage infrastructure that is required.

According to the English Nature research report ‘Green Roofs: Their existing status and potential

for conserving biodiversity in urban areas’, 71% of rain falling on a 100mm turf layer can be

retained within the turf layer, greatly reducing storm water runoff.

There are two main types of green roof: ‘Extensive’ and ‘Intensive’.

An Extensive green roof is a covering of the whole roof area with low growing, low maintenance

plants. They usually comprise of 25 – 125mm thick soil layer in which a variety of hardy, drought

tolerant, low level plants are grown. Extensive green roofs are designed to be self sustaining and

cost effective and can be used in a wide variety of locations often described as an ‘ecological

protection layer’.

Figure 5-1: Illustration of an extensive green roof at Schiphol Airport, Amsterdam

(Source: The Green Roof Research Programme)

An Intensive green roof is a landscaped area which can include planters or trees and are usually

publicly accessible. They may include irrigation and storage for rainwater. They often require

more maintenance and impose a greater load on the roof structure than extensive green roofs.

Some city parks are in fact intensive green roofs such as the parks within the Canary Wharf

Estate, Canada Square and West Ferry Circus in London.



Figure 5-2 - Illustration of an intensive green roof in London

(Source ‘Living roofs’ website)

Rainwater Harvesting

The rainwater harvesting process is essentially the collection of rainwater from roofs into

containers, which can be stored above or below ground. The stored rainwater can then be re-used

as and when required for every day non potable uses such as washing machines and toilets.

Alternatively, collected rainwater can be released into the sewerage system once the rainfall event

has subsided to reduce the risk of flooding and sewerage overflows.

Filter Drains

A filter drain is a trench lined with a geotextile layer and filled with gravel into which surface water

runoff is directed either from the drained surface or via a pipe system. The gravel in the filter drain

traps some sediment, organic matter and oil residues that can be broken down by bacterial action

through time. The runoff rate is reduced and runoff storage is also provided. Stored water can also

pass through the geotextile membrane and some filter drains need not lead to a watercourse at all.

Filter drain systems have been widely used by the highway authorities for roads drainage. Hybrid

infiltration systems and filter drains have been used for a variety of developments, including both

residential and industrial sites.

5.4 Combined Methods

Pervious Pavements

Pervious pavements allow rainwater to infiltrate through the surface into underlying construction

layers where water is stored prior to infiltration to the ground, reused or released to a surface water

drainage system or watercourse at an attenuated rate. Pervious surfaces can be incorporated into

soft landscaping and oil interceptors can be added to improve pollutant retention and removal.

Where pervious pavements are located within 5m of foundations or basements, an impermeable

membrane liner is required to prevent infiltration.



Figure 5-3- Illustration of a Pervious Pavement using Block Paving

(Source: CIRIA website)

Sub-surface Infiltration

Where permeable surfaces are not a practical option more defined infiltration systems are

available. In order to infiltrate the generated run-off to ground, a storage system is provided that

allows the infiltration of the stored water into the surrounding ground through both the sides and

base of the storage. These systems are constructed below ground and therefore may be

advantageous with regards to the developable area of the site. Consideration needs to be given to

construction methods, maintenance access and depth to the water table. The provision of large

volumes of infiltration/sub-surface storage has potential cost implications. In addition, these

systems should not be built within 5 m of buildings, beneath roads or in soil that may dissolve or

erode.

Various methods for providing infiltration below the ground include:

• Geocellular Systems

• Filter Drain

• Soakaway (Chamber)

• Soakaway (Trench)

• Soakaway (Granular Soakaway)

Swales

Swales are linear, grass covered depressions which direct surface water overland from the drained

surface to a storage or discharge system. Swales are shallow and relatively wide, unlike a

conventional ditch. They provide temporary storage for storm water and reduce peak flows. They

are located close to the source of runoff and can form a network within a development linking

storage ponds and wetlands.

A swale is designed to be dry during dry weather but in wet weather rainwater flows into it along its

length and moves slowly through the grass area. Sediment is deposited while oily residues and

organic matter are retained to be broken down in the top layer of soil and vegetation. The



underlying aquifer can be protected if required by placing and impermeable lining in the soil

beneath the swale.

During a rainfall event a proportion of the runoff can be lost from the swale by infiltration,

evaporation and transpiration. Overflows can be provided to allow conveyance during periods of

exceptionally heavy rainfall.

Some regular maintenance is required to keep a grass swale operating correctly, mainly mowing

during the growing season.

Infiltration Trench

An infiltration trench is a shallow, excavated trench that is lined with a geotextile and backfilled with

stone to create an underground reservoir. Surface water runoff flowing into the trench gradually

infiltrates into the subsoil. An overflow may be required for extreme rainfalls that exceed the design

capacity of the reservoir.

The performance of the trench is dependent on the permeability of the soil and the depth of the

water table. Infiltration trenches usually serve small catchment areas up to 2-3 hectares. The

closer the proximity of the trench to the source of the runoff, the more effective they are. The

operational life of the trench may be enhanced by providing pre-treatment for the inflow, such as a

filter strip, gully or sump pit, to remove excessive solids. Regular maintenance will be required for

most pre-treatment designs.

Infiltration Basin

Infiltration basins are shallow features where surface water runoff is stored until it gradually

infiltrates through the soil of the basin floor. An overflow may be required for extreme rainfall

events which exceed the capacity of the basin.

The performance of the basin depends largely on the permeability of the soil and the depth to the

water table. Infiltration basins can serve larger catchment areas than infiltration trenches because

a larger volume of water can be stored on the surface. Typically, they can serve catchments of up

to 10 hectares. As with the infiltration trench, the lifespan of the basin may be increased by

providing pre-runoff treatment, such as a filter strip, gully or sump pit to remove excessive solids.

Regular maintenance will be required for most pre-treatment designs. Larger basins need to be

carefully designed to prevent sediment being washed out during storms.

Strategic Flood Risk Assessment for Greater Manchester · Groundwater Vulnerability Maps 2’3...

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