Last Updated 04/09/2020 9:00am
Following the widespread flooding of 55,000 homes and businesses across England and Wales in June and July of 2007, the Pitt Review was published, making 92 recommendations for effective and integrated flood risk management. These recommendations were the main driver of the measures contained within the Flood and Water Management Act (2010), which, combined with the Flood Risk Regulations (2009), placed greater responsibilities on Local Authorities to manage flood risk.
The Flood Risk Regulations (2009) transposed the European Union Floods Directive (2007) into UK law and established upper tier and unitary local authorities as Lead Local Flood Authorities (LLFAs). Devon County Council is therefore the LLFA for its administrative area (Figure 1), with the Flood and Water Management Act (2010) setting out its responsibilities to include managing local flood risk from surface water and groundwater, and consenting and enforcement on ordinary watercourses.
In accordance with the Development Management Procedure Order (2015), each LLFA is now also a statutory consultee for all major planning applications within its area. This role took effect on 6th April 2015, when the Environment Agency handed over the responsibility, and ensures that technical surface water drainage advice is available to Local Planning Authorities (LPAs) when determining planning applications. Our advice is based on various statutory, non-statutory and industry best-practice guidance documents which provide a wealth of information in relation to the management of surface water, in accordance with the principles of Sustainable Drainage Systems (SuDS).
Consequently, this Guidance does not attempt to rewrite existing documents; it provides a summary of relevant information and signposts the reader to useful documents, whilst providing a local context. This Guidance is therefore intended for use by applicants, developers, architects, engineers and other professionals alike who are seeking advice on the standards and information required by the LLFA when reviewing planning applications.
The Water Framework Directive (2000) is a piece of European Union legislation which was introduced to improve and protect the quality of water bodies. It should ensure that water is more effectively and sustainably managed for the benefit of both society and the environment.
The Flood and Water Management Act (2010) was introduced in response to the concerns raised in the Pitt Review (2008) following the floods across the United Kingdom in 2007. The Act makes LLFAs responsible for coordinating flood risk management within their areas.
The National Flood and Coastal Erosion Risk Management Strategy (2011) was published by the Environment Agency under the above Act and should ensure that flood and coastal erosion risk is properly managed and coordinated. Importantly, the Strategy recommends the use of SuDS on development sites as an end to managing surface water flood risk.
Devon County Council’s Local Flood Risk Management Strategy (2014-2020) specifically focuses on local flood issues in Devon. Importantly, the Strategy recommends that all new developments should have an effective and robust surface water drainage management system, designed in accordance with the most recent SuDS principles, with the aim of reducing on-site flood risk, whilst also avoiding increasing flood risk elsewhere.
The National Planning Practice Guidance (2015) contains a Flood Risk and Coastal Change section which advises on how the planning and design process can take account of the risks associated with flooding and coastal change.
The Non-Statutory Technical Standards for Sustainable Drainage Systems (2015) outline the basic principles of sustainable drainage, and should be read in conjunction with the National Planning Policy Framework (2012), in addition to this Guidance.
The Devon Green Infrastructure Strategy (2015) was published in response to Paragraph 99 of the National Planning Policy Framework (2012) which requires green infrastructure to be incorporated into new developments if they are located in areas vulnerable to flooding. The Strategy provides guidance on how to successfully incorporate green infrastructure into new developments, and shows green infrastructure opportunities and deficiencies across Devon.
When rain falls on greenfield sites, the water may be intercepted by vegetation, evaporate, infiltrate into the ground, or flow overland to enter water bodies (Figure 2a). However, the way in which these sites hydrologically respond to rainfall is significantly modified when they are developed. Most notably, the introduction of impermeable surfaces results in a significant increase in the rates and volumes of surface water runoff from such sites (Figure 2b).
Traditionally, this surface water runoff has been managed in subterranean piped systems which are designed to prevent localised flooding by conveying surface water runoff downstream as rapidly as possible. An inherent disadvantage of these systems is the risk of downstream flooding resulting from the concentration of flows within a confined area, and the absence of control measures to manage exceedance events. Furthermore, sediment-associated pollutants and contaminants are not naturally managed within these traditional drainage systems, and nor do they provide aesthetically-pleasing visual amenity or biodiversity benefit.
In contrast to these traditional subterranean systems, sustainable drainage systems (SuDS) manage surface water on, or as is practicably close to, the ground surface, in a way that mimics natural hydrological processes. Managing surface water in this way controls the rate and quantity of surface water runoff, improves its quality, and also provides visual amenity and biodiversity benefits (Figure 3).
Generally, the use of a variety of above-ground SuDS components, which manage rainfall close to where it falls, provide the greatest environmental benefits, and often cost less than traditional piped systems.
An early assessment of a proposed development site is essential to identify what can and cannot be accommodated within the proposed layout, in addition to any environmental constraints which may preclude the use of particular SuDS components.
Drainage engineers must therefore work alongside architects and landscape architects to identify a range of SuDS components, such as those outlined below, which together will provide a surface water drainage management system which comprehensively addresses water quantity, water quality, visual amenity and biodiversity issues.
Rainwater harvesting systems collect and store surface water runoff from roofs or paved surfaces, prior to use on-site, typically for vegetation irrigation and toilet flushing.
Green roofs are planted soil layers on the roofs of buildings which are irrigated by rainfall and therefore reduce the rate and volume of surface water runoff.
Infiltration systems (including soakaways and basins) collect, store, and dispose of surface water runoff using overlying vegetation, underlying soils, and/or cellular structures.
Filter strips allow surface water runoff to flow across a densely vegetated surface and infiltrate into the ground, whilst also encouraging the natural removal of sediment-associated pollutants and contaminants.
Filter drains allow surface water runoff to be temporarily stored below the surface in a shallow gravel-filled trench, providing attenuation, conveyance and water treatment benefits.
Swales are shallow, open, vegetated channels which are used to convey and treat surface water runoff, whilst also providing a useable public space with biodiversity benefits.
Bioretention systems (including rain gardens) are shallow landscaped depressions which allow surface water runoff to temporarily pond on the surface, before filtering through overlying vegetation and underlying soils prior to collection or infiltration.
Trees can be used within formalised soil-filled pits to complement a range of infiltration SuDS components by collecting, storing and treating surface water runoff by filtration.
Pervious pavements allow surface water runoff to soak through structural paving to enable storage in the sub-base, or infiltration directly into the ground surface.
Attenuation storage systems are below-ground structures which can be used to temporarily store surface water runoff before controlled release or re-use.
Ponds, wetlands and detention basins are features with permanent pools of water, the levels of which increase following rainfall, enabling the attenuation and treatment of surface water runoff.
The SuDS Management Train (Figure 4) describes the use of a sequence of SuDS components across a development site to control the rate and volume of surface water runoff, reduce the concentrations of sediment-associated pollutants and contaminants to acceptable levels, and provide visual amenity and biodiversity benefits. This method ensures that natural hydrological processes are mimicked by managing surface water runoff at source (i.e. close to where the rain falls), with residual flows conveyed downstream to larger SuDS components:
Devon County Council’s Flood and Coastal Risk Management Team therefore requires all new surface water drainage management systems to provide a comprehensive SuDS Management Train. These systems should ensure that a development site’s surface water runoff is passed between a variety of source control, and site control, SuDS components, rather than being directly conveyed to one concentrated point (e.g. to one large attenuation pond).
Devon County Council’s Flood and Coastal Risk Management Team do not consider underground attenuation systems to be truly sustainable components within surface water drainage management systems because they do not inherently and naturally provide water quality, visual amenity and biodiversity benefits.
Underground attenuation systems will therefore only be permitted once robust evidence has been submitted which demonstrates that it is not viable to incorporate any above-ground SuDS components into the surface water drainage management plan. This evidence should be in the form of written statements which clearly justify the necessity to provide underground components, accompanied by sketches, drawings, or constraint plans, where the LLFA deems it necessary.
Despite the fact that underground attenuation systems may be the most suitable solution for sites where significant constraints exist, it is almost always feasible to incorporate some above-ground SuDS components into the surface water drainage management system in order to complement the other components (Figure 6)
Consequently, development sites must consider the use of a variety of above-ground SuDS components, and these must be incorporated into surface water drainage management systems wherever possible (Figure 5). This will ensure that all surface water drainage management systems within Devon County Council’s administrative area are both sustainable and easily maintainable.
The use of SuDS components to manage surface water runoff represents an important opportunity to improve both urban and rural environments, and help to meet the growing demands to deliver green infrastructure by creating open green spaces which encourage habitat creation (Figure 7).
Due to the fact that SuDS should exploit both green and blue corridor networks, above-ground components can significantly contribute to creating and maintaining biodiversity, in addition to providing public amenity, public health, education, and economic benefits.
It is therefore essential to consider SuDS as part of an integral part of the broad network of green infrastructure on developments, rather than stand-alone, bolt-on features (Figure 8).
Devon County Council’s Flood and Coastal Risk Management Team therefore strongly advises drainage engineers to work alongside architects and landscape architects when designing surface water drainage management systems.
This collaborative working will ensure that SuDS will not only provide flood risk benefits; they will be fully integrated into green infrastructure networks and contribute to safeguarding the local landscape character and distinctiveness.
Devon County Council’s Flood and Coastal Risk Management Team requires surface water drainage management systems for new developments to be designed in accordance with the following technical documents:
The introduction of impermeable surfaces onto greenfield sites will significantly increase the rates and volumes of surface water runoff. Consequently, significant changes to the natural hydrological regime must be mitigated.
When designing surface water drainage management systems, robust evidence must be submitted in order to demonstrate that flood risk will not be increased either upstream or downstream of the development site and, wherever practicable, betterment should be achieved.
Where the surface water drainage management system discharges to a surface water body that can accommodate uncontrolled surface water discharges without any impact on flood risk from that surface water body (e.g. the sea), the peak flow control standards (section 6.3) and volume control standards (section 6.4) need not apply.
If an applicant seeking planning permission believes that this exception applies to their development site, they must approach Devon County Council’s Flood and Coastal Risk Management Team with the relevant evidence before the surface water drainage management system is designed. The Team will then review the submitted evidence in order to determine whether an exception can be made, and will formally confirm the result of this in writing to both the applicant and Local Planning Authority.
Where formal approval for uncontrolled surface water discharges from a proposed development site is granted by Devon County Council’s Flood and Coastal Risk Management Team, the surface water drainage management system must still incorporate a variety of above-ground SuDS components, in accordance with the SuDS Management Train, to provide benefits to the quality of the surface water runoff discharged off-site (Figure 9).
Where not collected for re-use, surface water runoff should be discharged as high up the hierarchy of discharge solutions as is practicable:
1. Discharge into the ground (infiltration);
2. Discharge to a surface water body (with written permission from the riparian owner);
3. Discharge to a surface water sewer, highway drain, or other drainage system (with written permission from South West Water Ltd., Devon County Council Highways, or the riparian owner, respectively);
4. Discharge to a combined sewer (with written permission from South West Water Ltd.).
Discharge into the ground (infiltration) must therefore be explored as the primary method of surface water disposal from all development sites in the first instance (Figure 10).
However, if this, or any other method, is discounted when the surface water drainage management system is being designed, robust evidence must be submitted to justify moving down the hierarchy (e.g. detailed results of infiltration testing undertaken in strict accordance with Building Research Establishment Digest 365 Soakaway Design (2016)).
For developments on greenfield sites, the peak surface water runoff rate for the 1 in 1 year rainfall event, up to and including the 1 in 100 year rainfall event, must never exceed the peak greenfield runoff rate for the same event. This will ensure that the surface water runoff rate from developed areas matches greenfield conditions.
Devon County Council’s Flood and Coastal Risk Management Team only accept greenfield runoff rates calculated in accordance with the methodologies outlined in CIRIA’s SuDS Manual (C753). Furthermore, the runoff areas used in any of these methodologies must be consistent. Consequently, only the area being drained to the proposed surface water drainage management system should be represented in the greenfield runoff rate calculations. For example, if the surface water runoff from a landscaped area does not contribute to the drainage system, this must be excluded from the greenfield runoff rate calculations.
For developments on brownfield sites, peak flow control must still match the greenfield runoff rate. However, if this is robustly demonstrated as being unfeasible, the applicant must work backwards to achieve a betterment, with a surface water runoff rate as close to the greenfield conditions as possible, providing robust evidence of the calculations undertaken.
On development sites where the greenfield runoff rates are low, off-site discharge rates must still be as close as possible to the greenfield performance; the 5 l/s minimum discharge rate recommendation is no longer acceptable because well-maintained modern flow control structures can now facilitate lower discharge rates.
For these development sites, Devon County Council’s Flood and Coastal Risk Management Team will formally agree minimum off-site discharge rates on a site-by-site basis to ensure that any proposed flow control structure will remain fully operational and maintainable for the lifetime of the development. This will require applicants to submit evidence in order to demonstrate that the proposed off-site discharge rate and flow control structure design is as reasonable as is practicable.
For developments on greenfield sites, the volume of surface water runoff discharged off-site in the 1 in 100 year, 6 hour rainfall event, must never exceed the greenfield runoff volume for the same event.
For developments on brownfield sites, the volume of surface water runoff discharged off-site must still match the greenfield runoff volume. However, if this is robustly demonstrated as unfeasible, the applicant must work backwards to achieve a betterment, with a surface water runoff volume as close to the greenfield conditions as possible, providing robust evidence of the calculations undertaken.
Where infiltration is not used to dispose of surface water from a development site, long term storage must be provided to store the additional volume of surface water runoff generated by the increase in impermeable area, which is in addition to the attenuation storage required to address the greenfield runoff rates. The incorporation of long term storage into surface water drainage management systems will ensure that each SuDS component is appropriately sized, and must discharge at a rate not exceeding 2 litres/second/hectare.
In the United Kingdom, the prevalence of summer and winter precipitation events is increasing and it is likely that the intensity of these events will increase in the future. The Environment Agency has therefore issued advice which outlines the allowances for climate change which are to be used in the production of Flood Risk Assessments and surface water drainage management plans (Figure 11).
The advice is based on different climate change projections and carbon dioxide emissions scenarios, with allowances for various periods of time over the next century.
In order to ensure that surface water drainage management systems on new development sites across Devon are designed to manage increasing rainfall intensity in the future, Devon County Council’s Flood and Coastal Risk Management Team requires the upper end climate change allowance for the ‘2080s’ epoch to be used when calculating peak rainfall intensity.
|Applies across all of England||Total potential change anticipated for the ‘2020s’ (2015 to 2039)||Total potential change anticipated for the ‘2050s’ (2040 to 2069||Total potential change anticipated for the ‘2080s’ (2070 to 2115)|
Figure 11: the Environment Agency’s climate change allowances for peak rainfall intensity calculations will ensure that surface water drainage management systems are adequately designed to manage increasing rainfall intensity in the future. Credit: Environment Agency (2016).
Consideration must be given to surface water exceedance routes to ensure that life and property are not put at risk in the event of a surface water drainage management system failing, or during storms in excess of the design standard of such systems.
Consequently, unless an area is designed to hold or convey water, flooding within a development site should not occur:
Where surface water drainage management systems are designed to flood above the
1 in 30 year rainfall event, the depth of any standing water along access and egress routes must be demonstrated as being safe. Overland flows must also be managed by unobstructed exceedance routes which minimise the risk to life and property (Figure 12).
Dividing developments sites into sub-catchments to manage surface water runoff in smaller systems closer to the source of rainfall, in accordance with the SuDS Management Train, will ensure that the surface water drainage management system is more resilient to failure.
Several Flood Zone 1 areas across Devon have been identified by the Environment Agency as having critical drainage issues. These Critical Drainage Areas have stringent surface water drainage design standards which all new minor and major developments must adhere to:
To satisfy these standards, additional surface water attenuation storage areas will be required within the development site in order to contribute to a reduction in downstream flood risk.
All components of a development site’s surface water drainage management system must be designed to ensure the structural integrity of the entire system, as well as any adjacent structures or infrastructure, over the design life of the development, taking into account the requirement for reasonable levels of maintenance.
For all new major developments, a temporary surface water drainage management plan must be submitted to demonstrate how surface water runoff generated during the construction period will be managed. This plan, which may form part of a Construction Environmental Management Plan, must be approved and constructed before any other works are commenced. However, on large sites, phased construction may be requested and the temporary SuDS may be incorporated into the permanent surface water drainage management system.
In addition to surface water runoff, these temporary surface water drainage management systems must also manage any eroded sediment, preventing it from entering the permanent surface water drainage management system during the construction phase (Figure 13). On sites where the use of underground attenuation systems has been approved, this is particularly important because these systems are difficult to comprehensively clean prior to the developer leaving the site, which may compromise the efficiency of the whole system.
The submission of this information is required to ensure that:
The adoption and maintenance requirements of SuDS components are an important consideration during the design process, and it is the responsibility of developers to put in place suitable arrangements for the lifetime of the development.
Typically, SuDS components will either be maintained solely by the landowner, or by private companies which will maintain the surface water drainage management system alongside the site’s general landscape maintenance regime (Figure 14). This is possible because maintaining almost all SuDS components does not require specialist knowledge.
On occasion, developers may wish Devon County Council, as Highway Authority, to adopt part of a development site’s surface water drainage management system. The decision to adopt these systems is made on a site-by-site basis by Highway Development Management Officers who work outside of the Flood and Coastal Risk Management Team. Nonetheless, the design standard of these systems must at least meet the standards outlined in this Guidance, as well as any other requirements specified by the Highway Authority.
Developers may also wish South West Water to adopt part of a development’s surface water drainage management system, in accordance with Section 104 of the Water Industry Act (1991). Written evidence must be provided from South West Water’s Developer Services Planning Team to confirm that this is acceptable, and to satisfy their adoption criteria, these systems must be underground, sealed, and designed to the 1 in 30 year rainfall event.
Consequently, a separate surface water drainage management system must also be provided to manage the surface water runoff generated from rainfall in excess of this return period, up to the 1 in 100 year (+40% allowance for climate change) rainfall event, and this system must prioritise the use of above-ground SuDS components.
Where surface water drainage management systems are not proposed for adoption by the Highway Authority or South West Water, an operation and maintenance plan and timetable must be submitted to explain:
As a statutory consultee in the planning process, it is inherently important that Devon County Council’s Flood and Coastal Risk Management Team ensures that all information concerning the surface water drainage aspects of each planning application under consideration is shared with all concerned parties.
Consequently, if additional surface water drainage information is requested from an applicant, the Flood and Coastal Risk Management Team cannot provide a formal substantive response to the Local Planning Authority until the information has been submitted directly to the Planning Case Officer (Figure 15). This ensures that at all times, the Planning Case Officer is fully aware of any additional surface water drainage information submitted, which may impact other aspects of the application. Once any such additional information is submitted, the Planning Case Officer will be able to formally re-consult the relevant statutory consultees to provide another substantive response, usually within 14 days.
The surface water drainage management plan for any development site must be considered at the earliest possible stage; it is now not acceptable for applicants to address the surface water drainage aspects of development sites in discharge of conditions or reserved matters applications.
The level of information relating to surface water drainage management that must be submitted in support of planning applications should be proportional to both the scale of development and level of flood risk. Although not exhaustive, the lists contained within this section provide a guide as to the level of information required by the Lead Local Flood Authority when assessing the surface water drainage management aspects of most planning applications.
Site-specific Flood Risk Assessments are required for proposals:
Some of the information outlined in Sections 10.1 and 10.2 may be presented in a Flood Risk Assessment, and the National Planning Practice Guidance website contains detailed information, and a useful checklist, to aid in preparing these documents.
Under the Flood and Water Management Act (2010), LLFAs have the power to designate any feature which could have an impact on the flow or storage of water as a Flood Risk Asset. Surface water drainage management systems could be subject to this classification, and where one is designated as an Asset, permission must be sought from the LLFA to undertake any works to it.
The LLFA will inform the relevant LPA when it intends to formally designate a feature, and in the case of surface water drainage management systems, the following information could be requested as part of the designation process:
An Ordinary Watercourse is defined as any watercourse that is not designated as a Main River by the Environment Agency.
In accordance with the Land Drainage Act (1991), if any temporary or permanent works need to take place within such watercourses to facilitate any part of a development (e.g. an access culvert or bridge), Land Drainage Consent must be obtained from Devon County Council’s Flood and Coastal Risk Management Team prior to any works commencing.
Applicants must contact the Environment Agency to enquire about, and where necessary apply for, an Environmental Permit for flood risk activities if works are proposed:
Rainwater Harvesting System:
Bioretention Systems (Including Rain Gardens):
Attenuation Storage Systems:
Ponds and Wetlands: