1. Introduction
Surrey County Council as Lead Local Flood Authority (LLFA) is the risk management authority responsible for managing local flood risk from surface water, groundwater and ordinary watercourses. We are designated as LLFA under the Flood and Water Management Act (2010).
As the LLFA we receive site specific consultation requests from the Local Planning Authorities (LPAs) to provide comments on non-major planning applications in areas of increased surface water flood risk. This is to ensure that the cumulative effect of multiple smaller sites does not increase surface water flood risk by incorporating surface water mitigation techniques such as Sustainable Drainage Systems (SuDS). SuDS aim to reduce surface water flood risk, improve water quality and enhance amenity and biodiversity.
Non-major development is classified as:
- 1 to 9 dwellings or areas less than 1 hectare
- Commercial sites where the floor space is less than 1,000 square metres
As detailed within the National Planning Policy Framework (NPPF), SuDS should be considered on all developments and should be included in developments in flood risk areas (and all major developments), unless there is clear evidence that SuDS would be inappropriate.
This guidance provides a summary of the information required to show that SuDS have been satisfactorily considered within an application, in accordance with the NPPF and local planning policy. Meeting the requirements of this document will assist the LLFA in assessing the application and lead to a more streamlined assessment process.
This guidance does not cover in detail other considerations the LLFA may take into account such as works affecting ordinary watercourses or specific requirements from site specific Flood Risk Assessments (FRAs). These can be found on either the Surrey County Council website or Local Planning Authority (LPA) website.
For any queries relating to this advice or other SuDS matters please contact us at suds@surreycc.gov.uk
2. Our vision
SuDS are an approach to managing surface water runoff which seeks to mimic natural drainage systems and retain water on or near the site, as opposed to traditional drainage approaches which involve piping water off site as quickly as possible. SuDS offer significant advantages over conventional piped drainage systems in reducing flood risk by attenuating the rate and quantity of surface water runoff from a site, promoting groundwater recharge, providing biodiversity benefits, as well as improving water quality and amenity value.
There are opportunities to incorporate SuDS into all development, regardless of site size or previous land use. No two sites are the same; each site should be assessed and evaluated on a site-by-site basis when designing a drainage strategy. Site conditions (topography, geology, presence of surface water bodies, etc) will inform the drainage scheme.
3. Requirement for SuDS
A surface water drainage assessment should be carried out to demonstrate that the proposed development makes use of SuDS and will not increase the risk of flooding from surface water on site or to the surrounding area. The drainage design should be carried out in accordance with the NPPF and the Planning Practice Guide (PPG) and the appropriate Local Authority Plan.
NPPF paragraph 173 states (updated December 2023):
When determining any planning applications, local planning authorities should ensure that flood risk is not increased elsewhere. Where appropriate, applications should be supported by a site-specific flood-risk assessment. Development should only be allowed in areas at risk of flooding where, in the light of this assessment (and the sequential and exception tests, as applicable) it can be demonstrated that:
- 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 development is appropriately flood resistant and resilient such that, in the event of a flood, it could be quickly brought back into use without significant refurbishment
- it incorporates sustainable drainage systems, unless there is clear evidence that this would be inappropriate
- any residual risk can be safely managed
- safe access and escape routes are included where appropriate, as part of an agreed emergency plan
Non-major planning applications will be considered by the LLFA as a non-statutory consultee, with comments issued as advice/recommendations for the LPA only.
The LLFA would expect to receive as a minimum, a statement or plan which details how surface water will be controlled on site. This should demonstrate that the drainage hierarchy has been considered for the management of surface water and evidence that the proposals will not increase surface water flood risk. The statement can either form part of the site's FRA (if applicable) or can be presented as a standalone document and should include detailed plans of how surface water management is incorporated into the proposed works.
4. Design criteria
The drainage scheme must be designed to:
- manage flood risk to people and property on the development site
- prevent increases in flood risk outside of the development
- where possible maintain existing surface water flow routes
- provide a sustainable drainage system approach
The applicant must demonstrate that no above ground flooding will occur within the site, up to and including the 1 in 30 year (3.33%) rainfall event, outside of areas specifically designed to hold or convey water.
Surface water up to the 1 in 100 (1%) year rainfall event including an allowance for climate change, must be safely contained on site. There should be no flood risk to the development site users, third parties or to vital infrastructure up to this rainfall event.
4.1 Surface water discharge hierarchy
Surface water run-off not collected for re-use must be discharged according to the following discharge hierarchy set out in Part H of Building Regulations in order of priority:
- infiltration to ground
- attenuated discharge to a surface water body or watercourse
- to a public surface water sewer
- private surface water drainage system
- to a combined sewer where there are no other viable options, and only where agreed in advance with the relevant sewage undertaker
Pumped systems will only be accepted where it can be evidenced that no other practical alternative exists.
Clear evidence that the drainage hierarchy has been followed should be provided. Details of the proposed outfall type should be provided, which could include: soakaway type, flow control device, orifice plates etc.
4.1.1 At source reductions and water reuse
Systems which reduce the amount of surface water going into the drainage system should be considered, such as green roofs, rain gardens or rainwater harvesting. This may help achieve water neutrality and Biodiversity Net Gain requirements.
4.1.2 Infiltration to ground
Infiltration replicates natural greenfield conditions and benefits local groundwater recharge. These include soakaways, basins, swales and permeable paving etc.
Soakage tests should:
- be undertaken preferably in the winter period (December to March) at the location and depth of the proposed structures
- must be carried out in accordance with the Building Research Establishment (BRE) BRE Digest365, the Construction Industry Research and Information Association (CIRIA) CIRIA R156 or a similar approved method
If test results are not available to support a full planning application, then justification as to why these have not been provided should be included; examples may include: issues with site access i.e. the site is a working brownfield site and as such it is not possible to dig trial pits. Such reasons will be reviewed on a case-by-case basis. If infiltration has been discounted and infiltration tests are not available to disprove its suitability, intrusive ground investigations which conclusively show ground conditions are not suitable for infiltration are acceptable (eg due to clay, contamination, hazards etc).
Infiltration is not always appropriate, the advice of the Environment Agency (EA) should be sought for developments within Source Protection Zones (SPZs), in areas with a known history of land contamination, or in areas with known high seasonal groundwater levels.
If ground investigations show that infiltration may be feasible, but no soakage test results are available, a conservative infiltration rate of 1x10-6 metres/second (or other appropriate rate with justification) should be used. Additionally, the applicant should provide an alternative drainage strategy should future soakage test results show ground conditions are not suitable for infiltration.
If infiltration is proposed, the drainage strategy should demonstrate:
- that the base of the any infiltration feature is at least 1 m above the peak groundwater level
- the half drain times (in hours) of the infiltration feature are at least within 24 hours from the end of the 1 in 30 (3.3%) including Climate Change rainfall event
- the minimum factor of safety of 2 should be used in calculations
- the minimum distance of the infiltration feature from any foundations is greater than 5m
- any measures proposed to prevent siltation of the infiltration device
Deep bore soakaways should only be considered after other forms of infiltration have been explored and in all cases the applicant will be required to consult the EA before their inclusion in a drainage strategy.
4.1.3 Attenuated discharge to surface waterbody or watercourse
In areas where ground conditions are not suitable to infiltrate, surface water can be discharged to suitable nearby waterbodies such as wetlands, lakes, ponds and rivers. The discharge location should aim to mirror the greenfield hydrological situation. The ability of existing watercourses to carry water (and to function as an effective exceedance flow route, where appropriate) will always need to be maintained.
Evidence should be provided as to the suitability of the waterbody to receive flows. These could include a topographical survey showing banks and bed levels which confirm a gravity connection is achievable. Proposed use of existing ponds or lakes must be accompanied by calculations showing that the waterbody has appropriate capacity to receive the incoming flows without increasing flood risk and that a suitable outfall or infiltration rate is present or proposed.
Evidence confirming that water can freely flow within a receiving watercourse without obstruction or confirmation that the channel will be cleared prior to development should be provided.
The applicant should confirm that they understand that any connection or alterations made to an Ordinary Watercourse (including channel clearance) may require consent under Section 23 of the Land Drainage Act 1991. Please refer to section 5.1 for more details.
Where appropriate, third-party permissions should be sought and evidenced for connections crossing land outside the red line boundary of the application.
If watercourses are within or adjacent to the site, access for future maintenance should be included in the design.
4.1.4 Discharge to sewer
Where no suitable waterbody is present, attenuated discharge to a water and sewerage company maintained surface water sewer is acceptable.
If no surface water sewer is available then discharge to combined or foul sewer may be acceptable subject to the water and sewerage company agreement.
At the planning stage, confirmation from the sewerage undertaker should be provided confirming capacity is available within the existing system to receive flows from the site without increasing flood risk, and any maximum discharge rates. This should be a developer enquiry application or similar.
4.2 Catchment area and surface water runoff rates
SuDS should mimic the natural greenfield characteristics of a site. Drainage schemes should be designed to match greenfield discharge rates and follow natural drainage routes as far as possible.
The greenfield runoff should be calculated by an approved method. The engineering consultant HR Wallingford has produced a free online tool that can be used. Tools for SuDS design | UK SuDS
4.3 Discharge rates
Where possible the post development discharge rates should match pre-development greenfield runoff rates as calculated above. The greenfield Q1 or QBar rural rate (QBar is the peak rate of flow from a catchment for the mean annual flood) could also be used as the fixed discharge rate for the positively drained areas of the site.
4.3.1 Brownfield sites
Brownfield sites should be limited as close to greenfield rates as is reasonably practicable. If greenfield rates cannot be achieved, clear written evidence must be submitted as to why a lower rate cannot be achieved. Information will be considered on a site-by-site basis. For brownfield sites justification could include: analysis of storage requirements required to achieve greenfield runoff rates in comparison to a site area, a cost analysis, spatial or level constraints.
4.3.2 Low flow rates
In meeting the above, very low discharge rates (for example below 1.0 litres/second) may not be achievable, depending on self-cleansing velocities. However low flow control devices are available on the market to achieve such discharge rates. Through proper design and maintenance such devices should not be prone to blockage, if proposed discharge rates are greater than the equivalent greenfield runoff rates based on there being a greater risk of blockage, justification should be provided as to why blockage cannot be designed out and is considered to be a risk. Such sites will be considered on site-by-site basis.
4.4 Attenuation
The attenuation method required to achieve the proposed surface water discharge rate from the site should be clearly identified. Above ground storage devices that provide water quality, amenity and biodiversity benefits should be proposed where practicable. Justification should be provided if below ground attenuation devices such as crates, tanks or oversized pipes are proposed.
Attenuation devices that serve more than one property should always be located in accessible areas such as areas of open space, under access roads or shared parking areas. Consideration should be given to how maintenance and/or potential replacement of the attenuation devices would be carried out over the lifetime of the development.
4.5 Climate change
The EA has published climate change allowances for peak rainfall in England. The LLFA recommends using the upper end allowance value.
4.5.1 Commercial or time limited development
A lower provision for climate change may be appropriate for commercial development. The applicant should justify the percentage climate change allowance used based on the anticipated lifespan of the development.
4.6 Exceedance planning
Any areas where water is likely to pool or flow off site should be set out in a clear drawing and provided within the management plan to the future site owners. The design should ensure that it does not increase flood risk to people or property on and off site.
4.7 Management and maintenance considerations
At the application stage basic information should be supplied to confirm how the entire drainage system will be accessed and maintained in perpetuity.
5. Other approvals required
5.1 Watercourse regulation
If the site works propose to culvert, divert or create any other obstruction in a watercourse, the granting of planning permission does not represent permission to carry out these works.
Prior to construction, written consent under Section 23 of the Land Drainage Act is required from Surrey County Council for Ordinary Watercourses, the EA for main rivers or the Internal Drainage Board (where one exists). Where these works would affect development viability or site layout, agreement in principle should be sought at pre-planning submission stage to ensure it is likely to be acceptable.
5.2 Groundwater quality
If proposed works result in infiltration of surface water to ground within a Source Protection Zone, the EA will require proof of sufficient surface water treatment to achieve water quality standards.
6. Pre-application planning advice
Surrey County Council provide three levels of pre-application advice. The SuDS Planning advice webpage provides further information and associated costs.
7. Other data, design guidance and codes of practice
The following sources provide useful information on the level of surface water risk, national regulations and practical guidance on the design and implementation of sustainable drainage:
- Code of practice for surface water management for development sites BS 8585:2013
- The updated flood map for surface water and long-term flood risk information
- Building Research Establishment (BRE) Soakaway Design Digest 365 (BRE Digest 365 (2016))
- The Building Regulations 2010 Drainage and Waste Disposal Approved Document H, HM Government, 2015 edition
- Non-Statutory Technical Standards for Sustainable Drainage Systems, Practice Guidance, Local Authority SuDS Officer Organisation (LASOO)
- Sewers for Adoption 8th Edition, WRc plc, 2018
- SuDS Manual, C753, CIRIA, 2015
- Guidance on the Construction of SuDS, C768, CIRIA, 2017
- Water. People. Places: a guide for master planning sustainable drainage into developments, South East 7/AECOM, 2013
