Sustainable Drainage Systems

Sustainable Drainage Systems (SuDS)

On 18 December 2014, DCLG published a written statement to parliament explaining changes to the planning system in relation to major development: developments of 10 homes or more and 1,000m2 or more commercial development should ensure that sustainable drainage systems for the management of surface water run-off are put in place, unless demonstrated to be inappropriate.

The National Planning Policy Framework (NPPF) explains that sustainable development should not increase flood risk elsewhere, and it gives priority to the use of sustainable drainage systems. This is reflected in the Government's guidance on Flood Risk and Coastal Change updated in March 2015.

The Local Plan was adopted on the 11 September 2019 and include a new Policy CE2. The relevant section regarding surface water run-off and Sustainable Drainage Systems is:

 g: require major development to achieve greenfield run-off rates and minor development to achieve a reduction of 50% of existing rates, ensuring that surface water run-off is managed as close to its source as possible, through:

   i.       the increase of permeable surfaces;

   ii.       recognising opportunities for SuDS to provide other environmental benefits;

   iii.      factoring all flows into the sewer system (including swimming pools discharges, groundwater or other flows) in the calculations of greenfield run-off rates.

h.         require SuDS to have regard to DEFRA non-statutory SuDS standards and local guidance to ensure SuDS are adequately designed, built and maintained for the lifetime of development;

i.          resist impermeable surfaces in gardens and landscaped areas;

j.          encourage the retrofitting of SuDS in buildings even if the development will not have drainage implications;

[1] For the purposes of flooding and drainage, minor development includes up to 9 dwellings (under half a hectare); up to 999m2 of commercial floorspace (under 1 hectare); up to 9 Gypsy/Travellers pitches; household applications; change of use applications (no operational development) and listed building extensions and alterations.

How to meet our policy: minor developments

Our policy could be met through an increase in permeable surfaces, an attenuation of surface water run-off or a combination of both.

a) An increase in permeable surfaces: plans should show the permeable and impermeable surfaces for the whole site (rather than just the area where the development will take place), before and after development. If the site was totally impermeable pre-development and the proposal decreases the impermeability of the site by 50% our policy will be met. This could be challenging for some sites so further attenuation will be required to achieve a net reduction of 50% in surface water run-off rates. It should be noted that if other surfaces, instead of grass and vegetation are to be permeable (such as permeable paving, green roofs, etc.) further information will be required (points 3 and 4 below under Information required).

b) If the policy cannot be met through the introduction of permeable surfaces alone, then attenuation of surface water run-off will be required. The easiest way of attenuating surface water run-off will be to direct it to landscaped areas. This may be done by disconnectng rainwater pipes from the sewer system and conveying rainwater into landscaped areas. If a disconnecction of 50% or more of the site is achieved then the policy will be met. Plans showing the site drainage and how surface water will be conveyed into landscaped areas should be submitted .If surface water is directed to permeable surfaces other than landscaped areas further information about the permeable surfaces will be required (points 3 and 4 below under Information required).

If this can't be done, then an analysis of surface water run-off rates pre- and post-development should be submitted alongside details of the proposed SuDS. These calculations could be done following Chapter 24 of the CIRIA (Construction Industry Research and Information Association) manual:

  • Option 1 (Ciria Manual): the Wallingford procedure could be used to calculate the 1 in 1-year peak run-off rainfall intensity (1 hour duration) and to understand the attenuation volume required. SuDS should then be used to attenuate the excess of surface water to achieve a net reduction rate of 50%. If greenfield run-off can be met it will provide a considerable improvement.
  • Option 2: the Wallingford Greenfield run-off (IH124 method) could be used to calculate the rates
  • Option 3: round up figure. If calculations cannot be provided, a round up figure of 90 litres per square metre could be used to calculate the attenuation volume required to achieve a 50% net reduction for the whole site.

Information required

To ensure policy compliance the following information should be submitted:

  1. A detailed analysis of surface water run-off and attenuation volume required by policy CE2g (achieve a reduction of 50% of existing rates including climate change in the calculations and factoring in all flows into the sewer system including swimming pool discharges, groundwater or other flows). The storm events used in the calculations should include the 1 in 1-yr event.
  2. Information about the proposed SuDS types, their location, attenuation capacity, specification, structural integrity, construction, operation, access, and maintenance. More sustainable green SuDS should be favoured over attenuation tanks.
  3. Section/profile drawings of the SuDS if relevant (green roofs, blue roofs, sub-base attenuation, permeable paving, planters, species, etc.) and maintenance information.
  4. Drainage plans to show clearly how surface water run-off will be conveyed to the SuDS and any connections to the sewer system if necessary.

For planning applications which include basements, SuDS shall be provided in the same way as above. It should be noted that if 1 metre of top soil is provided on top of the basement, this will only be considered permeable if it is covered by grass, vegetation or permeable surfaces. Information about those surfaces should be submitted as explained above.

If this information is submitted at planning application stage this will reduce the need for pre-commencement conditions attached to the planning permission.

How to meet our policy: major development

For major developments, SuDS should be considered from the outset at a very early stage of the process (masterplanning) to ensure they are fully integrated with the topography and conditions of the site. We have produced the following SuDS guide based on the design process and aimed at promoting best-practice. This guide should be followed when designing SuDS in the Royal Borough.

A SuDS strategy should be submitted including the information below. Developers should also fill in the SuDS proforma to ensure that all the relevant points have been addressed. The SuDS strategy should show how the development will comply with policy CE2g and achieve greenfield run-off for all events (factoring in other flows into the sewer such as swimming pool discharges, groundwater and other flows). The storm events used in the calculations should include the 1 in 1-yr event. The strategy should take into consideration the drainage hierarchy referred to in the London Plan and include the following information:

  • the proposed SuDS types, their location, landscape integration strategy, attenuation capacity, specification, structural integrity, construction, operation, and access. More sustainable green SuDS should be favoured over attenuation tanks.
  • Section/profile drawings of the SuDS if relevant (green roofs, blue roofs, sub-base attenuation, permeable paving, planters, species, etc.).
  • Drainage plans to show clearly how surface water run-off will be conveyed to the SuDS and any connections to the sewer system if necessary.
  • Details of surface water management during construction.
  • Details of exceedance flow routes.
  • Foul drainage
  • Adoption of SuDS (ownership), long-term maintenance regime (including maintenance schedule, costs) and communication to residents
  • Compliance with the non-statutory SuDS Standards

Non-statutory SuDS Standards
 

  • Records of relevant consultations with other interested parties (Thames Water, Canal and River Trust, the Environment Agency when relevant)
  • A phasing plan if the scheme is delivered in different phases

SuDS pro-forma

RBKC SuDS Proforma.pdf

To fill in the Royal Borough of Kensington and Chelsea's proforma please see the London Sustainable Drainage Proforma website.

 

Useful SuDS websites

CIRIA (Construction Industry Research and Information Association) SuDS Manual which gives detailed advice regarding SuDS. Chapter 10, Designing for urban areas is particularly relevant. WSUD relates to water sensitive urban design.

Susdrain which has information about case studies

Livingroofs.org which has information on green and brown roofs

Designing Rain Gardens: A Practical Guide by Urban Desing London illustrates how to implement rain gardens on the highway.

Paving front gardens

When considering paving your front garden the Council encourages making use of permeable paving and a wide variety of materials are available in the market. You will need planning permission to hard pave your front garden if it is more than 5m2 in area. However, you can pave without the need of planning permission:

(1) if you use porous materials for paving;

or (2) make provision to direct water run-off from the hard surface to a permeable or porous area or surface within your property boundary.

If you would still like to hard pave your front garden in non-permeable paving, please apply for planning permission. In such cases Local Plan Policy CE2 (i) will apply. The policy resists impermeable surfaces in gardens and landscaped areas.

The Planning Portal has further information on paving front gardens.

The Government also developed guidance on the permeable surfacing of front gardens.

If you have any queries about this please contact the Policy Team at planningpolicy@rbkc.gov.uk.