Surface water management and Sustainable drainage systems (SuDS)
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What are Sustainable Drainage Systems (SuDS)? - "Slow the flow"
- SuDS collect surface water runoff and release it slowly, rather than discharging it all straight into the public sewer system or watercourse. This slower release reduces the risk of flooding.
- The main objective of SuDS is to minimise the impacts from any development on the quantity and quality of runoff and maximise amenity and biodiversity opportunities.
- Most SuDS dispose of the collected surface water by a combination of infiltration (seepage into the soil), evaporation (from a body of water), collection and gradual release of the water into the sewer system. Hence, they mimic the way that surface water run-off would have occurred if the land had not been developed.
- There are various examples of SuDS including permeable paving, soakaways, green roofs, swales, ditches, ponds, filter drains, filter strips, green roofs, rain water harvesting.
- The impacts of climate change are predicted to make extreme rainfall events more frequent. SuDS are believed as a proactive approach to adapt and mitigate the impacts of climate change. Also, there is potential to provide significant amenity benefits as part of the urban landscape. Creating a sustainable management of water resources. Complementing other strategies such as river enhancement and environment rehabilitation.
- The Mayor of London has developed sector-specific sustainable drainage (SuDS) guidance for schools and social housing, parks and green spaces, hospitals, commercial and Retail as part of the London Sustainable Drainage Action Plan. These documents can help you to make good decisions about inclusion of SuDS in the sector you are interested/involved in.
More information
Susdrain: the community for those involved in delivering sustainable drainage systems
Surface Water Management-Advice to Planning Applicants
London Borough of Barnet as a Lead Local Flood Authority (LLFA) is a statutory consultee on planning application for major development proposals. This is developments of 10 dwellings or more; or equal non-residential or mixed development. Which must also have surface water drainage implications in any flood zone.
London Borough of Barnet highly encourage the use of Sustainable Drainage Systems (SuDS) to achieve a sustainable approach of surface water management and contribute to the creation of infrastructure and communities that are safe from flooding for their intended lifetime.
SuDS and new developments
London Borough of Barnet (as the Lead Local Flood Authority, or LLFA) are the statutory consultees on planning applications relating to major development. Which are developments of 10 dwellings or more; or equal non-residential or mixed development. Which also have surface water drainage implications in any flood zone.
As part of this responsibility, LBB is required to ensure that Sustainable Drainage Systems (SuDS) for the management of surface water runoff are put in place, unless demonstrated to be inappropriate. Please click here for details of major and minor application definitions
When considering SuDS as part of a planning application, we need to satisfy ourselves that:
Surface water is managed in accordance with the surface water discharge hierarchy for discharge destinations. Generally, the aim should be to discharge surface run off as high up the following hierarchy of drainage options as reasonably practicable:
- into the ground (infiltration);
- to a surface water body;
- to a surface water sewer, highway drain, or another drainage system;
- to a combined sewer.
The aim of the surface water discharge hierarchy is to ensure that surface water runoff is treated at source and managed in a way which minimises the negative impact of the proposed development on flood risk and the water quality of receiving waters. Early consideration must be given to the use of rainwater harvesting systems and / or the use of green roofs, to both manage surface water runoff and deliver a source of non-potable water for the proposed development where practical.
A detailed surface water drainage strategy should be submitted which will set out the appropriateness of SuDS to manage surface water run off and provision of maintenance for the lifetime of the development which they serve. Major applications which do not meet this requirement will not be made valid.
The surface water drainage strategy for the proposed development must have evidence of Adopting Body(ies) accepting responsibility for the safe operation and maintenance of the proposed SuDS. The Adopting Body must demonstrate that sufficient funds have been held in reserve or will be raised to meet the operation and maintenance expense throughout the lifespan of the development. The Adopting Body shall be responsible for satisfying themselves of the suitability of the adopted SuDS prior to adoption. Keeping records of operation and maintenance activities, for possible inspection by the Council.
The New Design and Construction guidance (March 2020) which replaced Sewer for Adoption 7th Edition provides guidance on the types of SuDS components (not all) that can be adopted by Water and Sewerage Companies, hence paving the way for further use of SuDS. For more
The surface water drainage strategy should include detailed calculations to demonstrate that post-development surface water discharge rate are limited to greenfield runoff rates, for all return periods up to 1 in 100 year plus climate change allowance.
This is applicable for both Greenfield and previously developed sites. Greenfield runoff rate is the runoff rate from a site in its natural state, prior to any development.
Proposals for higher discharge rates should be agreed with the LBB ahead of submission of the Planning Application. Clear evidence should be provided with the Planning Application to show why greenfield rates cannot be achieved.
Further details are provided on the Barnet SuDS proforma which can be downloaded from the Mayor for London website. This proforma is required to be submitted along with your planning application.
We can offer free general advice up to 15 minutes of the officers time. We don't review Flood Risk assessments or detailed surface water strategies as part of this service; these should be requested via our formal pre-application advice service.
We also offer pre-application advice on the planning applications in relation to surface water drainage on a chargeable basis, you can contact us on swm@barnet.gov.uk.