Reducing flood risk in planning
Hampshire County Council is a statutory consultee on all major planning applications in Hampshire
Our duty is defined in The Town and Country Planning (Development Management Procedure) (England) Order 2010
- Hampshire’s role
The Council offers advice to the local planning authority (LPA) on surface water management strategies and Sustainable Drainage Systems (SuDS).
We offer pre-application advice in relation to surface water drainage. There is a charge for this service. See the surface water drainage pre-application advice charges.
If you need more information on a site's flood risk complete the pre-application advice request form. There may be a charge for this information.
- Sustainable Drainage Systems (SuDS)
New developments in the UK must consider SuDS as part of their surface water management strategy. The idea of SuDS is to encourage the most natural disposal of surface water back into the ground, as close to the source as possible. To find out more take a look at the surface water and sustainable drainage guidance.
- What you need for your planning application
We ask applicants for specific information, set out below. Hampshire County Council can then provide the local planning authority (LPA) with useful advice on proposed developments.
We need all the following data for every planning application:
- information on the proposed design. This includes plans of surface water drainage and any SuDS featured in the scheme
- evidence that the applicant understands the sensitivity of discharge points relating to the receiving water body. Where this is main river or discharging through contaminated land the LPA may have to consult the Environment Agency (EA)
- the existing flow paths of undeveloped (greenfield) sites
- the existing drainage network for previously developed (brownfield) sites
- areas that may have been affected by existing failures in the existing drainage regime
- evidence that the proposed drainage should follow the same pattern as the existing. This avoids directing more flow to another location
- information on the correct number of treatment stages in the system. For further information see the Ciria SuDS Manual (C753)
- where infiltration is used for drainage, we need evidence that a suitable number of tests have been completed. These need to be across the whole site; within different geologies and to a similar depth to the proposed infiltration devices. Tests must be completed according to the BRE 365 method
- existing and proposed run-off rate calculations completed according to a suitable method such as IH124 or FEH. Information is available from UK Sustainable Drainage: Guidance and Tools. Calculations must show that the proposed run off rates do not exceed the existing run-off rates. This must be shown for a one in one year event plus climate change and a one in one hundred year event plus climate change
- existing and proposed run-off volume calculations completed according to a suitable method such as IH124 or FEH. Calculations must show that, where reasonably practical, runoff volume should not exceed the greenfield runoff volume for the same event. This must be shown for a 1 in 100 year, 6 hour rainfall event
- evidence that enough storage/attenuation has been provided without increasing the runoff rate or volume. This must be shown for a 1 in 100 year plus climate change event (see note below)
- evidence that runoff exceeding design criteria have been considered. Calculations and plans must show where above ground flooding might occur and where this would pool and flow
- information on general maintenance regimes of SuDS features. Evidence that those responsible are in discussion with the developer
Surface water drainage proposals must be assessed in relation to the impacts of climate change. The appropriate allowances are available from the Environment Agency. The proposed system must be tested for the peak rainfall intensity. This is for the central and upper band in relation to the relevant development lifetime.
Urban Creep is the loss of permeable surfaces within urban areas creating increased runoff which contributes to flooding.
A suitable allowance for urban creep must be added. 10% on impermeable areas is suitable.