As a Statutory Consultee, the LLFA have a responsibility to provide the local planning authority with an informed response to planning applications for major development proposals, with respect to flood risk and drainage.
The flood map for planning can be used to find out which flood zone a location is in, as part of land use planning. This information can be used as part of a flood risk assessment for a planning application.
Flood Risk Development Guidance
Developers should be doing the following checks before submitting an application:
- Reviewing the flood map for planning.
- Completing the checklist
- Completing the Drainage Assessment Form
The Drainage Assessment Form is a mandatory requirement for all planning applications in the London Borough of Hounslow
The Lead Local Flood Authority have a duty to ensure that new developments in the borough are complying with the policies outlined below to ensure reduced flood risk in the borough and to manage development in high risk areas.
Developers should ensure that their applications adhere to the following policies.
- National Planning Policy Framework & National Planning Policy Guidance
- National Planning Policy Guidance – Climate Change Considerations
- Flood and Water Management Act
- The Non-Statutory Technical Guidance for SuDS
S1 and S2 For greenfield developments the runoff rate should not exceed the greenfield runoff rate for the same event. For brownfield developments the runoff rate should be as close as practicable to the greenfield runoff rate but should never exceed the rate of discharge from the development prior to redevelopment.
S4 and S5 For greenfield developments the runoff volume should never exceed the greenfield runoff volume for the same event. For brownfield developments the runoff volume musy be constrained to a value as close as reasonably practical to the greenfield runoff volume for the same event but should never exceed the runoff volume from the development site prior to redevelopment for that event.
S7, S8 and S9 The drainage system must be designed so that flooding does not occur on any part of the site for a 1 in 30 year rainfall event and so that flooding does not occur in a 1 in 100 year event in any part of a building or any utility plant susceptible to water. The design of the site must ensure that flows resulting from rainfall in excess of a 1 in 100 year rainfall event are managed in exceedance routes that minimise the risks to people and property.
- Visit the Mayor of London website to view the London Plan Policy SI 12
- Visit the Mayor of London website to view the London Plan Policy SI 13
Development should utilise sustainable urban drainage systems (SUDS) unless there are practical reasons for not doing so, and should aim to achieve greenfield run-off rates and ensure that surface water run-off is managed as close to its source as possible. There should also be a preference for green over grey features, in line with the following drainage hierarchy:
- Rainwater use as a resource (for example rainwater harvesting, blue roofs for irrigation)
- Rainwater infiltration to ground at or close to source
- Rainwater attenuation in green infrastructure features for gradual release (for example green roofs, rain gardens)
- Rainwater discharge direct to a watercourse (unless not appropriate)
- Controlled rainwater discharge to a surface water sewer or drain
- Controlled rainwater discharge to a combined sewer.
Refer to sections:
- EQ2 – Sustainable design and construction
- EQ3 – Flood risk and surface water management