Flood risk assessment in flood zone 1 and critical drainage areas
How to carry out a flood risk assessment so that you can complete your planning application.
You need to do a flood risk assessment if your development is in flood zone 1 and:
more than 1 hectare
in an area with critical drainage problems as notified by the Environment Agency
You also need to do a flood risk assessment if your development could be subject to other sources of flooding (eg surface water drains). This includes a change of use to an existing development that makes it more vulnerable to flooding.
Check with your lead local flood authority to see if your development would be affected by other sources of flooding.
Contact your local planning authority to check if your development is in an area with critical drainage problems as notified by the Environment Agency.
You also need to do a flood risk assessment if your development could be affected by other sources of flooding (eg surface water drains) or if the development is now classed as ‘more vulnerable’ following a change of use. Check this with your lead local flood authority - contact your local council to find out who this is.
Read the flood risk assessment for planning applications if you’re not sure if this guidance applies to you.
Your written flood risk assessment can be in any format but must include the relevant plans, surveys and assessments. Check with your local planning authority if they have any specific software requirements, eg for producing detailed hydraulic models.
Research your development site
Contact the following organisations for information about flood risk in your area:
Contact your local planning authority or check the planning section of their website for their strategic flood risk assessment if one has been adopted as part of the local plan. Refer to the strategic flood risk assessment in your own flood risk assessment.
Check if your development is within 20 metres (m) of a main river. Ask the Environment Agency for advice if it is.
Check with the Environment Agency if you need to get Environment Agency flood defence consent (permission to do work on or near a main river in England).
You need to provide a location plan showing:
any rivers, streams, ponds, wetlands or other bodies of water
other geographical features, eg railway lines or local landmarks such as schools or churches
You can buy a location plan from the Ordnance Survey.
You also need to provide a site plan showing:
the existing site
your development proposal
any structures that could affect water flow, eg bridges, embankments
You need to provide a survey showing:
- existing site levels
- the levels of your proposed development
Check with your local planning authority if you also need to show your site in relation to its surroundings.
If you do, you’ll have to put site levels in relation to the Ordnance Datum (the height above average sea level). You may be able to find Ordnance Datum information from the Ordnance Survey. If not, you’ll need to pay for a land survey carried out by a qualified surveyor.
Assess what the risk would be to your development if there was a flood. Consider flooding from other sources (eg surface water drains, a canal) as well as from rivers and the sea.
You should also consider climate change in your assessment.
Surface water drainage
You also need to assess surface water runoff on the site and provide:
an estimate of how much surface water runoff your development will generate
details of existing methods for managing surface water runoff, eg drainage to a sewer
your plans for managing surface water and for making sure there’s no increase in the volume of surface water and rate of surface water runoff
Surface water runoff describes flooding from sewers, drains, groundwater, and runoff from land, small water courses and ditches that occurs as a result of heavy rainfall.
Make sure your plans for managing surface water are in line with:
guidance on managing surface water runoff in your local planning authority’s strategic flood risk assessment
guidance from your lead local flood authority
Developments on or near main rivers
State in your assessment if you need Environment Agency flood defence consent and if you’ve applied for it if so.
Submit your flood risk assessment
They’ll review your flood risk assessment and tell you if it’s satisfactory.
Contact the Environment Agency
PO Box 544
Monday to Friday, 8am to 6pm 0370 8506 506
From outside the UK +44 1709 389 201