Guidance

Flood risk assessment in flood zones 2 and 3

How to carry out a flood risk assessment so that you can complete your planning application.

You need to complete a flood risk assessment as part of your planning application if your development is in flood zone 2 or 3.

Find out what flood zone you’re in.

If your development is in flood zone 2 or 3 and is either a minor extension or is classed as vulnerable, you may need to read the standing advice instead.

Read 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, for example 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 if you need to do a sequential test

Before you start a flood risk assessment, check if you need to carry out a sequential test. A sequential test compares your proposed site with other available sites to show which one has the lowest flood risk.

You need to carry out a sequential test if one has not already been done for the type of development you plan for your proposed site - check with your local planning authority.

If the sequential test shows there are not suitable alternative sites, you may need to carry out an exception test. The exception test shows how you’ll manage flood risk on and off the site.

You’ll need to carry out an exception test if your development is:

  • highly vulnerable and in flood zone 2
  • essential infrastructure in flood zone 3a or 3b
  • more vulnerable in flood zone 3a

Plans

You need to provide a location plan showing:

  • street names
  • any rivers, streams, ponds, wetlands or other bodies of water
  • other geographical features, for example 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, for example bridges, embankments

Surveys

You need to provide surveys showing

  • the existing site levels and the levels of your proposed development
  • a cross-section of the site showing finished floor or road levels and any other levels that inform the flood risk, for example existing raised banks and flood defence walls

You may also need to show your site in relation to its surroundings - contact your local planning authority to check if you need to do this.

Site levels need to be stated 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 get a land survey carried out by a qualified surveyor.

Assessments

You should consider the following aspects of flood risk in your assessment.

Assess flood risk

Assess what the risk would be to your development if there was a flood. Consider flooding from other sources (for example surface water drains, a canal) as well as from rivers and the sea and include an allowance for climate change.

State in your assessment the estimated level for your site, ie the 1 in 100 year river flood level or the 1 in 200 year tidal flood level.

You may be able to get this from the Environment Agency or your local planning authority. If not, you’ll need a flood risk specialist to calculate this for you.

You need to include an estimate of the:

  • duration of a flood
  • rate of surface water runoff
  • order in which areas of the site would be flooded
  • consequences for people living on or using the site

You should also provide details of past floods where this information is available. Contact:

Assess surface water runoff

You need to assess surface water runoff from the site and provide:

  • an estimate of how much surface water runoff (excess water that flows over surfaces) your development will generate - both the volume and the rate of the runoff
  • details of the existing methods for managing surface water runoff, for example drainage to a sewer
  • your plans for managing surface water and for making sure there’s no increase in the level 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:

Managing the flood risk

You need to state in your assessment:

  • details of existing flood resistance and resilience measures on your site - ask the Environment Agency or your lead local flood authority about these
  • the capacity of drains or sewers (existing and proposed) on your site - ask your local water company about this

State how your proposed design will reduce flood risk. Include details of how people will leave buildings during a flood and an explanation of how:

  • raised flood embankments or changes to ground levels could affect water flow
  • your development could affect rivers and their floodplain or coastal areas

Also explain what the residual risks will be to your site after any necessary flood defences have been built and how you plan to manage these risks.

Check if you need to carry out extra flood resistance and resilience measures to reduce flood risk and state this in your assessment if so.

Developments on or near main rivers

State in your assessment if your need an environmental permit and if you’ve applied for it if so.

Sites within the functional flood plain

If your site falls within the functional flood plain (land where water has to flow or be stored in times of flood), you need to state this.

Only water compatible developments or essential infrastructure developments that have met the requirements of the exception test are allowed in the functional floodplain.

Show that any water compatible or essential infrastructure developments have been designed to:

  • stay safe and operational during a flood
  • avoid blocking water flows or increasing flood risk elsewhere
  • avoid loss of floodplain storage (ie loss of land where flood waters used to collect)

Extra flood resistance and resilience measures

Areas at little or no risk of flooding from any source should always be developed in preference to areas at higher risk. You must make every effort to locate your development in an area that has little or no risk of flooding.

When developments cannot be located in a lower flood risk area, you need to consider flood resistance and resilience measures if you cannot raise your development’s ground floor levels above the estimated flood level for the site.

Which flood resistance and resilience measures you need to take depends on the estimated depth in metres (m) that flood water will reach in your building.

State that you have met the necessary requirements in your flood risk assessment.

Water depth up to 0.3m

Design your building or development to keep water out as much as possible. Do this by using materials that have low permeability (ie materials that water cannot pass through such as impermeable concrete).

Water depth from 0.3m to 0.6m

Design your building or development to keep water out (unless there are structural concerns) by:

  • using materials with low permeability to at least 0.3m
  • using flood resilient materials (for example lime plaster) and design (raised electrical sockets)
  • making sure there’s access to all spaces to enable drying and cleaning

Water depth above 0.6m

Design your building or development to allow water to pass through the property to avoid structural damage by:

  • using materials with low permeability to at least 0.3m
  • making it easy for water to drain away after flooding
  • making sure there’s access to all spaces to enable drying and cleaning

Submit your flood risk assessment

Submit your completed flood risk assessment with your planning application to your local planning authority.

They’ll review your flood risk assessment and tell you if it’s satisfactory.

Planning applications that do not have a satisfactory flood risk assessment may be refused.

Contact the Environment Agency

Environment Agency

PO Box 544


Rotherham
Yorkshire
S60 1BY

Published 1 April 2012
Last updated 27 February 2017 + show all updates
  1. Changed URL for Find out what flood zone you're in, to new Flood Map for Planning service. Changed URL for Check if your development is within 20 metres (m) of a main river, to point to Main River map.
  2. First published.