Environment Agency
This guidance is for planning applicants, developers and consultants on how to use modelling in flood risk assessments (FRAs) when applying for planning permission.
Hydrological and hydraulic modelling help you calculate the flood risk to and from a new development.
Before starting your FRA, you should also read:
- Flood risk assessments if youre applying for planning permission
- Flood risk and coastal change
- Hydraulic modelling: best practice (model approach)
- Flood risk assessment in flood zones 2 and 3
When to consider using modelling
You may need to use hydraulic modelling to produce a detailed FRA, depending on the scale, type, vulnerability classification and location of your development. You are more likely to need detailed modelling for:
- a new housing development or other large development, where the flood risk is high
- developing essential infrastructure
- developments that may increase the risk of flooding to surrounding areas, including buildings and infrastructure
- deciding if mitigation is required
Find out more about flood risk vulnerability classification.
You should find out what existing sources of information are available before starting any modelling. Read Check what information already exists.
Get modelling advice
The Environment Agency can provide a free, preliminary opinion on your development proposal.
You can also pay for modelling advice from the Environment Agency before submitting a flood risk assessment. You may need this advice before applying for:
- planning permission
- permission in principle
- technical details consent
- a marine licence
The benefits of using this pre-application service include:
- finding out what modelling is available, or being developed, and if its suitable for the scale and type of your development
- helping you understand complex hydraulic mechanisms and any historic flood events
- getting agreement with the Environment Agency on modelling approaches and hydrological assessment, to ensure you develop the model in line with current standards
- reducing the risk of time delays and wasted effort during the statutory planning process
The Environment Agency may also provide this advice during the application process and after you get consent.
Find out how to pay for environmental advice on your planning proposals.
Follow the modelling standards
The Environment Agency has published standards for flood risk modelling. These set out best practice techniques for producing evidence of flood risk.
Your model should comply with the appropriate flood risk modelling standards:
- River modelling: technical standards and assessment
- Coastal standards technical report (LIT documents number 56561)
- Flood estimation guidelines
- Coastal boundary conditions
- Climate change allowances
You may also need to refer to guidance by third parties, for example software user manuals.
Check what information already exists
Before starting your modelling work, find out what information is already available.
You should check:
- if your site is in flood zone 1, 2 or 3 on the Environment Agencys flood map for planning
- Environment Agency flood risk information to complete an assessment
- the local planning authoritys strategic flood risk assessment to find out if the site is in flood zone 3b, or at risk of flooding from any source, now or in the future
- if the Environment Agency already has suitable hydraulic modelling which could help you assess flood risk
You may also find historic flood risk information for the site from the:
- Environment Agency
- lead local flood authorities
- local planning authorities
- water and sewerage companies
- internal drainage board if youre in an internal drainage district
- local archived newspaper reports and images
- online resources and news stories
Understand limitations of using existing models
Its your responsibility to check the suitability of an existing model for your development.
If you use an existing model from the Environment Agency or another organisation, it is important to check if it:
- represents current risk
- uses the latest available datasets
- complies with current modelling standards
- is at a scale suitable for the assessment youre undertaking
- captures the detail required for a site-specific assessment
- makes use of current climate change allowances
You should be aware that:
- Environment Agency models are not designed to assess third party developments, so do not assume that they are suitable for your proposed development
- even if you use a recent model, you still need to review and possibly update it
- you should provide evidence of any modelling checks and subsequent updates you carry out and record these in the FRA model reporting
- you should refer to the Environment Agencys standard model and hydrology review templates when carrying out your model review
Write a method statement
You should write a method statement before starting to develop a model.
The method statement should cover:
- an assessment of information which already exists
- the approach youll take
- clear justification for the approach
Find out more about completing a method statement.
You can submit your method statement to the Environment Agency for pre-application advice.
Review and feedback at this early stage can help to:
- ensure that your proposed approach is in line with current standards
- reduce the risk of objections to, or queries about, your FRA later in the process
Develop the model
You should develop a baseline model which represents the current flood risk, before modelling the impact of the development youre planning.
Baseline models
Whether youre using an existing model or building a new one, it must be suitable for your proposed project.
You should:
- check the survey data is still valid for key areas and structures, if using old survey data
- carry out a new survey if changes have occurred since the last survey, and to find out if local refinement to flood flow routing is needed, for example, accurate representation of the crest level of side spills
- build and run models in the latest versions of software, or provide robust justification and testing if this is not possible
- include structures on the floodplain and in the channel which could affect the flood risk at the area of interest, document these and explain the reasons for any excluded structures in your report
- review the upstream and downstream boundaries to ensure they