Guidance

Using modelling for flood risk assessments

When to use hydrological and hydraulic modelling as part of a flood risk assessment for a planning application, and the expected standards.

From:
Department for Environment, Food & Rural Affairs and Environment Agency
Published
12 December 2023

Applies to England

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: 

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 fluvial or tidal risk 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 it’s 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.  

If you need advice on surface or ground water modelling you should contact your Lead Local Flood Authority (LLFA).

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: 

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: 

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 you’re in an internal drainage district 
  • local archived newspaper reports and images  
  • online resources and news stories 

Understand limitations of using existing models 

It’s 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 you’re 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 Agency’s 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 you’ll 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 you’re planning.   

Baseline models 

Whether you’re 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 are far enough upstream or downstream that they do not influence the results at the development site and that the model captures any changes to off-site flood risk 
  • check if the grid resolution is appropriate for your model and represents all potential flow routes 
  • consider how best to represent out-of-bank flows and flooding in the floodplain and review the suitability of any models built in 1D-only, 2D-only, or linked 1D–2D 
  • include any existing flood defences and the residual risk if they fail  

This is not a complete list. Find out more about best practice in hydraulic modelling. 

Boundary conditions 

You need to apply suitable boundary conditions to allow water to flow into or out of your model.  

Hydrology 

Your modelling should include a flood hydrology study. 

You should: 

  • review existing hydrology studies to ensure they are up to date and fit for purpose  
  • use the Flood estimation guidelines for new hydrology assessments or updates  
  • record the analysis in the Flood estimation report template (LIT documents number 65087) 

You should provide evidence and justify all your decisions to help the Environment Agency review your flood hydrology study.  

When using existing hydrology studies, you should: 

  • check if the previous study is suitable – for example, is it detailed enough for an FRA, does it consider all required sources of flood risk  
  • use the latest versions of hydrological methods and software to calculate estimates 
  • review and update input data and parameters – for example, include additional gauge data, modify rainfall-runoff parameters, check the latest climate change allowances, use the latest National River Flow Archive data 
  • update catchment descriptors from the Flood Estimation Handbook Web Service

Find out more about flood hydrology studies. 

Coastal and tidal boundaries 

You should define any coastal boundaries using the relevant guidance:  

For coastal models or models which are affected by tides, you should estimate water levels and tide curves using the Coastal flood boundary dataset and guidance.  

If you need to consider wave overtopping, you can use offshore and nearshore datasets produced for the State of the Nation coastal project. To request this data, contact your local Environment Agency team. 

Email: enquiries@environment-agency.gov.uk

You should carry out sensitivity testing to see if more detailed analysis is needed.

Depending on the results of your sensitivity testing, you may also need to carry out a joint probability analysis. This should assess the potential combined impact of 2 or more events occurring at the same time, for example a fluvial and tidal event.

Climate change allowances 

It’s important to include climate change allowances in your modelling to ensure your proposed development is resilient to flooding and coastal change.

Read Flood risk assessments: climate change allowances for more information. 

Post-development models 

Depending on what your baseline model shows, you may also need to produce a post-development model to include the impact of your development proposals. Your comparison of baseline and post-development model runs should cover the expected lifetime of the development.  

It should show: 

  • the flood risk to the proposed development 
  • any loss of floodplain storage by volume and level 
  • how floodplain compensation schemes will work 
  • the impact of potential blockage of significant structures – for example, debris blocking a bridge – on the development proposals 
  • the impact if water control assets fail, for example sluices and pumps 
  • the residual risk to the proposed development if flood defences fail (‘breach’) 

Contact the Environment Agency for advice on when you would need to provide a post-development model. 

Email: enquiries@environment-agency.gov.uk. 

Impact on off-site flood risk  

You may need to model the impact of your development on off-site flood risk if it could: 

  • reduce floodplain storage and you cannot provide ‘level-for-level’ compensatory storage 
  • obstruct flood flow routes 

The Environment Agency may object to planning applications with modelled increases in off-site flood risk.  

To avoid this, you should: 

  • mitigate for any increases to off-site flood risk through the design of your development  
  • look for opportunities to reduce the overall flood risk  
  • create model scenarios that assume flood risk management structures and features will work as designed 
  • model the off-site impacts of your development on residual risks, for example flood defence breaches or ‘exceedance’ (where excess water overflows the drainage system) 

This will minimise the risk of increasing off-site flood risk, including from the cumulative effect of multiple developments over time.  

You should calculate any change in off-site flood risk between a baseline and post-development model across several scenarios throughout the development’s lifetime.  

This could include changes to the flood:  

  • extent 
  • frequency 
  • depth 
  • velocity 
  • hazard 
  • speed-of-onset 
  • duration 

You should carry out this analysis using raw results, without including any allowance for model calculation error (‘modelling tolerance’). If you identify any change in flood risk as part of the model calculation error, you’ll need to provide robust technical analysis and reporting to support this. You must demonstrate that your development will not increase flood risk elsewhere.  

Quality assure the model 

You should build quality control and assurance into all stages of your modelling project. 

Model performance 

Your model needs to perform to the expected standards. You will need to: 

  • check hydraulic stability and results   
  • review stability metrics such as mass balance, oscillations, non-convergence and negative depths to identify improvements and implement them
  • demonstrate reasonable attempts to improve the stability of the model, providing evidence to show no significant impact on model results in the area of interest 
  • if the global parameters of an existing model have been modified to overcome stability issues, return these to the default values and try to improve the model to remove instabilities 
  • document all changes made to the model in the report 

Reduce and assess uncertainty 

All modelling contains uncertainty, from the input data to the outputs and decisions made in developing the model. 

It is important that you demonstrate and record: 

  • how you’ve minimised the uncertainty in your model build 
  • sensitivity tests of key model parameters 
  • which elements of your modelling produce the greatest uncertainty 
  • what levels of uncertainty are in the model build and how this varies throughout the model 
  • how uncertainty affects the conclusions of your assessment and the proposals 
  • what mitigation you will use to deal with uncertainty in the modelling 

Read more about sensitivity tests and model outputs in Hydraulic modelling: best practice (model approach).  

Represent reality 

To make sure the modelled results represent the real flood risk, you’ll need to calibrate and verify the model using relevant gauge data. If no gauge data is available, you should validate the modelled results instead. 

To validate your results, you should compare them to existing: 

  • Environment Agency flood mapping data 

  • historic flood information  
  • previous model results 
  • catchment information provided by the local Environment Agency team 

Read more about calibrating, verifying and validating your model. 

What information you need to provide 

Before submitting your model and reports, check that you have all the relevant information. This will reduce the risk of delays in the review process.  

Hydraulic model files 

You should provide: 

  • all baseline and post-development hydraulic model files, in a logical file structure which is easy to navigate and includes files only relevant to the latest model runs 
  • any calibration, validation or sensitivity testing data and model runs  
  • a hydraulic model development log with enough detail for the scale of the modelling   
  • all raw model results   
  • all post-processed model results  

To help interpret on and off-site flood risk, the raw model results must include: 

  • diagnostic files 
  • checks and logs 
  • gridded outputs, for example depth, height and velocity grids for each of your model run scenarios  

To show the impact of the development on flood risk, the post-processed model results should include: 

  • GIS outlines 
  • pre and post-development comparison plots, including composite depth, height and velocity grids 

Project reporting 

You’ll need to provide: 

  • a model report detailing all decisions made in the development of the modelling 
  • a flood estimation calculation record (LIT 66039) if you’ve carried out hydrological calculations 
  • flood estimation handbook catchment descriptors and calculations. 
  • survey data, including historical data from previous studies, if this is available 

Follow the advice on project reporting in the Hydraulic modelling: best practice guidance.  

Reusing existing models 

If you’ve used an existing model, you’ll also need to provide: 

  • a review which assesses its suitability, explains all changes made to the original model and justifies them 
  • reports for both hydraulic and hydrological approaches and survey data from the original study 

Submit your model for approval  

If your model is being reviewed as part of the pre-application service, you’ll need to send it to the Environment Agency.  

Email: enquiries@environment-agency.gov.uk

If you’re submitting a model to support a flood risk assessment for a planning application, you’ll need to submit it to the local planning authority with your application. 

Read Flood risk assessments if you’re applying for planning permission. 

How the review process works 

After you submit your model to the Environment Agency, they will do an initial screening to assess the application and its suitability.  

They will review your model along with all the evidence in your flood risk assessment, to check it adequately represents the fluvial and tidal flood risk. If they consider that the application is high risk or complex, they will carry out a more detailed review. The reviewer will record their comments on your model review templates. 

If you need any surface or ground water modelling reviewed, contact your Lead Local Flood Authority.

If the Environment Agency decides that the model is unsuitable, this may be due to: 

  • the quality of the model 
  • the quality of the reporting 
  • missing information  

It’s important to provide all the required information or provide a clear explanation for any omissions. 

Any objections to your flood risk assessment by the Environment Agency may affect your planning application. 

Responding to feedback 

If you need to respond to feedback on your modelling from the Environment Agency, you should add your comments to, and provide updated copies of, your review documents:  

Get help 

Contact the Environment Agency for copies of any of the documents referred to in this guidance: 

  • Flood estimation report template (LIT 65087) 
  • Flood estimation calculation review template (LIT 66039) 
  • Fluvial modelling standards (LIT 56326) 
  • Non-real time hydraulic model review template (LIT17617) 
  • Coastal standards technical report (LIT 56561) 
  • Estuary standards technical report (LIT 56527) 
  • HR Wallingford state of the nation phase 2 coastal datasets (LIT 16316) 

Remember to include the LIT number of the documents you need.  

Email: enquiries@environment-agency.gov.uk

Published 12 December 2023
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