Developing hydraulic models for flood risk

Before you develop a hydraulic model

Contact your local flood risk analysis team to discuss the scope of your hydraulic model and what data you will need before you begin work on it.

This will help make sure we can use your results to update our flood maps, plans and data.

Follow the guidance on this page to submit a model that is of sufficient quality that we can assess it. We will reject any models that aren't of sufficient quality. 

What you need to include in your submission

Hydraulic modelling work should be carried out by competent individuals and organisations.

Use our model checklist to make sure you’ve included key activities.

You should complete a:

  • scoping study to identify the data you need to build your hydraulic model
  • scoping report to summarise this data
  • hydrodynamically linked 1D/2D model (in most cases – in some cases ‘individual’ 1D or 2D model may be a better option). You might consider 3D models for complex cases, for example at hydraulic structures or where it is important to understand the vertical change in a body of water
  • model report and model log

Modelling packages

Use the Environment Agency Benchmarking to assess the suitability of your hydraulic flood modelling package. You can also use more recent bespoke testing by software, where relevant.

For the modelling package, you should consider:

  • the general availability of the software
  • whether the people who will review, interrogate and manage the model data have the skills to use the software

For the modelling approach, you should consider:

  • the scale of flood risk (to ensure the level of assessment is proportional to risk)
  • the source, mechanism and extent of flooding
  • the receptors vulnerable to flooding
  • the availability and suitability of existing flood risk information
  • maximising opportunities and achieving ‘wider’ benefits

Model boundaries

Hydrology (river flows and rainfall)

You must follow industry standards and best practice for estimating river flow and rainfall:

  • follow GN008 Flood estimation technical guidance for your hydrological calculations (contact us to request this)
  • send us a flood estimate calculation record for us to agree before you begin any detailed hydraulic modelling. This will help you avoid delays and extra work later

The hydrological assessment should be an iterative process, alongside the hydraulic modelling. For instance, you should review and update your estimates (where suitable data is available) as part of a calibration and verification exercise.

Coastal conditions

For extreme sea levels, use the updated Coastal Flood Boundary 2018 Dataset on the DeFRA Spatial Data Download portal.

The local flood risk analysis teams can give you additional guidance and advice about wave overtopping requirements.

Climate change

You must apply an allowance for climate change to estimates for :  

  • river flows
  • rainfall
  • extreme sea level

The Welsh Government’s Flood and coastal erosion risk management: adapting to climate change gives current allowances for climate change. There is a detailed breakdown of requirements in Model output requirements

Topographic and LiDAR surveys

You will need topographical surveys for:

  • the river channel
  • all in-channel structures that influence flood levels or flows
  • top of bank (at regular intervals and where there is notable change in level, gradient and direction/alignment)
  • formal flood defences and other ‘informal’ structures that influence flooding
  • any significant floodplain structures or features

You should follow the Environment Agency’s Standard Technical Specifications (Version5.0) for your survey.

If you model data based on existing surveys, you must:

  • ensure the survey data is up-to-date and suitable to re-use
  • validate existing data against new data
  • confirm there have been no changes since the existing survey data was collected, for instance man-made changes or natural processes
  • consider whether you need to update your data based on changes in OS datum levels (OSTN15) in 2016

If you are not sure about any of the above, you should complete a new survey.

LiDAR terrain data is available from DataMapWales. You can use this to represent the floodplain in 2D.

Model elements

You should include the following elements in your model:

  • model extent - is the model boundary (extent) appropriate and are the 1D and 2D domains in the correct areas? Does the model extent fully capture flood impacts to all affected areas (both on site and to adjacent areas)?
  • upstream and downstream boundaries - do they extend far enough to remove any uncertainty in the areas of interest?
  • river channel – have you spaced cross sections appropriately to reflect changes in channel geometry, gradient and alignment?
  • hydraulic structures - have you considered different modelling approaches and made a conscious decision about the most appropriate approach?
  • 1D and 2D linkages – have you set the boundaries between model domains appropriately? Have you fixed these using survey data?
  • ground model - is the model grid size and orientation appropriate? Does this realistically represent flow paths and restrictions? Have you added additional survey data to re-enforce key features?
  • building representation – have you considered the most appropriate way to represent buildings in the hydraulic model? Where you need depth data for damage calculations, have you given building threshold levels?
  • channel and flood plain roughness - does the model use consistent and realistic roughness values that represents the current, built environment? For the 1D domain, you should reference Manning’s ‘n’ roughness values (Chow, 1959). Please see indicative 2D roughness values
  • model calibration and verification – have you calibrated the model against known local flood events (using recorded or observed flood levels and extents) and for multiple events, where available? Have you reconciled differences where data is not available? Have you done a ‘sensibility check’ on model results?

Model scenarios

Baseline and proposed model

The baseline model represents the current, built environment.

You must create hydraulic models for the current ‘defended/baseline’ and ‘undefended’ scenarios (as applicable).

These will be the baseline for further assessment and we can use them to update the Flood Risk Assessment Wales map and the Flood Map for Planning.

You must treat the proposed or ‘planned’ model separately to the baseline model. For planned developments, you must consider third party impacts within the model.

Modelling for Flood consequences Assessments has more details about assessing these differences.

Blockage and breach

You do not need to assess blockage and breach as a direct requirement for updating national mapping data.

However, it is an important consideration for getting a full understanding of flood risk in an area, especially for development planning.

Your Scoping Study should identify the most appropriate scenarios to apply blockage and breach (for example, the 1 in 100, or 1%, Annual Exceedance Probability).

Contact the local flood risk analysis team to agree parameters and locations for blockage and breach assessment. You can also request our guidance on 'Modelling blockage and breach scenarios'. 

Sensitivity analysis

You should perform sensitivity analysis against key modelling parameters to understand how these affect the model and results, and in what way.

This must not be considered a ‘tick-box’ exercise but used to fully inform decision making at a particular site.

Your Scoping Study should identify the most appropriate scenarios to apply the sensitivity analysis (normally 1 in 100, or 1%, Annual Exceedance Probability).

Sensitivity analysis must include (but not be restricted to):

  • flow (minimum +/- 30%)
  • Manning’s n (+/- 20% in both 1D and 2D domains)
  • downstream boundary (for example +/- 20% change in downstream water level or gradient, coincidence of fluvial and tidal peaks)
  • building representation (for example, with or without raised building upstands and/or flow constriction)
  • key structures (modelling approach and/or structure coefficients)

Other parameters you should consider are:

  • model grid size and orientation
  • the influence of informal, or ‘de-facto’, flood defences
  • storm duration
  • known uncertainties or limitations in model input data and parameters.

You should consider the sensitivity analysis alongside Accounting for residual uncertainty: updating the freeboard guide.

Model results

There is detailed information on the required model outputs in Model output requirements.

You should agree with us how you’ll deliver against these requirements.

As a minimum requirement, your model must simulate floods with an Annual Exceedance Probability (AEP) of

  • 1 in 2 (50%)/ Qmed , 1 in 10 (10%), 1 in 30 (3.33%), 1 in 75 (1.33%), 1 in 100 (1%), 1 in 200 (0.5%) and 1 in 1,000 (0.1%) for Present Day conditions
  • 1 in 30 (3.33%), 1 in 100 (1%)/ 1 in 200 (0.5%) and 1 in 1,000 (0.1%) for Climate Change conditions.

This will ensure all your model results can feed into key flood risk datasets and flood map.

Your Scoping Study should set out the results you’ll need from the modelling. In most cases direct output from the hydraulic model should be:

  • depth
  • velocity (magnitude and flow direction)
  • hazard (based on the precautionary approach set out in Explanatory note for FD2320 and FD2321 and using the ‘conservative’ debris factor)
  • water surface elevation i.e. flood level
  • flood extent

You should consider creating animations in key locations for key scenarios.

Defended areas

You must create defended areas for new or existing defences, based on your model results and including an assessment of their Standard of Protection.

Contact our local flood risk analysis teams for more advice about this.

Updating our flood risk maps

You must create flood extents to update flood ‘likelihood’ information in the FRAW map and flood zones in the Flood Map for Planning.

Within six months of construction you need to send new model, mapping data and an 'as-built' survey to us to show the benefits provided by new flood risk management works. We will then update our flood maps.  

We will review all design data in your model against ‘as-built’ information to ensure the mapping data accurately represents construction.

More information about challenging our flood maps.

Reviewing your model

A senior modeller must review the hydraulic model separately before submitting it to us to ensure it:

  • follows best practise
  • uses data correctly
  • incorporates sensible model parameters (within defined limits)
  • is stable (within acceptable tolerance limits)
  • generates sensible results.

You must give evidence of this quality assurance in your modelling report and include any quality concern it identified.

If your model is likely to come under a high level of scrutiny, you may consider an independent quality assurance of the hydraulic model.

Submitting your model report

You must provide a comprehensive model report and a separate model log. The model report should clearly set out your:

  • project objectives
  • methodology
  • assumptions
  • model construction and stability
  • results (including description and interpretation of the model output)
  • recommendations and conclusions

You should highlight any limitations and areas of uncertainty in the report.

You should use a model log to track the changes between each model build and use this to clearly label the different model files.

Complete and include with your model the 'Flood map challenge change note'. This will give us a royalty-free, non-exclusive license to update national mapping and data. Contact the local flood risk analysis team for more information:

File format of documents

You should submit all your model data in a digital format:

You must submit:

  • all model input and output data with relevant run and check files. This must be the ‘final’ model with superseded or redundant files removed.
  • all modelling results in raw and post-processed format (.asc or .flt).
  • all supporting data you used to construct the hydraulic model in an agreed format, e.g. DWG for survey.
  • modelling report (PDF/Word), model log and supporting calculation records
  • GIS outlines (shapefile format).

Contact the local flood risk analysis team to agree delivery of your data via NRW Citrix Sharefile or equivalent.

You cannot send your model data through Dropbox or other services.

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