Guidance

River modelling standards: who they're for and how to use them

Updated 21 December 2023

Applies to England

This document is part of the flood modelling standards for river systems. There are 4 more documents that cover this topic. Read all the documents to make sure you have the information you need to start your modelling project.

Who these standards are for

The Environment Agency has created these standards for:

• hydrologists
• hydraulic modellers
• model reviewers
• project managers

This document assumes some existing knowledge of hydrological and hydraulic modelling.

What these standards are

The Environment Agency must produce evidence of flood risk. It must also advise others how to produce evidence of flood risk.

This evidence supports decision-making for:

• development and planning
• flood alleviation scheme proposals
• flood incident management
• emergency planning

Flood risk management decisions are based on various sources of information, but hydraulic models are particularly important. These are computer-based mathematical models of flood risk.

Construction of models requires skill and careful consideration. Models must put thorough quality assurance (QA) procedures to make sure they produce high quality and reliable outputs.

The Environment Agency must establish “how good is good enough?” for fluvial and integrated, non-real-time, hydraulic modelling. This also helps the Environment Agency to advise on future model developments.

This guidance provides a record of best practice techniques in the fields of hydrological analysis and hydraulic modelling. It uses existing literature and industry knowledge.

How these standards can help you

These standards are techniques that you can use in the fields of hydrological analysis and hydraulic modelling. Use this best practice guidance to help you:

• build hydraulic models
• review hydraulic models
• maintain consistency across the flood risk management industry
• improve the quality of your applications
• plan and prioritise targeted model updates

A model built following best practice relies on good quality data to give accurate results.

There will be times when following these standards is not possible because you do not have the right data. If you choose to use an alternative approach, you’ll need to provide evidence and justification to show it was a better option.

Use the fluvial model assessment tool to help you assess model quality

You should use these standards alongside the Microsoft Excel-based fluvial model assessment tool(LIT 56342).

The tool will help you assess the quality of a hydraulic model without completing a full technical review. You can then decide what the model is good for and where it could be used.

Using the tool does not exempt you from needing a technical review of your model. You’ll still need to put your model through a QA process before you issue it to the client.

Follow the source, pathway, receptor and model approach concept

The source, pathway, receptor (SPR) concept allows models to be split into 3 categories. A further category, the ‘model approach’, has been added to cover parts of modelling that do not fit well into the SPR categories.

Source

The source of flood water includes fluvial flows and direct rainfall. These are called model inputs.

This guidance uses annual exceedance probability (AEP) to describe design flood event rarity. Design flood events are hypothetical flood events with a specified probability of occurrence. A 100 year return period flood event is described as a 1% AEP flood - a flood with a 1% chance of occurring each year.

You need to work out how much water is entering the system from:

• fluvial inflows
• catchment rainfall runoff
• rainfall inputs
• downstream sources
• man-made inputs

You’ll need to consider:

• what hydrometric data is available and whether it is reliable
• what hydrological method you should adopt
• how to account for all inflows and outflows
• what the project’s climate change requirements are
• whether joint probability is needed
• whether the gauge is stationary or not

Pathway

Pathways can include river channels, pipe networks, floodplains, hydraulic structures, flood defences and downstream boundaries.

You need to represent pathways at each stage of the modelling process. This means creating a virtual model of the river channel and floodplain geometry to replicate flow.

You’ll need to consider:

• how to represent each pathway
• what data is available and how you’ll use it
• how to represent (discretise) the model in space - its cross-section spacing and grid resolution
• which channel and floodplain structures are important and how you’ll represent them
• how to represent frictional losses
• whether water will pass from the channel or pipe network to the floodplain
• whether flood defences are present
• how to represent downstream boundaries

Receptor

Receptors are residential and commercial properties, and infrastructure at risk from flooding.

Most receptors will be in the floodplain. To identify receptors, the model must represent the likely flow paths to predict the extent of flood events.

You’ll need to consider:

• how to represent receptors
• how reliable the available receptor data is
• whether you need to do post processing to establish the risk to receptors

Model approach

The model approach factors in the parts of modelling that are not actual features and inputs.

You’ll need to make decisions about hydraulic models that do not directly represent the system you are modelling. Your decisions should fulfil the model’s practical and mathematical requirements. You need to decide:

• the purpose of the model - for example, flood mapping, appraisal or flood risk assessment
• what hydraulic modelling software is most appropriate to use
• the most suitable dimension for the model - 1 dimensional (1D), 2 dimensional (2D) or 3 dimensional (3D)
• how you should discretise the model over time - when you’ll conduct calculations and extract results
• suitable modelling parameters
• whether the model is mathematically stable
• whether parameters can be calibrated and verified
• how you’ll measure uncertainties
• the required outputs

Existing model quality guidance

There are only a limited number of projects or research papers that offer a comprehensive set of modelling standards or review guidelines. The projects and research papers listed give the technical background to hydrological and hydraulic modelling decision-making. Research papers and projects relevant to this guidance include:

• LIT11327: computational modelling to assess flood and coastal risk, Environment Agency, 2010
• LIT 56527: standards for modelling and forecasting flooding in large estuaries, Environment Agency, 2015
• Flood Modelling Guidance for Responsible Authorities. Scottish Environment Protection Agency, 2016
• Rapid expert review for flood and coastal risk management appraisal modelling, Environment Agency, 2019

Send your work for approval: recommendations

To complete the hydraulic modelling process, you’ll need to send your model to the Environment Agency or project clients to approve or store for future use. Models are subject to the quality assurance processes.

Depending on the project type and scope specifications, you may need to include more outputs. You need to consider the scope of work before you complete your modelling tasks. This will make sure you set up the model to produce everything you need.

To make the approval process quicker and to make sure your model is suitable for future use, you should provide:

• an agreed hydrology method statement
• a Flood Estimation Handbook (FEH) calculation record hydrology report
• FEH calculations and results - if stored separately to the calculation record
• any topographic survey data you have collected for the project
• an agreed hydraulic method statement
• all hydraulic model files for final model version and required scenarios
• calibration and validation outputs
• sensitivity test outputs
• hydraulic model development log
• hydraulic modelling final project report, including section on assumptions and limitations
• post-processed outputs, such as flood outlines and depth or level grids

Request referenced documents

Email enquiries@environment-agency.gov.uk to request if you would like a copy of:

• a document referenced in this guidance
• a copy of the full PDF version of this guidance
• the Microsoft Excel-based fluvial model assessment tool

You should quote the reference number of the document you need, for example, LIT11327.