Technical Corner

When is an existing strategic-scale model appropriate for your project?

We regularly build hydraulic models from scratch, but this approach isn’t necessary for every project. The Environment Agency, Natural Resources Wales and other UK regulators hold a vast library of models which can be requested and used as a starting point for flood modelling studies. These are likely to be available for most towns and cities in the UK, with more sporadic coverage in rural areas.

If an existing model is available, then making use of this model can be a route to save time and cost for flood risk studies. This is particularly true when we are asked to look at small sites or individual properties – the cost of undertaking a bespoke model build in these instances can be disproportionate to the nature of the study. However, there are a few things to look out for when deciding whether it is appropriate; just because the model is supplied by the regulator doesn’t mean it is suitable for all applications.

Factors to consider

These are often strategic level models, designed to understand flood risk at a broad scale. This means that the level of detail at a local scale is inadequate for many purposes. In most cases, it’s usually desirable to integrate local topographic survey into the model to make sure that the representation of local features is sufficiently detailed.
Models held by the regulators were often developed some time ago and may not be built according to current best practice. This means that updates could be required, and it isn’t just a case of making use of what’s been supplied.
Similarly, it’s possible that an incoming model may have been built and run using earlier versions of hydraulic modelling software. Changes to the software means that modifications to the model can be required just to get them to run in the latest versions.
The user should be conscious of whether there have been changes in the catchment since the model was built and whether these changes could impact results at the site of interest. For example, if a new flood mitigation scheme has been built which changes how, when or where water moves.
Depending on the age of the model, the hydrological analysis may no longer use current datasets or follow the latest guidance. For some studies this might not be a problem, and the existing inflows can still be applied, but in other instances the regulators would expect to see an updated hydrological analysis.

So how can we decide whether to make use of an existing model? There is no single answer to this, and perhaps the most important consideration is quite a broad one – what question is the flood risk study ultimately trying to answer? The response to this dictates how a project should proceed.

If we’re making a high-level assessment of whether removing a weir might cause disbenefit to third parties, then a simple like for like comparison using an existing model and hydrological inflows could be sufficient for an early assessment. Conversely, if we’re asked to assess what Finished Floor Levels should be used at a proposed development then we are much more likely to need to integrate detailed topographic survey of the local watercourse, alongside undertaking a new hydrological analysis.

The answer will vary on a case-by-case basis and we would always advise reviewing the model upon receipt, as this can highlight potential issues at an early stage.

Please do contact us to discuss the requirements for your project.

Peter Aylett has worked at Edenvale Young since 2006 and is a Technical Director. He forms part of our technical team and is available to provide training and technical support.