How we evaluate your hydraulic model

Types of flood risk models

This page explains how we evaluate hydraulic models for flood risk from rivers and the sea. It does not cover how we evaluate models for other sources of flooding.

Do you need to submit a model?

In some cases, you will not need to produce a model. For example:

• very low risk developments or flood risk activities (you may only need simple calculations without the use of modelling software)
• if the model supports a planning proposal that conflicts with national planning policy (the Local Planning Authority will make a decision without our advice, unless they ask for it)
• if we decide your model is not fit for purpose without reading your model report (for example, if we own the model but we haven’t licenced it for use then it is not fit for purpose)
• if we can compare model outputs about similar developments or activities in the same area and determine that it is fit for purpose without an intermediate or detailed evaluation

In these cases, we will advise you about what you need to do, instead of evaluating the model.

Evaluating your model

When you submit a hydraulic flood risk model, we use a risk-based approach to decide:

• which models to evaluate
• the level of detail we need to evaluate

With a risk-based approach to model evaluation, we:

• do an initial evaluation to understand whether the model is fit for purpose
• ensure the evaluation is appropriate to the level of flood risk around a development, flood risk activity, or a flood scheme
• consider the impacts of doing a less detailed evaluation, for example can we make an informed planning decision?

Level of detail for evaluating your model

Our evaluation of your model will be:

• basic
• intermediate
• detailed

We consider which level of detail and the degree of certainty you will need to provide by looking at:

• what are you using the model for, for instance, to inform a planning decision?
• The level of flood risk to the area you have modelled (likelihood and consequence)
• The impact of flooding off-site from your development or activity

Likelihood of flooding

Generally, the higher the likelihood of flooding, the more detailed our evaluation will need to be.

Consequence of flooding

There is no nationally consistent way to define the consequence of flooding. We will consider:

• scale of development
• nature of development
• the sites at risk of flooding
• historic flooding in the area
• flood characteristics on and off the development site

Generally, the larger the development and the more vulnerable the sites affected by flooding, the more detailed our evaluation will need to be.

Basic evaluation

We generally use a basic evaluation for proposals that are:

• less than 1 hectare in area
• low vulnerability (development planning) or low risk (flood risk activity permit)
  - located in flood zone 2 (1 in 1000 year chance of occurring, including an allowance for climate change)

We will check your model report to ensure your methods are appropriate and the model is fit for purpose.

Intermediate evaluation

We generally use a basic evaluation for proposals that are:

• less than 1 hectare in area
• low vulnerability (development planning) or low risk (flood risk activity permit)
  - located in flood zone 3 (1 in 100 year chance of occurring, including an allowance for climate change)
• high vulnerability (development planning) or medium risk (flood risk activity permit)
  - located in flood zone 2 (1 in 1000 year chance of occurring, including an allowance for climate change)
• greater than 1 hectare in area
  - located in flood zone 2 (1 in 1000 year chance of occurring including an allowance for climate change)

We will check your model report and interrogate the model and hydrology. If we find any errors, we may decide to run the model ourselves to do some basic sensitivity testing.

Detailed evaluation

We generally use a basic evaluation for proposals that are:

• greater than 1 hectare in area
• high vulnerability (development planning) or high risk (flood risk activity permit)
  - located in flood zone 3 (1 in 100 year chance of occurring, including an allowance for climate change)
• greater than 1 hectare in area
  - located in flood zone 3 (1 in 100 year chance of occurring, including an allowance for climate change)
• a development for emergency services

We will check your model report and interrogate the model and hydrology. We will re-run the model to understand your outputs more fully and to understand how you developed the model.

This allows us to check the stability of your model and the results you have submitted.

In some cases, we may not have the software available to do a detailed assessment of your model. In this case, we will do as much checking as possible with the support of the consultant who produced the model.

Costs to evaluation the model

We charge for pre-application advice. 

There may be a charge if you ask us to review your model.

Our service to developers has a full list of charges.