Modelling for Flood Consequence Assessments

Flood Consequence Assessments

You may need a Flood Consequences Assessment (FCA) for:

planning applications for new developments in a flood risk area
Flood Risk Activity Permits (FRAP)

Flood Consequences Assessments demonstrate the flood risk:

to a new development
as a result of a new development or a flood risk activity

You can also use FCAs to demonstrate that your proposal has:

appropriate mitigation measures to reduce the flood risk and consequences
is as safe as possible
causes no increased flood risk elsewhere

What to include in your Flood Consequences Assessments

FCAs should provide evidence of flood risk from:

rivers and the sea
surface water and small watercourses
groundwater
reservoirs (if relevant)

If your development is in an area of combined risk of flooding from rivers and the sea, contact the local Flood Risk Analysis team to discuss which combinations of joint probability you should assess:

FRASouth@cyfoethnaturiolcymru.gov.uk
FRANorth@cyfoethnaturiolcymru.gov.uk

TAN15: Development and flood risk lists all the information you need to include in a FCA to support a planning application or a FRAP.

Flood risk model checklist

Use our flood risk model checklist to help you develop your model. We use this checklist for our quality assurance.

The checklist is a good starting point but you should also make further checks on:

local features
local conditions
limitations of the software you are using

You should complete the checklist and submit it to us with your model and reports.

Evidence to include in your assessment

You must fully assess flood risks and impacts in your FCA. These might include:

an appropriate and robust baseline hydraulic model that represents current conditions and includes the latest hydraulic modelling software (if you use a hydraulic model)
a hydraulic model that integrates your proposed permanent and temporary works with the baseline model
topographical surveys (in accordance with Ordnance Datum where possible) and the flood flow estimates you have used
outputs from both the baseline and proposed hydraulic model that calculate the flood risk in depth, velocity and extent for the flood flows you have chosen
a comparison of your baseline and proposal results
a range of flood scenarios “up to and including” the maximum design event
an allowance for climate change in line with current government guidance
an assessment of the impacts of flooding associated with a breach of defences and/or a blockage on a bridge/culvert, or pipe crossing a watercourse

Demonstrating no increased flood risk

To demonstrate that your proposed development or flood risk activity has not increased flood risk elsewhere, your FCA and any associated hydraulic modelling must show:

no increased depth, velocity, hazard or extent for flood water outside the river channel or agreed flood storage area
no change to the onset of flooding
no impact to infrastructure within or connected hydraulically to the river channel

Mitigation

You will usually need to show evidence of mitigation in your FCA to address any increased flood risk in another location.

Calculating flood risk

Your calculations must be appropriately precise and measured in metres:

depth and levels results to two decimal places
velocity, and hazard values to one decimal place

This is due to resolution in the model.

For example, if you calculated:

a baseline water level to 100.000m Above Ordnance Datum (AOD)
the proposed change in water level to 100.004m AOD

You would report this as 100.00mAOD. That is, no reported change.

But in the same example, if you calculate:

the proposed water level to 100.005mAOD

You would report this as a change in water level with a proposed water level of 100.01mAOD. This is a positive change in water level from the baseline.

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