Flood Risk Assessment in Northern Ireland

What is a Flood Risk Assessment?

A Flood Risk Assessment identifies all sources of flood risk to and from a proposed development and sets out how that risk will be managed and mitigated. The level of detail should be proportionate to the site and the risk. Many FRAs are based on the flood maps, a site visit and historic flood information. Where the site or DfI Rivers requires it, the assessment can also be supported by topographic survey and hydraulic modelling.

A good FRA does two jobs. It gives the decision-maker the evidence needed to apply planning policy, and it gives you a practical understanding of how flooding affects your layout, floor levels, access and drainage. Ideally it should be carried out early, before a layout is fixed, so that design work and planning fees are not spent on a scheme that cannot be approved.

When is a Flood Risk Assessment required in Northern Ireland?

Development in river and coastal flood plains (Policy FLD 1)

Under Policy FLD 1 of Revised Planning Policy Statement 15 (PPS 15) Planning and Flood Risk, development is not permitted within the 1 in 100 year fluvial flood plain (1% annual exceedance probability, AEP) or the 1 in 200 year coastal flood plain (0.5% AEP) unless it is an exception to the policy. For planning purposes these flood plains include an allowance for climate change.

The exceptions include, among others, previously developed land in defended areas, replacement buildings, certain agricultural, transport and utilities development that has to be in the flood plain, water-compatible uses, and outdoor sport, amenity and nature conservation uses. Minor development may also be acceptable. Where the principle of development is accepted, an FRA is required for all proposals, and it must show that:

• all sources of flood risk to and from the development have been identified; and
• there are adequate measures to manage and mitigate any increase in flood risk arising from the development.

FLD 1 also sets out measures that will not be accepted to make development in the flood plain possible. These include new flood defences, flood compensation storage works, and land raising within the undefended fluvial flood plain. Understanding these limits early is often the most valuable part of our advice.

Development near reservoirs (Policy FLD 5)

Policy FLD 5 applies where a site lies within the potential flood inundation area of a controlled reservoir, as shown on Flood Maps NI. New development will only be permitted where there is sufficient assurance about the reservoir’s condition, management and maintenance, and an FRA assesses the downstream flood risk from a controlled release, an uncontrolled release due to failure, and any change in flow paths. DfI Rivers’ Technical Guidance Note 25 explains how it applies this policy.

Surface water and other sources

Outside flood plains, surface water (pluvial) flood risk is dealt with under Policy FLD 3, usually through a Drainage Assessment. Where a site is affected by more than one source of flooding, we can combine the FRA and Drainage Assessment into a single, consistent report.

What our Flood Risk Assessments include

Every FRA is scoped to the site and the level of risk, but it will typically cover:

• a review of the proposal against Revised PPS 15, the SPPS and any relevant policies in the council’s adopted Plan Strategy;
• a review of DfI Rivers’ Flood Maps NI (present day and climate change layers) and any other DfI Rivers information for the site;
• a site visit to confirm watercourses, structures, flow paths and existing drainage;
• historic flood information for the site and the surrounding area;
• identification of all sources of flood risk to and from the development;
• surface water and drainage considerations, including SuDS and discharge arrangements (with a Schedule 6 consent application where discharge is to a watercourse);
• mitigation, such as recommended finished floor levels, safe access and egress, flood resilience measures and, where appropriate, a flood management and evacuation plan;
• a clear report for the planning application, with conclusions against the relevant policy tests.

Where the site or DfI Rivers requires it, we can also provide:

• topographic survey, including drone LiDAR or photogrammetry survey using our own equipment, to establish ground levels accurately;
• hydrological analysis and 1D/2D hydraulic modelling of the watercourse using InfoWorks ICM or HEC-RAS.

Developments we assess

Frequently asked questions

How do I know if my site needs a Flood Risk Assessment?

Check the site on DfI’s Flood Maps NI. If it lies within or close to a mapped river, coastal or reservoir flood extent, or the council or DfI Rivers has asked for one, you are likely to need an FRA. If you are unsure, send us the site location and we will advise.

What is the difference between a Flood Risk Assessment and a Drainage Assessment?

An FRA mainly addresses river, coastal and reservoir flooding under Policies FLD 1 and FLD 5. A Drainage Assessment addresses surface water flood risk and site drainage under Policy FLD 3. Many sites need elements of both.

Can I build in a flood plain in Northern Ireland?

Only if the proposal is an exception to Policy FLD 1 (or meets the equivalent policy in your council’s adopted Plan Strategy), and a Flood Risk Assessment shows the risk can be managed without increasing flooding elsewhere.

Do all Flood Risk Assessments need flood modelling or a survey?

No. Many FRAs can be completed using DfI Rivers’ Flood Maps NI, historic flood information, a site visit and any existing site levels. A topographic survey or hydraulic model is only needed where the site or DfI Rivers requires it, for example where the flood extent or levels need to be defined more precisely, or where the mapped flood extent is being challenged. We will tell you at the outset whether either is likely to be needed, and we keep the assessment proportionate to the risk.

What modelling software do you use?

Where an FRA needs hydraulic modelling, we use InfoWorks ICM or HEC-RAS, choosing the one that suits the watercourse and the questions the assessment needs to answer.