Drainage Assessment in Northern Ireland

What is a Drainage Assessment?

A Drainage Assessment (sometimes called a Drainage Impact Assessment) is a statement of the drainage issues relevant to a development and the measures proposed to deal with them. It addresses Policy FLD 3 of Revised PPS 15 Planning and Flood Risk, which covers development and surface water (pluvial) flood risk outside flood plains.

Its purpose is to demonstrate that adequate measures will be put in place to mitigate flood risk to the development and from the development elsewhere. The level of detail should be proportionate to the size and nature of the proposal.

When is a Drainage Assessment required?

Under Policy FLD 3, a Drainage Assessment is required for any development proposal that exceeds any of these thresholds:

• a residential development of 10 or more dwelling units;
• a development site of more than 1 hectare;
• a change of use involving new buildings and/or hard surfacing of more than 1,000 square metres.

A Drainage Assessment is also required for any proposal, except minor development, where:

• the site is in an area with evidence of a history of surface water flooding; or
• surface water runoff from the development may adversely affect other development, or features important to nature conservation, archaeology or built heritage.

A planning authority may also ask for one where evidence of drainage problems comes to light, even if the thresholds are not met. Where a council has adopted its Local Development Plan Plan Strategy, the policies in that Plan Strategy also apply. We check which policies apply to your site at the outset.

Even where no Drainage Assessment is formally required, if the surface water layer of Flood Maps NI shows potential flooding on your site, it remains the developer’s responsibility to assess and manage that risk.

What our Drainage Assessments include

Each assessment is tailored to the site, but typically includes:

• a review of the proposal against Policy FLD 3 of Revised PPS 15, the SPPS and any relevant policies in the council’s adopted Plan Strategy;
• a review of DfI Rivers’ Flood Maps NI, including the surface water layer, and a desk study of watercourses, geology and existing drainage;
• a site visit to confirm drainage features, outfalls and overland flow routes;
• historic flood information for the site and the surrounding area;
• identification of the sources of flood risk to and from the site, particularly surface water;
• surface water and drainage considerations: existing (greenfield or brownfield) and post-development runoff rates, attenuation volumes for design storms including a climate change allowance, and evidence that a feasible discharge route exists, whether to a watercourse, the public sewer or the ground;
• mitigation, including a preliminary drainage layout showing that the storage and flow controls fit within the site, an assessment of exceedance flows and overland flow paths, and a SuDS maintenance and management outline;
• a clear report for the planning application.

Where the site or DfI Rivers requires it, we can also provide topographic survey, including drone survey using our own equipment, to define catchments and hard-surfaced areas accurately. Where a site also has river or coastal flood risk, we can combine the Drainage Assessment with a Flood Risk Assessment, including hydraulic modelling in InfoWorks ICM or HEC-RAS where the site or DfI Rivers requires it.

Overland flow and exceedance

Drainage systems are designed for a specific storm. Larger events will exceed them, so a robust Drainage Assessment also considers where water goes when that happens. We map overland flow routes across and around the site, using drone-derived terrain data where available, to make sure exceedance flows are directed away from buildings and do not increase risk to neighbours. This can help set finished floor levels and shape the site layout. It is often cheaper to design these routes in early than to add storage later.

Frequently asked questions

Does my development need a Drainage Assessment?

If it is 10 or more homes, exceeds 1 hectare, or involves more than 1,000 m² of new buildings or hard surfacing through a change of use, it almost certainly does under Policy FLD 3. Smaller schemes may also need one where there is a history of surface water flooding or where runoff could affect others.

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

A Drainage Assessment deals with surface water flood risk and site drainage under Policy FLD 3. A Flood Risk Assessment deals mainly with river, coastal and reservoir flooding under Policies FLD 1 and FLD 5. Some sites need both, and we can combine them into one report.

What discharge rate will be allowed from my site?

DfI Rivers and NI Water commonly limit discharge to the greenfield (pre-development) rate, with attenuation for larger storms. The allowable rate depends on the site and the receiving watercourse or sewer, and we agree it as part of the process.

Do you provide a drainage layout?

Yes. We produce a preliminary drainage layout that demonstrates feasibility for planning.