Renewable Energy Flood Risk Assessment & Drainage

A renewable energy flood risk assessment is the site-specific report that shows a solar farm, an onshore wind farm or an anaerobic digestion plant can be built and run without raising flood risk on the site or anywhere downstream of it. It is the evidence a local planning authority, and often the Environment Agency or Natural Resources Wales, needs before it will grant permission where flooding is a material concern. On most renewable schemes it travels with a drainage strategy, because how the site handles rainfall is usually the first thing a drainage consultee asks about.

We have prepared flood risk and drainage evidence for renewable energy developments since 2014, on schemes ranging from 500kW community installations to 20MW export sites. Ground-mounted solar, onshore wind and anaerobic digestion each behave differently in a flood, and each draws a slightly different question from the regulator. What they share is that the flood risk work is rarely the reason a good scheme fails. Getting it to say what the consultee actually wants to see, before the layout is fixed, is where projects are won or lost.

Get the flood risk and drainage evidence wrong on a renewable scheme and you rarely just lose time. You can lose generating capacity, and with it the income the site was built to produce.

What is a flood risk assessment for a renewable energy development?

It is a technical report that identifies every source of flooding that could affect a renewable energy site, tests the proposal against them, and sets out the mitigation and drainage that keep the development safe over its lifetime without pushing water onto neighbouring land. In England it is prepared to the National Planning Policy Framework and the accompanying Planning Practice Guidance; in Wales the equivalent is a Flood Consequence Assessment under TAN15.

The underlying flood data is the same data any development uses. What changes for renewables is scale and layout. These sites are often large, frequently rural, and regularly sit on the kind of low-lying or riverside land that a housing developer would walk away from but a solar or anaerobic digestion project, tied to a grid connection or a feedstock supply, sometimes cannot. A generating station also has expensive, immovable plant that does not cope well with standing water, which puts the emphasis firmly on protecting infrastructure and controlling runoff rather than on evacuating people.

Which renewable energy projects need a flood risk assessment?

The trigger is set by national policy, not by the technology. A flood risk assessment is required whenever a site meets any of the following:

• Any site in Flood Zone 2 or 3. All development in the medium and high probability zones needs an assessment, whatever its size.
• A Flood Zone 1 site of one hectare or more. This is the limb that catches most renewable schemes. A ground-mounted array or a wind development easily exceeds a hectare, so it needs an assessment even in the lowest-probability zone.
• Smaller Flood Zone 1 sites with other flood risk. Surface water risk shown on the Flood Map for Planning, a critical drainage area, or an SFRA flag can all pull a sub-hectare site into scope.

If you are not sure which zone your site sits in, our free flood risk map postcode checker screens a location against the Environment Agency datasets in a few seconds, the Government's check long-term flood risk service gives the everyday picture, and the statutory Flood Map for Planning gives the formal planning zone. For the full picture of when policy bites, see our guides on when you need a flood risk assessment and flood zones explained, or the detail on our flood risk assessment for planning page.

Choose your project type

Each renewable technology raises its own flood risk and drainage questions, so we have written a dedicated page for each. Start with the one that matches your scheme. Each page covers the classification, the drainage issue and the regulatory route specific to that technology.

Building something that does not fit these three, such as battery storage, an EV charging hub or a grid connection compound? The same policy tests apply, and our note on flood risk and drainage for EV charging infrastructure is a useful starting point. Send us the details and we will tell you what is needed.

What a renewable energy flood risk assessment and drainage strategy covers

Whichever technology you are building, the assessment works through the same core questions. The weighting shifts from site to site, but a robust report for a renewable development almost always addresses each of these:

• Surface water management. Controlling runoff so that no more water leaves the site after development than before it, and nothing is concentrated onto a neighbour or a watercourse.
• Compensatory flood storage. Where any part of the development sits in a functional floodplain, replacing the flood storage it displaces, level for level.
• Protecting the plant. Raising transformers, inverters, control buildings and other key electrical infrastructure above the design flood level, including the Environment Agency's climate change allowances.
• Keeping floodwater moving. Laying the site out so floodwaters can pass across it freely rather than banking up against rows of panels, foundations or bunds.
• A sequential approach within the site. Putting the most vulnerable elements on the highest ground and accepting water where it does least harm.
• Sustainable drainage. Swales, infiltration trenches and attenuation that slow flows and encourage water back into the ground, designed to the current SuDS standards.
• Safe access and a plan for the worst case. Statutory and bye-law distances from watercourses maintained, and a flood warning and evacuation plan where people need to reach the site safely during an event.

The drainage side is often a document in its own right. Our surface water drainage strategy and wider drainage strategy pages set out what a council's drainage consultee expects, and the SuDS hierarchy explains how a discharge point is justified. Where infiltration is on the table, our free ground conditions checker gives an early read on the geology. Where the national maps are too coarse for a riverside site, flood modelling or a groundwater flood risk assessment may be needed to answer the question properly.

On a generating station the priority is rarely people. It is keeping expensive plant dry and making sure the scheme does not move flood risk somewhere it did not exist before.

Is this the right type of flood risk assessment for you?

Renewable energy sits inside a wider family of flood risk work, and people often land here when another report is what they actually need. Use this to point yourself at the right page:

• A standard development in England. Housing, commercial or mixed-use rather than a generating station. Our flood risk assessment for planning page is the place to start.
• A site in Wales. Welsh applications need a Flood Consequence Assessment under TAN15, not an English FRA. See flood consequence assessment for Wales.
• Told to pass the Sequential Test. A large greenfield renewable site in Flood Zone 2 or 3 may need one. Our Sequential and Exception Test reports cover it.
• The national maps do not fit your site. Detailed hydraulic evidence comes from flood modelling.
• Chasing a sustainability rating. A building-led scheme targeting credits needs a BREEAM flood risk assessment.
• An insurer or lender has raised flood risk. That is a job for a flood risk assessment for insurance.

Not sure where you fit? Tell us about the site and we will point you to the right report, even if it is not this one. Our flood risk consultants handle the whole family of work in one team.

Renewable schemes in England and in Wales

The border matters more than most applicants expect. In England the framework is the NPPF, applied by the local planning authority with the Environment Agency as statutory consultee on flood risk, and our NPPF flood risk guidance walks through what it requires, at what site size and in which flood zone. In Wales the framework is TAN15, reviewed by Natural Resources Wales, and it turns on the consequences of flooding rather than probability alone. The TAN15 guidance and Welsh Government's TAN15 set out the detail.

Wales adds a second hurdle for drainage: larger sites need approval from the SuDS Approving Body against the national standards, a statutory step that runs separately from planning and can catch a scheme out if it is left until the application is nearly in. Whichever side of the border your scheme sits, the vulnerability classification and the climate change allowances shape the answer. See our guides on vulnerability classification and climate change allowances.

What drives the cost of a renewable energy flood risk assessment

There is no fixed price, and any consultant quoting one before seeing the site is guessing. The fee follows the work the site genuinely needs, which comes down to a handful of things:

• Flood zone and flood sources. A low-risk Flood Zone 1 site is a shorter job than a site in Flood Zone 3 exposed to rivers, surface water and groundwater at once.
• Site size and layout. A few hectares of ground-mounted panels is a different exercise from a single turbine compound or a plant with digestate tanks to protect.
• Whether modelling or compensation is needed. A riverside site that needs a hydraulic model or level-for-level floodplain compensation carries more work than a desk-based assessment.
• How many technologies are on the site. A co-located scheme with solar, storage and a grid compound has more moving parts than a single-use site.

We quote against what your site actually requires, and we will tell you where a lighter scope will do. Our guide on what drives the price of a flood risk assessment explains the drivers in more depth.

Why choose Unda for a renewable energy flood risk assessment

• Track record across the technologies. More than 5,000 flood risk assessments since 2014, including solar, onshore wind and anaerobic digestion schemes from 500kW to 20MW.
• Chartered expertise. A CIWEM Business Partner with chartered consultants, and every report signed off by a senior flood risk consultant before it goes out.
• One team, one coordinated package. Flood risk assessment, drainage strategy, modelling and groundwater work sit together, so the evidence does not contradict itself.
• Proportionate scope. We scope the smallest assessment that credibly answers the question, and we say so when a site does not need modelling at all.
• Local knowledge. Detailed familiarity with catchments, watercourses and each council's own requirements across our area, river and local authority guides.

When a scheme has already stalled, we pick it up as ordinary work. See our notes on resolving an Environment Agency objection or an LLFA drainage objection.

Frequently asked questions

To talk it through, call our experienced flood risk consultants on +44 (0) 1293 214444, email enquiries@unda.co.uk, or start a quote and we will come back to you within 60 minutes on a working day.