Flood Risk Assessment for Solar Farms and Renewables

The rapid expansion of solar farms and other renewable energy installations across the UK has brought flood risk to the forefront of the planning process for energy developers. Solar farms, by their nature, are land-hungry — a typical utility-scale solar farm requires 5-8 acres per megawatt of installed capacity, and sites of 50MW or more are becoming common. Finding sufficient land inevitably means that some sites will be located in areas with flood risk.

This is not necessarily a problem. Solar farms can coexist with flood risk in ways that many other development types cannot. But the planning framework requires developers to demonstrate this through a robust Flood Risk Assessment, and there are important technical and policy considerations that are specific to renewable energy installations.

Vulnerability Classification

The starting point for any flood risk assessment is the vulnerability classification of the proposed development. The NPPF classifies development types into vulnerability categories, and the category determines what is appropriate in each flood zone.

Where Do Solar Farms Sit?

The NPPF vulnerability classification is set out in Annex 3 of the NPPF. Renewable energy installations, including solar farms, are classified as “essential infrastructure” where they are needed for the generation and distribution of energy.

However, this classification is not automatic. The PPG clarifies that whether a renewable energy installation qualifies as essential infrastructure depends on the circumstances:

• Nationally Significant Infrastructure Projects (NSIPs) — solar farms over 50MW in England (the threshold was increased from 50MW by the Energy Act 2023) are NSIPs and are generally treated as essential infrastructure.
• Sub-NSIP solar farms — solar farms below the NSIP threshold may or may not be classified as essential infrastructure, depending on the LPA’s assessment. The PPG suggests that smaller renewable energy projects should be considered on a case-by-case basis.

Some LPAs classify all solar farms as essential infrastructure; others treat them as “less vulnerable” development. The distinction matters because essential infrastructure can be permitted in all flood zones (subject to the Exception Test in Zones 3a and 3b), while less vulnerable development cannot be permitted in Zone 3b.

In practice, most solar farms are classified as essential infrastructure, reflecting the national policy imperative to increase renewable energy generation. But this should not be assumed — it should be confirmed with the LPA at the pre-application stage.

The Compatibility Matrix for Essential Infrastructure

Flood Zone	Essential Infrastructure
Zone 1	Appropriate
Zone 2	Appropriate
Zone 3a	Exception Test required
Zone 3b	Exception Test required

This means that essential infrastructure can, in principle, be permitted in any flood zone, including the functional floodplain (Zone 3b), provided the Exception Test is passed.

The Sequential Test

The Sequential Test requires the LPA to consider whether the proposed development could be directed to a site at lower flood risk. For solar farms, the Sequential Test presents some distinctive challenges.

The Area of Search

The Sequential Test requires the LPA to define an appropriate area of search — the geographic area within which alternative sites are considered. For housing, this is typically the LPA’s administrative area or a relevant part of it. For solar farms, the area of search may be different.

Solar farms have specific locational requirements that constrain the area of search:

• Grid connection — proximity to a viable grid connection point is a critical factor. Grid capacity is limited, and the cost of grid connection increases with distance. A site 5km from a substation with available capacity may be commercially viable; a site 50km away may not.
• Land availability — solar farms require large areas of relatively flat, unshaded land with willing landowners. Such land is not available everywhere.
• Irradiance — while solar irradiance varies relatively little across the UK compared to other countries, it is a factor in site selection.
• Agricultural land quality — national policy discourages solar farms on Best and Most Versatile (BMV) agricultural land (Grades 1, 2, and 3a). This further constrains the available land.
• Landscape and visual impact — solar farms need to be sited to minimise landscape and visual impact, which may rule out otherwise suitable sites.

Given these constraints, the area of search for a solar farm’s Sequential Test is often defined by the grid connection opportunity. The developer should identify the relevant grid connection point, define a reasonable search area around it, and demonstrate that there are no suitable alternative sites at lower flood risk within that area.

Demonstrating the Sequential Test

The FRA should include a Sequential Test assessment that:

1. Identifies the grid connection opportunity and explains why it defines the area of search
2. Maps all available sites within the area of search
3. Assesses the flood risk at each site
4. Explains why the proposed site was selected (considering all relevant factors, not just flood risk)
5. Demonstrates that there are no reasonably available alternative sites at lower flood risk that could accommodate the development

This assessment should be agreed with the LPA at the pre-application stage if possible, as disagreements about the area of search or the availability of alternative sites are a common cause of delay and refusal.

The Exception Test

Where the Sequential Test cannot direct the development away from Flood Zone 3 (or where the LPA accepts that the proposed site is the most appropriate), the Exception Test must be passed. The Exception Test has two parts:

Part 1: Wider Sustainability Benefits

The development must provide wider sustainability benefits to the community that outweigh the flood risk. For solar farms, this is generally straightforward to demonstrate:

• Clean energy generation — the national policy imperative to achieve net zero by 2050 and increase renewable energy capacity is a powerful sustainability benefit
• Energy security — domestic energy generation reduces dependence on imported fossil fuels
• Economic benefits — construction jobs, business rates, landowner income, community benefit funds
• Biodiversity net gain — many solar farms deliver significant biodiversity enhancements, including wildflower meadows, hedgerow planting, and habitat creation beneath and around the panels

Part 2: Safe Development

The development must be safe for its lifetime, taking account of the vulnerability of its users, without increasing flood risk elsewhere, and where possible reducing flood risk overall.

For solar farms, this part of the Exception Test is where the FRA needs to provide detailed technical evidence.

Technical Considerations for Solar Farm FRAs

Panel Design and Flood Levels

Solar panels are typically mounted on frames that elevate the panels above ground level. The design flood level — and the associated climate change allowance — determines the minimum clearance required:

• Panel base height — the lowest point of the panel (in its lowest tilt position) should be above the design flood level. The Environment Agency typically requires a freeboard of 300mm above the 1% annual probability flood level (with climate change allowance) for river flooding, or 600mm in some circumstances.
• Tracker systems — some solar farms use single-axis tracker systems that change the panel tilt angle throughout the day to maximise energy generation. The FRA must consider the panel position at its lowest point, not just the static design position.
• Inverters and transformers — these are the most flood-sensitive components of a solar farm. They must be located above the design flood level or within flood-protected enclosures. Inverter damage is the primary cause of flood-related losses at solar farms.

Flood Storage and Conveyance

One of the key advantages of solar farms in flood zones is that, if properly designed, they do not significantly reduce floodplain storage or impede flood flows:

• Panels are elevated — the mounting frames allow floodwater to pass beneath the panels
• No buildings — unlike conventional development, solar farms do not introduce significant buildings to the floodplain (the transformer station and control building are typically small and can be located in the lowest-risk part of the site)
• Minimal foundations — panels are typically mounted on driven steel piles or screw anchors that occupy a negligible volume of the floodplain

The FRA should quantify the impact on flood storage and demonstrate that it is negligible. For sites in Flood Zone 3b (functional floodplain), this is particularly important, as any loss of flood storage must be compensated.

Surface Water Runoff

The impact of solar panels on surface water runoff is a topic of ongoing research and some debate:

• Panels concentrate rainfall — rainwater runs off the panel surface and drips from the lower edge, creating a concentrated line of water input along the drip line. This can cause localised erosion if not managed.
• The ground beneath panels may be partially sheltered — reducing direct rainfall on the soil surface. This can alter the soil moisture regime and affect infiltration rates.
• Access tracks — solar farms include access tracks for maintenance vehicles, which introduce impermeable surfaces.

The FRA should assess the impact on surface water runoff and recommend mitigation measures. Common measures include:

• Maintaining or establishing grass cover beneath the panels to promote infiltration and prevent erosion
• Installing gravel or grass-reinforcement systems on access tracks
• Providing swales or other SuDS features to manage runoff from access tracks
• Installing drip-line attenuation features (gravel strips or shallow channels) to manage concentrated runoff from panels

Climate Change Allowances

Solar farms have a typical design life of 35-40 years. The FRA must apply climate change allowances appropriate to this lifetime:

• Peak river flow allowances — using the Environment Agency’s published allowances for the relevant river basin district and time horizon
• Peak rainfall intensity allowances — for surface water drainage design
• Sea level rise allowances — for sites at risk of tidal flooding

The climate change allowances can significantly affect the design flood level. For a solar farm with a 40-year design life starting in 2026, the relevant time horizon extends to 2066, and the applicable allowances should be selected accordingly.

Access and Egress

Solar farms are not permanently occupied, which significantly reduces the access and egress requirements compared to residential or commercial development. The FRA should address:

• Maintenance access — can maintenance personnel safely access the site during a flood event? In most cases, the answer is that maintenance should not be carried out during a flood, and the FRA should include a flood management plan that suspends site access when flooding is forecast.
• Emergency access — is emergency vehicle access available in the event of an equipment fire or other emergency during a flood?

Ecological and Geomorphological Impacts

Solar farms in floodplains can have significant ecological implications:

• Shading — panels shade the ground beneath them, which can affect vegetation growth and the ecological value of the floodplain
• Fencing — security fencing around solar farms in floodplains can trap debris during floods, which can impede flow and cause localised flooding. The EA typically requires that fencing in functional floodplains is designed to allow the passage of floodwater (e.g., using open-rail fencing rather than close-boarded or mesh fencing)
• Geomorphology — in areas of active river geomorphology (meandering, bank erosion), the FRA should consider whether the solar farm could be affected by channel migration over its lifetime

Battery Energy Storage Systems (BESS)

Many modern solar farms include co-located Battery Energy Storage Systems (BESS). These present additional flood risk considerations:

• Batteries are highly flood-sensitive — inundation can cause battery failure, thermal runaway, and release of hazardous materials
• Environmental contamination risk — damaged batteries can release electrolyte and other chemicals into floodwater
• Fire risk — battery fires are difficult to extinguish and can be exacerbated by water contact

The FRA for a solar farm with BESS should:

• Locate the BESS in the lowest-risk part of the site
• Ensure the BESS is elevated above the design flood level with an appropriate freeboard
• Include a flood management plan that addresses battery safety during flood events
• Consider the environmental contamination risk and include appropriate containment measures

Wind Farms and Other Renewables

While solar farms are the most common renewable energy project type encountered in flood zones, other renewable energy installations also require FRAs:

Onshore Wind Farms

Wind turbines in flood zones present fewer challenges than solar farms, as the turbine tower elevates the generating equipment well above any conceivable flood level. The FRA should focus on:

• Access tracks and their impact on surface water runoff
• Substation and transformer locations
• Crane pads and temporary construction areas
• Cable routes and their interaction with watercourses

Hydropower

Small-scale hydropower installations are, by definition, located on or adjacent to watercourses. The FRA should address:

• The impact of the installation on flood flows and flood levels
• The design of the intake and outfall structures to avoid increasing flood risk
• The safety of the installation during flood events

Biomass and Energy from Waste

These facilities are typically classified as “less vulnerable” development and require FRAs in the same way as other industrial development.

Working with the Environment Agency

For solar farms in Flood Zone 3, the Environment Agency is a statutory consultee and will provide a bespoke response to the planning consultation. The EA’s main concerns are typically:

• Flood storage — ensuring the development does not reduce floodplain storage
• Flood flow conveyance — ensuring the development does not impede flood flows
• Safe design — ensuring electrical equipment is above the flood level
• Access — ensuring a flood management plan is in place
• Environmental protection — particularly for BESS installations

Pre-application engagement with the EA is strongly recommended for solar farms in flood zones. The EA can provide detailed flood data (Product 4) and guidance on their specific requirements, which can save significant time and cost during the planning process.

Conclusion

Solar farms in flood zones are eminently achievable, provided the flood risk is properly assessed and the design addresses the identified risks. The key is early engagement with the LPA and EA, a robust FRA that addresses the Sequential and Exception Tests, and a design that keeps flood-sensitive equipment above the flood level while maintaining floodplain storage and conveyance.

At Aegaea, we have extensive experience of preparing FRAs for solar farms and other renewable energy projects across the UK. We work with energy developers from the site selection stage through to planning submission, providing the flood risk and drainage expertise that these projects require.

If you are developing a solar farm or other renewable energy project and need flood risk advice, contact our team for a no-obligation discussion.