Solar farm development flood guidance

This guidance note has been produced to assist in planning response and for applicants who wish to submit a planning application for the development of solar farms.

Solar farms may be substantial in size and have the potential to interrupt overland flow routes across a development site. Siting of solar arrays should account for any potential flow routes or highly impermeable soils which may require an additional allowance.

Solar farms' influence on flood risk

The common setup of solar farms means sites are usually considered 95% permeable, with the other 5% being impermeable for associated infrastructure. However, without the incorporation of basic drainage the development may lead to increases in flood risk elsewhere.

Rainfall will drain freely off the panels onto the ground beneath the panels where the surface remains permeable. Thus, the total surface area of the photovoltaic array is not considered to act as an impermeable area and the impact is assumed to be nil, expect for the provision of required infrastructure such as substations, access roads, etc. However, the nature of the underlying groundcover and antecedent conditions can have a demonstrable influence on the surface water run-off characteristics of a site. Permeable land surfaces underneath and between panels should be able to absorb rainfall as long as they are not compacted and there is some vegetation to bind the soil surface.

To ensure that proposed schemes will not increase the surface water run-off rate, volume, or time to peak compared to the pre-development situation. It should be ensured as part of any proposed scheme that:

• formalised perimeter drainage, as a minimum, should be incorporated into the design to capture exceedance and retain on site
• grass and wildflower cover should be incorporated and well-maintained across the site
• soil organic matter levels should be retained, or enhanced, compact areas of soil should be loosened
• existing land drainage is retained
• incorporating bunds, filter drains or other measures to interrupt flows of water between rows of solar arrays to disperse water flows over the surface and promote infiltration into the soils
• incorporating wide grassed filter strips at the downstream side of blocks of solar arrays and maintaining the grass at a long length to interrupt water flows and promote infiltration
• incorporating gravel-filled filter drains or swales between blocks of solar arrays to help infiltrate run-off (where ground conditions allow)
• a drainage strategy should be provided for any large infrastructure on site, e.g. impermeable substation, access roads, compound areas, etc.

Mitigating the impact of development upon flood risk

Regardless of potential permeability we expect overland flow paths and site water to be controlled and released in line with our standard policies. We encourage the inclusion of measures within the development to reduce the impact of channelised flows and wherever possible promote the use of infiltration to discharge surface water from the site, refer to list above.

In the areas of the site which are proposed to be developed with impermeable surfaces, i.e. access roads and substations, surface water should be collected within a formalised drainage system. These systems will require controlled outflows, identified discharge locations and potentially land drainage consent. Details of this should be provided with the application.

Access roads have the potential to be constructed using permeable surfaces, (e.g. a proprietary grass paving system or gravel medium) which would mimic existing surface conditions and not require additional formal drainage. Vehicular movements on site should be restricted to designated access tracks. In doing so, the risk of soil compaction is minimised and limited to specific locations.

Other areas accessed by machinery should be maintained to ensure rutting of the surface is managed and repaired where necessary. These ruts in themselves can collect water into channelised flow paths.

It will generally be acceptable to secure drainage arrangements for a solar array development with appropriate planning conditions unless detail is provided at the outset that removes the need for conditions to be required.

Any formal drainage on a site may utilise connections to local watercourses. Any works on site that have the potential to affect an ordinary watercourse, including works within the stream, culverting and outfalls are likely to require Land Drainage Consent under the Land Drainage Act (1991). Early consultation with the internal drainage board should be sought for consent.

Questions

Please email floodrisk@centralbedfordshire.gov.uk if you have any additional queries or site-specific requirements that may need addressing.