Solar farms have the potential to interrupt overland flow routes, reduce the amount of rainfall absorbed into the ground and increase the rate and volume of surface water runoff. All solar farm applications should provide a drainage strategy as well as a land management strategy as these are crucial when assessing the surface water flood risk associated with these sites.

Sites should be configured or selected to avoid the need to impact on existing drainage systems and watercourses. Culverting existing watercourses/drainage ditches should be avoided. Where culverting for access is unavoidable, it should be demonstrated that no reasonable alternatives exist and where necessary only temporarily for the construction period.

The rows of solar panels should be arranged parallel to the site contours wherever possible. This arrangement would facilitate natural drainage i.e. sheet flow of the surface water runoff. If the rows of solar panels are arranged perpendicular to the site contours then channelised flows would occur causing higher velocities as well as greater soil erosion possibilities.  Where channels are likely to be formed, cut-off drains filled with gravel or berms at regular intervals should be used to spread the flows and create sheet flow conditions.

Mitigation measures and further guidance are detailed in the SUDS design guide on solar array development.