Solar farms may be considered to have a relatively low risk in relation to surface water flooding, however this is only the case when there are surface water flood risk mitigation measures in place. 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.

Solar farms, just like any other development, have the potential to cover large areas, whereby they can interrupt overland flow routes, reduce the amount of rainfall absorbed into the ground and increase the rate and volume of surface water runoff.  Research shows that the panels themselves are only likely to contribute to a small increase in total runoff. For example a study found that over a total length of 225 m with 30 solar panels, the runoff increased when assessing simply the introduction of the panel cover by 0.26 m3, which was a difference of only 0.35% (Cook and McCuen, 2013). While this means when assessing the panel cover that smaller sites are likely to have a limited impact this could lead to large additional volumes of water affecting downstream flood risk from a large site. 

Significant ways that discharge rates and volumes are increased further is through the subsequent creation of drainage channels and/or poor land management.

The surface water usually flows from the surface of the solar array to the areas in between the rows with an increased kinetic energy. This leads to an increased concentration of surface water and erosion in these areas and has the potential to create channelised flows, eroding the soil further and increasing the volumes and rates of surface water discharge. This can be further exacerbated by lack of maintenance and further erosion/compaction from vehicles such as maintenance vehicles. Therefore, necessary mitigation is required to combat this effect and it should be demonstrated that whatever land management techniques are being used to ensure that the land maintains or improves its current infiltration potential include small amounts of storage too.

 Examples of the mitigation of channelisation:

• The use of SuDS features such as buffers, swales, filter strips, and filter drains to interrupt and slow the channelised flows, enhance and promote the infiltration and interception capacity, and to help spread the water over a greater surface area.
• Maintaining vegetative areas in between the solar arrays at a long length to help interrupt and slow the channelised flows, reducing erosion and also enhance and promote the infiltration and interception capacity. Where possible bare ground or gravel should be avoided.
• Where ground conditions allow, the inclusion of infiltration basins and strips to capture any additional flows carried by the created channels which would have previously been infiltrated.
• The incorporation of bunds to help slow and interrupt the surface water flows.
• An enforceable and robust soil, grass, and/or land management plan to keep land in good condition is key. If the ground becomes bare due to lack of maintenance the peak discharge has the potential to increase significantly.

 Any roads, or structures should follow guidance found elsewhere in this guide. This includes, following the drainage hierarchy, restricting discharge rates to ensure surface water flood risk is not increased, promoting interception storage, making sure any necessary storage is sized correctly (and located in above ground green features where possible), and making sure surface water is treated of any pollution before discharge.

 If works are to be undertaken involving an ordinary watercourse then consent must be given by the LLFA. Please see the ordinary watercourse section.

 After construction the soil should be chisel ploughed, or similar, to mitigate soil compaction during construction. This will ensure that the site can infiltrate to its potential. Furthermore, during the first few years it is important to hold frequent inspections of the planting and soil to ensure it is growing properly, isn’t bare and isn’t compacted. Any remedial work should occur as soon as possible.

 

Reference:

• Hydrologic Response of Solar Farms, Lauren M. Cook, S.M.ASCE1; and Richard H. McCuen, M.ASCE2, 2013