The proposed site for Old Hayes Solar Farm is in the floodplain of the River Cole.
The floodplain performs an important water storage function, with implications for the River Thames c3km downstream to the north. The water quality and flow of the River Cole and its three tributaries which flow across the site, and the hydrology of the floodplain, are likely to be impacted by the proposed Solar Farm.
Flooding is of two types, River Flooding and Surface Water Flooding. Both occur here. New maps show the extent of flooded land on this proposed site, with projections for increased flooding to 2040-2060.
Surface Water Flooding is likely to be dominant.
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Surface Water Flooding is rainwater which sits on the land and cannot infiltrate or flow off. New maps showing both River and Surface Water Flooding have recently been published and are available on www.gov.uk/check-long-term-flood-risk
The maps show Flood Zones 1, 2, 3a and 3b. Flood Zone 1 has low probability of flooding; Zone 2 has medium probability and Zone 3a high probability. Zone 3b is a Functional Floodplain, storing water from rivers during flooding or allowing water to flow through in periods of flood. Much of the land in the proposed site is in Flood Zones 2 and 3.
The published maps also project the likely scenario for future flooding in 2040-2060. Not surprisingly, due to climate change, the flooded area will increase.
Flooded sites are not suitable for a solar farm for many reasons. Indeed the whole site was under water or waterlogged for 4 months in the 2024/2025 winter.
There are many impacts of a Solar Farm on river, stream and floodplain hydrology. The result an include severe reduction in the water-storage function of a floodplain, and damage to the soils.
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A list of possible impacts of a Solar Farm on river, stream and floodplain hydrology include the following:
- A large expanse of impermeable surface created by the solar panels would intercept rainwater and hence soil and watercourse flows.
- In summer low-rainfall or drought periods this reduction in water reaching the ground and infiltrating will be particularly acute; soil-drying has been recorded under panels in summer.
- New drainage ditches would probably be necessary, again affecting soil moisture and watercourse and river flows.
- Access tracks with hard surfaces would change natural water movement pathways.
- The effect on soil hydrology by the installation of thousands of panel mountings inserted 3-4m into the ground is probably not well studied, but could dry out the soil by speeding transfer of water down the soil profile.
- A well-rooted grassland capable of intercepting pollutants and storing carbon could take a long time to establish on this site – possibly many years.
- Vegetation growth can be poor in the shade beneath solar panels, resulting in poor organic matter delivery, poor soil carbon and increased compaction (Carvallho et al, 2025. https://eprints.whiterose.ac.uk/221644/ ). These factors affect water movement through the soil.
- Construction is likely to yield excessive amounts of silt and fine-particle sediment which would impact the water quality in the river and streams. While the water quality is not at present good, it could be expected to decline further.
- There may be extra demand for water for cleaning the solar panels and site maintenance.
All these factors would compromise the water-storage capacity and full functioning of this important floodplain – with consequent downstream impacts on the River Thames, as would the very major disturbance to the whole site during construction.
‘Standard practice’ when preparing a Solar Farm site results in topsoil loss, compaction and sometimes use of gravel and/or geotextiles, greatly reducing the ability of the site to absorb storm water.
On flooded sites solar panels may have to be raised up to 4m above ground, with major visual impact on the landscape.