Hydrology, Hydrogeology & Flood Risk
The hydrology assessment has reviewed the existing water environment from publicly available data and through consultations with the Environment Agency and Wokingham Borough Council. Rainfall, predicted flood levels, water quality data, abstractions and discharges have been collated from the various sources, and an overview of the underlying soils and geology was undertaken.
The site is located within the River Loddon valley, and includes the floodplain. As such there is a high probability of flooding. According to the Environment Agency website the majority of the site lies within Flood Zone 3, representing 1% annual probability of fluvial flooding. There are numerous drains across the site and areas prone to saturation. A separate Flood Risk Assessment (Appendix 13.1 to the ES) has been undertaken in compliance with Planning Policy Statement 25: Development and Flood Risk (2006).
The development has the potential to generate adverse effects from sediment in run-off from excavations, from changes to the drainage that might affect flooding regime, and from accidental spillages or leaks from construction vehicles that might affect water quality.
To address the potential impacts on water quality, standard environmental procedures and best practice will be followed. Environmental controls will be implemented for the stripping and storage of vegetation and soils during access track, cable laying and turbine foundation construction to prevent significant effects on water quality. Silt traps, settlement ponds (if required), and suitable drainage from swales and gravel surrounds will be implemented across the site. Controlled wash down areas on the site will be used for managing dust and silt from construction vehicles. Spill response measures will be in place and bunded storage areas will be provided for oils and lubricants. Therefore, it is considered that this potential effect on water quality will not be significant.
The Flood Risk Assessment (FRA) shows that 55m3 of attenuation (for drainage) will be required which will be built in the form of gravel surround soakaways to the wind farm elements, such as crane pads, new access tracks and compound buildings, to control and treat surface water run-off. The attenuation has been calculated for a 1 in 100 year rainfall event plus 10% increase in rainfall to allow for climate change, so that run-off is limited to the existing greenfield rates.
The proposed location for Turbine 4 is within the high-risk flood area (Flood Zone 3) of the River Loddon. Turbine 2 is located on the cusp of this area although for the purposes of the assessment has also been considered to be within the high-risk area. The estimated depth of flood waters above ground level in a 1 in 100 year flood event plus 20% (to account for potential climate change effects) at Turbine 4 is estimated to be up to 1.33m, and up to 0.13m at Turbine 2.
Subject to detailed design specifications it has been determined that the total reduction in the flood storage area due to the turbine bases, towers and crane pads might be in the region of 750m3, which in the context of the storage capacity of the wider flood plain is considered by the FRA to represent a minimal displacement of flood water. Nevertheless, indicative calculations have been undertaken in the FRA which shows that there is sufficient area to create the required compensatory storage in the immediate vicinity of these two turbines. These initial calculations will be updated with further detailed calculations during the detailed design stage to confirm both the requirements for and proposed location of compensatory storage. There will also be further, more detailed, consideration of the proposed landform and geotechnical factors.
With the proposed environmental and mitigation measures the assessment concludes that the potential effects will not be significant.