Contents Page

Summary of Environmental Effects

Water Environment

Chapter 11: Water Environment of the PEIR assesses the effects of the Project, on all aspects of the water cycle including: flood risk, surface water drainage, geomorphology, water quality, groundwater resources, water supply and wastewater.

A baseline assessment of all sources of flood risk and surface water drainage has been undertaken. The findings are reported in a Flood Risk Assessment in accordance with planning practice guidance and the National Planning Policy Framework. The Flood Risk Assessment considers flood risk to the Project site from all sources, including fluvial, surface water, groundwater, flooding from reservoirs and sewer/ water supply flooding. The assessment is primarily based on site-specific fluvial hydraulic modelling that has been developed by GAL, in partnership with the Environment Agency.

A geomorphological walkover survey of the site study area was undertaken to develop an understanding of channel characteristics on the watercourses which are potentially impacted by the Project.

Gatwick Airport is located in the Thames River Basin District and within the Upper Mole catchment. The River Mole flows through the airport, passing under the existing main and northern runways in culvert. Tributaries of the River Mole, including Crawter’s Brook, the Gatwick Stream and Westfield Stream all run through or adjacent to the Project site.

There are areas classified as being within Flood Zone 3 (areas at risk of flooding in a 1% (1 in 100 annual probability) and Flood Zone 2 (area at risk of flooding in between a 1% and 0.1% (1 in 100 to 1 in 1000 annual probability) within the Project site. These are associated with the River Mole, Westfield Stream, Man’s Brook and Crawter’s Brook on the western and southern sides of the airport and with the Gatwick Stream on the eastern side.

According to the Environment Agency’s Risk of Flooding from Surface Water mapping, surface water flooding occurs in several areas of the airport. Areas at high risk (greater that 3.3% (1 in 30 annual probability) are predominately associated with areas around existing watercourses or drainage features, although there are isolated pockets of high risk likely to be the result of rainfall filling local depressions rather than overland flow paths. Areas at medium risk (between 3.33% and 1% (1 in 30 and 1 in 100 annual probability) are generally small and adjacent to the areas at high risk. A large area at medium risk is located near the River Mole and south of the existing main runway. There are larger areas predicted to be at low risk (between 1% and 0.1% (1 in 100 and 1 in 1000 annual probability) within the airport, particularly to the south of the main runway and in proximity to existing terminal buildings.

British Geological Survey mapping identifies that there is susceptibility to groundwater flooding throughout areas of the Project site underlain by superficial deposits (ie superficial deposits flooding), with a moderate level of confidence. Based on the Crawley Brough Council Strategic Flood Risk Assessment there have been only two occurrences of groundwater flooding recorded in the Crawley area. These are not located near the airport.

In terms of water quality, the River Mole upstream of Horley is classed as ‘Heavily Modified’ with a current potential status of ‘Good’; and overall objective of ‘Good’, as defined by the Water Environment Regulations.

A number of measures have been designed into the Project to reduce the potential for impacts on the water environment. Mitigation measures would include the following:

Potential Significant Effects

Overall, the significance of flood risk effects from the Project on all sources of flood risk has been assessed to be (at worst) negligible or minor adverse and therefore not significant in terms of EIA regulations, taking into account the proposed mitigation measures. The Project would therefore be safe for its users and would not increase flood risk elsewhere. Fluvial hydraulic modelling results for the Project, as shown in the figure below, show that for certain receptors, the Project would result in an improvement in terms of flood risk, which would be a significant beneficial effect in some cases.

Zoom in
Fluvial hydraulic model results (showing the 1 per cent (1 in 100) Annual Exceedance Probability (AEP) event, including a 35 per cent climate change allowance)

There would be very limited adverse effects throughout all phases of the Project. The Project would require modifications to the alignment of the River Mole, including the re-meandering and restoration of the natural channel morphology, and improved channel diversity and floodplain coupling. In the long term this would deliver an overall improvement to the geomorphology of the watercourses resulting in an overall beneficial effect for this watercourse. 

No significant cumulative effects on the water environment have been identified in the assessment.

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