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Club News

New stadium: flood risk?

22 January 2015

An update on meeting flood risk requirements

 Following the release of the Environment Agency’s report on the joint Galliard and AFC Wimbledon planning application, we are publishing a short article explaining the issues in more detail and how they have been satisfied.

As regards the additional information and detail requested by the EA, our consultants believe they will be able to resolve the outstanding concerns. We welcome the EA’s response to the plans, which was published yesterday, because it has confirmed that there are no issues of principle which remain to be addressed.

Executive summary

When the development is complete it will be safe for occupants:

•  The flood risk does not preclude development of the site.

•  The flood risk will be no greater that it is at present.

•  Existing problems with surface water will be reduced.

•  The measures to manage and reduce flood risk have been discussed in depth with the Environment Agency and satisfy the requirements of planning policy.

1. Location of the development site within a flood zone

The development site lies within an area designated by the Environment Agency as Flood Zone 3a and 3b, based on the flood plain maps for the River Wandle. The area is also at risk of surface water flooding and is highlighted as a Critical Drainage Area by the London Borough of Merton. However, this does not in itself preclude any development on the site, provided certain conditions are met, which they have been.

Within planning policy there is applied what is known as a Sequential Test. Essentially, this asks whether there are any alternative, lower-risk sites where the proposed development could take place. If there are none, the test is passed, regardless of the proposed site being a flood zone. This test is applied either at the strategic site allocation stage or at the application stage, not both.

In the present case the test was applied at the former stage, when the site was allocated for sporting intensification with the potential for supporting residential development. The Sequential Test has therefore been passed, and there is no requirement to apply it again at the time of the planning application. This position has been agreed by both the London Borough of Merton and the Environment Agency.

2. Flood Risk Assessment

Planning policy also requires that any development in an area where there is a flood risk must pass the Exception Test, which sets out that the proposals should not increase the risk of flooding in neighbouring properties, and should preferably reduce that risk, and also that the development should be safe for its future occupants. A Flood Risk Assessment for this site has been produced, and has been considered by the Environment Agency and the London Borough of Merton. The assessment deals with flood water storage, surface water drainage and safety.

Its main points are as follows:

•  The site is at risk of flooding from the River Wandle and currently suffers from inadequate surface water drainage. However, the site has not flooded from the river since 1968.

•  To avoid any floodwater being displaced to neighbouring properties, the Environment Agency requires that calculations are done to show that the current capacity for storing floodwater on the site is maintained by the proposed development. The proposed scheme will use areas below buildings to store floodwater, and the required calculations have been made and agreed in principle with the Environment Agency. There is, therefore, no increase in flood risk to neighbouring properties.

•  To reduce the potential for surface water flooding, the current rate of run-off needs to be reduced by the proposed development. The proposed scheme significantly reduces the rate of surface water run-off, which in turn reduces the existing risk of surface water flooding. There is, therefore, a reduction in flood risk to neighbouring properties.

•  To ensure that the new residential properties do not flood, they will be raised on podiums to a level above the 1-in-100 year flood level, with an allowance for climate change. None of the new properties would flood during this large flooding event.

•  For the vast majority of the time, access to and from the proposed development would be unaffected by floodwater. At the peak of any large flooding event the neighbouring road network would be impassable for 6 to 12 hours, during which time access and egress would be managed by the emergency services. A flood management plan is being prepared in conjunction with the emergency planners from the London Borough of Merton.

•  The stadium itself will have its own emergency plan to manage the safe evacuation and dispersal of fans during a match. If there is any significant risk of a major flood event occurring, any scheduled match would be postponed.

In their initial response to the FRA, the Environment Agency have not raised any objections in principle to the proposed approach, but have requested additional detail to ensure that any consented scheme will provide the maximum benefits to existing and future residents. We anticipate that we will be able to resolve the outstanding concerns.

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