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Flooding Report - Raising the Site Level by 300mm

Raising the Site Level by 300mm

The developer has recognised that this area is subject to extensive flooding, as he has proposed raising the development site by at least 300mm to attempt to provide some protection to the new homes.

The developer has also proposed a flood relief channel, quote “To minimise the flood risk of the culverts on the northern boundary of the Site a flood relief channel is proposed, near to the northern boundary. The developer has managed to quote 3 different sizes for this channel in 3 different documents.

Design and Access Statement : size:- 3.5m wide with no depth or side slope.
Environmental Statement: size:- 4.0m wide x 0.75m deep with 1 in 2 side slope.
Flood Risk Assessment: size:- 4.0m wide x 0.75m deep with 1 in 4 side slope – a mathematical impossibility.

Taking the largest case, the total capacity of this channel along the northern boundary is less than 1,000m3. This would fill within 7 minutes with the floodwater from Ash Lodge Drive/Loddon Way and in possibly only 3 minutes with the 100 year +30% event.

If the developer raises the site level by at least 300mm the surface flooding from the north would not be able to overflow into the fields and properties along Ash Lodge Drive, Kennet Close, Loddon Way, Colne Way, Lea Close, Wandle Close, Southlands Close and Southlands Road will be extensively flooded. This is in direct contravention of the requirements of NPPF TG clause 2 which states that any development must not increase the flood risk elsewhere.

Raising the development site by 300mm would actually raise most of the site to a higher level than Ash Lodge Drive and Loddon Way. Using the developers topography survey the addition of 300mm would mean the site would be 200mm higher near Loddon Way, 50mm higher near the entrance to FP350 and 40mm higher near the Beverley Close end. This is a sure recipe for increasing the flood risk, and existing properties along Ash Lodge Drive, Kennet Close, Loddon Way, Colne Way, Lea Close, Wandle Close, Southlands Close and Southlands Road will be extensively flooded.

Depending on the rainfall it is likely that an additional 100 existing residential properties would be subject to internal flooding as a direct result of raising this development site by 300mm. Note that in all the flooding incidents so far the rainfall has only been a fraction of the 100year +30% event.

Flooding from the North (Ash Lodge Drive has overflowed into field parcel 1)

Another view of Ash Lodge Drive overflow into field

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