Flood Risk Assessment – Harrow

Southwest Environmental Limited have recently prepared a Flood Risk Assessment for a site in London.

The site was in flood zone 1, and was under the area (1 hectare) that would ordinarily trigger the requirement for a Flood Risk Assessment.

It was our opinion that the Flood Risk Assessment was requested because the site is:

• A risk from surface water flooding.
• Includes a basement. 

Surface Water Flow Velocity

Surface Water Depth

The report recommended that as long as a few simple measures were adopted such as:

There is surface water flooding highlighted as close to site of the extension and basement. Careful consideration will have to give to the avoidance of flow path creation. For example: the ground level at the rear of the building should be maintained so as to prevent surface water flowing towards the rear of the house. This would include depressions created for external stairways. An up an over stair crest may be required to maintain this level.

Removable flood boards should be fitted in all external doorways, up to a height a 50cm above threshold. Levels (front threshold + 50cm) should be matched at rear of property for stair crest.

Flood resilient construction should be used in basement level (see below table). Clear access should maintain from basement to ground floor level. New External and Internal Doors should hinge inwards.

Centralised service nodes such as boilers and consumer units should be situated on the ground floor level.