Flood Risk: 3 house development, North London

We undertook a FloodSmart Plus investigation to assess the nature and potential significance of flood risk to land upon which a three house residential development was being proposed in North London.

The site had been identified as lying within the Environment Agency’s Flood Zone 2 and we contacted the Agency to obtain the exact flood elevations which may impact on the site.

We were able to provide advice on the minimum floor level for the proposed development in order to satisfy the Local Authority that the property will not be damaged by flooding during a river flooding event.

The increase in surface water runoff from the site, due to the impact of the proposed development, was also considered and the flood risk assessment followed the standard outlined in the National Planning Policy Framework (NPPF, 2012).

A review of the data reported that the site had been identified as lying within the Environment Agency’s Flood Zone 2, where the risk of river flooding is between a 1 in 100 years and a 1 in 1000 years event.

In such instances the Environment Agency is contacted to obtain the exact flood elevations from their hydrological model which may impact on the site. The flood elevation was compared with the site topographic data to provide advice on the minimum floor level for the proposed development in order to satisfy the local authority that the property will not be damaged by flooding during a river flooding event.

Additionally, the flooding history showed no reported floods at or within 250 metres of the site, and the site was not at risk due to a breach of flood defences as there were none within the surrounding area. It was also identified that there were no areas within 50 metres of the site that are at risk of groundwater flooding.

The increase in off-site flood risk due to increased surface water runoff from impermeable surfaces was tackled using a Sustainable Drainage System (SuDS), ensuring the existing site drainage had the capacity to manage the increase in runoff.

Suitable SuDS options were recommended in the form of a permeable pavement for the footpaths and driveway, a retention pond or an infiltration/soakaway system.