30 Causeway, Lakeside
We were commissioned to prepare a Flood Risk Report (FRA) to support a permitted development application for three buildings located at 30 Causeway in Staines.
The site currently comprises three large office buildings, and associated car park areas. The proposed works involved the conversion of the three office buildings from office to residential use. The site is located in Flood Zones 2 and 3a, with fluvial flood risk associated with the River Thames. In addition to the FRA report, we prepared a Flood Management Plan for the buildings, detailing the actions to be taken following receipt of a flood warning and the flood evacuation route.
Key details
Project name
30 Causeway
Client
Stonegate Homes
Location
Staines
Services provided
- Hydrology
- Drainage
Challenge
It is a requirement to provide safe access and egress for all new residential developments for a 1 in 100-year fluvial event, including allowances for climate change. Whilst the site meets this, the proposed safe route would fall within the flood outline for the 1 in 100 plus 35% climate change event.
Initially it was proposed to install a flood level gauge within the boundary of the site and to evacuate the occupants of the buildings when the flood level exceeds the critical level, which is 300mm below the maximum predicted flood level in the area for the 100-year event. The Lead Local Flood Authority (LLFA) did not accept this approach and requested a demonstration of safe access and egress route with “very low” hazard rating during the 1 in 100-year plus climate change flood event.
The area around Staines War Memoria, Flood Zone 1, l was proposed as a congregation point. However, the LLFA officers did not accept this solution since the area is in the dry island surrounded by areas which were in Flood Zone 2 or 3.
Solution
We carried out a topographical survey of the Causeway along the proposed evacuation route in order to precisely reflect the details of the terrain. From comparing the modelled flood levels with the ground levels we estimated the predicted flood depth, as well as the flood velocity results. This allowed us to demonstrate that whilst sections of the proposed evacuation route were within the floodplain, the flood hazard was predominantly “very low” with a couple of short sections being at “low” hazard with flood depth of below 200mm.
In order to demonstrate that the proposed congregation point is at safe location, we analysed the modelling flood level time series results at two structures; one near the location of the proposed local gauge and the second at the High Street along the War Memorial where it passes under the railway. The results indicated that there were 30 hours difference between the point in time when the flood level will reach the evacuation trigger level at the local gauge at the site, and the time when the flood level will start to rise at the underpass.
During this period of time, the link between the Staines War Memorial and London Road leading towards the area entirely outside of flood outline will be completely dry and safe for people to walk through. This will provide plenty of time for the building management to initiate an emergency evacuation of the site, firstly through to the War Memorial site and then onwards as necessary.
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