Bisgood Cottages, Somerset Bridge

Flood Risk Assessment – Bisgood Cottages

RMA worked alongside Polden Planning, to prepare a Flood Risk Assessment for a two new dwellings on land at Bisgood Cottages on Somerset Bridge in Bridgwater, Somerset.

The EA’s flood map for planning for the area showed that the proposed dwellings are located within Flood Zone 3 (high risk) and is considered to be at risk from both fluvial flooding and tidal flooding.   However, the site was shown to be currently defended from River Parrett Estuary defences.

When comparing the sites topography to the modelled fluvial flood levels provided by the EA, it was determined that the proposed dwellings would remain free from flooding during the 1% and 0.1% AEP fluvial events.  During the 1% AEP plus 30% climate change event, the south-eastern corner of the northern plot and the southern plot would flood.  Therefore, level-for-level flood compensation storage was calculated for the areas at risk of flooding.  It was proposed for the area to the east of the northern dwelling to be lowered in 0.2 m increments to provide the flood compensation and this area was proposed to be profiled to allow flood water to freely drain in and out of the floodplain.

With regard to tidal flood risk, data was extracted from the EA’s Wessex North Coast Model (2012). It was concluded that the site would not flood during the current day 0.5% AEP event and is provided protection from the River Parrett Estuary tidal flood defences.

From reviewing The Bridgwater Tidal Barrier – Sites 4 and 5 Residual Flood Risk Assessment, the proposed dwellings would remain free from flooding during the current and future 0.5% and 0.1% AEP events with the construction of the Bridgwater Tidal Barrier in 2027/2028.  This assessment also confirmed that the proposed dwellings would remain free from flooding for all of the modelled breach scenarios.

However, consultation with the EA recommended that a climate change allowance of 1.21 m should be applied to the undefended 0.5% AEP tidal flood level as the River Parrett is tidally influenced in the vicinity of the site and this flood level should be used as a proxy for a breach event of the defences in the vicinity of the site.  However, the likelihood of a breach in the defences occurring adjacent to or in the vicinity of the site is considered to be extremely unlikely.

The Finished Floor Levels (FFLs) of the proposed dwellings were raised to 7.00 mAOD which is above the fluvial 1% AEP plus climate change flood level.   As a precautionary measure, the first floor of the proposed dwellings have been raised to 9.60 mAOD which is more than 300 mm above the 0.5% AEP tidal event for the year 2125.  This will ensure that should sufficient warning of a fluvial event not be available or a breach of the defences occurs, a safe refuge is available within the dwellings.

This FRA demonstrated that the proposed development will be safe and that it would not increase flood risk elsewhere. The development was approved by Somerset Council and the EA and further details on these applications can be found using the planning references: 37/24/00020 and 37/24/00029.