RMA worked alongside Flowers & Hayes Ltd and Della Valle Architects, to prepare a Flood Risk Assessment and Surface Water and Foul Drainage strategy for a residential development of 46 dwellings on land to the north of Walrow in Highbridge, Somerset.

The EA’s flood map for planning showed that the site is located within Flood Zone 3 (high risk) and is at risk from fluvial and tidal flooding.   However, the site was shown to lie within an area benefitting from River Parrett Estuary coastal flood defences as well as natural earth banks located along the River Brue.

Comparing the topographic survey to the modelled fluvial in-channel flood levels provided by the EA concluded that the site was elevated significantly above than the 0.1 % AEP fluvial flood level and it was therefore concluded that the site would not flood during a defended fluvial scenario.

In-channel fluvial flood levels for the River Brue in an undefended (or breach) scenario were assessed.  This confirmed that the site was located at an elevation that is 1.95 m above the 0.1% AEP modelled water level and therefore the site was not at risk of flooding from a fluvial breach event during the 0.1% AEP event or any of the analysed flood events.

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 0.5% AEP event and is provided protection from the River Parrett Estuary tidal flood defences.

From reviewing Level 2 SFRA mapping for the area, it was concluded that the site would remain free from flooding for all modelled tidal breach scenarios.  This is largely due to the railway embankment located to the west of the site which acts as a secondary defence during a breach scenario.  An assessment was undertaken on the structural integrity of the railway embankment in relation to its function as a secondary flood defence.   This assessment confirmed that with the implementation of Network Rail’s Asset management policy, the railway line is maintained to a good standard with a low likelihood of failure. Therefore, the proposed development will be protected and free from flooding during a tidal breach event for its operational lifetime.

The undefended scenario was also assessed and consultation with the EA confirmed that, should the future worst-case 0.5% undefended scenario occur, then a safe refuge for future occupants within all the proposed dwellings will be provided at first floor level.

It was concluded that the proposed development passes the Sequential Test based on a review of reasonably available alternative sites within the agreed search area of Burnham-on-Sea and Highbridge.

It was proposed to discharge surface water runoff via an attenuation-based system at a limited greenfield rate to the Worston South Culvert located along the eastern boundary of the site.  Attenuation on site would be provided through a combination of geo-cellular storage tanks and flow control devices within the parking spaces and hardstanding areas of the site.

Consultation with Wessex Water confirmed that the public foul sewer that runs through the centre of the site has sufficient capacity to accommodate the proposed wastewater discharges from the proposed development.

The 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 following no objection from the EA or LLFA; further details of this application can be found using the planning reference: 11/19/00128.

Project Manager – Rob Murdock

Technician – Melissa Seymour