- 22nd September 2017
- Posted by: Joanne Woodhouse
- Category: Projects
We undertook a Flood Risk Assessment (FRA) for a proposed residential development outside Gainford, County Durham. The St Peter’s School site lies on the northern side of the River Tees and consists of a range of buildings that have been empty for some time.
The Environment Agency’s Flood Map for Planning showed the site was in Flood Zones 2 (the 1,000-year flood outline) and 3. However, the mapped outline contained an unrealistic straight edge immediately to the west of the St Peter’s School site. For the development to progress it was necessary to challenge the Flood Zones.
Revised Flood Zones were produced for the St Peter’s School site. The outlines were produced by:
- Building a HEC-RAS model of the River Tees based on LIDAR data
- Calibrating the HEC-RAS model to an accuracy of less than 0.2m using surveyed wrack marks from a flood event in January 1995.
- Checking the sensitivity of the HEC-RAS model to increased roughness and raised downstream boundary conditions
- Plotting flood outlines using peak levels simulated in the HEC-RAS model.
The finalised model showed the St Peter’s School site is raised at least 1.3m above the one in 1,000-year flood outline. We are therefore confident that the St Peter’s School lies inside Flood Zone 1.
The Flood Zones were the major blocker to progressing the site development. The study used freely available LiDAR data to challenge the existing Flood Map. This provided a cost-effective solution to overcoming the Flood Zone hurdle.
Our client now has confidence to progress other aspects of the site development process. The subsequent flood risk assessment has been reduced to a surface water management study with no need to undertake the Sequential Test.