- 5th February 2018
- Posted by: Joanne Woodhouse
- Category: Projects
Our Specialist Flood Risk Services (SFRS), developed with Future Climate Info (FCI), help homebuyers and commercial property purchasers gain a greater understanding of the risk of flooding to a property.
In this example, an FCI report for a bungalow in Chester recommended a further action with respect to surface water flooding. A Report Review, the initial specialist flood risk service, was commissioned to explore this further.
The property is in a predominantly rural area, set back from the road. From online photography, there is no obvious threshold to the house though the view was partially obscured by trees.
Comparison of our surface water flood map (as shown in the FCI report) and the Environment Agency’s map showed slightly different patterns of flood risk. The JBA map indicated a significant flood risk to the house with depths of between 0.3 metres and 1 metre within a 1 in 75-year frequency. The Environment Agency’s surface water map showed some flood water ponding on the road and to the rear of the house during a 1 in 100-year event but no flood risk to the house itself.
Both maps are generated by routing rainfall over a Digital Terrain Model (DTM). DTM data is gathered using aerial survey techniques and then filtered to remove buildings and trees to generate ground levels. JBA and the Environment Agency apply different assumptions when filtering the data which accounts for the different flood outlines at this site. This filtering process can generate artificial dips in the landscape, especially where there is dense vegetation.
Aerial images showed that the flooded areas coincided with the presence of trees and buildings.
While the risk of surface water flooding cannot be totally ruled out we concluded that the predicted flood risk had probably been exaggerated due to DTM filtering. Therefore, the Report Review concluded that the bungalow had a reduced risk of flooding. The client therefore had the option to amend the original FCI flood report to reflect this and deduced that no further property resilience measures were needed at the site.