BWB Consulting - Integrated Engineering and Environmental Consultants

Digital Elevation Map

Modelled Floodplain Depth Contours

Following the imposition of planning objections to a proposed Regional Control Centre development, on the grounds of flood risk, BWB Consulting Ltd was appointed to investigate the flood risk in greater detail and coordinate consultations with the Environment Agency.

Indicative floodplain mapping available from the Environment Agency and access to their one-dimensional model data, suggested that the site was located within the 100-year floodplain of the River Trent. However, due to the complexity of the floodplain and the need to prove the adequacy of emergency access and egress routes over a significant area, BWB Consulting recommended a two-dimensional hydraulic analysis of the catchment to gain a more accurate representation of the floodplain extents and the flood risk.

BWB Consulting´s Water Division worked closely with the Environment Agency to define the scope of the modelling exercise to ensure that the model results would be acceptable. The Water Division constructed a Digital Elevation Map (DEM) using Environment Agency LIDAR data comprising approximately 10km of the River Trent, River Derwent and associated floodplain.

Using the DEM and specialist analysis software TUFLOW our hydraulic modelling team were able to analyse the distribution of flood flows, breaching the defences of the rivers, across the floodplain for extreme storm events.

From this analysis it was established the site was naturally defended from flood flows by elevated ground features within the floodplain. This information was presented to the Environment Agency and independently audited. Consequently the Environment Agency removed their holding objection and planning permission was granted.

The advantages of 2d modelling are:

• More realistic and potentially less onerous than extrapolating 1d channel flood levels into the floodplain.
• Provides an understanding of depth and velocity of flow in the floodplain, to check on access and egress routes.
• Excellent tool for flood defence breach analysis enabling definition of sudden inundation zones and quantifying the extent of the area affected by a breach.
• Powerful GIS and animated results presentation assists in demonstrating the true level of risk to regulators and managing the degree of conservatism applied to the ´precautionary approach´.

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