Natural Flood Management - Flood Mitigation

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The key project requirements were as follows:

• Build a fully distributed basin scale model of the Bydemill Brook; including soil parameters that have been calibrated to SPRHOST for observed rainfall events as well as baseflow.
• Use surveyed cross section data to develop the 1D component of the model in ESTRY
• Model all design options outlined in proposal, including a baseline scenario;
• Provide an assessment of the most effective NFM measure according to the project objective.

A suite of potential NFM measures were supplied and include:

1. Addition of a meander upstream of the culverted reach,
2. Daylighting the existing culverted channel and replacement with a meandering open channel,
3. Combination of Options 1 and 2
4. Incorporation of an engineered option which combines a meandering reach upstream of cross section 12 with the further addition of a 10m culvert through a new earth bund and spill to constrict flood flows in order to reduce flood risk downstream, giving rise to a new flood storage area.

Model results upstream of the railway are shown in figure 1 below.

Key challenges encountered during the modelling process include the options provided by the client providing minimal benefit to a reduction in peak flows to the main railway line. This provided the opportunity to develop a range of alternative natural flood management options that were more effective.