Leafield Stream

Client: Bristol Avon Rivers Trust

Categories: Geomorphology, Natural Flood Management, River Restoration

When: 2018

Location: Corsham, UK

The Leafield Stream is located in Corsham, Wiltshire; which is a tributary of the Bydemill Brook. The river is partially culverted and therefore natural flood mitigation measures including day-lighting of the culvert were provided by the Bristol Avon Rivers Trust.

The project undertook a distributed hydrological model including design rainfall and soil calibration. Soils were calibrated to SPRHOST generated by FEH CD Rom catchment descriptors for the January 2018 event for the gauge at Thingley.

More Details

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.

Figure 1: Peak flows downstream of natural flood management measures for the 4% AEP event.

 

 

 

 

 

 

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.

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