SRA Annual Report 2023-24: Wellhams Brook water storage study
In March 2022, the SRA Board agreed to fund a study assessing the feasibility of creating a 4-acre storage pond on land near Wellhams Mill in the Wellhams Brook catchment between the Houndstone and Lufton side of Yeovil and Martock. Water from Wellhams Brook (8.88km long, catchment size 26.61km2) flows down to the River Parrett.
In autumn 2022, for the SRA, the Farming & Wildlife Advisory Group SouthWest (FWAG SW) appointed consultants JBA to model catchment flows and examine the practicalities, and potential flood risk reduction benefits, of a large new water storage area.
JBA completed their report in November 2023. Instead of recommending one single new facility, capable of holding so much water it would qualify as a reservoir (a classification which imposes strict legal obligations), a series of ponds was proposed. Namely, five storage areas up to two metres deep with a total surface area of 1.94 hectares and a total capacity of around 38,000m2. Estimated cost, including a 60% risk allowance: £734,400. Estimated flood risk reduction benefits: £619,000, rising to £976,642 when benefits to nature were included.
Modelling suggested the five ponds would result in various slight delays to peak flood flows of up to one hour and various small reductions to flood extents in some areas. They would reduce average modelled flood depths in a range of scenarios (from every two-year type floods up to every 100-year type floods) by up to 0.9 centimetres (from 19.2cms to 18.3cms in an every 100-year type flood). In the case of an every five year type-flood they would increase it from 16.2cms to 16.5cms. Figures varied a little if a tilting weir was added to the final pond.
SRA technical group members, in a discussion about this study in March 2024, felt the benefits of creating these five ponds would be marginal. It was agreed that Somerset Council’s Flood and Coastal Management team should look more closely at the modelling and cost-benefit analysis, to assess what should be done next.