SRA Annual Report 2024-25: A38 Blackbird Bends
An A38 Blackbird Bends Catchment Assessment: Hydraulic Modelling and Options Report was completed in September 2024. Funded by Somerset Rivers Authority, this report was commissioned by Somerset Council from consultants WSP.
Its purpose: to study why part of this road between Taunton and Wellington – one of the busiest and most important roads in Somerset – is so prone to flooding. It is, for example, expected to flood in the kind of rainstorm that has a 50% chance of happening every year.
The diagnostic picture is complicated by catchment modifications made by people since at least the late 19th century (such as weirs and sluices), farming practices (such as livestock grazing), and coarse geological features (such as pebbles and cobbles).
However, the main causes of flooding around the A38 bridge near the garden centre are now well understood, thanks to this new report.
Put simply, channel capacity downstream of the bridge is limited by several factors, notably a build-up of materials that enables agricultural vehicles to move between fields. This means less water can flow downstream of the bridge, so it backs up, and there’s a rise in water levels upstream of the bridge and along about 280 metres of the Hockholler Stream. This rise leads to overtopping at a low point along the right bank of the Hockholler Stream, and water then flows through the garden centre towards the A38. Further upstream, Hockholler Stream overflows on its left bank. Haywards Water mostly overflows on its right bank. In bigger storms, these flows combine and directly flood the A38 bridge.
WSP propose several ideas for reducing flooding. These include building a flood embankment, increasing capacity under the bridge, removing channel sediment, creating a bypass channel, and creating storage areas to hold back water upstream.
The SRA funded de-silting under the bridge in 2024-25, as a temporary measure to increase its capacity. Somerset Council is considering future possibilities, which will depend upon a range of factors such as funding and co-operation with landowners.