SRA Annual Report 2020-21: A38 Blackbird Bends flood alleviation study

The Blackbird Bends section of the A38 is about half a mile north-east of Chelston, between Wellington and Taunton. Flooding in this area has centred around the Hockholler Bridge. It has fairly often closed one lane of the A38, sometimes both. As the A38 is a busy road, and is used for diverted traffic if the M5 is closed, it is important to keep the road open.

On behalf of the SRA, in 2019 Somerset County Council’s Highways commissioners engaged WSP as consultants to investigate ways of reducing flood risks. It was originally suspected that the infrastructure in place for taking water under the road – the main Hockholler Bridge and two secondary culverts – would prove to be inadequate. In fact the problem has turned out to be less straightforward.

Hockholler Bridge is immediately downstream of the confluence of Haywards Water and Hockholler Stream. WSP collected data about these watercourses and the local area from the Environment Agency, Wessex Water, Skanska and Somerset County Council’s historic flood records, CCTV surveys and Ordnance Survey mapping.

In May 2020 a site visit was made to survey local structures, and new modelling was produced. The modelling showed that there should be enough capacity in the system to prevent the kind of annual flooding that has been occurring. But the survey also found that in practice the system’s capacity was restricted, predominantly by silting-up in Haywards Water and at the downstream side of the bridge. It was deduced that silting-up was the main reason why, during periods of heavy rain, a significant amount of water had not been going under the A38 but overtopping it instead and causing flooding.

What was then less straightforward was trying to establish the cause of the silting-up. The silting-up observed was surprising because the system had recently been cleaned out. Subsequent investigations found that silt was building up and not being washed away because of slow channel flow rates. These slow rates of flow could in their turn have several different interlinked causes and possible solutions.

Next steps are now being considered by the SRA and partners.

A bridge, a channel, with a scour hole between them, and vegetation growing either side of the channel. — A scour hole upstream of Hockholler Bridge, along the A38 at Blackbird Bends near Wellington. Observations by consultants WSP suggest the scour hole is caused by high-velocity flows from two upstream watercourses. The transition from narrow channels to one wider shallow channel, in conjunction with the presence of vegetation, reduces speeds beneath the bridge, resulting in the deposition and build-up of sediment on the downstream side. This sediment deposition then reduces velocities further. This is one factor in a complicated situation.