Enhancing Straightened River Channels (River Somer)

About the city. Midsomer Norton is an ancient market town located in the Somer river valley. The river flows through the high street and forms a link to the surrounding countryside.

Goal

The aim of the project is to improve an over-wide and heavily silted reach of the River Somer running through Midsomer Norton High Street.

Implementation period. The project was implemented in 2011.

Fact

The reach of the Somer river running through Midsomer Morton had been a concern for many years. The complete absence of vegetation, flooding of the dam and accumulation of large amounts of silt seriously affected the habitat and amenity potential of the channel. This contrasted sharply with the natural brook up and downstream of the city.

Solutions

The new sinuous channel design was constructed by forming berms to create a low-flow channel, with higher flows able to surpass these features. This allowed the necessary power to be provided. The flood relief channel maintained the current level of flood defence on the High Street and allowed for temporary diversion of flows from the reach during construction.

The low flow channel width and the distance between the riffle sequence of the pool were informed by suitable reference reach from the Somer catchment.

Silt accumulations were removed from the channel and stored in place for use as backfill in the new design. Three weirs were removed, retaining only the two sides of each structure that were incorporated into the channel berm. This was done at the request of the Council, who wanted to be able to install temporary boards across the channel to preserve the water depth if severe dry conditions occurred.

The berms were constructed from “white lias” limestone blocks from a nearby quarry. They were placed manually to mark the edge of the new low-flow channel, allowing gaps to be created for existing drainage pipes. The blocks varied in size to a maximum of about 0. 40mx0.3mx0.15m. The planting bays along the edges of the new channel were lined with a heavy coconut fibre and filled with retained silt.

A carefully selected range of local coastal plants such as Brookline (Veronica beccabunga), water forget-me-not (Myosotis scorpioides) and water mint (Mentha aquatic), were purchased from a professional supplier as plugs. Consideration was given to plants whose flowers would be attractive in the urban setting and species which would encroach into the channel to adjust the width, but flatten during flood flows to ensure they would not reduce conveyance capacity. Plants that would enhance the physical integrity of the berm were also selected. A 40-60 mm angular gravel mix from a local supplier was used to finish the top of the berm and create riffles.

Challenges

• The scope of work was limited by several on-site factors. There are culverts at both ends of the High Street. Vertical stone walls line the reach, with various surface water drainage pipes discharging into the channel. The channel is crossed by two small pedestrian bridges and three low weirs. The bed consisted mainly of natural rock with some concrete screed to provide a level surface during the construction of bridge piers and weirs. The pre-restoration reach had a mean water depth of 0.5 m and an average channel width of 4.5 m.
• After implementation, there were several high flow events but the berms were intact and in a good condition.

Team

Midsomer Norton Town Council; B&NES Council; Woodland, Water and Gardens

Timeline

Since completion, many wildlife species have benefited, with otters now inhabiting the site, as well as an increased variety of aquatic organisms, including brown trout. The floods did not cause any damage, as low berms and grassy native vegetation were designed to cope with flooding. The canal is now self-restoring and there has been no more silt accumulation, which has reduced the need for dredging.

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