Hobson's Conduit Trust

De-silting the Conduit

For some time the Trustees and those who take a close interest in Hobson’s Brook and Conduit have been concerned about the volume of silt that has accumulated downstream from the Memorial Bridge where the main spillway to Vicar’s Brook takes off a large portion of the flow from Hobson’s Brook.

Achieving a reduction in the volume of silt is particularly challenging because access to the Conduit is very constrained. Removal of a portion of the silt was achieved in 2020 along the Botanic Garden bank. The silt extracted was used to backfill the trench created to take the steel piling inserted along much of the bank to stop the leaks from the Conduit through the bank into the Garden.

Further upstream we have only the relatively narrow margins of the Conduit available to take the silt, and otherwise we would need to transport the silt off site. This is made more difficult because we need to remove water from the silt before it can be taken away, and despite a sustained effort to find an acceptable approach to de-watering, this remains elusive. We continue to investigate how this could be achieved without the use of potentially noxious chemical flocculants. Limited space precludes natural drying of the volumes of silt envisaged.

The Trustees have also been concerned not to cause any unnecessary disruption to the ecosystem. The dimensions of the Conduit mean that the smallest amphibious machinery available would only just fit within the Conduit’s margins.

Recent work on bank repairs by the Memorial Bridge has shown that a small 1.8 tonne Hitachi excavator can operate from the path, and with the use of wheelbarrows and a moveable temporary ‘bridge’ it is possible to remove a worthwhile volume of silt and place some of it onto the eastern bank. Some silt was used to complete bank repairs and these areas will soon be planted with carefully chosen native species.

We have therefore decided to continue with this experimental silt removal method, and in February we aim to excavate a channel some 50 cms wide and 40 cms deep between the Memorial Bridge and the Accordia Bridge.

The objective will be to improve flow, and encourage the higher flows that are experienced in spring and early summer to scour more silt. The initial experimental silt removal has already started the scouring process downstream from the Memorial Bridge. We have reopened the penstock close to the Conduit Head which takes water from the Conduit down Fen Causeway to Coe Fen. This has improved the flow along the Botanic Garden and Brookside, and will enhance the water habitat on Coe Fen.

We have been advised by the City’s Biodiversity Officer, and we will cease work in time for the bird nesting season.

If February’s experiment is successful, we intend to resume silt removal using a similar method in late autumn and winter 2022-23 with the object of achieving a similar impact in the section between the Accordia Bridge and Brooklands Avenue. Because the path along this section is relatively narrow, a path closure will be required, but the designated footpath around the Empty Common allotments will be available as the alternative route.

One of the consequences of the build-up of silt is that the bed of the Conduit has been raised, and the water level is therefore higher, which contributes to leaks. Other consequences are the shallow depth of water and slow, sometimes imperceptible flow. This is not conducive to a good, rich and diverse habitat, but it is likely that some invertebrates benefit from shallow water.

We are therefore aiming to achieve a balance between different depths of water, and open up a main channel along the Conduit that will make the principal flow both deeper and faster.

The whole Conduit between Brooklands Avenue and the Memorial Bridge has been expertly surveyed for the presence of water voles by MKA Ecology Ltd. Currently the water voles appear to occupy several locations from the Memorial Bridge to just north of the Accordia Bridge. We are therefore using a de-silting approach that will minimise interference with the water voles’ habitat.

Cambridge City Council are supportive of this programme, and we are grateful for the enthusiastic interest and support that we have received from the Accordia residents, and the Empty Common allotment holders and community garden.

A further update will be posted in February once the current work is complete, and again later in the year once our programme for the sections of the Conduit downstream from the Accordia Bridge has been settled.