Hobson's Conduit Trust

A successful new desilting method

In 2021 the section of the Conduit in front of the University Botanic Garden was partially de-silted by Miles Water Engineering, and the silt was used to remake the bank where some 400 tonnes of steel piling were inserted to stop leaks that were damaging the Botanic Garden. 

In early 2022 we embarked on an experimental de-silting of the section from the Trustees’ Memorial Bridge, (where much of the flow to Vicar’s Brook leaves the Conduit over a spillway), down to the Accordia Bridge.

This experiment was successful and proved the technique.   Our contractors are SP Landscapes, who already had a contract for weed and vegetation clearances.  Their 1.8 tonne Hitachi excavator can fit along the path, without needing formal closure of the path, and by using a Youngman moveable bridge and wheelbarrows, their team are able to de-silt a narrow channel some 50 cm by 50 cm and place the silt mainly onto the Eastern bank.  This gives the natural flow a ‘helping hand’, and promotes further scouring of the Conduit where depths of over 1 metre of silt had accumulated in many places.

The Trustees have been looking for several years for a suitable method to enable de-silting to be carried out.  The constraints of lack of space and the poor load bearing capacity of the ancient bank structure, together with the complexities of disposing of the silt removed, have together made this a very challenging project.

The depth of anaerobic silt in the Conduit creates both poor habitat for flora and fauna including fish, and had reduced the depth of the water to an unhealthy degree.  The resulting lack of freeboard also brings the water level up towards the top of the Conduit bank, where it is most porous, resulting in the potential for leaks, particularly across and under the path along the Conduit into the allotments.

Our time window for activity is limited by bird nesting, and we stopped work at the end of March, finishing close to the Brooklands Avenue bridge.

We carried out water vole surveys in both 2021 and 2022 before commencing de-silting, with known vole burrows flagged and avoided.

We will resume the de-silting process in winter 2023/24 in specific locations, but we have now completed an initial de-silting and channel cutting along the Conduit from the Memorial Bridge down to the Botanic Garden inlet near to Bateman Street.

The bed of the Conduit increasingly includes traces of sand and gravel that are attracting fish.  Natural continuation and extension of this improvement is of course dependent on there being sufficient volume and velocity of flow.  

There are a number of places along the path where the bank has been eroded, and is insufficiently high.  A serious leak in winter 2021-22 caused flooding of several allotments.  We are therefore undertaking a programme of bank repairs.

As a priority task this Spring we commissioned a bank repair at the Zig of the Zig Zag by the Bench.  This involved insertion of plastic piling 1.8 metres long, reinforced by wooden stakes, and backfilled with puddle clay to make the repair impermeable.  The bank at this curving section, where time had taken its toll on earlier attempts at reinforcement, had virtually disappeared, and erosion was threatening further flooding.  A sinkhole had appeared on the path.  The method was an experiment, but so far has been judged a success.  The piling was made from recycled plastic.

We have also carried out bank repairs either side of the Memorial Bridge using ‘Soluform’ bags.

We will be progressively using both methods where each is best suited to deal with the various locations identified along the path by the allotments where erosion and inundation of the bank is evident.

We are fortunate that SP Landscapes have been able to commit to these projects.  Their team is fully experienced and very meticulous.  We have been grateful for continuing support from Cambridge City Council officers, and from MKA Ecology who carried out the water vole surveys.

With our Volunteers, the Trustees have planted snowdrops and English bluebells, together with a number of native perennials along the bank to seek to encourage invertebrates, as well as to bring an attractive increase in variety to the marginal plants for the pleasure of users of the path.  These plantings are experimental but will be ongoing.