Cambridge University Botanic Garden, Cambridge City Council and Hobson’s Conduit Trust are working together to repair the leaks in the bank of Hobson’s Conduit that forms the western boundary of the Botanic Garden. Leaks into the Botanic Garden have been causing concern about potential damage, and have reduced the available flow of water to the Conduit Head and further into Cambridge. All the extensive efforts that have been made to stem them have proven fruitless, and therefore a more radical approach has become necessary.
Hobson’s Conduit Trust and other interested parties have become increasingly concerned about the volume of silt that has built up along the Conduit, particularly in its lower reaches. A vigorous and prolonged debate has taken place among the Trustees and, the Trustees’ decision is that a programme of desilting is urgently required. The Trust’s advisors in the City Council agree.
It is unclear when desilting last took place, but there is little doubt that in earlier times the Conduit was cleared on a regular basis, reflecting its importance as a source of drinking water for Cambridge. Research also indicates that the overhanging trees and adjoining foliage that are now a valued and enjoyable element of the scene along the Conduit and Brook were not present 200 years ago.
The consequent build-up of silt is therefore likely to be much more a feature of the past century than those that went before. During the almost unprecedented period of very low flows in the Conduit in the second half of 2019 the level of the silt was such that there was a risk of the Conduit ceasing to carry water. These circumstances were viewed with great alarm both by the Trustees, and by those who love the Conduit.
Working together with the University and the City, Hobson’s Conduit Trust is aiming to make use of the presence of suitable mechanical equipment in the Botanic Garden to clear a channel through the Conduit leaving a ‘V’ shaped profile. A portion of the silt near the surface will be left undisturbed. In this way the flow along the Conduit beside the Botanic Garden should be enhanced whilst to the best extent possible preserving the conditions favoured by invertebrates and other wildlife. The target depth for the desilted channel is 60 cm.
Piling going in May 2020
The method being used by Miles Water Engineering Ltd. (who were responsible for the successful dredging of the Botanic Garden Lake three years ago), to reinforce the bank, is to excavate a trench along the bank into which steel piles at least 3 metres long are being driven. The piling will be covered over, in part by using the silt to be excavated from the Conduit. Re-planting of the bank will be undertaken in due course by the University Botanic Garden. The tops of the piles will be below the level of the bank and will be invisible, and the bank will be re-profiled but will look very similar after the piling is in place.
Work is expected to continue until late summer of 2020. The work programme was originally intended to take place during the winter of 2019-20, but various factors have impacted negatively on timings. Unfortunately the delays will mean some disturbance to invertebrates during their peak activity period, but this should be mitigated by the retention of silt on either side of the channel.
It is intended that water will remain in the Conduit throughout the project, but it seems highly unlikely at present that it will be possible to operate the Runnels in Trumpington Street this year.
Cambridge City Council is participating, including giving environmental advice, and all possible steps are being taken to preserve this historic site’s ecology and heritage.