The dredging will help reduce flood risk and will be carried out on the stretch of river between Cromwell Road and New Haven Terrace, including a 200m section between the railway culverts.
Previously Environment Agency officers have explained that no silt has built up in the channel in the last 40 years and was therefore was not increasing flood risk, meaning there was no need to dredge.
However, after looking back even further into the history of the river, they found that some silt had built up between the time the river was last improved in the 1950s and the introduction of modern maintenance techniques in the 1970s.
Adrian Clack, Asset Performance Team Leader said:
We carry out regular reviews to make sure we’re taking the best approach to maintenance, based on all the available evidence.
Dredging where silt has built up will restore the profile of the river and help water flow freely, reducing flood risk.
We’ll also continue our regular maintenance programme and will continue working with our professional partners, who are looking at other ways of improving the river’s condition.
In areas ear-marked for dredging, up to 600mm (2 feet) of silt accumulated before the 1970s. Other parts of the river have seen only about 100mm of silt build-up – approximately the height of a street kerb – and will be cleared by weed-raking.
Weed-raking is a part of normal maintenance that was introduced in the 1970s and prevents further silt from building up in the river.
Other parts of the river have not seen silt accumulate and therefore do not need additional maintenance above the routine weed-raking the Agency already carries out.
Dredging work is expected to start in late autumn and finish in the new year so as not to disturb protected nesting birds and other wildlife.