SRA Annual Report 2023-24: Dredging and silt monitoring
In January 2024 a new combination of dredging methods was successfully tried out along parts of the River Parrett between Burrowbridge and the M5 bridge. This activity was organised for Somerset Rivers Authority by the Parrett Internal Drainage Board (IDB). In selected spots where build-ups of silt were judged to be the biggest nuisance, excavators scooped sediments from the upper bank of the river (called its shoulder) and put them down into the path of a Water Injection Dredging (WID) vessel working in the river (pictured below). The WID vessel used powerful jets of water to agitate and disperse the sediments placed in its way, so they were washed out on the tide.
This dredging was done because the Parrett, as a tidal river, gets enormous amounts of sediments coming in from the sea. Vast quantities also enter from the Parrett’s catchment (which is 770 square kilometres, or 478 square miles).
The build-up of sediments in the Parrett reduces the river’s cross-sectional area and hence its effectiveness as a flood channel. The removal of consolidated sediments reduces flood risks for people, properties, roads and land, particularly in the area around Moorland which was so badly hit in 2014.
More than eight years of SRA-funded experimentation and knowledge-building went into the combined dredging trial of January 2024. It sought to blend the most useful elements of several important strands of work.
Firstly, silt monitoring twice a year since 2015. SRA partners have amassed a detailed panoramic picture of how sediments come, go and get to stick around in the tidal section of the Parrett between Burrowbridge and the M5 bridge. Thus trends can be observed and target areas selected.
Secondly, the SRA has funded various techniques of dredging with long-reach excavators (on the Parrett and elsewhere), both for major re-profiling of river banks through so-called pioneer dredging and for less drastic works of maintenance dredging designed to preserve channels’ capacity to convey water.
Thirdly, the SRA has also funded various different techniques of Water Injection Dredging, working through the Parrett IDB with contractors Van Oord. Water Injection Dredging is much cheaper, much quicker and much less disruptive than traditional dredging using excavators. Water Injection Dredging targets what is known as the thalweg of the river, the central part of its lower channel, six metres wide in the case of the Parrett. Scientific studies in 2016 and 2017 found this method to be effective in removing sediments, also that it could lead to unwanted build-ups on river banks being washed away through a process of steepening, slumping and natural scouring. However, it had become increasingly noticeable over the years that silt and vegetation (including trees) were encroaching on some upper banks, on the shoulder of the river.
Reviews of data by the Parrett IDB in 2023 found that cross-sectional areas of the river were reducing over time, even with annual Water Injection Dredging. Working with partners and contractors (Van Oord, WM Longreach), the IDB therefore devised and implemented the targeted fusion of methods described above.
It was seen to produce improved results: partners invited onto the dredging vessel during the works were unanimously impressed. However, the full extent of the works’ success could not immediately be established. This was because post-dredging survey works had to be delayed due to high water levels from March through to May. Lessons learned from the January 2024 trial will be used to inform the approach taken to combined Water Injection Dredging and Shoulder dredging works over the impending winter of 2024-25.