Scientists study effects of Water Injection Dredging in Somerset

Because nothing quite like it has been done before in the UK, water injection dredging on the Parrett is being studied by scientists from Loughborough and Nottingham universities. Research team leader Dr Andrew Pledger (pictured above right) here outlines some of the fascinating work being done and gives a taster of what is being discovered.

This is an extract from the SRA Annual Report 2018-19.

Background

Dredging of the River Parrett has historically been achieved by mechanical extraction of sediment from the river, using heavy construction equipment such as excavators with grabs, buckets or draglines. Whilst these extraction dredging techniques do increase channel size allowing more flood water to flow through, they also tend to be expensive and disruptive operations that risk ecological damage to river habitats. Somerset Rivers Authority and Somerset Drainage Boards Consortium (SDBC) have therefore been trialling water injection dredging (WID), in an effort to find a more cost-effective and environmentally sensitive method of sediment management in the River Parrett.

What is Water Injection Dredging?

WID is very different to land-based mechanical dredging. It does not remove sediment from the river but instead uses boat-mounted pumps to inject high volumes of low-pressure water into the river bed. This mobilises fine-grained sediments (particularly sand-, silt- and clay-sized particles collectively known as “fines”) that are transported downstream by the flow, away from the areas being dredged. As sediments are not extracted and placed on land, WID is considerably cheaper than mechanical dredging. Being vessel-based, WID also has a limited impact on bankside habitats, especially as water jets are targeted at the river bed, not the banks.

Despite wide application of WID globally, mostly in marine and estuarine environments, very little is known about its effects on ecological communities and on the physical environment (e.g. channel depth, sediment condition and water chemistry where the method is applied). We also know very little about how long the effects of dredging last, especially in a fines-rich river system like the Parrett.

Greater knowledge is important.

It would allow us to manage rivers and estuaries more effectively, so correct methods can be applied at correct times to provide maximum benefits, with negligible impacts on wildlife.

Experiments and ecological surveys

Experiments and ecological surveys were therefore run alongside water injection dredging on the Parrett between 2016 and 2018. Scientific investigations have been led by myself in collaboration with colleagues from Loughborough and Nottingham universities, SDBC, Fishtrack Ltd and a range of other partners. Our aim was to gain a better understanding of the impacts of WID through space and time within the River Parrett. In 2017, experiments sought to quantify the effects of WID on fish, diatom (algae) and invertebrate (aquatic insects) communities, and on water chemistry, channel depth and sediment condition. In 2018, the impacts of WID on fish movements and behaviours were assessed using a range of acoustic fish survey techniques.

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Final reports are being written and will be submitted for publication in peer-reviewed scientific journals (where papers are reviewed/scrutinised by academic peers prior to publication) later this year. Therefore, while we cannot share too much information just yet, we can reveal some initial findings and we will provide non-technical summaries in future SRA publications.

Initial findings

Initial findings suggest WID is an effective sediment removal technique for the surveyed reach of the Parrett between Burrowbridge and the M5 bridge, although its effects can be short-lived, lasting less than 10 months, meaning regular repeat dredging can be necessary to maintain channel depths.

Results indicate the method failed to influence the size of bed sediments and given the highly tidal nature and associated high levels of suspended fine sediment typical of the studied area, WID’s effects on some measured water chemistry parameters were similar to the effects of the tide. For example, turbidity (a measure of the quantity of suspended fine sediment in the water) is naturally high, particularly during high spring tides (when fine sediment is washed upstream from the marine environment), and turbidity peaks during dredging were not dissimilar to those observed during high tides without dredging. This finding is of ecological importance and suggests species naturally present are likely to be resilient to rapid changes in water chemistry and to sub-optimal conditions, as observed during dredging and under high tides without dredging.

Conferences and scholarships

Findings have been presented at several conferences, including the American Geophysical Union General Assembly in Washington DC, and the British Hydrological Society’s 13th national symposium (Hydrology: advances in theory and practice) in London. The research team have gratefully received financial support from a range of funders, including SDBC and the Fishmongers’ Company, in the form of two MSc student scholarships. These scholarships were awarded to Ethan Sylvester and Oscar Newman (Geography and Environment, Loughborough University) who are completing projects on the effects of WID on diatom communities and fish behaviours and movements, respectively.