SRA Annual Report 2019-20: The first scientific study of water injection dredging on the River Parrett
Last year’s annual SRA report outlined some of the fascinating work being done by scientists to assess the effects of water injection dredging works on the River Parrett. We promised that when full studies were published, their findings would be summarised for Somerset readers. Here is the first.
The study and its authors
In March 2020, the Journal of Environmental Management published a 15-page article titled ‘Characterising the geomorphological and physicochemical effects of water injection dredging on estuarine systems’. ‘Geomorphological’ refers to the channel’s physical features (e.g. depth, width, sediment characteristics), ‘physicochemical’ to water quality.
The paper’s authors are Andrew Pledger of Loughborough University, Matthew Johnson of the University of Nottingham, Phil Brewin of Somerset Drainage Boards Consortium in Highbridge, John Phillips of the Environment Agency in Bridgwater, and Sarah Martin and Dapeng Yu of Loughborough University.
The team studied the effects of two water injection dredging trials carried out for Somerset Rivers Authority by the Parrett Internal Drainage Board in October-December 2016 and December 2017.
The trials focused on the reach between Burrowbridge and the M5, and the paper describes the different methods used to collect data during the trials. For example, a Beckman-Coulter LS230 laser particle size analyser was used to perform grain size analyses on sediment samples collected before and after dredging.
The three main aims of the research were – put simply:
- to measure what influence water injection dredging would have on the Parrett’s channel and flow
- to see how long the effects would last
- to assess the impacts of mobilised sediment on places downstream
The short answers are:
- can be more effective than traditional methods of maintenance, potentially better for the environment and cheaper
- around 10 to 12 months, but it varies, so regular work may be required to maintain channel shape
Text continues below photographs.
Why water injection dredging can be “more cost-effective” and “more environmentally sustainable”
Some more detail. A key finding for the SRA is this: water injection dredging (WID) “has the potential to deliver maintenance outcomes in a more environmentally sustainable manner and it is more cost effective compared to bankside excavation and land disposal. Costs can be further reduced through recognition that natural fluvial scour can in some years be more effective in channel maintenance than dredging – thus to reduce costs as much as possible, dredging programmes should be evidence-based and adapt to inter-annual variation in accretion rates. Another advantage of WID over extraction methods is that dredged sediment remains within the system.”
Water injection dredging in the Parrett is focused on the central 6 metres of the deepest part of the channel, called the thalweg zone. Working in this zone to maintain the river’s capacity is better than using excavators to extract silt from riverbanks because it avoids disturbing bankside habitats of significant ecological importance. The article says:
“Findings demonstrate that the WID method was effective in removing the required volume of sediment from the ecologically-poor thalweg without directly impacting the relatively ecologically rich inter-tidal bank face, which is a significant disadvantage of some extraction methods.
“Some localised loss of inter-tidal bank material and bank slumping was noted following both WID trials where the toe of the bank had effectively been over-steepened by the deeper thalweg zone created by WID.
“This was an anticipated outcome and supports one of the aims of the WID trials – that is, the required loss of bank volume is achieved through enhanced fluvial scour processes rather than a more damaging direct impact on habitat and species through physical excavation.”
How long the effects last
Put another way, it makes sense for the SRA to preserve important bankside habitats and target the relatively ecologically poor thalweg zone, minimising the ecological impact of dredging.
Further, by working with not against natural processes, we can produce the results wanted, while accepting that such efforts may need to be ongoing.
The effects of the first trial lasted for less than ten months, the second for over 12 months.
The article states this was “almost certainly caused by differences in fluvial flows and marine sediment delivery”.
Higher flows of water in the months following the second trial meant that less silt was deposited and more was eroded from the bed.
Dr Pledger and his team expect the future to intensify challenges. A warmer climate could reduce summer river flows so more fine sediment accumulates, reducing the channel’s ability to carry water. Fewer, heavier, more intense bursts of rainfall could increase flood risks. Sea level rises could also push normal tidal limits upstream, impacting flows and increasing rates of sediment deposition there.
The article states: “In the context of the River Parrett estuary it is reasonable to assume river base levels may be raised, decreasing stream power in the lower reaches and so, increasing sedimentation there and penetration of tidal flows upstream.”
What effects the building of the Bridgwater Tidal Barrier could have on this process are being separately investigated by the Barrier team.
Long-term management strategies will be needed to deal with any changes in rainfall, sedimentation and river flows. “Furthermore, cost-effective and environmentally sustainable catchment-wide approaches to land management are required to increase field soil-water retention and, reduce fluvial sediment loadings in rivers where this represents a significant source of fine-grained sediment.”
Water injection dredging technology was developed in the Netherlands in the mid-1980s, partly in response to the rising costs of silt disposal. Until now, it has been little studied. However, Dr Pledger and his team intend to publish four more scientific papers. So the SRA’s work on the Parrett is the basis for what will soon (to our knowledge) be the best body of research into water injection dredging in the world.
CONTACT DR PLEDGER BY EMAIL IF YOU HAVE ANY QUESTIONS ABOUT HIS TEAM’S STUDIES, GYAGP@LBORO.AC.UK