Hydromorphological and ecological impacts of water injection dredging in the River Parrett, Somerset Levels, Somerset, UK [Poster]

The Somerset Levels (‘The Levels’ hereafter) are a low-lying and flood-prone agricultural landscape in South West England, UK. The Levels drainage network is heavily managed for navigation and flood relief purposes and there is a legacy of dredging, to help mitigate flood risk. Despite widespread application of the technique internationally, knowledge of the short to medium-term effects of dredging in the wider ecosystem remains limited. Furthermore, the potential impacts of new dredging technologies, including water injection dredging, have rarely been considered or quantified, nor have the effects of these operations on instream biota. A programme of work therefore investigated the hydromorphological (bathymetry, bed material grain size, water physicochemistry) and ecological (macroinvertebrate, diatom, fish) effects of water injection dredging temporally and spatially within the River Parrett, Somerset. Hydromorphological and ecological sampling occurred at a control site (upstream of the injection site), within the managed area (injected) and downstream of the dredge site on multiple occasions before and after dredging. Additionally, during dredging, the sediment plume was surveyed via pelagic trawling and captured fish were inspected for mortality or signs of stress or ill health. Approximately 33,900 cubic metres of sediment were mobilised from the injection site, with management activities coinciding with a 39% increase in channel roughness. With regard fish, all 236 individuals captured during the dredging operation were alive, showed no obvious signs of dredging-induced stress and were returned unharmed post-processing. There was a significant decrease in fish abundance following operations at the injection site, with mean values declining from 15 to 3, possibly due to fish avoidance of the injection site. No other parameters or biotic indices were significantly affected at the dredge site. Studies such as these that utilise robust experimental designs and consider a range of aquatic organisms are integral for understanding the factors that contribute to successful restoration and flood risk management and form an important bridge between academic research to underpin practice.