ABSTRACT
Enhanced levels of fine sediment entering our rivers have several negative impacts on the functioning of the system. The recent flooding of the Somerset Levels, UK in the Winter 2013/14 highlighted the potential impacts of reduced channel capacity as a result of sedimentation. Over 6500 hectares of agricultural land flooded for approximately 54 days. Fi-ne sediment accumulation in river channels has also been found to degrade fish spawning and invertebrate habitats. Nutrients and heavy metal contaminants are often associated with fine sediment, which reduces water quality. Increases in sediment delivery to river channels from changes in land use and cli-mate in the future are projected to increase the severity and occurrence of these issues.
Catchment managers need to put into practice measures which will reduce the quantity of fine sediment that is delivered to river channels. To do this they need to know where the sediment is coming from i.e. source and either prevent it being eroded or disconnect the pathway between source and channel.
The River Eye, Leicestershire is a typical UK lowland sand bed river with relatively homogenous geology. The 180 km2 catchment is dominated by agricultural land use on heavy clay soils. The town of Melton Mowbray centered within the catchment has a long history of flooding. In 2002, the Melton Mowbray Flood Alleviation Scheme was implemented to reduce flood risk in light of the 1998 flood event. Online silt traps were installed to reduce fine sediment deposition within the town. Since installation, fine sediment is still a persistent problem for catchment managers within the River Eye, though its sources are unknown.
22 suspended sediment samplers were deployed throughout the catchment at strategic locations (upstream and downstream of the silt traps, confluences and Brentingby Dam) to identify which locations were delivering the most sediment. These devices were regularly emptied and analyzed.
The silt trap situated on the main channel was monitored to investigate its influence on local and reach scale flood risk. Four monitoring devices were installed to continuously measure river stage. These combined results provide an insight into the success of silt traps as natural flood defenses. Location of Silt Traps in the River Eye Catchment. https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315623207/4fbc492d-6678-4a12-aaf6-5c2b8ea38e5f/content/fig17_1.tif"/>
To determine the sources of fine sediment in this catchment, the approach of sediment fingerprinting was used. Originating in the 1990’s, this technique has seen substantial recent attention, which start to question some of the basic assumptions behind the general approach, such as quantifying the uncertainty in the predictions, the non-conservativeness of some of the sediment properties, and the validity of correction factors and weightings.
The sampling strategy has been carefully de-signed to account for all the known land uses and at different levels of complexity. Standard laboratory tests were carried out on both source and channel samples, to distinguish between different land use categories in seemingly homogenous geology. This process provided an extensive database for the statistical assumptions of the sediment fingerprinting approach to be questioned and to identify sources of fine sediment. This knowledge can then be used by catchment stakeholders i.e. Environment Agency to inform flood risk management.
