Cleansing of fine-grained sediment (FGS) from polluted gravel-bed rivers under conditions of reduced sediment supply.

Fine-grained sediment (FGS) comprising material < 2 mm is an important component of natural fluvial systems, influencing a range of hydraulic and geomorphological processes and contributing to ecosystem form and function. In recent decades, however, land use intensification and change has significantly increased the supply of FGS to many streams and rivers with deleterious effects on water quality, riverine ecology and the provision of ecosystem services. The effects reflect issues of both sediment quantity and quality and are therefore wide ranging, impacting on all life stages of fish, macro-invertebrate populations and macrophyte communities.

Under the EU’s Water Framework Directive, UK surface waters have to attain good ecological status by 2027. Attempts to mitigate the ecological impacts of FGS in lotic environments have focused on soil and water conservation though the development of community-based erosion control initiatives as exemplified by DEFRA’s Catchment Sensitive Farming and Water Friendly Farming programmes. Although rarely stated explicitly, the ultimate aim of such initiatives is to reduce the flux of sediment to watercourses, thereby facilitating a process of passive

stream restoration whereby water quality and riverine ecology gradually improve as FGS stored within channel systems are evacuated downstream. However, our understanding of the potential timescales and trajectories of channel restoration is severely constrained by a lack of knowledge concerning processes of FGS transport in alluvial systems. For example, current relations developed to predict the transport rates of sand-gravel mixtures fail to predict sand transport accurately when it is present in the interstices of coarser gravel frameworks and further work is needed to improve model predictions of the depths to which FGS may be eroded from gravel substrates.

The aim of this project is to develop our understanding of FGS transport dynamics in fluvial systems with a focus on understanding the behaviour of surficial and interstitial fines under conditions of reduced sediment supply. This information will be combined with existing knowledge to develop a conceptual framework for understanding potential channel responses to soil and water conservation measures currently being introduced to mitigate the adverse effects of FGS on water quality and riverine ecology.