About the Project
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 function1. In recent decades, however, land use intensification and change has significantly increased the supply of FGS to many streams and rivers2,3 with deleterious effects on water quality, riverine ecology and the provision of ecosystem services (Figure 1). 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 communities3,4.
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 Farming5 and Water Friendly Farming6 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 mixtures7 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 substrates8
References
1Owens, P.N. , Batalla, R.J., Collins, A.J., Gomez, B., Hicks, D.M., Horowitz, A.J., Kondolf, G.M., Marden, M., Page, M.J., Peacock, D.H., Petticrew, E.L., Salomons, W., Trustrum, N.A. (2005). Fine-grained sediment in river systems: environmental significance and management issues. River Research and Applications, 21, 693–717, doi: 10.1002/Rra.878.
2Evans, R. (2006). Land use sediment delivery and sediment yield in England and Wales. In Owens, P.N. and Collins, A.J. (Eds) Soil Erosion and Sediment Redistribution in River Catchments : Measurement, Modelling and Management. CAB International, Wallingford, 70-84.
3Stone, P. & Shanahan, J. (2011). Sediment matters: A practical guide to sediment and its impacts in UK rivers. Environment Agency, Research Project No: SC070024.
4Wood, P.J. & Armitage, P.D. (1997). Biological effects of fine sediment in the lotic environment. Environmental Management, 21, 203-217, doi: 10.1007/s002679900019.
5Natural England, (2015). Catchment Sensitive Farming: reduce agricultural water pollution. https://www.gov.uk/catchment-sensitive-farming-reduce-agricultural-water-pollution.
6Biggs, J., Stoate, C., Williams, P., Brown, C., Casey, A., Davies, S., Grijalvo Diego, I., Hawczak, A., Kizuka, T., McGoff, E., & Szczur, J., (2014). Water Friendly Farming. Results and practical implications of the first 3 years of the programme. Freshwater Habitats
Trust, Oxford, and Game & Wildlife Conservation Trust, Fordingbridge.
7Wilcock, P.R. & Crowe, J.C. (2003). Surface-based transport model for mixed-size sediment. Journal of Hydraulic Engineering, 129, 120-128, doi: 10.1061/(ASCE)0733-9429(2003)129:2(120).
8Kuhnle, R.A., Wren, D.G., & Langendoen, E.J. (2015). Erosion of sand from a gravel bed. Journal of Hydraulic Engineering, doi: 10.1061/(ASCE)HY.1943-7900.0001071.
9Cui, Y. (2007). The unified gravel-sand (TUGS) model: Simulating sediment transport and gravel/sand grain size distributions in gravel-bedded rivers. Water Resources Research, 43, W10436, doi:10.1029/2006WR005330.
10Detert, M. & Parker, G. (2010). Estimation of the washout depth of fine sediments from a granular bed. Journal of Hydraulic Engineering, 136, 1615-1626, doi: doi.org/10.1061/(ASCE)HY.1943-7900.0000263.
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