Application of a regional procedure to assess the risk to fish from high sediment concentrations

Periods of high suspended sediment concentration in rivers can affect the behaviour, health and habitat of freshwater fish. A simple regional model relating daily mean suspended sediment concentration in rivers to flow and percentage of cropped and urban land has been applied to the Lower Swale, UK. The model uses a GIS catchment coverage of hydrological response units (HRUs) of area 5-8 km2, each of which is assumed hydrologically independent and drains to an identified river reach. The sediment delivery properties of each HRU are a function of its land use characteristics determined from existing databases, and hydrological conditions determined from effective rainfall measurements. Daily mean suspended sediment concentrations for each river reach were derived by cumulating simulated HRU flows and sediment loads down successive reaches. A map of the modelled 10% quantile sediment concentrations is presented for both HRUs and river reaches. The map is substantially different from existing erosion risk maps because the delivery model is driven by flow in addition to land use, rather than by soil type. Frequency curves showing the exceedance probability of mean simulated sediment concentrations over 1-6-day durations have been derived using peaks-over-threshold techniques. Maps of the risk of harm to fish, based on the probability of 1- and 4-day sediment concentrations exceeding 80 mg l(-1), are presented. Such maps can be used to assess the impact of sediment on fish and for setting appropriate river water quality objectives.