Agriculture fine sediment :sources, pathways and mitigation

Abstract

This project utilises study sites in slowly permeable soils with artificial tile drainage typical of the lowland study region in the North East of England. The three selected sites investigate sediment loss processes at a range of scales, from the field, to catchment. Sediment monitoring and in a 0.6 km2 farm ditch catchment indicated that the majority of sediment losses occur during a small number of large runoff events. Source fingerprinting suggested that over 50% of the sediment is derived from arable tile drains, and approximately 10% from the farm track. A small outlet sediment trap was partially effective, with 37% sediment removal on average. It is estimated that a trap of 100-200 m3 volume may be effective for a catchment of this size. Detailed plot study of tile drain sediment losses under winter cereals showed conventionally tilled land generated mean sediment losses of 333 kg.ha-1 over a single winter. Over 80% of total sediment loss was via tile drain rather than surface flow. The use of a shallow cultivation minimum tillage system significantly reduced mean sediment losses to 183 kg.ha-1. Buffer strip treatments were not effective, particularly as they were unable to mitigate the majority of sediment loss occurring through the subsurface pathway. At the catchment scale the importance of topographically controlled concentrated runoff pathways was recognised as a key sediment loss issue. Channel erosion sediment losses of up to 750 kg.ha-1 were estimated for one field in the catchment during a single runoff event. The retention of c. 1 t of sediment was recorded during the same event in a field with an engineered flood retention barrier. A novel field method of event sediment loss estimation is presented. An investigation of a wider catchment area demonstrated that tile drainage exceedance during flood events is a major source of field erosion and sediment loss. While surface runoff events are infrequent in this landscape, they are of high magnitude and the relatively small capacity of the drainage network results in concentrated flows of high erosive potential. The effectiveness of buffer strips is likely to be limited in these landscapes due to the role of concentrated runoff and subsurface sediment pathways. Grassed waterways and in-field soil conservation techniques are likely to be more effective against these two pathways respectively. Engineered structures also appear to be effective, and appear to be most justified where runoff control is also desirable.