Transport Characteristics of Tile-Drain Sediments from an Agricultural Watershed

The use of tile drains for subsurface drainage in agricultural watersheds has created concern for the delivery of sediment to receiving waters and potential undesirable effects on surface and subsurface water quality. In this study, transport characteristics of sediment from tile drains in an agricultural watershed of the Thames River, near Kintore, Ontario, Canada were tested in a 5 m diameter, rotating circular flume located at the National Water Research Institute in Burlington, Ontario. Tile drain sediments were collected and mixed with river water at different speeds in the flume to study transport processes such as deposition, erosion and flocculation as a function of bed shear stress. During deposition and erosion experiments, water samples were collected to determine changes in the concentrations of cations, anions and dissolved organic carbon. The results show that tile drain sediments have a tendency to flocculate when subjected to a range of shear stresses. The median diameter (D₅₀) of the floc size distribution reached a maximum value at a shear stress of 0.169 Nm−2 which can be considered an optimum shear stress for flocculation for this sediment. The critical shear stress at which all of the initially suspended sediment deposited to the flume bed was measured as 0.056 Nm−2. The pH and cation concentrations remained relatively constant during erosion and deposition experiments. Anion concentrations were more variable, most likely due to the presence of bacteria which could have also played a role in the flocculation mechanism of tile drain sediment.