Thresholds of erosion and sediment movement in bedrock channels

Abstract

This study examines the erosive resistance of the discontinuous, loose joint blocks of various sizes that comprise the bed material of the Etobicoke Creek, an incised bedrock channel found in an urbanized setting within the city of Toronto, Ontario, Canada. In order to accomplish this, both field and laboratory experiments were conducted on the bed material found in this channel. The field work involved repeated detailed surveying of the channel bed and estimations of distances of movement of various clasts during many sediment transport events. We found that rates of erosion and/or deposition can be significant during the snowmelt spring events and that there was a greater loss of bed material over the study reach than was gained in several spots, with bed lowering > 0.10 m in some locations. The field work also showed that the distances moved by the rock clasts scaled directly with shear stresses observed during rain events and that average distances of transport tend to be short. The frequency distributions of distance of transport scaled with particle size is log-normally distributed. Flume experiments demonstrated that the clasts offered different levels of resistance to movement, depending on their orientation. The platy nature of the rock fragments, clast orientation, and their imbrication significantly affect the threshold of erosion. The results have significant implications for the understanding of bedrock rivers from a management perspective, especially in the case of those rivers experiencing the effects of urban development in their drainage basins.