The Dynamics of Glacial Till Erosion: Hydraulic Flume Tests on Samples from Medway Creek, London, ON

English Abstract:

The erosion of till material from the river bank of Medway Creek in London, Ontario was studied to determine the erosion mechanisms and critical shear stress of the till, and to understand how the alluvial cover, particularly the gravel particles, impacts the erosion process. Samples were collected from Medway Creek and were tested under a unidirectional current in a hydraulic flume at McGill University under a unidirectional current. Samples were tested under three separate sets of conditions: samples at their natural moisture content in clear flow conditions, air-dried samples in clear flow conditions, and samples at their natural moisture content with large gravel particles present in the flume. The two latter tests were performed to determine any effects that weathering and the presence of alluvial material may have in the erosion process. The results show that mass erosion was the dominant form of erosion, occurring around natural planes of weakness and irregularities, such as gravel particles, within the material. The critical shear stress was observed to be approximately 8 Pa. The effect of drying on the erosion process was extreme – the critical shear stress dropped to below 1 Pa and the structure of the cohesive material disintegrated. The presence of gravel particles led to increased surface erosion due to impacts and a more rapid progression of the erosion. In summary, the till erodes around planes of weakness and irregularities, till exposed to a wetting-drying is most at-risk of erosion, and the presence of gravel does not provide a protective layer over the till and instead lowers the critical shear stress.