A Comparison of Time-Induced Stability Differences Between a Framework-Supported and a Matrix-Supported Gravel:Sand Mixture

It is well established that the critical threshold of entrainment decreases rapidly with sand content over the transition from a framework-supported bed to a matrix-supported bed in gravel-sand bed rivers. Also, it is recognized that the proportion of fines in the bed surface may be related to flood history. Seeking to combine this knowledge, recent ‘stress history' experiments have shown that graded beds increase in stability when subjected to prolonged periods of sub-threshold flow durations. Flume experiments are presented that compare fine and coarse grain size distributions response to exposure to two flow phases: (i) an antecedent period of 0, 10, 20, 40 and 60 minutes duration using an applied sub-threshold bed shear stress of magnitude 50% that of the critical entrainment of the D50; (ii) a stability test in which flow was increased to establish the new critical entrainment threshold of the bed. As expected, results indicate that the critical shear stress for the fine distribution was up to 39.8% lower than that of the coarse. However, the fine bed was up to 3.5 times more responsive to stability induced during the flow history period. Discussion focuses on explaining these stress history effects with reference to recent literature. Keywords: Graded, Sediment, Entrainment, Bedload, Stability, Stress History