Stress History Effects on Graded Bed Stability

Abstract

This paper presents the first detailed examination of the dependence of graded bed stability on antecedent flow conditions (stress history). Thirty-three experiments, including repetitions, were undertaken where a bimodal sediment bed ( D50=4.8mm , σg=2.1 ) was conditioned for between 30 and 5,760min . The antecedent shear stress ranged from 53 to 91% of the critical shear stress for the D50 of the sediment bed. Data indicate that higher antecedent shear stresses reduce bed stability due to selective entrainment of the fine matrix; conversely, extending the duration of the antecedent conditioning phase increases bed stability due to local particle rearrangement. Analysis of the competitive effects indicates that particle rearrangement may be of greater relative importance than compositional change. Overall, the paper demonstrates the significance of antecedent flow conditions for hydraulic engineering and research, including the modeling of bed-load transport during flood events and the need for standardizing the flume-based experimental procedure.