Understanding the influence of slope on the threshold of coarse grain motion: Revisiting critical stream power

This paper investigates the slope-dependent variation in critical mean bed shear stress for coarse grain motion, and evaluates stream power per unit bed area as an alternative threshold parameter. Explanations for observed slope dependency and existing approaches for predicting the critical stream power per unit bed area are reviewed. An analysis of secondary bed-load transport datasets is used to examine the strength of associations between stream power per unit bed area, mean bed shear stress and mean velocity, with bed-load transport rate. Data from an original flume study are combined with secondary data from similar flume experiments to investigate the effect of slope on both critical stream power per unit bed area and critical mean bed shear stress. Results suggest that stream power per unit bed area is most closely correlated with bed-load transport rate, and also that critical stream power per unit bed area is less variable with slope than critical mean bed shear stress. Alternative solutions to approximating critical stream power are explored. These include: (1) modifying existing expressions for critical stream power to account for higher critical mean bed shear stresses at higher slopes, and (2) applying a constant dimensionless critical stream power criterion.