Scour in Low Gradient Gravel Bed Streams: Patterns, Processes, and Implicationsfor the Survival of Salmonid Embryos

摘要

A field investigation of scour depth in eleven gravel bed stream reaches between100 and 300 m long, with slopes between 0.001 and 0.01, showed that scour assessments need to consider two distinct bedload transport mechanisms. Substrate disturbance is caused by (1) bedload layer motion, and (2) spatial and temporal imbalances in sediment transport rate. This work evaluates both mechanisms in the context of predicting scour depth and salmonid intragravel survival. Measurements of the maximum depth of substrate disturbed by a moving bedload layer ranged between approximately 1.5 times the 50th (D(sub 50)) and twice the 90th (D(sub 90)) percentile particle sizes of the streambed grain size distribution. The upper bound was also approximately equal to 1.5 times the competent grain size, and became independent of flow strength once the largest particles present were mobilized. Disturbance depth did not increase with bedload transport rate because of large stresses needed to mobilize two or more layers of the bed; surface particles may instead move faster to effect a greater transport rate. Reach-average disturbance depth increases with shear stress primarily because a greater bed area becomes active.