Scour in low gradient gravel bed streams: patterns, processes, and implications for the survival of salmonid embryos

Abstract

A field investigation of scour depth in eleven gravel bed stream reaches between approximately 100 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 (i) bedload layer motion, and (ii) 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 (D50) and twice the 90th ( D90) 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.Larger magnitude scour depths resulted from three forms of sediment transport rate imbalances. They are, in order of increasing spatial scale: (i) scour and fill of transient, finer grained bedforms located downstream of partial flow obstructions causing differential mobility; (ii) at the pool and riffle scale, where scour depth depends on inter-riffle distances and riffle deposit size and morphology; and (iii) at the reach scale in response to temporal and spatial variability in sediment supply to the channel.Salmonids may have adapted to these processes by burying eggs greater than 2 to 2.5D90 deep, and by constructing redds in locations of the channel least likely to experience significant sediment transport rate imbalances. Adverse effects of floods on intragravel survival may occur indirectly through scour-related fine sediment intrusion, rather than directly through redd scour.The findings suggest that scour depth in spawning beds is controlled strongly by the size and quantity of gravel and cobble in a reach, and weakly by flood magnitude and duration.