SEDIMENT SUPPLY IN NATURAL RIVERS: IMPACTS ON GRAIN-PROTRUSION AND THE THRESHOLD FOR MOTION?

Critical Shields stress (τ*_c), is a dimensionless parameter that characterizes the riversurface shear stress required to initiate sediment motion. τ*_c is typically assumed constant in transport models, yet compilations of field data have shown that τ*_c can vary wildly, causing sediment transport models to over- or under-predict fluxes by an order of magnitude or more. Understanding the processes driving this variability in τ*_c is essential to improve model performance. As a result, recent studies have explored bed structure (i.e., protrusion, intergranular friction), sediment supply, and stress history (i.e., the sequencing of high flow events) as potential controls on this scatter. While promising, to our knowledge, no study has connected all three factors in a field setting, nor taken flow conditions (e.g., a snowmelt- vs rainfall-dominated hydrograph) into account when exploring the relationship between τ*_c and sediment supply at various sites, making it difficult to discern if differences in τ*_c are attributed to sediment supply or the stress history.

To determine if τ*_c varies systematically with sediment supply, we measured the normalized resisting force (a proxy for τ*_c) and grain protrusion (height of a grain above the mean bed elevation) in 12 different rain-dominated streams in northwestern Washington State, USA, spanning a range of low, medium, and high sediment supply. Our initial results suggest that the relationship between normalized resisting force, sediment supply, and protrusion is quite complex, with protrusion and resisting force varying substantially between sites in the same sediment supply category. We find no statistically significant difference in grain protrusion or normalized resisting force between sediment supply categories, suggesting that sediment supply may not act as a systematic control on τ*_c. However, normalized resisting force does vary systematically with protrusion, with higher protrusion leading to a lower resisting force, consistent with findings from recent studies. While τ*_c does not systematically vary with sediment supply in this initial dataset, our findings support the idea that bed structure acts as an important control on the threshold for motion and should be accounted for in sediment transport models.