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Does sediment supply impact the threshold for initial sediment motion in natural, gravel bedded streams?
Date Permissions Signed
Date of Award
Department or Program Affiliation
Master of Science (MS)
Mitchell, Robert J. (Geologist)
Yager, Elowyn M.
Sediment transport in river channels control channel morphology, streamflow, and benthic ecosystems. Predicting sediment transport rates through a channel is required for sediment management for stream restoration and aquatic habitat assessment. The critical Shields stress (τ*c), is a dimensionless parameter used in sediment transport models that characterizes the river bed surface shear stress required to initiate sediment motion. The 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. Recent studies have explored bed structure (e.g., protrusion, intergranular friction), sediment supply, and stress history (i.e., the sequencing of high flow events) as potential controls on τ*c scatter. To my knowledge, no study has connected all three factors in a field setting, nor taken flow conditions (e.g., streams with similar hydrograph) into account when exploring the relationship between τ*c and sediment supply between sites, making it difficult to discern if differences in τ*c are attributed to sediment supply or the stream channel stress history.
To determine if τ*c varies systematically with sediment supply, I used two independent field methods to estimate τ*c in 12 different rain-dominated streams in northwestern Washington State, USA, with either a low or high sediment supply. The first method uses a hand-held force -gauge to measure the normalized resisting force (a proxy for τ*c) and grain protrusion (height of a grain above the mean bed elevation). The normalized resisting force (normalized Fr) is converted into τ*c estimates using a model. The second method estimated τ*c from the movement of spray-painted rocks (tracer particles) after a storm event for three long-term study sites with a low and high sediment supply. The results suggest no systematic or statistically significant relationship between sediment supply, bed structure (protrusion), and τ*c in natural streams with a similar stress history. This is surprising since prior field and flume studies have found a systematic relationship between sediment supply, τ*c, and protrusion. The lack of observed relationship between sediment supply and τ*c may suggest that other physical parameters have more control on τ*c, or the variation in morphologic and hydrologic characteristics between watersheds is greater than expected and obscures the relationship between sediment supply and the threshold for motion. Furthermore, this study found that normalized Fr varies systematically with protrusion, with higher protrusion leading to lower resisting force, consistent with findings from recent studies. While τ*c does not systematically vary with sediment supply in this data set, findings support the idea that bed structure acts as an important control on the threshold for motion.
Western Washington University
Subject – LCSH
River sediments--Washington (State); Sediment transport--Washington (State); River channels--Washington (State); Fluvial geomorphology--Washington (State)
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Loucks, Emily, "Does sediment supply impact the threshold for initial sediment motion in natural, gravel bedded streams?" (2023). WWU Graduate School Collection. 1151.