Experimental Insights Into the Threshold of Motion in Alluvial Channels: Sediment Supply and Streambed State

Abstract

Multiple bed surface characteristics, including surface grain size distribution (GSD), grain protrusion and surface roughness, and development of coarse-grain clusters, respond to water and sediment supply in ways that generally enhance bed stability. As the sediment supply decreases, the gravel-bed surfaces coarsen and bed stability increases (Dietrich et al., 1989; Gessler, 1970; Nelson et al., 2009). This armoring (surface coarsening relative to the subsurface or the sediment supply) occurs when transport capacity exceeds sediment supply, resulting in an immobile armored bed surface. Armoring can also reflect a bed adjustment so that the GSD of sediment transported out of a reach matches the sediment supply GSD from upstream, producing a mobile armor (e.g., Parker & Klingeman, 1982; Temple & Wilcock, 2005). Early work on the organization of gravel beds showed that the process of bed coarsening is accompanied by the formation of structural features, including clustering of the relatively large and less mobile grains that further reduces the overall mobility of the bed surface sediments and increases roughness (e.g., Brayshaw, 1984; Brayshaw et al., 1983). A wide variety of bed structures is now known to occur that form coherent patterns of clasts in gravel-bed rivers (see review in Venditti et al., 2017). These features are larger than individual clasts and generally smaller than channel-scale features (e.g., bars or step-pool features). The bed features include clusters (e.g.

Effect of stress history on sediment transport and channel adjustment in graded gravel-bed rivers

“…Previous researchers (Haynes and Pender, 2007;Masteller and Finnegan, 2017) have suggested that an exponential function can be implemented to describe such a decrease of sediment transport rate under conditioning flow. Additional analysis is implemented in the Supplement to fit REF2 (15) and REF6 (15) (which have the longest duration of conditioning phase) against a two-parameter exponential function.…”

“…To further study such "memory" effects of antecedent flow on the sediment transport during a subsequent flood, a number of flume experiments and field surveys have been conducted in the past decade, and different terms have been proposed, including "stress history effect" (Monteith and Pender, 2005;Paphitis and Collins, 2005;Haynes and Pender, 2007;Ockelford and Haynes, 2013), "flood history effect" (Mao, 2018), and "flow history" (Masteller et al, 2019). The difference in the terminology could be partly due to the available data and the chosen approach in different research works.…”

“…They found that with a longer and higher antecedent flow, the critical bed shear stress increases and the total bed load flux decreases. The work of Paphitis and Collins (2005) was extended by Monteith and Pender (2005) and Haynes and Pender (2007) to consider bimodal sand-gravel mixtures. They found that for a graded bed, longer periods of antecedent flow increase bed stability due to local particle rearrangement, in agreement with Paphitis and Collins (2005), whereas higher magnitudes of antecedent flow reduce bed stability due to selective entrainment of the fine matrix on bed surface, counter to Paphitis and Collins' (2005) conclusion based on uniform sediment.…”

“…They found that for a graded bed, longer periods of antecedent flow increase bed stability due to local particle rearrangement, in agreement with Paphitis and Collins (2005), whereas higher magnitudes of antecedent flow reduce bed stability due to selective entrainment of the fine matrix on bed surface, counter to Paphitis and Collins' (2005) conclusion based on uniform sediment. Haynes and Pender (2007) further analyzed the two competing effects and concluded that particle rearrangement may be of greater relative importance than the winnowing of the fine sediment, as it affects subsequent sediment transport. By using high-resolution laser scanning and statistical analysis of the bed topography, Ockelford and Haynes (2013) also demonstrated that the response of bed topography to stress history is grade-specific: bed roughness decreased in uniform beds but increased in graded beds with an increase length of an antecedent flow period.…”

“…Research to date has shown that antecedent flow can stabilize the river bed, thus influencing the threshold of sediment motion and bed load flux. However, most of the previous research about stress history is either under conditions with relatively low sediment transport or with relatively short durations of sediment transport in order to capture the threshold of sediment motion (Monteith and Pender, 2005;Paphitis and Collins, 2005;Haynes and Pender, 2007;Ockelford and Haynes, 2013;Masteller and Finnegan, 2017;Ockelford et al, 2019). On the other hand, other researchers have found that exceptionally high-discharge events can reduce critical shear stress by disrupting particle interlocking and breaking of bed structure (Lenzi, 2001;Turowski et al, 2009Turowski et al, , 2011Yager et al, 2012;Ferrer-Boix and Hassan, 2015;Masteller et al, 2019).…”