Experimental study of sediment supply control on step formation, evolution, and stability

Abstract

Abstract. We present results from an experimental campaign conducted in a steep flume subject to longitudinal width variations and different sediment feed rates. The experiments were designed to study how sediment supply influences step formation, step location, and step stability. Our results show that steps are more likely to form in segments of the channel where the width narrows because of particle jamming, and these steps are also more stable. Sediment feed increases particle activity which generates a dynamic channel morphology with steps forming and collapsing. A comparison with experiments without sediment feed shows that sediment supply does not inhibit step formation. Time series of step formation, evolution, and destruction show that the maximum number of steps is achieved when the sediment feed is larger than zero but smaller than the transport capacity. We summarize this outcome in a conceptual model where the dependence of step frequency on sediment supply is expressed by a bell curve. Sediment yield measured at the channel outlet followed the sediment feed at the inlet closely, even when we fed 50 % more and 50 % less than the calculated transport capacity. This outcome challenges the applicability of the concept of transport capacity to steep channels and highlights the key role played by sediment feed in dictating sediment yield and channel response. Finally, we detected a positive correlation between sediment concentration and step destruction, which stresses the importance of particle interactions for step formation and stability.

On How Episodic Sediment Supply Influences the Evolution of Channel Morphology, Bedload Transport and Channel Stability in an Experimental Step‐Pool Channel

“…The experiments were conducted in a 5 m long and 8% steep flume (Figure 1a) in the Mountain Channel Hydraulic Experimental Laboratory at The University of British Columbia. We created systematic longitudinal width variations by inserting six concrete elements in the flume to obtain wide, narrow, widening, and narrowing segments (the same flume geometry used by Saletti and Hassan (2020a, 2020b). The wide sections were 0.4 m wide, the narrow sections were 0.2 m wide, and these sections were separated by narrowing and widening segments, resulting in an average flume width of 0.31 m (Figure 1a).…”

Experimental insights into the effect of event sequencing and sediment input texture on step‐pool channel evolution

“…The feed rate was 129 g/s/m for R1 and R9, 172 g/s/m for R2 and R8, 258 g/s/m for R3 and R7, and 516 g/s/m for R4 and R6 (Figure 1b). These values represented respectively 1.5, 2, 3, and 6 times the reference transport capacity (86 g/s/m) used in the constant sediment supply experiments by Saletti and Hassan (2020a) under the same flow rate of 36 L/s/m in the same flume. The reference transport capacity was estimated using Wilcock and Crowe (2003) applied to the bulk GSD.…”

“…The experiments were conducted at the Mountain Channel Hydraulic Experimental Laboratory at The University of British Columbia in a 5‐m long flume with a slope of 8% (Saletti & Hassan, 2020a, 2020b; Wang et al, 2021). The flume was artificially narrowed using concrete blocks to create longitudinal width variations, with 0.4‐m wide segments and 0.2‐m narrow segments separated by narrowing and widening segments, resulting in an average flume width of 0.31 m. The initial bed had a 15‐cm deep layer of poorly sorted sediment (i.e., with grain size between 0.5 mm and 64 mm, Figure 1a), with a median size of the bed material , and geometric standard deviation .…”

“…Channel stability in steep streams is also enhanced by bed structures and bedforms such as step‐pools (Chin, 1989; Chin, 1998; Montgomery & Buffington, 1997), which are influenced by sediment supply (Recking et al, 2012; Saletti & Hassan, 2020a). Low sediment supply or sediment starved conditions are favorable for step‐pool formation (Grant et al, 1990) and stability (Church & Zimmermann, 2007; Comiti & Mao, 2012).…”

“…Field evidence (Recking et al, 2012) also suggests that step‐pool channels directly receiving sediment supply from lateral hillslopes are less stable than similar channels disconnected from sediment sources. More recently, flume experiments under constant sediment feed conducted by Saletti and Hassan (2020a) showed that an average value of sediment supply (greater than zero but below transport capacity) maximized step frequency. Previous studies have mainly focused on the effect of sediment supply magnitude on step dynamics and channel stability, whereas the effect of episodic sediment supply regimes on the overall adjustment of the step‐pool channel system remains to be explored.…”