Investigating step‐pool sequence stability

Abstract

[1] Step-pool units, common features in steep, narrow streams, are highly dynamic systems that adjust during high flows and active sediment transport conditions. Consecutive step pools in a reach form a step-pool sequence, and though these features are prevalent in nature, quantifying the stability of such systems is challenging. This study focuses on the statistical relationships between 445 stable sequences of three or more steps and nine geometric and resistance-based parameters.Step sequence stability is parameterized through a sequence stability parameter, a metric of how long a given step-pool sequence exists relative to the total run time. Despite variability in the strength of the statistical relationships, the results allow identification of dominant trends between sequence stability and both roughness and geometric parameters. The sediment concentration ratio provides a means for comparing the relative strength of these statistical relationships. As sediment concentration ratios increase, the relations between flow resistance parameters and step stability change from dominantly inverse to dominantly direct. Thus, at higher sediment concentration ratios, sequence stability is more likely to increase with flow resistance, but the reverse is more likely at low sediment concentration ratios. A connection exists between bed morphology, flow resistance, and stability. For the majority of stable step sequences, resistance parameters increase over time, indicating some sequences adjust their geometry to increase stability during flood events. The influence of sediment concentration over the stability-geometric and stability-resistance parameter relationships may enable the use of the sediment concentration ratio as a predictor of step sequence stability during a flow event.

Influence of Bed‐Load Transport on Flow Resistance of Step‐Pool Channels

“…For equation (5), the coefficients of the fitted equation deviate also from the original coefficients A = 6.25 and B = 5.75 of Hey (). The slightly superior performance of the VPE is in accordance with previous field and lab studies (Rickenmann & Recking, ; Waters & Curran, ). Evidently, the data for bed‐load feed scatter above the trend lines, indicating a lower flow resistance for the same relative flow depth (Figure ).…”

“…Again, these results are limited to plane beds, uniform, or narrow grain size distributions consisting of sand and gravel fractions. Koll and Dittrich (2001), Waters and Curran (2012), and Johnson et al (2015) shifted research to step-pool channels and observed that an increase in bed-load transport caused bed instabilities in step-pool systems. Waters and Curran (2012) state that high bed-load transport rates led to accumulations of sediment at the step, thus increasing step height, what then led to deeper scour and a steeper step.…”

Testing models of step formation against observations of channel steps in a steep mountain stream

“…Here, however, the interaction of adjacent steps may play an additional role (Waters and Curran, 2012). For example, after exceptional floods that break up the majority of channel steps, we can advance new hypotheses on the chronological development of new channel steps.…”

Temporal variability and memory in sediment transport in an experimental step‐pool channel

“…When these large grains are dislodged, they mobilize significant quantities of sediment which propagate downstream. Step‐pool channels have been studied extensively in recent decades, focusing mainly on channel and step geometry [ Curran and Wilcock , ; Milzow et al ., ; Chartrand et al ., ], hydraulics [ Comiti et al ., ; Zimmermann , ; Wilcox et al ., ], bed morphology [ Zimmermann and Church , ; Weichert et al ., ], step formation, and channel stability [ Zimmermann et al ., ; Waters and Curran , ].…”