A Design Procedure for Sizing Step-Pool Structures

Thomas, Dave; Abt, Steven R.; Mussetter, R. A.; Harvey, M.

doi:10.1061/40517(2000)340

Cited by 14 publications

(10 citation statements)

References 12 publications

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“…The manually constructed steps were completed to mimic the naturally interlocked steps by three arrangements: (1) large stone with b-axis of 103 mm was put in the middle of the step as the keystone (KS), as suggested by Thomas et al (2000); (2) the jammed state, which was considered as the dominant mechanism for step-pool formation in narrow stream sections (Church and Zimmermann, 2007;Zimmermann et al, 2010;Golly et al, 2019), was created inside the step model (Zhang MORPHOLOGICAL AND HYDRAULIC ADJUSTMENTS OF STEP-POOL UNIT et al, 2018), with the jamming ratio = 4.85; and (3) grains sized of 30 to 45 mm (30-45% comparing with the KS) were imbedded at the step toe to provide support to the step stones from downstream. By the first two arrangements, we combined and applied the KS and jamming model for step formation in the step model (Golly et al, 2019).…”

Section: Experimental System and Step-pool Modelmentioning

confidence: 99%

Experiment on morphological and hydraulic adjustments of step‐pool unit to flow increase

Chendi

Hassan

et al. 2019

Earth Surf Processes Landf

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Exceptional flood events with a return period of about 50 years can be destructive to step‐pool channel segments. However, field investigations and flume experiments have not examined the hydraulic and morphological feedbacks of step‐pool morphology during unsteady hydrographs of exceptional flood events. We performed a series of flume experiments with a manually constructed step model, perturbed with three hydrographs that varied in the rate of water supply change. The bed texture, topography, flow regimes, surface flow field and water depth were characterized and measured as the flow rate was increased during the experiments. A distinct pool feature emerged downstream of the manually constructed step when the flow rate exceeded the threshold scaled to the peaks of ordinary flood events in well‐graded mountain streams. The pool feature was modified in several different ways with flow rate increase. The bed surface steadily coarsened, micro‐bedforms developed and became more pronounced, the bed topography became more spatially complex based on analysis using the Hurst exponent, and last, pool depth steadily increased. Pool modification was also linked to the flow regime: the impinging jet regime led to grain size segmentation in the pool while the jump regime contributed to decelerating flow velocity. The steeper rising limb of hydrograph led to a less developed pool feature, with smaller sized micro‐bedforms in the pool bottom to outlet, and higher discharge threshold for distinct coarsening and scouring in the pool. The estimated energy dissipation within the step‐pool unit decreased as a power function from low to high flow, quantified as the ratio hc/HS, where hc is the critical water depth and HS is scour depth. Our results highlight the interaction between morphology, hydraulics, and energy dissipation of step‐pool unit and the crucial role of hydrograph shape on the interaction during flow increase © 2019 John Wiley & Sons, Ltd.

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Section: Experimental System and Step-pool Modelmentioning

confidence: 99%

Experiment on morphological and hydraulic adjustments of step‐pool unit to flow increase

Chendi

Hassan

et al. 2019

Earth Surf Processes Landf

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“…In contrast to transverse structures, several authors specify step orientation as oblique, 70-80 degrees (Grant et al, 1990), or diagonal (Zimmermann and Church, 2001). Grant et al (1990) and Thomas et al (2000) observed that some steps form V-shaped patterns pointing upstream. Hayward (1980) found that boulder steps are sometimes arranged in a curved line across the channel.…”

Section: Step-pool Geometrymentioning

confidence: 99%

Bed morphology and generation of step–pool channels

Weichert

Bezzola

Minor

2008

Earth Surf Processes Landf

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Flume experiments have been carried out to study the formation processes and the bed morphology of step-pool channels. From the experiments different step types and step configurations could be distinguished depending on the stream power. These step types can be seen as an image of the generation mechanisms of step-pool systems. These results suggest that the bed roughness geometry develops towards a condition that provides the maximum possible bed stability for a given grain size distribution. In contrast to a variety of other studies, antidunes did not contribute to the generation of the step structures. However, the data of the presented study fits well into the region of antidune formation proposed by Kennedy for sandbed rivers. This observation points out that step-pool field-data located in the Kennedy region do not inevitably prove that antidunes played a role in step development. It is rather proposed that in Kennedy's region of antidune formation there exist hydraulic conditions where the flow resistance is maximized. It is suggested that such maximum flow resistance is associated with an optimal distance between the bedforms and their height, independently of whether these are antidunes in sand-and gravel-bed rivers or step-pool units in boulder-bed streams. The considerations of the Kennedy region of antidune formation and the analysis of planform step types depending on stream power both suggest that steep channels have a potential for self-stabilization by modifying the step-pool structure towards a geometry that provides maximum flow resistance and maximum bed stability.steeper and flatter sections are denoted as riffle-pool sequences and are often associated with bar features. As width is often constricted in steep open channels, riffles and pools in connection with alternate bars tend to be absent from or poorly developed in boulder-bed streams. Nevertheless, bar features as well as meandering tendencies have also been described for boulder-bed channels (Leopold et al., 1964).In the present paper the results of the flume study are presented that focus on bed morphology and the generation of step-pool channels. In contrast to a number of other flume studies, the generation and configuration of steps and pools was studied with regard to its dependence on flume width. R. B. Weichert et al.

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“…Furthermore, the importance of granular interactions for step formation and stability has been previously recognized (Church and Zimmermann, 2007;Saletti et al, 2016) and it has been suggested that steps are more stable than predicted because of the emergence of force chains in the transversal direction that keep them in place even when subjected to higher shear stress (Bouchard et al, 2001;Church and Zimmermann, 2007;Saletti and Hassan, 2020a). The basic questions of how and where steps form and under which conditions remain stable is paramount for practitioners who are often asked to design steps (or similar structures) to stabilize steep channels while maintaining their ecological value and visual appeal (e.g., Chin et al, 2009;Thomas et al, 2000). Existing design criteria consider only flow variables (e.g., flow rate and flow depth), grain size, and channel geometry, ignoring factors that might strongly impact the stability of artificial step pools (e.g., sediment supply and longitudinal width variations).…”

Section: Introductionmentioning

confidence: 99%

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

Saletti¹,

Hassan²

2020

Earth Surf. Dynam.

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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.

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A Design Procedure for Sizing Step-Pool Structures

Cited by 14 publications

References 12 publications

Experiment on morphological and hydraulic adjustments of step‐pool unit to flow increase

Experiment on morphological and hydraulic adjustments of step‐pool unit to flow increase

Bed morphology and generation of step–pool channels

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

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