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

Abstract

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.

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

“…It is important to note that sediment feed rate and longitudinal channel width variations are only two of the variables that influence step frequency. Flow rates and the hydrological regime (e.g., Zhang et al, 2019Zhang et al, , 2018, grain-size distribution for the availability of large grains acting as keystones (e.g., Hohermuth and Weitbrecht, 2018), and channel geometry and slope (Chartrand et al, 2011) are also expected to be important controls. Sediment supply can also vary not only with respect to the magnitude but also in terms of duration and frequency (e.g., Hassan et al, 2020).…”

“…Several studies in the last decades have increased our knowledge on how step-pool systems function, especially with regard to the step-forming mechanisms (Chin, 1999;Curran, 2007;Golly et al, 2019;Saletti et al, 2016;Zimmermann et al, 2010), the stability of steps (Waters and Curran, 2012;Zhang et al, 2019Zhang et al, , 2018Zimmermann et al, 2010), the links between channel and hillslope dynamics (Golly et al, 2017;Molnar et al, 2010), and the relations between flow magnitude, flow resistance, and sediment transport (Comiti et al, 2009a;Hohermuth and Weitbrecht, 2018;Saletti et al, 2015;Turowski et al, 2009;Zimmermann, 2010). Field studies and flume experiments highlighted how boulder protrusion (Yager et al, 2018(Yager et al, , 2007, grain clustering (Johnson, 2017), and the supply of fine sediment (Johnson et al, 2015) impact flow resistance and therefore channel stability and sediment transport in step-pool channels.…”

On how sediment supply affects step formation, evolution and stability in steep streams: an experimental study

“…It is important to note that sediment feed rate and longitudinal channel width variations are only two of the variables that influences step frequency. Other variables are expected to be important, such as flow rates and the hydrological regime (e.g., Zhang et al, 2019Zhang et al, , 2018, grain-size distribution for the availability of large grains acting as keystones (e.g., Hohermuth & Weitbrecht, 2018), channel geometry and slope (Chartrand et al, 2011). Sediment supply can also vary not only with respect to https://doi.org/10.5194/esurf-2020-30 Preprint.…”

Small-Scale Flume Investigation of the Performance of Step-Baffle Drainage Channels in Mitigating Debris Flows

“…This self-organized bed configuration can enhance channel resistance, decrease velocity, and extensively dissipate flow energy (Chin 2002;Comiti et al, 2009;Wilcox et al, 2011;Golly et al, 2019;Saletti and Hassan 2020;Zimmermann et al, 2022). In addition, stepped cascades (Figure 2B) are known to significantly dissipate hydraulic energy in hydraulic engineering (Chanson 2001;Matos and Chanson 2006;Bung et al, 2012;Felder et al, 2019;Zhang et al, 2019). Inspired by these two naturally occurring structures, a similar system, stepbaffle drainage channels were introduced to mitigate debris flows (Figure 2C).…”