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Influence of feed timing on sediment transport and bed evolution during hydrographs Mitchell, Alexander Jonathan

Abstract

Sediment transport in gravel-bed streams is an important component for river ecology, infrastructure design, and hazard assessment. Yet making accurate predictions of sediment transport remains elusive despite many decades of field, lab, and numerical modeling research. Most lab and modeling experiments use either constant discharge, constant feed, or both are constant. While these works have helped discover many aspects of sediment transport, they do not reflect the variability in discharge and sediment supply found in natural settings. We performed several flume experiments to challenge an assumption made by Parker, Hassan, and Wilcock (2007) that feed timing during a hydrograph does not matter for sediment transport behavior and bed evolution. We used mixed grain sizes for both the initial bed and feed (ranged between 0.5mm and 32mm; D 50 = 7.83mm), a symmetrical stepped hydrograph, and five different feed scenarios: no feed, constant feed, rising-limb only feed, falling-limb only feed, and capacity-scaled feed. All feeding scenarios had the same total mass fed. We show that the assumption made by Parker et al. is wrong. Feeding on the rising limb strongly controlled sediment transport rates, overall bedload yield of the hydrograph, and bed scouring severity, but not bedload grain sizes. Feeding on the falling limb controlled bedload grain sizes, bedload transport rates in the early portion of the falling limb, and recovery of the bed towards a pre-flood morphology (e.g. elevation and slope). Shifting the feed timing towards different parts of the hydrograph highlighted different processes and influenced the overall hysteresis trends.

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