Channel stability in steep gravel–cobble streams is controlled by the coarse tail of the bed material distribution

Abstract

Researchers have associated channel-forming flows with reach-average shear stresses close to the entrainment threshold for the surface D 50. We conducted experiments using a model of a generic steep, gravel-cobble stream to test this association. Our results suggest that channel-forming flows fully mobilize the D 50 , and produce shear stresses close to the entrainment threshold for the largest grains in the bed. The channel dimensions were set by flows capable of mobilizing between 85% and 90% of the bed surface, which produced a brief period of lateral instability lasting about 1h, followed by a prolonged period of relative stability during which modest adjustments occurred, but during which the reach-average hydraulics remained about the same. The adjustments during the unstable phase of the experiments are characterized by rapid bank erosion, extensive deposits on the channel bed and a restructuring of the major morphologic elements of the stream. The adjustments during the stable phase of the experiments involved barform migration and bed surface coarsening but did not appreciably modify the physical template established by the end of the unstable phase. The behaviour we observed is not consistent with the concept of a dynamic equilibrium associated with a formative flow that is just capable of entraining the bed surface D 50. Instead, it suggests that rapid adjustments occur once a stability threshold is exceeded, which creates a template that constrains channel activity until another event drives the system across the stability threshold, and re-sets the template. While we believe that it is probably too simplistic to associate a channel-forming discharge with the entrainment threshold for a single grain size, our results suggest that the D 95 is a more logical choice than the D 50

Floods on Alluvial Fans: Implications for Reworking Rates, Morphology and Fan Hazards

“…Eaton et al. (2020) further showed that sediment transport scales with the volume of erosion in laterally active streams. It is perhaps unsurprising then, that we observed a non‐linear relation between flow rates and volumes of geomorphic change.…”

“…Many bedload transport formulae predict sediment transport as a non-linear function of some flow metric (Barry et al, 2004;du Boys, 1879;Meyer-Peter & Müller, 1948;Parker, 1990;Schoklitsch, 1962;Shields, 1936;Wilcock & Crowe, 2003;Wilcock & Kenworthy, 2002;Wong & Parker, 2006). Eaton et al (2020) further showed that sediment transport scales with the volume of erosion in laterally active streams. It is perhaps unsurprising then, that we observed a non-linear relation between flow rates and volumes of geomorphic change.…”

“…The sensitivity of F n and M at high flows may also reflect the crossing of stability thresholds set by coarse grains. Experiments in a laterally mobile stream by Eaton et al (2020) showed that as flow increased and as much as 80% of the bed material was mobilized, it was only once flows were great enough to mobilize the largest grains present that channel dimensions were modified. Consequently, they postulated that overall channel stability reflects the stability of a small population of immobile or partially mobile large grains.…”

Floods on alluvial fans: implications for reworking rates, morphology and fan hazards

“…Many bedload transport formulae predict sediment transport as a non-linear function of some flow metric (Barry et al, 2004;DuBoys, 1879;Meyer-Peter & Müller, 1948;Parker, 1990;Shields, 1936;Schoklitsch, 1962;Wilcock & Kenworthy, 2002;Wilcock & Crowe, 2003;Wong & Parker, 2006). Eaton et al (2020) further showed that sediment transport scales with the volume of erosion in laterally active streams. It is perhaps unsurprising then, that we observed a non-linear relation between flow rates and volumes of morphologic change.…”

Modulating the lateral migration of a gravel bed channel using the coarse tail of the bed material distribution

“…Large grains stabilize the bed by causing a greater proportion of the bed surface to remain immobile; Eaton et al. (2020) found that channels will remain laterally stable where at least 20% of the bed surface is stable. Although the exact mechanism through which large grains impart stability remains under investigation, it is likely structures such as pebble clusters, ribs and stone cells that form through interactions between individual large grains near their threshold of motion act to shield finer, more mobile material from entrainment and displacement (Church et al.…”