Using experiments and theory to explain why dryland flash flood channels are morphodynamically different

Abstract:

Flow in relatively steep dryland channels is commonly intermittent and dominated by flash floods. For a given flood discharge, these channels usually transport much more sediment than do mountain rivers with perennial flow. Dryland channel beds tend to be sand-rich, poorly sorted and unarmored even in channels with abundant gravel, in contrast to well-armored gravel beds typical of upland rivers in temperate climates. To explore how unsteady flow and sediment supply influence dryland channel morphodynamics, we first propose a simple model of a developing boundary layer to predict how basal shear stresses change during flash flood hydrographs. Results from the boundary layer model are compared to experimental constraints on bore shear stresses, and are used to predict transport rates for different grain size classes. However, transport depends not only on fluid stresses, but also on granular interactions among particles. Our flume experiments show that bores can disrupt bed grains and cause rapid kinematic sieving and surface coarsening, although shear stresses are high enough to easily transport all available grain sizes. While flash floods can efficiently transport grains both large and small, we show that perennially-flowing channels are typically more supply-limited with respect to fine sediment, because base flow is often sufficient to remove large amounts of sand from the system. Transport rates increase greatly with surface and subsurface sand fraction; we put bounds on the effect of sand abundance on transport efficiencies in flash flood channels. Finally, the comparison to dryland channels also provides insight into “typical” gravel-bed rivers with well-developed surface armoring. While large floods that exceed gravel thresholds of motion cause the most coarse bed load transport, these channels are able to evolve to states of armored stability not because of big floods, but because of the persistent, sand-transporting effects of base flow.