Steep Bedload-Laden Flows: Near Critical?

Abstract

Steep gravel-bed rivers sometimes experience floods that dramatically rework river bed structure and topography. Hazard assessments and paleo-event reconstructions require better knowledge of such phenomena. This paper explores morphodynamic evolution of steep channels carrying bedload-laden flows, using a generic Froude-scaled model. Bedload-laden floods were introduced in a narrow flume and spread into a 5 times wider unconfined area with a 0.1 steep slope (m/m). Image analysis enabled measurements taken at an unprecedented level of accuracy on unconfined flows laden with bedload. A flow reconstruction procedure was used to compute depth, Froude (Fr) and Shields (τ*) numbers on millions of pixels based on a friction law and measurements of surface velocity, slope, and roughness. Despite the steep slope, Froude numbers proved to be mostly subcritical in all but the regions experiencing the most active sediment transport. Competent flows, identified by the transport stage higher than unity (τ*/τcr* > 1), were near critical and seldom had Fr> 1.3–1.5. This demonstrates that, providing that bed width and structure can adjust, hydraulic features such as standing waves, hydraulic jumps, and lateral shock waves dissipate energy very efficiently in addition to adjusting channel features. These competent flows also tend to rework channels to approach the energy minimum of near-critical flows. This hypothesis was postulated by Gordon Grant (1997, https://doi.org/10.1029/96WR03134, Wat. Resour. Res. 33(2):349-358) but demonstrated here for the first time at this scale. Considering near-critical flows during discharges high enough to be clearly competent in laterally unconfined reaches seems reasonable as a first approximation in steep channels.