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*false/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.

Modelling of massive bedload deposition in a debris basin: Cross comparison between numerical and small scale modelling

“…Well-defined and highly mobile channels were observed during the experiments. Efficient grain size sorting, along with processes of armoring and armor breaking or diversion of channel after formation of coarse grain clusters were systematically observed in the physical model as in Piton et al (2018Piton et al ( ,2019b. Such processes obviously cannot be capture in the single grain size formulation used in TELEMAC 2D-SISYPHE in the present work.…”

Can Lava Flow Like Water? Assessing Applications of Critical Flow Theory to Channelized Basaltic Lava Flows

“…At‐a‐point measurements showed that flows locally alternated between supercritical and subcritical conditions, with the channel centerline average very close to Fr = 1. Recent flume experiments have largely confirmed this hypothesis and extended it to coarse bed channels (Piton & Recking, 2019).…”

Review of Pelletier Controls on the hydraulic geometry of alluvial channels: bank stability to gravitational failure, the critical-flow hypothesis, and conservation of mass and energy

“…This condition applies irrespective of grain size, although the author rightly points out that the actual bed features that set up this interaction are different for sand, gravel, and even boulder bed channels. A recent paper tests this idea in the flume and articulates the mechanism well (Piton and Recking, 2019). Most of the streams in the Dunne and Jerolmack data set have slopes much less than 0.01, and consequently much lower Fr, as the data shown in the paper point out.…”