Laboratory rivers adjust their shape to sediment transport

Abstract

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Laboratory observations on meltwater meandering rivulets on ice

“…In the laminar regime, a wide range of slope values are possible. For example, laminar river analogs have been observed to have 1.5-2.5 times higher slopes than their natural counterparts (Malverti et al, 2008) and others have also reported values such as 0.1 (Delorme et al, 2018) and 5 × 10 −3 (Abramian et al, 2020). In the context of our experiments, such analyses suggest that temperature gradients in laminar flows can have values that differ from the fully turbulent analogs but nevertheless still recreate specific morphologies.…”

“…The quintessential example of a meandering channel is the alluvial river, which forms and evolves by erosion and deposition of sediment (e.g., Fisk, 1944;Seminara, 2006) and interaction with riparian vegetation (e.g., Braudrick et al, 2009). Meandering channels also form in bedrock where the driving processes include dissolution (e.g., Veress and Tóth, 2004;Allen, 1971;Zeller, 1967), abrasion by sediment particles (Sklar and Dietrich, 2004), plucking of bedrock (Chatanantavet and Parker, 2009;Whipple et al, 2000), slaking (Johnson and Finnegan, 2015), and weathering (Pelletier and Baker, 2011).…”

The importance of threshold in alluvial river channel geometry and dynamics

“…In Parker's formulation, channel geometry is fundamentally determined by hydraulics. An alternative model was recently proposed by researchers from the Institut de Physique du Globe de Paris (IPGP), in which channel shape, under laminar flow conditions, is adjusted to achieve a balance between lateral diffusion of bed-load flux toward the channel margins, and inward sediment motion due to gravity 51 . In this formulation, raising the imposed sediment discharge drives increases in channel aspect ratio (W /H) and slope, away from the threshold state associated with no sediment flux (Fig.…”

“…Early research linked fluid mechanics with alluvial channel geometry [45][46][47] through the development of flow resistance relations in threshold canals, designed to convey water while never exceeding the entrainment threshold. Building on canal theory 46,48,49 , a family of models has been advanced in which sediment transport is formally treated as a mathematical perturbation to the threshold state [50][51][52] . Though different in detail, these models indicate that alluvial rivers at bankfull organize their geometry such that fluid shear stresses at the channel center only slightly exceed the entrainment threshold.…”

“…A viable theory for alluvial channel geometry must: (i) explain, to leading order, the shape of channel cross sections; (ii) explain how the channel maintains this state under natural (highly stochastic) forcing; and (iii) directly link channel shape to the mechanics of sediment transport. While a multitude of models are capable of predicting channel cross-section geometry, only the near-threshold model has been demonstrated to achieve all three criteria 6,51,52 . This Review examines the physical basis and surprising explanatory power of the near-threshold model, and attempts to clarify misconceptions regarding its formulation and application to natural rivers.…”