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Effect of Froude Supercritical Flow on Fluvial Facies, Geometries and Architecture


Froude subcritical flow is considered the geomorphically formative flow in rivers, as reflected in fluvial facies models, morphodynamic river discharge and sediment dispersal models, flood mitigation strategies, and in how we read the stratigraphic record. Froude supercritical flow is commonly assumed a transient occurrence, or limited to steep bedrock rivers with shallow flow. Yet, the sedimentary record of modern and ancient rivers with highly variable discharge displays an abundance or even dominance of Froude supercritical flow deposits. This paper shows that Froude supercritical flow may exert first-order control on river morphodynamics, such as the nature of the small- and large- scale bedforms and thus the resultant stratigraphy, as well as on sediment transport mode and rate. Such rivers characteristically lack well developed barforms, as supercritical flow is not advected by bars and bar migration thus not maintained. Transition to Froude supercritical flow significantly increases the proportion of sandy and gravelly sediment carried in suspension. Suspension transport critically increases downstream sediment transport rates, as bedload transport rates are linked to downstream bedfrom migration rates that are only a small fraction of the mean flow velocity. This paper discusses the effects of supercritical flow on small- and large-scale bedform migration and the resultant depositional architecture in rivers where supercritical flow is the geomorphically formative flow.