FROUDE SUPERCRITICAL FLOW IS GEOMORPHICALLY FORMATIVE FLOW IN SOME VARIABLE DISCHARGE RIVERS (Invited Presentation)

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 assay anticipates to be a starting point to new research into the role of Froude supercritical flow as a geomorphically formative flow, and exemplifies such hydrodynamic conditions and the resultant river deposits. The implications range from the need to reconsider flood mitigation strategies in such rivers, to reassessment of their potential as archives of landscape evolution and climate forcing, to the need for new morphodynamic and stratigraphic models.