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Supercritical Bedforms Created by Density Flows


Subaerial bedforms are now relatively well understood but their counterparts in deep-water remain elusive due largely to increased flow complexity and the limited number of experimental studies. Extrapolation of equilibrium regime diagrams developed for subaerial bedforms to the deep-water realm has been widely practiced without testing. Here we present results of more than 500 experiments of saline density currents and some dilute turbidity currents running over low-density plastic sediment, in a 7-m long and 15-cm wide flume submerged in a large fresh water tank. Experimental currents spanned a wide range of conditions with water discharges from 3-18 gpm and initial slopes of 1o-10o, producing subcritical, approximately critical, and supercritical flows (Fr = 0.7 – 2.5). Results confirm some similarities between subaerial and deep-water bedforms both in process and product, but also reveal some interesting differences. For example, ripples and dunes form under both sub and supercritical density currents while supercritical currents yield both small-wavelength, downstream-migrating, and long-wavelength, upstream-migrating antidunes, where the latter may transition to cyclic steps. Supercriticality of the flow, the proportion between bedload and suspended load, and the sediment caliber were the major controls on the prevailing bedform observed. To confirm the character of these new types of bedforms (i.e. supercritical dunes), detailed analyses of flow structure over the bedforms were performed using PIV technique. Outcrop examples are used to support (and demonstrate) that the observed experimental gravity flow bedforms, in particular those that have not been described before, have their counterparts in the field. Our findings underscore the rich spectum of potential bed states produced by dense underflows and their deviation from bed behavior in open channel flows. As a result, inversion of gravity flow bed features based on known subaerial bedform regimes is potentially misleading. New gravity flow bedform regime diagrams are presented that will improve our ability to determine the flow conditions at the time of deposition.