Author(s): Gary Parker; Xudong Fu; Yuanfeng Zhang; Jessica Zinger; Kory Konsoer
Keywords: Dunes; Antidunes; Plane bed; Bankfull Discharge; Rivers; Submarine Channels; Turbidity currents
Abstract: Dunes are a common feature of sand-bed rivers. Dune stratification has also been occasionally observed in turbidite outcrops, i. e. ancient deposits emplaced by deep -sea turbidity currents, but is much rarer than in fluvial outcrops. A standard view in fluvial hydraulics is that dunes form at modest flood flows, but then give way to upper-regime plane bed or antidunes at higher flows. This behavior is observed in. for example, the Rio Grande River, USA. Yet in large sand -bed rivers such as the Mississippi River, USA, dunes are known to persist at bankfull, and even above-bankfull flows. Here the Vanoni bedform phase diagrams and the Wright-Parker formulation for sediment transport and resistance in sand-bed rivers are used to explain this difference in behavior. For many (but by no means all) smaller, high-slope sand-bed streams, the Froude number of the flow are biased toward larger values. As discharge increases, flow depth and Froude number increase in tandem, until the Froude number reaches a value sufficiently high for the dunes to wash out. The analysis is applied to the Yellow River, China, to explain the presence of dunes at lower flows, but their absence at higher flows. In contrast, the Froude number is biased toward smaller values in larger, low-slope sand-bed streams. Again flow depth and Froude number increase in tandem as discharge increases, but the Froude number rarely reaches as the critical value for dune washout, even at bankfull conditions. The analysis is applied to the Mississippi River to explain the persistence of dunes at all flows. In the case of channels sculpted by turbidity currents, the Froude number is biased toward relatively high, but nevertheless subcritical values at channel-forming (bankfull) flows. The analysis suggests that dunes may form under sub-channel-forming (bankfull) conditions, and then be preserved by a subsequent net-depositional flow. At bankfull conditions in submarine channels, however, dunes are not likely to be present. This may explain in part the observation that dunes are seen less frequently in ancient turbidite outcrops than in outcrops of ancient fluvial deposits.