Abstract
AbstractBedforms related to Froude‐supercritical flow, such as cyclic steps, are increasingly frequently observed in contemporary fluvial and marine sedimentary systems. However, the number of observations of sedimentary structures formed by supercritical‐flow bedforms remains limited. The low number of observations might be caused by poor constraints on criteria to recognize these associated deposits. This study provides a detailed quantification on the mechanics of a fluvial cyclic step system, and their depositional signature. A computational fluid‐dynamics model is employed to acquire a depth‐resolved image of a cyclic step system. New insights into the mechanics of cyclic steps shows that: (i) the hydraulic jump is, in itself, erosional; (ii) there are periods over which the flow is supercritical throughout and there is no hydraulic jump, which plays a significant role in the morphodynamic behaviour of cyclic steps; and (iii) that the depositional signature of cyclic steps varies with rate of aggradation. Previous work has shown that strongly aggradational cyclic steps, where most of the deposited sediment is not reworked, create packages of backsets, bound upstream and downstream by erosive surfaces. Here, the modelling work is focussed on less aggradational conditions and more transportational systems. The depositional signature in such systems is dominated by an amalgamation of concave‐up erosional surfaces and low‐angle foresets and backsets creating lenticular bodies. The difference between highly aggradational cyclic steps and low‐aggradation steps can be visible in outcrop both by the amount of erosional surfaces, as well as the ratio of foreset to backset, with backsets being indicative of more aggradation.
Highlights
Large quantities of sediment are transported by high-discharge events, such as floods or jo€kulhlaup (Nordin & Beverage, 1965)
The depth-resolved numerical model allows a unique insight into the mechanics of a cyclic step system, and the modelling results can be used to link the mechanics to the depositional signature
The simulated cyclic steps generally adhere to existing conceptual models, with Froude-supercritical flow over the lee side and Froude-subcritical flow over the stoss side of the bedform
Summary
Large quantities of sediment are transported by high-discharge events, such as floods or jo€kulhlaup (Nordin & Beverage, 1965) Such events are prone to Froude-supercritical flow, where surface waves cannot migrate upstream because the flow velocity exceeds the wave-. Froude-supercritical unidirectional sediment-laden flow over an erodible sediment bed leads to the formation of bedforms, such as antidunes (Kennedy, 1969; Alexander et al, 2001), and at higher Froude numbers cyclic steps (Winterwerp et al, 1992; Parker, 1996; Taki & Parker, 2005; Kostic et al, 2010; Cartigny et al, 2014). The large number of observations of Froude-supercritical flow related bedforms on the sea floor, mainly found in submarine canyons and steep delta slopes, reaffirms the prevalence of Froude-supercritical flows in marine settings (Symons et al, 2016)
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