Morphodynamics and depositional signature of low-aggradation cyclic steps: new insights from a depth-resolved numerical model

Abstract

Bedforms 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 constrains on criteria to recognise 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.

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