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 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.

Pyroclastic dune bedforms: macroscale structures and lateral variations. Examples from the 2006 pyroclastic currents at Tungurahua (Ecuador)

“…Additionally, the extreme steepness of the backset beds contrasts with the experimental evidence of bedding related to Froude‐supercritical flows, which are generally at low angle and cross‐cutting one another (e.g. Alexander et al ., ; Cartigny et al ., ; Vellinga et al ., ). As already highlighted from their surface expression, the geometrical relationships of the dimensions and steepness of the bedforms are not compatible with Froude‐supercritical flows (Douillet et al ., ), an argument further exacerbated by the internal patterns evidenced here.…”

Response of unconfined turbidity current to deep‐water fold and thrust belt topography: Orthogonal incidence on solitary and segmented folds

“…The finite-volume method derives from the conservation laws describing fluid motion and the finite-difference method is based on the Taylor expansion (FlowScience, 2012). The mass-conservation and momentum-conservation equations are time-averaged, and a turbulence model is used to account for all scales of flow vorticity (Vellinga et al, 2018). Flow turbulence is modelled by the renormalization group (RNG) of equations with explicitly derived constants (Yakhot & Orszag, 1986).…”

Pyroclastic dune bedforms: macroscale structures and lateral variations. Examples from the 2006 pyroclastic currents at Tungurahua (Ecuador)

“…Additionally, the extreme steepness of the backset beds contrasts with the experimental evidences of bedding related to Froude-supercritical flows, which are generally at low angle and cross-cutting each other (e.g. Alexander et al 2001, Cartigny et al 2014, Vellinga et al 2018. As already pointed from their surface expression, the geometrical relationships of the bedforms' dimensions and their steepness are not compatible with Froude-supercritical flows (Douillet et al 2013b), an argument further exacerbated by the internal patterns evidenced here.…”