Sedimentation from forced wall jets versus free gravity flows: integrating tank experiments and field data into a physics-based depositional model
Final Report Abstract
The morphodynamics of expanding supercritical flows have a large impact on the dispersal of sediment and the resulting depositional architecture. Wall jets, expanding from enclosed conduits, and density flows, expanding from lateral confinements, represent end-member classes of such flows. Momentum-dominated jets are generally Froude supercritical at the orifice and evolve into subcritical or supercritical gravity-dominated density flows. The objective of this project was to develop a generic, physics-based depositional model for successions related to expanding Froude supercritical flows by the integration of observations from tank experiments and outcrop sections. In the experiments, the transition from jets to density flows occurred at a short distance from the orifice. Hydraulic jumps were absent in the jets and at the transition from jets to density flows. The bedform successions can be subdivided into bedforms related to jets and bedforms related to density flows. Jet deposits included early-stage bedforms, scours and mouth bars. Bedform trains related to density flows were dominated by symmetric antidunes, which partly displayed superimposed asymmetric ripple-like bedforms. The formation and evolution of the experimental bedforms was controlled by the initial flow density, bed slope, grain size and sediment-supply rate. The studied outcrop sections represent deposits of high-density turbidity currents in channellobe transition zones (CLTZ) and lobes. In all sections, sinusoidally stratified sandstones interpreted as antidune deposits are the most common facies, forming the bulk of the proximal lobe deposits. Convex-up cross-stratified conglomerates and structureless or backset crossstratified pebbly sandstones represent mouth bars and scour fills, respectively, which were deposited in the CLTZ or proximal lobe. The succession of bedforms and facies observed in the experiments and outcrop sections, respectively, are interpreted as representing morphodynamic successions. Each morphodynamic succession comprises (i) an initial expansion zone characterised by intense scouring and rapid deposition, potentially causing mouth-bar growth, (ii) a zone of unconfined Froude supercritical flow, where aggrading antidunes are the dominant bedforms, and (iii) a marginal zone of dissipation characterised by thinning, fining and finally pinch-out of the deposit.
Publications
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(2019) Bedform successions formed by submerged plane-wall jet flows. Marine and River Dune Dynamics – MARID VI, Bremen, Germany
Lang, J., Fedele, J.J. & Hoyal, D.C.
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(2019) Facies architecture of deepwater lobes deposited by expanding supercritical density flows. 34th International Meeting of Sedimentology, Rome, Italy
Lang, J., Postma, G., Bayliss, N., Demko, T., Fedele, J.J., Gutierrez, M., Hoyal, D.C., Lentsch, N. & West, L.
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(2019) Facies architecture of perched lobes deposited by expanding supercritical density flows – field examples from the Eocene Sant Llorenç del Munt fan-delta complex (Ebro Foreland Basin, NE Spain). EGU General Assembly, Vienna, Austria
Lang, J., Postma, G., Demko, T., Fedele, J.J. & Hoyal, D.C.
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(2019) New bounding surface hierarchy methodology for recognition of supercritical flow bedforms in outcrop. 34th International Meeting of Sedimentology, Rome, Italy
Postma, G., Hoyal, D.C., Demko, T., Fedele, J.J. & Lang, J.
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(2019) Supercritical jet flows and their transition to density flows – insights from tank experiments. 34th International Meeting of Sedimentology, Rome, Italy
Lang, J., Fedele, J.J. & Hoyal, D.C.
