Joris T. Eggenhuisen

Morphodynamics of submarine channel inception revealed by new experimental approach

Submarine channels are ubiquitous on the seafloor and their inception and evolution is a result of dynamic interaction between turbidity currents and the evolving seafloor. However, the morphodynamic links between channel inception and flow dynamics have not yet been monitored in experiments and only in one instance on the modern seafloor. Previous experimental flows did not show channel inception, because flow conditions were not appropriately scaled to sustain suspended sediment transport. Here we introduce and apply new scaling constraints for similarity between natural and experimental turbidity currents. The scaled currents initiate a leveed channel from an initially featureless slope. Channelization commences with deposition of levees in some slope segments and erosion of a conduit in other segments. Channel relief and flow confinement increase progressively during subsequent flows. This morphodynamic evolution determines the architecture of submarine channel deposits in the stratigraphic record and efficiency of sediment bypass to the basin floor.

Dynamic deviation of fluid pressure from hydrostatic pressure in turbidity currents

Turbidity currents are large sediment-carrying submarine avalanches that flow over the continental slope and through submarine channels to the deeps of the world9s oceans. For more than 50 years researchers have extended the hydrostatic pressure simplification, which is valid in most hydraulic analyses of flow in rivers and channels, to analyses of turbidity currents. In this paper we present the first measurements of fluid pressure beneath experimental turbidity currents. Basal fluid pressures are much lower (30%–45%) than those predicted from bulk flow density, in proportion to the kinematic energy density of the flow, a parameter usually referred to as the dynamic pressure. This invalidates the commonplace assumption of a hydrostatic pressure regime in turbidity currents, and necessitates the incorporation of dynamic fluid pressure as a physical parameter in the analysis of turbidity currents. The contrast between turbidity currents and free surface flows is caused by the difference in their density structure. We highlight current enigmas and gaps in our knowledge of submarine currents, for which our result opens new opportunities of investigation.

A Classification of Clay-Rich Subaqueous Density Flow Structures

This study presents a classification for subaqueous clay-laden sediment gravity flows. A series of laboratory flume experiments were performed using 9%, 15%, and 21% sediment mixture concentrations composed of sand, silt, clay, and tap water, on varying bed slopes of 6°, 8°, and 9.5°, and with discharge rates of 10 and 15 m3/hr. In addition to the characteristics of the boundary and plug layers, which have been previously used for the classification of open-channel clay-laden flows, the newly presented classification also incorporates the treatment of the free shear layer. The flow states within the boundary and free shear layers were established using calculation of the inner variable, self-similarity considerations, and the magnitude of the apparent viscosity. Based on the experimental observations four flow types were recognized: (1) a clay-rich plug flow with a laminar free shear layer, a plug layer, and a laminar boundary layer, (2) a top transitional plug flow containing a turbulent free shear layer, a plug layer, and a laminar boundary layer, (3) a transitional turbidity current with a turbulent free shear layer, no plug layer, and a laminar boundary layer, and (4) a fully turbulent turbidity current. A connection between the emplaced deposits and the relevant flow types is drawn and it is shown that a Froude number, two Reynolds numbers, and a dimensionless yield stress parameter are sufficient to associate an experimental flow type with a natural large-scale density flow.

Keynote Speech - Taking Physical Modelling of Deepwater Depositional Systems Forwards

We show how new scaling considerations, which we term Shields scaling, have opened up new avenues of investigation in the physical modelling of deepwater depositional systems. We demonstrate the added value of the new approach with two examples: A) Channel-levee relief development. B) Depositional patterns in a break-of-slope setting. The flow dynamics results of the experiments are consistent with the extensive body of previously published physical modelling results in both examples. The sediment distribution, however, shows a clear departure from the draping depositional behavior encountered predominantly in previous work. In contrast, the experimental deposits presented here display a subtle spatio-temporal interplay between erosion, transport, and deposition of sediment by turbidity currents, which results in a morphodynamic evolution that is a much better analogue for deepwater system development.

Process-based Modelling of Turbidity Currents - From Computational Fluid-dynamics to Depositional Signature

Turbidites are amongst the largest deposits on earth, but linking fluid processes and sedimentary processes to deposits can be difficult. Here we show that it is feasible to study the link between flow dynamics and flow deposits by the use of a numerical model. This method is able to produce detailed facies maps, to find generalised depositional signatures of flows with different characteristics. Additionally, the method aides a better understanding of processes shaping bed-morphologies and deposits, as flow-conditions at the flow-bed interface are known over time and space. We envision that such process-based modelling approaches can in future be used to improve geostatistical reservoir-modelling tools