Turbulent diffusion modelling of sediment in turbidity currents: An experimental validation of the Rouse approach

Abstract

The margins of submarine channels are characterized by deposits that fine away from the channel thalweg. This grain-size trend is thought to reflect upward fining trends in the currents that formed the channels. This assumption enables reconstruction of turbidity currents from the geologic record, thereby providing insights into the overall sediment load of the system. It is common to assume that the density structure of a turbidity current can be modelled with simple diffusion models, such as the Rouse equation. Yet the Rouse equation was developed to describe how particles should be distributed through the water column in open-channel flows, which fundamentally differ from turbidity currents in terms of their flow structure. Consequently, a rigorous appraisal of the Rouse model in deep-marine settings is needed to validate the aforementioned flow reconstructions. The present study addresses this gap in the literature by providing a robust evaluation of the Rouse model's predictions of vertical particle segregation in two experimental turbidity currents that differ only in terms of their initial bed slopes (4° versus 8°). The concentration profiles of the coarsest sediment, which is suspended predominantly in the lower part of the flow, is accurately reproduced by the Rouse equation. Significant mismatches appear, however, in the concentration of finer grained sediment, especially towards the top of the flow. This problem is caused by the mixing with clear water at the top of turbidity currents.Caution is therefore advised in applying a Rouse model to levee overspill and leveecrest deposits. Nonetheless, the Rouse model shows good agreement with laboratory measurements in the lower regions of the flow and for the coarser grains that are predominantly transported in the lower sections of submarine channels. K E Y W O R D SGrain-size segregation, Rouse modelling, simplified sediment transport modelling, turbidity current reconstruction 204 | EGGENHUISEN Et al.

Flow‐process controls on grain type distribution in an experimental turbidity current deposit: Implications for detrital signal preservation and microplastic distribution in submarine fans

“…were therefore more concentrated in the upper flow with respect to coarser grains) and were more able to overspill the channel on to the levees (Figure 13). These observations of coarser channel‐fills and finer levees have been corroborated both experimentally, and from both modern and ancient natural systems (Altinakar et al, 1996; Eggenhuisen et al, 2020; de Leeuw et al, 2018a; Hansen et al, 2015; Jobe et al, 2017). Garnet was deposited primarily in the channel axis immediately down‐dip of the inlet pipe, indicating it was not able to be held in suspension by the flow, which was strongly depositional (Figures 11 and 13).…”

Novel Acoustic Method Provides First Detailed Measurements of Sediment Concentration Structure Within Submarine Turbidity Currents

“…Much smaller increase in K ta are observed near the bed in the sustained bodies of the longer flows. This indicates that there is likely a greater proportion of coarser material in the grain size distributions nearer the seafloor, as would be expected for Rouse-type sediment concentration profiles (Eggenhuisen et al, 2019;Rouse, 1937). This effect would cause the sediment attenuation coefficient to decrease with the increasing grain size near the .…”

Entangled external and internal controls on submarine fan evolution: an experimental perspective

“…These growing levees would normally be predicted to progressively confine the flows due to the flows becoming smaller with respect to the channel form (Hodgson et al, 2016;Shumaker et al, 2018). Instead, the levees continued to be overtopped; probably due to flows becoming progressively larger, experiencing more turbulent mixing and decreased grain-size stratification as the sediment supply rate was increased between runs (Rouse, 1939;Kneller and McCaffrey, 1999;de Leeuw et al, 2018a;Eggenhuisen et al, 2019). When sediment supply rate was reduced, overbank deposition lessened (<5 L in each run) and deposition in the channel axis increased as the flows of runs 4 and 5 were now substantially underfit with respect to the new evolved channel dimensions (Figures 6 and 7; de Leeuw et al, 2018b).…”