Sediment deposition from turbidity currents in simulated aquatic vegetation canopies

Abstract

A laboratory flume experiment was carried out in which the hydrodynamic and sedimentary behaviour of a turbidity current was measured as it passed through an array of vertical rigid cylinders. The cylinders were intended primarily to simulate aquatic vegetation canopies, but could equally be taken to represent other arrays of obstacles, for example forests or offshore wind turbines. The turbidity currents were generated by mixing naturally sourced, poly‐disperse sediment into a reservoir of water at concentrations from 1·0 to 10·0 g l−1, which was then released into the experimental section of the flume by removing a lock gate. For each initial sediment concentration, runs with obstacle arrays with solid plant fractions of 1·0% and 2·5%, and control cases with no obstacles, were carried out. The progress of the current along the flume was characterized by the array drag term, CDaxc (where CD is the array drag coefficient, at the frontal area of cylinders per unit volume, and xc is the position of the leading edge of the current along the flume). The downward depositional flux of sediment out of the current as it proceeded was measured at 13 traps along the flume. Analysis of these deposits divided them into fine (2·2 to 6·2 μm) and coarse (6·2 to 104 μm) fractions. At the beginning of their development, the gravity currents proceeded in an inertia‐dominated regime until CDaxc = 5. For CDaxc > 5, the current transitioned into a drag‐dominated regime. For both fine and coarse sediment fractions, the rate of sediment deposition tended to decrease gradually with distance from the source in the inertial regime, remained approximately constant at the early drag‐dominated regime, and then rose and peaked at the end of the drag‐dominated stage. This implies that, when passing through arrays of obstacles, the turbidity currents were able to retain sufficient sediment in suspension to maintain their flow until they became significantly influenced by the drag exerted by the obstacles.

Fragmented Canopies Control the Regimes of Gravity Current Development

“…The inertial regime was observed as the first phase of the gravity current development in all the runs carried out. This is in accordance with the results obtained in the initial development phase of a gravity current (Hatcher et al, ; Rottman & Simpson, ; Tanino et al, ; Soler et al, ). Furthermore, in agreement with Rottman and Simpson (), the self‐similar phase was observed after the inertial regime for the cases of SPF = 0%.…”

“…The denser the stem distribution was, the slower the gravity current travelled through the vegetation. This result coincides with the results from Soler et al (), where the temporal evolution of a gravity current through an array of cylinders of SPF = 2.5%, was slower than for SPF = 1%. This result demonstrates the buffering capacity vegetation has in attenuating currents, as stated by other authors (Bouma et al, ; Järvelä, ; Leonard et al, ; Morris et al, ).…”

“…For the nonimpeded experiments, the evolution of the leading edge with time depended on the sediment concentration, which is in accordance with the results found by other authors (Gladstone et al, ; Soler et al, ), in such a way that the greater the sediment concentration the faster it travelled along the flume. The same results were found for the experiment carried out with vegetation.…”

“…Tanino et al () found that the transition from inertial to drag‐dominated regime occurred at C D aL T = 7, whereas Soler et al () found the transition occurred at C D aL T = 5. The C D aL T obtained in this present work was greater than those obtained by Tanino et al () and Soler et al ().…”

“…The cases with the presence of stem distribution, and when the drag term in equation dominates over both inertia and viscous forces the temporal evolution of the front follows (Hatcher et al, ; Soler et al, ; Tanino et al, ): where ϕ = (π/4)ad is the volume fraction of the obstacles, a = Nd/A is the frontal area of the cylinders per unit volume, N is the number of cylinders, A is the total bed area occupied by the cylinder array, and d is the stem diameter. In this regime, the velocity of the gravity current varies as (Zhang & Nepf, ) …”