LABORATORY EXPERIMENTS AND NUMERICAL SIMULATION OF SEDIMENT-WAVE FORMATION BY TURBIDITY CURRENTS

Abstract

This paper presents the results of laboratory experiments and numerical simulations carried out in order to understand the mechanism of sediment-wave formation by turbidity currents. Experimental turbidity currents were generated in a 10-m-long laboratory flume, so as to investigate the topographic effects of slope and ridges on turbidite deposition. The results indicate that preferential deposition occurs on the upstream side of the ridges. This preferential deposition is considered to result in possible upstream migration of the topography, a common feature of deep-sea sediment waves, provided that such deposition were repeated. The preferential deposition occurred under a subcritical turbidity current, implying that antidune flow conditions (0.844<Fr<1.77) are not necessarily required for upstream migration of the bedforms. A numerical simulation using layer-averaged Navier-Stokes equations is successfully applied to the laboratory experiments, and then to deep-sea turbidity currents. The numerical model predicts the formation of a wavy structure in a sequence of thousands of turbidites, with wave dimensions and internal architecture similar to deep-sea sediment waves. The wavy structure is interpreted to form as a succession of mounds, each of which grows individually rather than simultaneously as a sinusoidal wave. Each mound is developed by preferential deposition downstream of a slope break, which in turn generates a new slope break on its downstream side flank. Upstream migration of the waveform results from differential deposition between the upstream and downstream sides of the mounds. These results indicate that neither antidunes nor lee-waves are necessary for the formation and upstream migration of the wavy structure. Wave formation as a series of individual mounds can be considered as an origin for relatively small sediment-wave fields, because no more than 4-5 crests are formed in the model predictions. It is suggested that more extensive sediment-wave fields can be developed by turbidite deposition on an initially undulating bottom, which results in upstream migration by differential deposition, and downstream extension of the wave field by mound formation progressively further downstream.