The Influence of Particle Size on Turbulence and Stratification in Wave-Supported Gravity Flows

Wave-supported gravity flows (WSGFs) play an important role in cross-shelf fine sediment transport. Recent studies suggest that the sand fraction of the bed may play an important role in the dynamics of WSGFs through armoring and the generation of bedforms. In this work, we use laboratory experiments to investigate the bed response in WSGFs with varying sand fractions. The experiments were performed in a U-tube wave tank using sediment with 1% and 13% sand fraction. We find that the larger sand fraction alters the near-bed flow structures, turbulence and concentration profiles through the formation of ripples as well as bed erodibility, relative to the finer mixture. For the 13% sand fraction, ripples formed under low wave forcing and enhanced turbulence in the wave boundary layer. However, as wave forcing increased, ripple steepness decreased. For the 1% sand fraction, ripples did not form for low or high wave forcing but did form under intermediate forcing. We hypothesize that ripples formed under these conditions as a result of winnowing and the collection of sand particles on the bed surface. This mechanism is consistent with the observation that the timescale for ripple formation in the 1% mixture was long relative to the 13% mixture. Finally, we show that van Rijn’s (1984) threshold for suspension (u_*/w₀ = 4/d_*), as applied to the sand fraction, successfully predicts the initiation of the high concentration layer. This result, along with the observed concentrations and turbulence in the wave boundary layer, supports the conclusion that the sand fraction controls the generation of WSGF via armoring and bedform generation. This theory is also tested using field data from a predominantly sandy shelf in the North Sea.