The influence of particle size on near-bed turbulence and bed response in wave-supported gravity flows

Abstract:

Wave-supported gravity flows (WSGFs) play an important role in down-slope sediment transport across continental shelves. Prior work has shown that the dynamics of WSGFs are sensitive to the sand content of the sediment bed. In particular, high sand fraction increases near-bed turbulence and sediment concentration through the formation of ripples relative to a lower sand fraction sediment mixture, especially in low wave energy conditions. While the sand content increases turbulence through ripple formation, it may also armor the bed and decrease erodibility. In this work we perform a series of laboratory experiments to investigate the bed response in WSGFs with different sand fractions. The experiments were carried out in an oscillatory water tunnel using bed sediment with 1% sand and 13% sand fraction. We observe that, even in mud-dominant mixtures, the threshold for sediment suspension is primarily dependent on the sand fraction. We present a modified criterion that provides one control on the formation of a high concentration layer based on the sand fraction in laboratory and field observations. Results from the bed measurements also show that bed erosion is not linear; the erosion rate decreases as the bed erodes. Finally, new high resolution measurements of near-bed dynamics are in progress, which will be used to evaluate the relative importance of bed and stratified flow controls on the generation of the high concentration layers in high wave energy conditions. These results extend the understanding of WSGFs and may guide subsequent numerical simulations of WSGFs.