A comparison of the shear stress distribution in the bottom boundary layer of experimental density and turbidity currents

Abstract

The internal stress distribution within weakly depositional turbidity currents has often been assumed to be similar to saline gravity currents. This assumption is investigated by analyzing a series of experiments to quantify and compare the shear stress distribution in the bottom boundary layer (BBL) of saline and particle-laden gravity currents. Vertical profiles of Reynolds stresses, viscous stresses and turbulent kinetic energy (TKE) were obtained from the mean downstream velocity profiles and turbulent velocity fluctuations, and were broadly similar in both flow types, suggesting that saline gravity currents are a good analogue to turbidity currents. Maximum positive Reynolds stresses occur where the velocity gradient is largest in the BBL but below this maximum, the Reynolds stresses decrease significantly and are balanced by an increase of viscous stresses. The bulk drag coefficients CD is defined for both flows using three methods, (i) a log-fit method based on the law of the wall, (ii) the observed maximum total stress and (iii) direct measurements of turbulent velocities. The CD values of both flow types were broadly similar but each method led to CD values of different orders of magnitude. The log-fit method yielded the largest drag coefficients of O(10−2) whereas measurements of turbulent velocities gave relatively small values of O(10−4). The best correlation with drag coefficients observed in field measurements of O(10−3) was obtained by using the maximum total stresses next to the wall. The variation of CD is discussed in relation to parameterization methods in experimental and numerical modeling.