Turbulence Modulation in Clay-Laden Flows over Ripples and Dunes

Jaco H. Baas and James L. Best
University of Leeds, Leeds, United Kingdom

Flume experiments investigated the effect of cohesive sediment concentration on flow behind current ripples and dunes. This work expands similar experiments using flat beds (Baas & Best, J.Sed.Res. 2002), showing that turbulence is modulated in clay-laden currents at remarkably low concentration, and that the transitional regime from turbulent to laminar flow has unique dynamic properties.

The experiments used a 10m long, 0.3m wide hydraulic flume, filled with kaolin suspensions (concentration: 0-13vol%) flowing at ~0.3-0.4m/s over a straight current ripple (length: 0.15m, height: 0.015m) and a train of straight dunes (length; 0.65m, height: 0.045m).

Time-series of downstream velocity and turbulence intensity (approximated by root-mean-square velocity) at different heights in the flows show that the recirculation vortex in the bedform troughs contracts rapidly with increasing clay concentration. This may result from progressive changes in feedback relationship between shear-induced turbulence and cohesion. In the turbulent regime, cohesive forces are relatively small (or absent), hence the vortex is fully developed. In the transitional regime, a cohesive network of clay particles is established, which is regularly broken up by turbulence. This results in unique transitional flow dynamics. In the laminar regime, the cohesive network is sufficiently strong to consistently suppress turbulence.

Interestingly, maximum turbulence is stronger in transitional than in turbulent flow, probably because of an internal shear layer, positioned just above the bedform crests and troughs. The suspensions in the lower half of the bedform troughs appeared stationary during laminar flow. The implications for natural flows and deposits of all above observations are discussed.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005