The role of physical cohesion on ripple dynamics under combined wave and current flow

We present the results of flume experiments, conducted in the Total Environment Simulator at the University of Hull, that aim to provide a fuller understanding of the influence of physical cohesion (abiotic clay content) on ripple development under combined wave and current flow Experiments including three runs were performed with wave-dominated combined flows, with maximum orbital velocity of 0.3 m/s and depth-averaged current velocity of 0.2 m/s. The bed in each run consisted of three channels side-by-side with common flow conditions. Run 1 as a control experiment used three beds of well-sorted sand (D50 = 440 ). Beds with homogenous mixing Kaolinite clay with the same sand were in Run 2 and 3, with initial cohesive clay contents ranging from 6% to 13.4%. A suite of state-of-the-art instruments was deployed to quantify the interactions of near-bed hydrodynamics and sediment transport over rippled beds formed by combined waves and currents. An Ultrasonic Ranging System (URS) was mounted on an automated traverse to acquire cross-section profiles of the evolving bed, capturing the development of the ripples and quantifying how planform geometries (2D or 3D) evolved. URS transducers at fixed positions were also used to detect ripple migration and thus determine bedload transport rates. Moreover, sediment cores were collected from ripple crests and troughs in each channel during the experiments to quantify how clay content varied with time and depth as the ripples evolved. The experimental results show the significant influence of the amount of initial cohesive clay in the substrate on ripple evolution under combined flows. Higher cohesive clay contents in the bed dramatically slowed down the rate of ripple development and evolution. Furthermore, the dimensions and steepness of the ripples decreased with increasing clay content, particularly for bed clay contents above 10%. The implications of the results for bed roughness calculations and the process interpretation of interference ripples in sedimentary environments will be discussed.