Ripple generation under the combined influences of waves and currents on the Canadian continental shelf

A multi-parameter instrument package ( Ralph) was deployed for 15 days in 22 m of water on Sable Island Bank, Scotian Shelf. The instrument successfully recorded mean current velocity, statistics on near-bed wave motion, wave height, period and propagation direction, and time-lapse photographs of the seabed. The seabed at the deployment site was composed of well-sorted, fine sand (0.23 mm mean diameter) which moved during peaks in tidal flow and during periods of high wave activity. Eight distinct bed types were seen in time-lapse imagery: (1) wave ripples; (2) straight-crested current ripples; (3) linguoid current ripples; (4) wave and current ripples; (5) transitional wave ripples; (6) transitional current ripples; (7) poorly developed (biodegraded) ripples; and (8) flat bed. Each bed type occurred at well-defined combinations of near-bed wave motion, mean current speed and biodegradation. Wave-current ripples intermediate in form and orientation, were not seen. Bedform types were found to be well separated in plots of wave Reynolds number (U mb·d 0/ν) and dimensionless mean flow (U 100/W s). A better separation was found if the wave Shield's parameter (after Grant and Madsen, 1979 , Journal of Geophysical Research, 84, 1797–1808) and the current Shield's parameter (after Sternberg, 1972 , Shelf sediment transport, process and pattern, pp. 61–83) were used. Bedform stability plots showed that wave ripple threshold is influenced by currents, and current ripple threshold is influenced by waves. Despite this, ripple type is defined by the partitioned, wave and current components of stress and not the total stress as no intermediate (wave-current) ripple type was observed. Thresholds for sand transport and the generation of well-developed bedforms were influenced by the combined, near-bed motions of waves and currents even when the bed exhibited wave ripples or current ripples only. The threshold for sand transport under combined flows is vague, but is adequately represented by a line drawn between the threshold for transport under unidirectional flow based on the modified Shield's curve of Miller et al. (1977 , Sedimentology, 24, 507–527) (θ c= 0.04) and the threshold specified for pure wave motion of Komar and Miller (1973 , Journal of Sedimentary etrology, 43, 1101–1110) (θ w = 0.04). A first approximation to a threshold criterion for fine sand is proposed as [θ w + θ c]crit = 0.04 .