Suspension of coarse and fine sand on a wave-dominated shoreface, with implications for the development of rippled scour depressions

Abstract

Measurements of waves, currents and sediment suspension from a 35-day deployment of three instrumented tripods on the wave-dominated shoreface off Tairua Beach, New Zealand, are presented. Two tripods were deployed at 22-m depth, one within a coarse-sand rippled scour depression (RSD), and the other nearby on the surrounding, relatively featureless fine-sand plain. A third tripod was deployed at 15-m depth, also on fine sand. To distinguish between suspended fine and coarse sand at each site, a method of simultaneously inverting two-frequency acoustic backscatter data was applied. Background, long-period swell was interrupted by two 6-day periods of high waves. At the 15-m/fine-sand site, only fine sand appeared in suspension. However, at the 22-m/fine-sand site, coarse sand appeared in suspension together with fine sand when the waves were highest. This suggests that coarse sand resuspended from the RSD can be dispersed on the adjacent fine-sand plain, at least along the same isobath. At the RSD, there was no coarse sand in suspension under background swell, but there was a fine-sand “washload”, presumably advected from the surrounding fine-sand plain, even though the coarse bed sediment in the RSD itself was not in motion. Under high waves, both fine and coarse sands were in suspension over the RSD. The fine sand appeared again as a washload, but the coarse sand was confined relatively close to the bed in accordance with a sediment diffusivity that decreased linearly with elevation above the bed. Measurements of fine-sand reference concentration over both fine-sand beds (15 and 22-m depth) plot against wave-induced skin friction in one cluster under the background swell and in another cluster under the high waves, which suggests a change in bedforms, possibly to a hummocky bed, that is related to wave conditions and that alters the suspension dynamics. The coarse-sand reference concentrations over the RSD fall into the “hummocky cluster”, which supports this interpretation. The fine-sand washload is indicative of an unfavourable settling environment over the RSD, which could constitute a positive feedback that causes RSDs to self-organize from initial perturbations. Under high waves, the suspended-sediment load impinging from the surrounding plain will be high, but turbulence will also be more energetic on the RSD, thus more effectively inhibiting deposition. Under low waves, deposition may be less inhibited on the RSD, but the suspended-sediment load arriving from the surrounding fine-sand plain will also be lower. Thus, the fine-sand deposition rate on the RSD will be small, even though conditions are more favourable for settling. Modelling could be expanded to consider the balance between inhibition of settling and tendency for burial by the sediment load that impinges from the area surrounding an RSD, and the way wave climate and water depth might mediate the basic self-organization process. Escape of coarse sand from RSDs under high waves may seed the growth of new RSDs.