Effects of currents and waves on the morphologies of coastal sandy clinoforms: sediment mobility calculations based on current meter and wave data from Southern California, U.S.A.

Abstract

In some wave-exposed coasts, sandy clinoforms occur with rollovers (locations where their surfaces steepen) at ∼ 20–60 m depth. They have been suggested to have formed from sand mobilized by strong wave agitation in shallow water that has deposited in more tranquil deeper water beyond the rollover, although other suggested origins of clinoforms might also apply. In situ information on active sediment transport is needed to address their origins. Here, we assess sediment transport across a sandy clinoform rollover at 25–30 m depth using legacy data from current meters installed across the Southern California shelf near Del Mar in depths of 15, 30, and 60 m. Although lasting only 25 hours, the data captured conditions during the passage of a cyclone, which occurs frequently along this coast. Information from a global meteorological model reveals that waves were 1–2 m in height during the cyclone passage. Using the mean particle size from vibracores (0.129 mm), sand at the 15-m-depth site would have been continuously agitated above its threshold of motion during the 25-hour period, whereas sand at the 30-m site was mobile over 17 hours. Total-load fluxes of sand estimated from the waves and currents were strong at the 15-m site and comprised mostly suspended particles. The cyclone winds drove water, including bottom water, mostly parallel to the coast, but that movement was accompanied by a downwelling component over the first ∼ 10 hours. That downwelling likely moved suspended particles from the topset of the clinoform onto the foreset. A later upwelling component may have partially reversed that particle movement but was unlikely to have been so effective. The results suggest how wave effects, coupled with downwelling currents, dominated the sand transport and deposition over this short 25-hour period, contributing to the clinoform morphology.