Observations of sediment transport on the continental shelf off the mouth of the Waiapu River, New Zealand: Evidence for current-supported gravity flows

Abstract

Instrumented tripods deployed at depths of 40 and 60 m on the shelf off the mouth of the Waiapu River on the east coast of New Zealand's North Island recorded data on waves, currents, and sediment fluxes from May 22 to August 10, 2004. Three major flood events and several wave events occurred during the deployment. Data from acoustic Doppler velocimeters and profilers revealed that downslope sediment fluxes accompanied a flood event of late June, during which near-bed downslope current speeds approached 0.5 m s −1. The most pronounced downslope transport within the benthic layer occurred the day after peak flood but coincided with strong isobath-parallel currents. Suspended sediment concentrations about 1 m above the bed were on the order of 2–4 g l −1 at times of maximum seaward flow. Suspension of freshly discharged sediment within the hyperpycnal layer over much of the profile was maintained by the high bed stresses associated with the strong benthic currents, in contrast to the wave-supported sediment gravity flows recently reported on other river-nourished shelf systems. Nonetheless, observed concentrations and velocities were largely consistent with the dynamics of critically stratified sediment gravity flows based on equations previously applied elsewhere to wave-supported cases. Despite lower concentrations in current-supported gravity currents, the greater thickness results in similar total loads. In further contrast to earlier results, our data suggest that as sediment off the Waiapu flowed into deeper water across the seaward steepening bottom profile, auto-suspension may have aided in thickening and accelerating the gravity current.