Empirical validation of sediment transport algorithms along the East Australian inner Continental Shelf.

Abstract

This paper presents an investigation into the offshore dynamics of shelf sediments transported northward along the East coast of Australia. Dominant southeast winds and waves force a net northerly littoral drift in the surf zone, forming sediment sinks within estuary mouths, sand islands and the dune system. During severe storms and strong East Australian Current eddy events, sediments can also be transported further offshore and deposited on the inner shelf.

Coastal managers of Gold Coast City, a popular tourist location within a dynamic sediment transport system, have undertaken various engineering and management strategies to help stabilise the impacts of coastal processes on development. Encroachment upon the dune system to accommodate infrastructure such as roads, boardwalks and enhanced sea views has reduced the sand reserve needed during extreme events plus construction of walls and groynes has necessitated regular dredging programmes to remove sediment from the entrances. The dredged material is currently used to provide nourishment for depleted beaches but could be stored on the inner shelf region ready for remedial use following extreme events.

Sediment dynamics studies generally focus within the surf zone leaving the inner continental shelf area from the outer surf zone to the 25-30m depth edge of the photic seabed, not as well understood. By measuring parameters such as fluid velocity, depth, bottom roughness and grain size, empirical sediment transport relationships are being established to support theoretical approaches.

To validate theoretical approaches to local conditions, Griffith University has deployed an offshore observatory, moored at a depth of 30m, to investigate the specific conditions of this inner shelf region of the East Australian coastline. This developing structure currently comprises an ADCP measuring discrete vertical cell velocities and back scatter; a thermistor string measuring temperature throughout the water column coupled to sediment traps capturing suspended sediment deposition. Located 10m above the seabed is a Seapoint turbidity sonde, cleaned by a ZebraTech wiper assembly, attached to an RBR 420 CTD plus a string of 10 HOBO lux meters record irradiance readings from the seabed to the surface. A side-scan sonar survey sonogram has provided seabed morphological detail verified by diver observations, Van Veen grab sampling and a novel procedure providing measurements of ripple slope and direction. The paper concludes with an analysis of the data including an overview of GAM (Generalized Additive Model) analyses used to explore the functional relationships between various measured covariates.