Coastal parallel sediment transport on the SE Australian inner shelf : a study of barrier morphodynamics

University essay from Lunds universitet/Institutionen för naturgeografi och ekosystemvetenskap

Abstract: In SE Australia, sediments are moved northwards by longshore currents and deposited as barriers, dunes, and estuarine or inner shelf deposits. According to Roy & Thom (1981), the magnitude and complexity of the deposition morphology increase from south to north. Modelling by Cowell, Roy, and Jones (1995) shows that the steepness of the regional substrate is the primary influence on coastal morphodynamics. It is even more important than slow sea level changes. More sediment is available for transport where the shoreface slope is gentle (a larger volume of sand between any two depths), simply because waves will entrain more sand across a greater distance of the inner continental shelf. Correlation between the size of coastal sediment deposits and the inclination of the inner continental shelf (between 20 and 60 metres depth) for the coastline between Fraser Island (Lat. 25º S) and Cape Howe (Lat. 37.5º S) suggests that the direction of long-term residual sediment transport on the inner continental shelf is along, i.e. northward, and towards the coast during stable to semi-stable relative sea level. The dataset consists of 698 two kilometre wide (north-south) segments of coast. The size of the sand deposits and the local substrate inclination for each segment are extracted from a GIS, created through the interpretation and digitisation of geologic, topographic, and bathymetric maps. A simple linear correlation shows that the size of a barrier is partly controlled by the size of the potential sediment supply directly offshore. Added lag shows that much more can be explained by littoral transport. The correlation improves when the size of a deposit is explained by the steepness of the substrate south of its location. To see if the result can be reproduced on parts of the data, the coast is divided into four geologically separable sections. They are Fraser Island - Byron Bay, Byron Bay - Port Stephens, Port Stephens - Beecroft Head, skipping Jervis Bay, and St. Georges Head - Cape Howe. These four sections are in turn divided, yielding nine new zones that are analysed also. There are variations in the strength of the correlation that seem to reflect the different sizes of the coastal cells. This can be seen when the correlation is plotted on one axis and the lag on the other. More importantly, each section has a different “lag correlation pattern”, which seems to give insight into the many different scales involved, i.e. we can see that the sand is transported from one embayment to the next, that the sand is transported passed the next major headland, and so on.

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