GEOMAT - Modelling of Continental Shelf Sediment Mobility in Support of Australia's Regional Marine Planning Process

GEOMA T is a geological-oceanographic computer modelling project which aims to enhance our understanding of the processes controlling sediment mobilisation on the Australian continental shelf. This report describes tidal and surface ocean swell-wave models and their application to studies of shelf sediment mobilisation. The work has been carried out over the past 2 years by a team of collaborators from AGSO, the University of Tasmania, the Australian Bureau of Meteorology and Kort & Matrikelstyrelsen, Geodetic Division in Denmark. Our models predict that swell wave energy is sufficient to mobilise fine sand (0.1 mm diameter), on at least one occasion during the year March 1997 to February 1998, over 63.5% of the Australian continental shelf. The largest and most powerful waves were able to mobilise fine sand up to a water depth of 148 m in the Great Australian Bight. Tidal currents are capable of mobilising fine sand at least once per semi-lunar cycle (ie. -2 weeks) over about 56.4% of the shelf. Overlaying the wave and tide threshold exceedence maps demonstrates that there are areas on the shelf where one process dominates, some areas where tides and waves are of relatively equal importance and still other areas where neither process is significant. We defined 6 shelf regions of relative wave and tidal energy: zero (no-mobility); waves-only, wave-dominated, mixed, tide-dominated and tides-only. The relative distribution of these regions varies with grain size. Inclusion of estimated mean grain size is being undertaken at the present time and this will enhance the usefulness of the regionalisations. GEOMA T provides a predictive, process-based understanding of the shelf sedimentary system. It helps to explain the distribution patterns of surficial sediments and will probably be useful for mapping biological habitats and communities, although further work is needed to better define these relationships. GEOMA T provides a useful tool that will assist with marine environmental management in general, and with the National Ocean's Office regional marine planning process in particular. It has demonstrated applications to marine engineering projects where shelf sediment mobilisation is of concern and to regional studies of pollution dispersal and accumulation.