The morphology and sediment transport dynamics of the Seven Mile Beach spit

Sandy coasts and dune systems are dynamic and changing environments. A study on the morphology and sediment transport dynamics of the Seven Mile Beach spit in south eastern Tasmania supports this notion. The Seven Mile Beach spit is a mid-bay spit that projects into the waters of Frederick Henry Bay and Pitt Water, the mouth of the drowned Coal River Valley. The spit contains significant coastal dune features, including bare mobile dune ridges and over-steepened dunes covered with marram grass. A combination of aerial photography and GIS analysis, demonstrated that significant changes in shoreline position and land cover type have occurred to the spit during the past fifty years. During this period, it was found that the toe of the spit was migrating east, while episodic retreat on the southern side and deposition on the northern side occurred. Analysis of land cover change indicated that marram grass has been particularly successful in colonising the area. Marram grass cover increased during the fifty year period from 15% to over 50% of the study area, displacing areas of native vegetation and bare sand. Field monitoring techniques provided primary data on sediment transport dynamics at the study site. Results indicate that there were significant differences between sand transport at bare and marram grass covered sites, and in the amount of sand transported at different elevations above the surface. The proximity of the sample sites to the active dune system also influenced the amount of sand deposition. Erosion pin data and the monitoring of a highly mobile dune slip face illustrated that sediment transport is influenced by wind direction and strength and the presence of vegetation. This study highlights that coastal dune systems are naturally unstable and dynamic environments, and that this complexity should be considered in the management of these features.