The influence of wind flow over a long-walled asymmetric coastal parabolic dune : morphodynamic feedback, evolution and migration

Mobile transgressive dune systems are often overlooked in the study of coastal morphodynamics and sediment budgets. Inundation bK sand of abutting agricultural land and infrastructure has recurred throughout the 2ot 1 century, frequently accompanied by requests by farming communities to the government to stabilise the dunes. In addition, the association of sea-beach-coastal dunes provides an attractive setting for residential I holiday developments on dune systems. Yet little is known about the pattern of dune mobility in response to changing atmospheric conditions (rainfall, air temperature and wind speed) and their effect on dune mobility and the rate of actual and predicted dune migration. The work presented in this thesis addresses the evolution of a parabolic dune, dune mobility and migration. Quantitative data were collected to determine process-response in an actively mobile long-walled asymmetric parabolic dune located on the western coastline of northeast Tasmania, Australia. The data were acquired over a period of four years from a composite of geomorphic survey techniques that incorporated repeated surveys of the ground surface topography using kinematic GPS and arrays of erosion pins, which were linked to analysis of wind flow patterns acquired from an on-site 2 m high meteorology station, and analysis of sediment that forms the dune. Wind shear velocity (u*) and roughness length (zo) were measured close to the ground on windward slopes, and speed-up ratios were determined. Analysis oftemporallyspaced ortho-rectified and georeferenced aerial photography extended the duration of the survey by fifty years to establish the longer term evolution of the parabolic dune, including the downwind and lateral migration rates of the dune system. The level of accuracy (planimetric accuracies typically in the range of 2-5 cm and vertical accuracies of 3-10 cm) achieved in the six epochs of kinematic GPS surveys provided reliable GPS data that were used to construct three-dimensional topographic models of the dune system on 5 m x 5 m grids using kriging. The topographic models displayed the general patterns of displacement and accretion of sediment that maintain dune form. Using cross-sectional profiles, at representative locations across the dune, the pattern of process-response between wind speed I wind direction on the direction of sand movement and on morphodynamics was clearly evident. A series of morphological parameters were extracted from the kinematic GPS data to quantify dune migration rates, dune volumes, and dune reconstitution time. The measured migration rate of the parabolic dune head was 21.5 m/yr (mean value) measured over 4 years, and 27.3 m/yr measured from a fifty-year sequence of orthorectified, georeferenced, digitised aerial photography. The head of the dune reconstitutes at the rate of 8.9 years, with a volume of 100,800 m3. Volumetric change for the study site over an interval of thirty months was in the range of 656,400 m3. The majority of sand transport is generated by moderate velocity wind flows which have a higher frequency of occurrence. Forty one percent of wind events occur in the moderate velocity category (4 to< 12 m/s), twenty seven percent occur in the high velocity category of> 12 to> 20 m/s. Measurement of flow close to the ground on windward slopes indicated that shear stress does not progressively increase from toe to crest of slope due to interaction between flow and bed form. Some seventy percent of wind events are topographically aligned; across dune flow from the SW sector results in lateral migration of the trailing arms, with the smaller south trailing arm migrating ENE at more than twice the rate of the north trailing arm. This rate of lateral displacement maintains the 3: 1 length to width ratio of the asymmetric parabolic dune by maintaining the dimensions of the deflation plain which in turn maintains the dynamic equilibrium of the aerodynamic envelope of the dune form. This integrated approach has provided quantitative evidence that demonstrates the dynamic feedback between the prevailing wind regime and seasonal and discrete wind flow events on the dune bed, and the influence of atmospheric conditions on dune mobility