The Cresmina dune is a transgressive loose sand body of approximately 300m wide by 230m long, that is moving from NNW to SSE along the Guincho-Oitavos dunefield. This dunefield, located near Cascais, in the west coast of Portugal, can be classified as an headland bypass dunefield. The sand enters the system from two beaches at north, migrates on top of a marine abrasion platform, cut into cretaceous hard rocks, returning to the sea at south.
In order to determine the trend of evolution, the advance rate and the resultant sand drift that is occurring, a four year campaign (2000–2004) for monitoring the precipitation ridge of the transgressive dune and the contiguous area, is being carried out with one detailed topography survey per year.
Topography surveys are made using a Trimble DGPS with 1.5cm and 2.5cm of horizontal and vertical accuracy respectively. ArcView GIS is used to process the data and display the results. Because subtraction of 3D surfaces relative to different years is our goal, a very large effort is made to survey all the elevations on the studied area. Detail surveys of the 43,067m2 have been done with approximately 10m spaced measurements in flat smooth topography but 0.15m in rough topography.A 10cm grid is calculated using the measured points and the resultant Digital Elevation Models show elevations below 10cm.
The comparison between surfaces obtained from the 2000 and 2001 surveys clearly show the areas affected by deflation and accumulation. A volume of transported sand into the study area of 14,249m3, approximately 39.7m3year−1 per meter of dune section, and a variation of the precipitation ridge advance of 0.5m to 10m, depending on dune high and relative position were determined. Asymmetric evolution of the dune and influence of leeward relief in sand drift are clear.
