Assessing Shoreline Change Rates in Mediterranean Beaches

Abstract

Shoreline change rate constitutes an essential parameter for coastal areas management and monitoring in order to map erosion/accretion areas and to forecast the future shoreline position. Here, shoreline rates were assessed in a heavily human influenced coastal sector of the Mediterranean coast located at Almeria province, Spain. In order to evaluate shoreline rate change assessment in Mediterranean beaches, a comparison was carried out between three methods applied throughout 2009 to 2011 period. In this sense, two kinds of sources were used in order to derive shoreline positions: (i) digitizing the high water line (HWL) through orthoimage interpretation and (ii) automatically extracting a contour level from a LiDAR-derived coastal elevation model (CEM). Shoreline extraction quality was studied by comparing HWL and two datum-based contours, one extrapolated up to 0 m and the other interpolated at 0.75 m above mean sea level (Spanish altimetric datum). It was found a significant bias between HWL and datum-based shoreline positions which had been qualified as negligible in other previous studies carried out in microtidal areas. Since HWL and 0.75 m contour-based shorelines showed a similar distribution, although presenting an added offset, and the 0 m contour was too noisy because of extrapolation errors, it was concluded that the 0.75 m contour-based shoreline was the most stable and accurate proxy datum for multitemporal shorelines comparison. Finally, a high variability of shoreline position could be tested when HWL was used as a proxy for shoreline, being HWL less accurate than CEM-derived shorelines except for the case of using poorly accurate photogrammetrically derived CEMs (e.g. based on very old aerial flights).