Preliminary Results on High Accuracy Estimation of Shoreline Change Rate Based on Coastal Elevation Models

The shoreline is one of the most important features on the Earth’s surface, representing a critical indicator of coastal evolution and vulnerability for any Coastal Geographic Information System. In this sense, a new methodological proposal for high accuracy shoreline mapping is outlined along the present work. Briefly, this methodology starts from a coastal elevation model (CEM) obtained by LiDAR or stereo-protogrammetric technologies and vertically geo-referenced to a geodetic datum. The second step would consist on mapping the contour line corresponding to the 0 m elevation to draw the continuous line that represents the physical intersection between land and the reference geoid, i.e. the Mean Sea Level (MSL). The problem is that most of time the boundary between water and land is not clear because LiDAR or stereo-photogrammetric data may be contaminated by waves and runup. So those data should be first eliminated by removing all of data points that lie seaward of that theoretical boundary. Then we suggest the use of extrapolation methods to cope with this problem, presenting and evaluating the performance of two algorithms for extrapolating the MSL contour line from CEMs. The first one is based on a classic approach named Cross-Shore Profile method where linear regression is computed along different cross-shore profiles to extrapolate the corresponding shoreline position as the intersection between the regressed straight line and the selected tidal datum. The second method is a new iterative grid-based data technique that expands the Elevation Gradient Trend computed for every grid point to extrapolated grid points with unknown heights. The process is repeated till the new grid point reaches the level just below the chosen tidal datum. After that, the border which separates grid points (above and below the reference height) is joined to map the corresponding tide-coordinated shoreline. From the analysis of the obtained results, both quantitative and qualitative, the new grid-based approach can be strongly recommended because its precision, local slope acquisition, robustness regarding the presence of noise and outliers, and capability to deal with very curved and even closed coastal features. The preliminary results also indicate that, though the global rate-of-change for the whole coastline between 2001 and 2009 may be catalogued as relatively low (0.55 ± 0.50 m/year of net accretion), the local results for every one of the 29 homogeneous units considered have been extremely variable and statistically significant, ranging from 3.85 ± 0.61 m/year accretion to 3.97 ± 0.56 m/year erosion. In this way, many local phenomena, registered in a short-term period and mainly due to human intervention, may strongly affect the shoreline evolution in certain and localized areas. * Corresponding author. This is useful to know for communication with the appropriate person in cases with more than one author.