Shoreline change assessment using remote sensing and GIS techniques: a case study of the Medjerda delta coast, Tunisia

Eight scenes of Landsat Multispectral Scanner, Thematic Mapper, Enhanced Thematic Mapper Plus, and Operational Land Imager sensors, covering the period between 1972 and 2013, were used to demarcate shoreline positions and estimate shoreline change rates of the Medjerda delta coast, northeastern Tunisia. The method relies on image processing techniques using the IDRISI software, and the Digital Shoreline Analysis System, a free extension for ArcGIS software, which provides a set of tools permitting transects-based calculation of shoreline displacement. First, the Landsat images were radiometrically and geometrically corrected. Second, band ratioing, reclassification, raster to vector conversion, and smoothing techniques are applied successively to detect and extract the multi-temporal shoreline data. Third, these data are overlaid and the changes are calculated using the end points and linear regression methods. The results indicate significant shoreline changes ranging from 8.6 to -42.6 m/year, while some parts remained unchanged. The estimated shoreline change rates are comparable with those obtained through in situ measurements and from the analysis of multi-date aerial photos and toposheets. The main causes of erosion in particular are related to the natural shifting of the Medjerda River course and mouth, damming of Medjerda and its tributaries, construction of Ghar El Melh port, and the destruction of the small bordering dunes in addition to the wave-induced longshore currents, relative sea level rise due mainly to accelerated coastal subsidence, and sand mining.