Automated techniques for quantification of beach change rates using Landsat series along the North-eastern Nile Delta, Egypt

摘要

Ten scenes of Landsat sensors (MSS, TM and ETM+) at unequal intervals spanning 35-year period between 1972 and 2007, were analyzed to quantify erosion and accretion pattern along the North-eastern coastline of Nile Delta, from Gamasa to Port Said. Rates of shoreline changes were calculated from automated waterline positions generated at 852 locations using a Digital Shoreline Analysis System (DSAS) version 3.2 programs. To assess impacts of coastal structures on the beach morphology the shoreline positions are divided into two groups. The first group (1972 - 1990) is designated to calculate rates of shoreline retreat approximately before protecting the coastline and the second one (1995 - 2007) after construction protection works. Rates of shoreline changes estimated from three statistical approaches of DSAS (the end point rate, the Jackknife and a weighted linear regression) are validated with ground observations of beach profile survey data at the same corresponding positions. Comparison of shoreline rates of beach change obtained from Landsat data with that previously estimated from beach profiles shows that the method used is reasonably accurate with a correlation coefficient value of 0.76. Results indicate that the general alongshore erosion/accretion pattern is locally disrupted bythe construction of protective engineering structures. The erosion at the tip of the Damietta promontory is terminated due to the construction of the 6 km seawall built in the year 2000; erosion was originally -43 m/yr before construction of this wall. The 8 km sand spit that has been formed from the eroded zones at the promontory tip before construction of the seawall is now under erosional processes due to deficiency of sediment supply. Further west andprior to protectionof Ras El Bar resort, erosion (-10 m/yr) is spatially replaced by a formation of salient accretion (15 m/yr) following emplacement of the detached breakwaters between 1991 and 2002.However, local adverse erosion has been resulted in at the western end of the breakwater system, averaging -5m/yr. This erosion has resulted from the interruption of thewesterly longshore sediment transport by these breakwaters.The seasonal reversal of the NNE waves isresponsible for generating of thiswestward-flowing longshore current along Ras El Bar coastline.