The integration of remotely sensed data and gis in monitoring the senegal river estuary : responses of the sand spit to anthropogenic driven changes

The coastal zone of the Senegal River Estuary is a complex geological feature, which is generally under various natural and anthropogenic pressures. As a result, its morphodynamics changed rapidly following the construction of Diama and Manantali dams. To assess the responses of the river to regulation, nearly fifty years of aerial photos (MAS-1954 ＆ 1960; OMVS-1978 ＆ 1980 and JICA-1989) and remotely sensed data (Spot-1999, 2000, 2002 ＆ 2003, Landsat-1972, 1988 ＆ 1998 and JERS 1-1995 ＆ 1996) were processed and analyzed. The evaluation of morphologic changes in the "Langue de Barbarie" sand spit shows various changes of the coastline during the last decades involving a reduction of it beaches. The most important changes occurred between 1963 and 2002 with an erosion rate of 1 my^＜-1＞, when the system responded shapely to human activities that interfered with the longshore transport and the retention of sediment sources. Erosion processes was rapid up to the early 1950s, then moderated with differential migrations of the sand spit during pre-dam and post-dam periods. These stop-and-go phases were accompanied with long (4 to 7 kilometers) and short (200 to 600 meters) jumps of the spit breaking positions. From 1986, the sand spit extended southwards (±400 my^＜-1＞). The composition of recent sediment and their accumulation processes are mainly under the "Ekman transport" phenomenon and littoral drift influences as shown by particle size distribution patterns. The long and medium terms evolution analysis cope with the annual net drift amount and inter-annual precipitation index, however the short term evolution analysis revealed a particular behavior of beaches and the sand spit in response to local hydrodynamic and human activities