Estimating Coastal Slopes Vulnerability in Dakar Peninsula, West Senegal

Dakar Peninsula, where most of the economically active places in Senegal are concentrated, has been repeatedly subjected to rockfalls and coastal erosion on the stretch of coastline between Pointe des Abattoirs and Pointe de Dakar. Coastal scientists, engineers and political leaders are increasingly being called upon to assess the physical and socio-economic impacts of such slope degradation, and hence explore appropriate response strategies. The objective of this terrain hazard mapping study was to collect information to enable the prevention and control of slope failure and slope degradation to be made on a rational basis. The area to be investigated was defined and sub-areas were further subdivided into 12 individual sections and assigned hazard and risk categories or levels derived from the field data.The present study evaluated the risk of slope failure within a conventional probabilistic framework based on slope outcrop inventory and structural discontinuities measurements, geomorphological mapping combined with air photograph interpretation, review of pre-existing geotechnical data as well as the sampling of rock specimen for laboratory testing and analysis. Detailed sea level rise analysis reported elsewhere leads to the conclusion that the background geology of the area should be looked as the cause to the several coastal hazards, among which slope instability poses the most important urban management problems in Dakar. From the results of the study it was possible to infer that local rockfalls and degradation of rock slopes are closely linked to the geological setting of the area, notably local rock type and structure, local hydrogeological conditions, local topography and geomorphological processes, as well as a poor city planning and overpopulation. The results also showed that geomorphological mapping can provide an effective tool to the engineering geologist in anticipating the slope failure processes which may otherwise go unnoticed. Eventually the probabilistic method has been found to work consistently for this case study and therefore suggests that the orientation and distribution of structural discontinuities play a key role for assessing the risk of slope failure with acceptable degree of accuracy.