AN ASSESSMENT OF LANDSLIDES AND DEBRIS FLOWS DISTRIBUTION IN THE JAZAN AREA, SAUDI ARABIA USING REMOTE SENSING AND GIS TECHNIQUES

We adopted an integrated (field and remote sensing) approach to map the distribution of landslides and to identify their controlling factors in the Jazan area, Red Sea Hills, Saudi Arabia. Our approach depends on: (1) Identification and characterization of known landslide locations using field and/or high resolution remote sensing data sets, (2) extraction of criteria to enable identification of the known landslide locations using remote sensing data sets, (3) using the selected criteria to identify areas where similar landslide types could occur elsewhere in the study area. To conduct steps 1 through 3, we produced several derived image products that were used to identify areas that are prone to, or witnessed mass movement. For example, areas prone to movements have low vegetation (NDVI image) and steep slopes (slope image). Areas that witnessd mass movement are less vegetated than their surroundings and hence are bright compared to their vegetated dark surroundings on reflectance images and show evidence of disruption on contour maps. Investigating the intensity of vegetation in an area was enabled by examining NDVI images which were extracted from spectral reflectance images of SPOT multispectral images. Contoured elevation maps (contour interval: 10 m) were derived from available landuse maps. Additional products were generated to evaluate the hazards associated with the road network and houses. The study identified three causes for the landslides. First, landslides caused by debris flows within ephemeral valleys that could be hazardous to the roads and buildings they intersect. We found 3455 intersections of streams with roads and 430 houses were found to be within the stream zones. Second, Landslides caused by debris flows related to overland flow, which occur in areas that do not have terraces, posses steep slopes (> 40°), and are sparsely vegetated (NDVI<0.09). Finally, Landslides caused by failure along fracture planes occur when the following conditions are met: (1) the dip direction of the planner discontinuity is within 20° of the dip direction of the slope face, (2) the dip of the planner discontinuity is less than the dip of the slope face, and (3) the dip of the planner discontinuity is greater than the angle of the friction of the surface (40°). We found 13942 picture elements that satisfy these conditions.