Application of geomorphologic knowledge for erosion hazard mapping

An erosion hazard map was elaborated using geomorphologic and lithological information; this was the base to characterize the erodibility of the territory. The aim of the proposed methodology is to define the areas where more detailed studies are necessary (e.g., to estimate rates of soil erosion, mitigation measurements, land use) to prevent future problems. Field work and remote sensing data (study of historical aerial photographs and satellite images) were used to understand the geomorphologic evolution and the current processes taking place in an area; this information was used to group the units according to its lithology, dynamic and slope inclination. The map was processed using the geographical information system and categorized in zones of very high, high, moderate, low and null fluvial erosion hazards. The map covers the Metropolitan Area of San Salvador, which is experiencing serious problems of mass wasting processes, collapse and settlements of foundations. Most affected areas belong to the Tierra Blanca Joven tephras which are unsaturated and cover most of the surface; nowadays, the urban projects and infrastructure resting in this material are suffering from extensive damage. The geotechnical information on the tephras shows a decrease in strength and collapsible behavior when saturated. Due to this, the use of Quickdraw tensiometers (suction) and TMS3 (soil moisture content) is proposed for monitoring. The methodology of erosion hazard mapping correlates well with mass wasting reported in the studied area, and for this reason, it could be a good way to protect the natural resources and improve the land use.