The landslide hazard and human vulnerability in La Paz City, Bolivia

Landslides in the city of La Paz, Bolivia are complex in space and time. Their distribution within the city is differentiated by geographical variations in slope gradient, the nature of overlying surface deposits, and drainage density patterns. When mapped, the distribution of the most landslide-prone locations in the city coincides with the most mobile surface deposits on the higher and steeper slopes of the city. The timing of landslides is triggered when slope materials become saturated with moisture by rainfall, stream water, water seepage from high surrounding water tables, and from domestic sources. Landslide frequencies over the last 40 years show the role of inter-annual variations in summer rainfalls, especially the late summer period (Jan-Mar), and rising water tables connected to Lake Titicaca. Associations with ENSO events, both warm (El Nino) and cold (La Nina), prove negative, pointing to the importance of moisture sources from the north, south and east (Amazon) but not from the west (Pacific). The most vulnerable group exposed to the landslide hazard comprise the inhabitants of the self-built informal housing areas who occupy the more elevated steeper slopes of the northern part of the city. But societal vulnerability to earth movement in the city is widespread and interconnected. Higher income groups in the southern part of the city are affected by slower, if less dramatic, forms of land failure, and rich housing in downstream areas of the city are impacted by flash flooding encouraged by deforestation on the northern hill slopes where the poor live. Two case studies highlight the vulnerability and plight of the inhabitants of the self-built settlements in the aftermath of a major landslide, and a brief discussion of hazard mitigation and the importance of building hazard resilient communities concludes the paper.