Geologic hazards of the Ocoña river valley, Peru and the influence of small-scale mining

Abstract

The Ocona River Valley, in the Arequipa Department in southern Peru, extends over 150 km from the Pacific coast to its headwaters in the Andes Mountains. While traditionally sparsely populated with farmers and crawfish fishermen, the valley has seen a large population growth in the last 20 years as small gold mines have developed. This change poses the opportunity to evaluate three research questions: How do geologic hazards and risks increase from mining activity? How does unplanned urban development affect risks? How do landforms restrict development and thereby increase risks? The combination of climate, topographic, and hydrologic conditions result in widespread hazards from debris flows, collapsible soils, erosion, rockfall, flooding, and liquefaction. We have developed maps of these hazards for three communities where mines have recently developed, Secocha, Alto Molino, and San Juan de Chorunga, and then we conducted a risk analysis for changing conditions, termed “Risk–Risk” analysis. Because of the lack of vegetation on mountain slopes, mining operations typically only affect rockfall hazards, mostly in the immediate vicinity of the mines and in the nearby runout zones. However, increased population and associated settlement infrastructure is constrained by steep slopes and regularly-flooded valleys, so residents live in fan and terrace areas with substantially increased risks from debris flows and collapsible soils and moderately increased risks from flooding and rockfall.