Methodology for assessing exposure, vulnerabillity and risk due to mass flows in the city of Arequipa, Peru

Assessing hazard and risk in large cities exposed to multiple hazardous events has long been a challenge. The case of Arequipa (south Peru), the largest South American city exposed to multiple hazards from El Misti volcano, earthquakes and flash floods, is no exception. A multidisciplinary project aims to address hazards, exposure, the physical vulnerability of habitats and infrastructure, as well as community risk perception of flash floods and debris flows along two of the five ravines that drain the ring plain and cross the city. Arequipa has been affected by mass flows every 2 to 5 years on average over the past 60 years. Using simulated volcanic and non-volcanic mass flows, the goal is to contribute to scenario-based hazard zone mapping and mitigation procedures usable by Civil Defense. Three scenarios for a range of mass flows including flash floods, hyperconcentrated flows (HCF), and debris flows were derived from recent disastrous events ̶ c. 100,000 m³ for the 8 February 2013 HCF through to simulated extreme events c. 350,000 m³ for pre-historic lahars. Simulations using high-spatial resolution DEMs and numerical codes show that mass flows commonly spill over bends from sinuous ravines that cut down through the volcaniclastic fans of El Misti and avulse in channels that have been artificially filled and constructed. Exposure and physical vulnerability of constructions were investigated using surveys at the city block scale together with remotely sensed Pléiades imagery. Numerical simulations showed that flow dynamic pressures act differently on parallel, angular and perpendicular house walls, which affects vulnerability to impacts. Experimental lab tests have defined the resistance of lapilli stone, several types of brick, ignimbrite, reinforced concrete, and confined frame buildings. Seismic design is scarce and construction procedures usually poor in the most populous districts located along the ravines upstream of the city center. Physical vulnerability increases from the high terrace to the ravine channel; where the weakest habitat is located without property titles. Demographic factors, together with risk perception insights, have been collected from residents in at least 30% of the houses. Surveys suggest that risk perception exists, but hazard knowledge remains low.