Abstract

During the extraordinary hydro-meteorological event that occurred in March 2015 between Tuesday 24 and Thursday 26 more than 50 mm of rain fall in the Atacama Desert (Chile) between 22o and 32o S and a maximum range of 50 to 90 mm fell between 26o-29o S (Atacama Region). These intense rains caused flashfloods in 17 ravines of this region that affected more 7 municipalities. The most extensive damage occurred in the cities of Chañaral and Copiapó. The city of Copiapó (~160,000 inhabitants) is the administrative capital of this Chilean Region and it is located at the confluence between Quebrada Paipote and Copiapó River. The run-off generated in the 3 upper subbasins of the Copiapó watershed (18,400 km2) was stored at the Lautaro dam. However, Quebrada Paipote subbasin (6,600 km2) that also drains the Andes Cordillera has its junction with the river further downstream. This tributary is dry under normal climatic conditions but during this event a large volume of water mixed with fine sediments arrived to the Copiapó River at its confluence where the city of Copiapó is located. This process involved very rapid surging flow (2.5 - 5.5 m/s) that carried on a huge volume of sand, clay and silt transported by high density and viscosity flows that buried the city under a 31 cm mean thick layer of massive sandy mud. Although in other ravines debris flow occurred, neither Quebrada Paipote nor the Copiapó River transported large boulders. Mudflows are characterized by their transport capacity and the main problem they caused in urban areas is the accumulation of the sediment they transport once the energy decreases. A total area of 12.2 km2 was flooded what corresponds to the 72% of the urban area of Copiapó. Four sectors of the city had flood heights higher than 2 m with a maximum measurement of 3.8 m close to the confluence of Quebrada Paipote and a mean flood height of 45 cm across the city. The estimated volume of accumulated sediment in the city is 2.2 million m3 that infilled houses, sewerage, etc. This event highlights the need of developing accurate hazards maps that combined with vulnerability analysis of the population and the infrastructures makes possible to evaluate the risk of flooding in the city as well as the direct and indirect impacts caused by this process.