Assessing strategies to mitigate debris-flow risk in Abancay province, south-central Peruvian Andes

During the 2011–12 austral summer, the Abancay province (south-central Peruvian Andes) experienced unprecedented, persistent precipitation in the records extending back locally to 1982. As a consequence, a large debris flow was triggered in the upper part of the Sahuanay creek on March 18, 2012, with catastrophic consequences in the highly populated downstream districts of Tamburco. After this event, structural measures were built in the form of an artificial channel and two sediment traps, so as to mitigate the negative impacts of future events. Here, we reconstruct the 2012 event and assess the reliability of the new countermeasures during future events of similar size, with the final aim to obtain response times for inhabitants to reach safe areas. To this end, we employed a field-based assessment, numerical models (i.e., RAMMS) and GIS modelling based on a Statistic Least-Cost Distance (LCD). This assessment suggests that the implemented countermeasures are indeed able to retain most sediments of future, 2012-like events, yet the model also suggests that the new channel would be overflown at the same locations where avulsion occurred prior to their construction. Our findings also indicate that people living between the upper part of the torrent and the Maucacalle stadium would not have enough time to evacuate between the beginning and the arrival of the debris flow. We also show that at least 380 buildings could be affected by future events and despite the existing mitigation structures, thereby illustrating that the measures will not provide the expected protection.