Residual slope stability in low order streams of angangueo mining area (Michoacán, Mexico) after the 2010 debris flows

Mexico, largely a tropical mountainous region, is continually subjected to natural hazards like landslides induced by heavy rainfalls. At the end of January 2010, two cold fronts and low-pressure storms moved over the State of Michoacán (central Mexico) and in February the town of Angangueo, in the eastern sector of the Monarch Butterfly Biosphere Reserve, was devastated by a huge muddy debris flow. The in situ observations after the event showed that considerable material was removed from the countryside surrounding the urban area. In this framework two low order streams were indicated as the main suppliers of granular material to the Angangueo River: Melon and Catingón creeks. To evaluate the residual risk and slope stability along these two torrential watercourses, specific investigations were carried out. A GIS-based map of the most relevant hazardous features was performed at high level of detail by means of targeted field surveys that distinguished natural and anthropogenic elements like: (i) riverbanks and slopes with new signs of instability and already collapsed areas; (ii) large accumulations of debris and plant remains in channels and slopes; (iii) deposits of mining waste and abandoned mining areas; (iv) hydraulic works along the riverbeds and (v) cultural elements potentially at risk. In both circumstances, a generalized criticality emerged; moreover, all the riverbanks along the Catingón creek are almost still under unstable conditions, whereas Melon creek has precarious riverbanks only for about 1/3 of the considered slopes. In both cases, the depth of their rupture surfaces seems directly related to the different geotechnical properties of existing materials with some influence from gradients.