Geological-geotechnical analysis of a rock-toppling prone canyon in Furnas, Brazil, after a fatal event

Abstract. When a disaster related to a natural phenomenon occurs in areas dependent on Geotourism, restoring tourist confidence can be a challenge. An example is the rock-toppling event that occurred on January 2022 in one of the four canyons located in the Furnas reservoir in Brazil, which caused 10 fatalities. Visitation fear and their temporary closure severely impacted the economy of the surrounding municipalities that rely on tourism. To support a safer operation of the canyons to visitation, our study investigates the factors that can lead to landslide events in the region, based on the combination of field investigations, rock-mass quality evaluation (RMR14) and kinematic analysis. We hypothesize that the assessment of rock-mass quality can successfully identify specific areas in the bedrock that are prone to rockfall and rock toppling, supporting risk management strategies in tourist regions. Our results indicate that the 2022 rock-toppling event occurred due to the combination of different factors, such as rainfall infiltration in the unfavorably-oriented joints of the bedrock and reservoir water-level fluctuations. Moreover, the long-term erosion at the base of the slope, caused by the nearby waterfall flow and water-level variations, weakened rock-mass support. The RMR14 method adapted to open rock slopes successfully supports the estimation of the bedrock’s geomechanical properties, identifying structural zones in the rock mass that are prone to slope failure(s) and, as a consequence, should be monitored. The kinematic analysis further indicates that the four canyons are highly susceptible to planar failures and, less so, to toppling, although specific locations in the slopes show a higher rock-toppling susceptibility, especially where two perpendicularly-oriented fault zones (NW-SE and NE-SW strikes) intersect. The consideration of geomechanical properties in hazard evaluation is recommended as a risk management strategy, supporting the delimitation of regions near the rock slopes that should be restricted and of specific portions in the bedrock that should be retained. Our study was fundamental to establishing visitation procedures in the canyons, so that tourists and workers are more protected and aware of the existing geohazards.