Rockfalls are a common natural hazard in mountainous regions, especially in the Andes, where steep slopes, seismic activity, and climate change can trigger them.
They can cause damage to roads, railways, pipelines, power lines, and buildings, as well as injuries and fatalities to people.
Therefore, quantifying the risk associated with rockfalls is essential for planning and implementing mitigation measures.
A new method to assess rockfall risk in the Andes
(Photo : MIGUEL MEDINA/AFP via Getty Images)
A team of researchers from the University of Chile, the University of Lausanne, and the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL) has developed a new method to assess rockfall risk in the Andes.
The method combines field observations, remote sensing data, numerical modeling, and statistical analysis to estimate the frequency, magnitude, and impact of rockfalls in different scenarios.
The researchers applied the method to two case studies in Chile: the Cajón del Maipo valley, near Santiago, and the Elqui valley, in the Coquimbo region.
They collected data on the characteristics and distribution of rockfalls, such as size, shape, volume, and location, using drones, laser scanners, and cameras.
They also used satellite images and historical records to identify the factors that trigger rockfalls, such as earthquakes, rainfall, snowmelt, and temperature changes.
Using a software called Rockyfor3D, the researchers simulated the trajectories and velocities of thousands of rockfalls under different conditions, such as slope angle, friction, and vegetation cover.
They then calculated the probability of rockfalls reaching and damaging different elements at risk, such as roads, buildings, and people, using a statistical model called Bayesian Network.
Rockfall Mitigation Measures and Their Effectiveness
To mitigate the hazard of rockfall, broadly two types of techniques are used, namely active protection and passive protection techniques.
Active protection measures are those which prevent the rockfall from occurring, whereas the passive protection measures reduce the damage caused by the falling rocks. These include interventions on the slope that reduce the likelihood of a rockfall occurrence.
These can be accomplished by removing loose rocks in a controlled manner (removal), by changing the configuration of the slope (re-sloping), by securing the rock blocks in place (slope reinforcement) or by slope drainage.
Removal refers to partial or complete removal of loose or unstable rocks to reduce the occurrence of rockfall.
It may include the modification of the slope profile; horizontal benches are very effective as they reduce the tensional forces of the slope, decelerate erosion rate, and stop falling blocks depending on the width of the bench.
Meanwhile, slope reinforcement aims to secure prone rock blocks in place, reducing the occurrence of rockfalls. Various types of slope reinforcement are available, these are: dowels, bolts, and anchors that can be used for individual blocks or for rock masses.
Slope drainage aims to reduce the pore water pressure within the rock mass, which can increase the stability of the slope.
Passive protection measures include interventions at the toe of the slope or along the path of the falling rocks that reduce the impact or divert the trajectory of the rockfalls.
These measures can be classified into three categories: barriers, rock sheds, and catchment areas. Barriers are structures that intercept and stop the falling rocks before they reach the protected area.
The protected area and allow the falling rocks to slide over them. They can be made of concrete, steel, or timber.
Catchment areas are zones that collect the falling rocks and prevent them from reaching the protected area. They can be natural or artificial, such as ditches, berms, or forests.
The effectiveness of these measures depends on several factors, such as the size, shape, and velocity of the falling rocks, the geometry and material of the slope, the design and maintenance of the structures, and the environmental conditions.
Therefore, it is important to conduct a comprehensive risk assessment and select the most suitable and cost-effective measure for each case.
Also Read: Beneath the Andes Mountains: Earth's Crust 'Dripping' Into the Planet's Interior
The results and implications of the study
The results of the study showed that rockfall risk varies significantly depending on the location, the season, and the triggering factor.
For example, in the Cajón del Maipo valley, the risk is higher in winter than in summer, due to the effect of snowmelt and freeze-thaw cycles on the stability of the rocks.
In the Elqui valley, the risk is higher in spring than in autumn, due to the influence of rainfall and temperature fluctuations on the rock fractures.
The study also revealed that the most vulnerable elements at risk are the roads, which are exposed to frequent and large rockfalls, especially in narrow sections of the valleys.
The researchers estimated that the annual expected loss due to rockfall damage to roads is about 1.5 million US dollars in the Cajón del Maipo valley and about 0.5 million US dollars in the Elqui valley.
They hoped that their method can help decision-makers and stakeholders to evaluate and reduce rockfall risk in the Andes and other mountainous regions.
Furthermore, they suggested that the method can be used to identify the most critical areas and seasons, to design and implement effective mitigation measures, such as rockfall barriers, nets, and fences, and to improve the emergency response and recovery plans in case of rockfall events.
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