Landslide Susceptibility Mapping and Rainfall Threshold Analysis of Chishankhugadhi Rural Municipality Wards 1, 4, 5 and 7, Eastern Nepal: A Gis Based Frequency Ratio Approach
This study utilizes a GIS-based Frequency Ratio approach to generate a landslide susceptibility map for four wards of Chishankhugadhi Rural Municipality in eastern Nepal, identifying high-risk zones associated with specific geological and topographical factors while analyzing local rainfall patterns against global thresholds to inform land-use planning and geotechnical investigations.
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In the rugged hills of eastern Nepal, the ground is never truly still. The landscape is shaped by a slow, powerful collision of tectonic plates that has pushed mountains upward for millions of years, leaving behind rock that is fractured, folded, and often weak. When heavy monsoon rains soak into these slopes, the water adds weight and reduces the friction holding the soil and rock together, causing them to slide downhill. This process, known as a landslide, is a frequent and destructive force in the region, threatening homes, roads, and lives. Scientists who study these hazards do not just look at where a landslide has already happened; they try to understand the specific conditions that make a slope likely to fail in the future. By mapping factors like the steepness of the hill, the type of rock beneath the surface, and how close a slope is to a road or a stream, researchers can create a picture of risk. This work is essential for communities trying to decide where it is safe to build and where they need to be most careful.
A team of researchers from Tribhuvan University recently applied this approach to Chishankhugadhi Rural Municipality, a community nestled in the Lesser Himalaya. They focused on four specific administrative wards where the terrain is steep and the geology is complex. To build their picture of risk, the team first created a detailed inventory of twenty landslides in the area, identifying exactly where they occurred, what type of movement they involved, and what materials they were made of. They then used computer software to layer this information over maps of the local environment. They examined seven key factors that influence slope stability: the angle of the slope, the direction the slope faces, the curvature of the land, the underlying geology, and the distance from streams, roads, and geological faults. By comparing the locations of the known landslides against these environmental layers, the researchers calculated a susceptibility score for every part of the landscape, grouping the area into zones ranging from very low risk to very high risk.
The resulting map revealed a clear pattern. The areas most likely to experience landslides are concentrated on slopes with angles between twelve and thirty degrees, which are steep enough to be unstable but often covered in soil or loose rock rather than solid bedrock. These high-risk zones are predominantly found on hillsides made of weathered phyllite, a type of rock that is common in the region and prone to breaking down into weak, clay-like material when exposed to rain. The map also showed that landslides are frequently associated with areas near roads and streams, where human activity and water flow can weaken the ground. Interestingly, while the researchers expected roads to be a major trigger, the statistical model showed that landslides were actually more common further away from the road network. The authors explain that this does not mean roads are safe; rather, it suggests that roads in this region are often built on gentler, more stable ground, while the steepest and most dangerous slopes remain off the main routes. However, field investigations confirmed that where roads do cut into steep hillsides without proper support, they do create local points of failure.
To understand why these slopes fail, the team collected soil and rock samples from eleven different landslide sites and tested them in a laboratory. The results painted a picture of materials that have very little natural strength to hold themselves together. The soil samples showed extremely low cohesion, meaning the particles do not stick to each other well, and moderate friction angles, indicating that the material slides easily when wet. In some places, the moisture content of the soil was as high as twenty percent, suggesting that the ground is often saturated. This combination of weak, water-logged material and steep terrain creates a perfect setup for failure. The researchers also looked at rainfall data from a nearby weather station over the last eleven years. They found that the region receives an average of 1,668 millimeters of rain annually, with more than eighty percent of that falling during the monsoon season between June and September. The rain does not just fall gently; it arrives in intense bursts, with some single days recording over one hundred millimeters of rain. While the team could not link specific rain events to specific landslides because the exact dates of many historical slides are unknown, the data confirms that the region experiences rainfall intensities that are known to trigger shallow landslides in similar mountainous environments.
The study concludes that the risk of landslides in Chishankhugadhi is not caused by a single factor but by the interaction of weak geology, steep topography, and heavy seasonal rain, all of which can be worsened by human activities like road construction and farming on steep slopes. The susceptibility map they produced is not a perfect prediction of where the next slide will happen, but it serves as a valuable screening tool. It helps local authorities identify which areas are generally more dangerous and need closer inspection or better drainage planning. The researchers emphasize that while their map provides a broad overview, it cannot replace detailed engineering investigations for specific sites. For the people living in these wards, the findings offer a clearer understanding of the landscape they inhabit, highlighting that the ground beneath them is dynamic and that careful planning is necessary to live safely alongside the forces of nature.
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