Living on landslides: seasonal rainfall effects on rates of movement in highly urbanized unstable areas in Gangtok, Sikkim Himalaya

The Himalayan State of Sikkim is prone to some of the world's largest landslides which have caused catastrophic damages to lives, properties and infrastructures in the region. The state capital, Gangtok, has experienced rapid population growth over the last decades. The rate of urban expansion has led to encroachment and development on unstable slopes and unplanned construction and frequent violation of building by-laws and regulations. Significant areas of the city experience constant displacement due to the presence of relatively weak rock formations comprising of schists and phyllites. While some urban areas have been completely abandoned due to the structural damages in residential housing, schools, and office buildings, often these buildings are simply repaired or replaced.

Gangtok is draped over several relatively steep hillsides. In this study, we have used synthetic aperture radar interferometry (InSAR) to understand the patterns of displacement better, highlighting areas prone to landslides. The rates of movement of these highly urbanized unstable areas have been measured using data collected by the Sentinel-1 satellites between 2015 and 2021. Field investigations have also confirmed the ongoing ground surface displacements shown in the InSAR results.

Discretely bounded areas in Gangtok are moving at rates sometimes exceeding 12 cm/year. In this study, we concentrate our analysis on three landslide areas where people are residing: Tathenchen, Chanmari and Upper- and Lower-Sichey. In each of these areas, movement is continuous throughout the year. However, distinct periods of acceleration and deceleration are clearly linked to seasonal monsoon rainfall. For example, the velocity of the Upper Sichey landslide varies from about 3 cm/year to 7 cm/year, with peak velocity being reached shortly after peak precipitation each year. In addition, less than half of households in the region are connected to a wastewater network, resulting in significant amounts of water seeping into the local ground.

The type of displacement information obtained by InSAR monitoring is helpful for developing effective mitigation strategies that can limit landslide damage. For example, while rainfall cannot be controlled, better drainage networks can mitigate the local effects. In a broader perspective, the data can be used within urban development planning to identify risk areas and monitor potential zones of catastrophic collapse.