Quantifying Antecedent Rainfall Effects on Landslides in the Garhwal Himalayas

Landslides are one of the recurrent precarious geological hazards that may prove fatal to life and property. In the Indian Himalayan region, the primary triggering factor contributing to landslides is rainfall. Recent advancement in rainfall threshold related studies have contributed significantly to a better understanding of the problem and the development of more accurate models at the local, regional and global levels. Although there are well-established studies on role of antecedent rainfalls and its criticality in the initiation of landslides, at present, there is no uniformly accepted method to consider effect of antecedent conditions or rainfall duration on stability of slopes. The antecedent period considers the influence of both soil moisture and groundwater on slope once the rainfall has ceased, since its effect is delayed due to hydrological attributes of the soil. Study from Uttarkashi region indicate that 15-day antecedent rainfall of around 109 mm can activates about 99% landslides in the area, highlighting the need to quantitatively estimate the likelihood of landslide incidents. For the present study, a decadal data on rainfall and landslide were curated from the Uttarkashi district of Uttarakhand state in India which comes under the Garhwal Himalayan region. These data were utilized to assess the influence of daily rainfall and antecedent rainfall on slope stability and to develop an empirical equation that predicts the probability of slope failure. The equation can be used as landslide warning for vulnerable zones if forecast precipitation values are available.