Runout Characteristics of Rainfall-Induced Debris Flow: A Case Study from Sonapur, Meghalaya, India

Extreme rainfall and human activities in mountainous terrains lead to slope instability and pose threats to infrastructure and lives. A notable example is the catastrophic debris flow on 14th June 2023 at Sonapur, Meghalaya, India, which disrupted NH-6, a critical corridor connecting Guwahati to Mizoram and Tripura, affecting similar to 4.9 million people. This study examines the factors initiating the devastating Sonapur debris flow event. The Atterberg limit test on collected field samples revealed a liquid limit of 27.14%, plastic limit of 15.50%, and free swell index of 30%. RAMMS::Debrisflow simulation analyzed flow characteristics for three hypothetical debris volumes (V-1 = 7343 m(3), V-2 = 14,686 m(3), V-3 = 22,030 m(3)). Peak flow height was observed at 150 s for all volumes after flow initiation. Additionally, the analysis of diffusion flow momentum (DFM) revealed flow behavior in two distinct phases (rapid phase and gentle phase). The rapid phase was characterized by sharp decline in DFM between 75 and 175 s with curve slope of similar to 82 degrees, similar to 71 degrees, and similar to 66 degrees for V-3, V-2, and V-1, respectively. The faster decrease of DFM in the rapid phase can be due to the steep slope and the sudden release of saturated debris material with high velocity. The gentle phase was characterized by the gradual decrease in DFM between 175 and 600 s with a curve slope of similar to 11 degrees similar to 8 degrees, and similar to 4 degrees for V-3, V-2, and V-1, respectively. The larger surface area interaction of the debris material with the underlying terrain reduced the debris flow velocity in the gentle phase. The findings from this study enhance the understanding of how flow volume and slope morphology influence the movement characteristics of debris flow.