Understanding the formation mechanism of rainfall and snowmelt jointly induced Bicharh Nallah debris flow, North Pakistan

Abstract

Heavy and intense rainfall commonly triggers debris flow events, but the relationship between gentle rainfall, snowmelt water, and freeze–thaw erosion in high-altitude cold regions of northern Pakistan remains unexplored. A devastating debris flow from Bicharh Nallah hit the village of Sherqilla in northern Pakistan on July 5, 2022, and killed 8 people and destroyed 10 houses completely and 250 houses partially. The Bicharh Nallah debris flow (Sherqilla village) in Ghizer district is used as a case study to find out what makes this size debris flow happen in a cold region and how it is affected by rainfall, snowmelt water, and freeze–thaw erosion. This work was done through fieldwork investigation, laboratory work, and statistical analysis. The debris flow dynamic characteristics such as peak discharge, velocity, and density were calculated as 430.82 m³/s, 5.11 m/s, and 1.74 g/cm³, respectively. We determined that rainfall acts as a direct triggering factor. Snowmelt water was quantitatively calculated, which contributed to triggering the debris flow. This study revealed that gentle rainfall and snowmelt water are direct triggering factors in the debris flow outbreak in Bicharh Nallah of Sherqilla village. Long-duration, extreme, and severe freeze–thaw erosion provided enough loose soil conditions for the triggering and formation of debris flow. The regional tectonics, geology, and local topography indirectly contributed to the development and amplification of Bicharh Nallah debris flow.