This is a technical case study report demonstrating monitor, assess, and quantify high mountain geohazard risks for government agencies based on the flooding and landslides in Melamchi watershed in Nepal. On June 15, 2021, the Melamchi river experienced massive flooding and caused disastrous damages in the downstream. The overall situation was intensified by the heavy rainfall and possible glacial lake outburst. A second flooding event equally severe as the first one was observed on August 1st. Following the flooding, the World Bank initiated three activities to support the national disaster risk reduction and management authority (NDRRMA) and other governmental agencies to plan for future actions. The drone survey was carried out from July 6th to August 30th, 2021, and covered a river stretch of approximately 100 km. The survey provided timely documentation of the damages from the event that can be used to calibrate and validate the flood modeling. The products derived from the drone survey include georeferenced orthorectified images, 3d-point cloud, digital terrain model (DTM), and approximate estimates of erosion and deposition along the channel. The flood was modeled using hydrologic engineering center’s river analysis system (HEC-RAS) as a two-dimensional non-Newtonian flow with the rainfall data collected from the global precipitation measurement (NASA), and a high-density mix of water and debris (10 percent debris in water) was selected for the final non-Newtonian dam breach model. The close coordination and data share with partners equipped the government and other stakeholders with useful quantitative information to better understand the complex and disastrous flood event that occurred in the Melamchi river and provided a basis for future risk reduction and management.