Integrated debris flow hazard and risk assessment using UAV data and RAMMS, a case study in northern Pakistan

The debris flow is one of the most devastating geological hazards. This study evaluates debris flows in the mountainous region of Balakot, northern Pakistan, with a focus on the Hassa debris flow. Employing the advanced RAMMS-DF numerical simulation tool, a comprehensive hazard and risk assessment was undertaken, utilizing the Voellmy-Salm model. Calibration of the model was achieved through a meticulous back-analysis of a notable debris flow event on October 24, 2023, using a DJI Inspire II unmanned aerial vehicle (UAV), facilitating the determination of crucial frictional parameters (= 0.07, = 550 m2/s). These parameters were instrumental in conducting hazard assessment for three potential release areas. Evaluating the NRMSE of UAV-DSM yielded 92.21%, indicating that the collected topographic data is accurate. Based on the geomorphology, the debris flow is categorized into three zones: depletion, accumulation, and transition zones. The results highlight potential scenarios where debris flows could affect the Hassa village community and infrastructure, with calculated economic losses of more than USD 20 million. This study also integrated landslide hazard, vulnerability, and risk assessment on a larger scale, with 44.73% of the area in the moderate, 34.98% of the area in the high, and 15.5% of the area in the very high hazard zone, with more than 479 houses structures, including schools and hospitals, at significantly high risk of debris flow. The economic losses estimated in the area due to risk may range from 41624.40 USD to 752402.54 USD which constitutes around USD 02 million. The significance of this study lies in its ability to offer numerical insights into flow intensity factors, especially in regions with limited historical data. By combining hazard assessment with vulnerability analysis, the study provides a holistic perspective on the potential impacts on surrounding communities and infrastructure. This research emphasizes the significance of proactive risk mitigation techniques in light of the identified threats. To improve overall resilience and safety for the local population and infrastructure, the findings are intended to direct relevant agencies in the development and implementation of efficient strategies to limit the effects of debris flows in the mountainous region of northern Pakistan. In light of the identified risk, this research underscores the importance of proactive risk mitigation strategies.