Assessing pipeline vulnerability to lahar flows in Umbul Wadon, Mount Merapi, using integrated HEC-HMS and HEC-RAS models

Volcanic debris flows, or lahars, triggered by intense rainfall pose a significant threat to critical infrastructure, particularly in regions surrounding active volcanoes like Mount Merapi. This study utilizes an integrated hydrological-hydraulic model to assess the vulnerability of a water distribution pipeline system in the Umbul Wadon area to lahar hazards. The analysis focuses on the lahar event of December 1, 2021, which caused widespread pipeline damage. Using existing HEC-HMS 4.8 and HEC-RAS 5.0.6 models, we simulated the event based on rainfall data, field surveys, and digital elevation models. The model was calibrated with field observations, successfully reproducing the event's timing and confirming a peak discharge that coincided with the reported pipeline failure. To project future risks, a 10-year return period design storm (184.3 mm) was simulated, indicating an average lahar flow depth of 2.18 meters. A subsequent spatial vulnerability analysis revealed that a substantial portion (73%) of the pipeline network is situated within these high-risk areas. The findings provide quantitative data essential for risk-informed infrastructure planning and underscore the urgent need for targeted mitigation strategies, such as pipeline reinforcement or rerouting, to enhance resilience in lahar-prone regions.