Dam break analysis of the Nagmati and Dhap dams using HEC-RAS

ABSTRACT This study examines the risks, vulnerability, and potential impacts of dam breaches, focusing on the Dhap and Nagmati dams in Kathmandu, Nepal. These dams are constructed to enhance river flow, but pose a risk of breaching, potentially causing severe damage, loss of life, and inundation of UNESCO World Heritage Sites. Despite these potential consequences, these dams have not been comprehensively investigated and no detailed scientific analysis has been conducted. This study aimed to assess the effect of Nagmati and Dhap dam breaches under the overtopping mode of failure and prepare flood inundation and flood hazard vulnerability maps. The analysis employs the Hydrologic Engineering Center-River Analysis System to simulate unsteady flow corresponding to the probable maximum flood, with flood hazard vulnerability mapping based on general flood hazard vulnerability curves guidelines. The results show peak discharges of 27,835 and 1,064 m³/s and velocities of 27.2 and 7.27 m/s for the Nagmati and Dhap dam breaches, respectively. World Heritage Sites fall under the H6 and H5 hazard zones after the Nagmati breach, with breach height being the most sensitive parameter. The finding highlights the impact of dam breaching and helps in land use planning, emergency response, and flood mitigation to reduce loss of life and property.