Integrated Numerical Assessment of Rapid Drawdown Stability of Kulekhani Rockfill Dam, Makawanpur Nepal Using GeoStudio Software

Abstract

The Kulekhani Rockfill Dam at Makawanpur District in Bagmati Province is the Nepal’s one and only large storage hydropower reservoir project that plays a critical role in country’s peak energy generation. The long-term safety assessment of such dam is very crucial because seepage conditions, reservoir operation and rapid drawdown may significantly influence its embankment stability and deformation behavior over time and continuous operations. This study presents an integrated numerical investigation of seepage, slope stability and deformation characteristics of the Rockfill Dam using the GeoStudio software where SEEP/W was used to evaluate seepage behavior and transient pore-water pressure response while SLOPE/W to assess its either slopes’ stability under steady-state and rapid drawdown conditions and SIGMA/W to investigate the deformation characteristics. The results from the analyses indicate that the embankment dam remains stable under normal operating conditions with factors of safety (FOS) of 2.006 and 1.627 for the upstream and downstream slopes respectively. Under rapid drawdown conditions, the factor of safety progressively decreased and reached a minimum value of 1.793 which indicates that rapid drawdown represents the governing loading condition for the safety of dam. The sensitivity analyses over different parameters showed that the friction angle of the rockfill shell is the most influential parameter controlling the slope stability of the dam. Introduction of an upstream stabilizing berm showed improvement in the factor of safety from 1.750 to 1.930, an increase of approximately by 10.3%. The model validation has demonstrated satisfactory agreement between numerical predictions and available design values of the Project. The study confirms that integrated numerical modelling provides an effective framework for evaluating the hydraulic characteristics, stability and deformation performance of rockfill dams. The findings highlight the importance of reservoir operation control and provide practical recommendations for future safety management of Kulekhani Dam and similar type of embankment dams in Nepal or elsewhere.