DETERMINATION OF THE PERMISSIBLE RAPID DRAWDOWN RATE FOR EMBANKMENT DETENTION DAMS

Rapid Drawdown Rate (RDDR) is larger in detention dams than storage dams due to faster flood routing in their reservoirs. It can be several meters per day, and is usually higher than the permissible RDDR proposed by existing standards. The main objective of this research is to determine the permissible RDDR based on the permissible factor of safety (FOS) for rapid drawdown analyses. In this research, rapid drawdown analyses were carried out using SEEP/W and SLOPE/W software programs. Further, FOSs were compared for static water level (SWL) and Rapid Drawdown (RDD) analyses, and by 8 slope stability analysis (SSA) methods in three Stability Analysis Groups (SAGs). SAG1, SAG2 and SAG3 are the type of stability analysis methods that satisfy all equilibrium conditions, all equilibrium conditions except horizontal forces and all equilibrium conditions except moment equilibrium, respectively. Furthermore, reduction percentage of the factor of safety (D%) is defined to determine reduction of FOS by the RDD analyses. The result of SSA for SWL analysis disclosed that simplification assumption and equilibrium condition satisfaction have a major influence on the variation of FOS. Moreover, the variations of FOS with initial water level in RDD analyses were amplified by increasing RDDR compared to SWL analysis. However, similar to SWL analysis, SAG1 and Bishop methods estimated FOS appropriately by using complete satisfaction of equilibrium conditions, avoiding overestimation of SAG3 methods and conservative estimation of Janbu and Ordinary methods. In addition, since D% usually exceeds permissible value, RDD analysis is more important for SSA of the upstream slope of embankment dams. A permissible RDDR of 3 meters per day is suggested for the presented case study. This research shows that the current standards (USBR and IDNR) used for permissible RDDR can be improved, and in any case study, RDD analyses is needed to decide permissible RDDR.