The Investigation Of Okhissa Dam Using A Real-Time Monitoring System

According to the National Inventory of Dams (NID), the United States has approximately 87,000 dams. Many of these dams are more than 50 years old and need to be investigated to ensure their safety to the public. A real time monitoring system can obtain a more reliable evaluation of the dam’s performance as well as serve as an early warning system to inform officials of a dam’s condition. Okhissa Dam was selected for investigation using a real-time monitoring system. Okhissa Dam is located in a region that contains both a phreatic aquifer and a confined aquifer. Prior to this investigation of Okhissa Dam artesian conditions were present. The Artesian conditions were alleviated by placing additional fill material on the downstream side. It is presumed these high piezometric water levels resulted from pressure in the aquifer due to communication from Okhissa Lake. The goal of this investigation is to determine if the reservoir is influencing the piezometric water levels and how these water levels can be used to evaluate slope stability and provide a real time warning. The comparison of water levels recorded by the shallow and deep piezometers to reservoir water levels show the reservoir has no influence on the shallow phreatic aquifer. However, the reservoir water levels are potentially affecting the deeper confined aquifer water levels. The largest fluctuation in the groundwater levels occurred with the intentional reduction of two feet in the reservoir level on December 16, 2014. This would have been the best predictor of variation in groundwater elevations at the dam, but the reservoir data was not collected due to sensor damage. Using the software GeoStudio Slope/W® Student Version, the dam is analyzed for slope stability. Okhissa Dam was analyzed for slope stability under conditions of steady state seepage and rapid drawdown. The results of steady state seepage analysis exceed the minimum design factor of safety of 1.5. The analysis for rapid drawdown is approximated due to insufficient knowledge of the behavior between the piezometric line and the upstream slope soils. The real time factor of safety readings show Okhissa Dam is stable