The Yelm Hydro Electric Project services the City of Centralia, Washington. The Yelm Hydro Project is located along the pristine Nisqually River. The Yelm Hydroelectric Project is a run-of-the-river hydropower generation facility consisting of a diversion dam on the Nisqually River, an intake structure at the dam, 9.1 miles of diversion canal with three spillways connected to the adjacent Nisqually River, a forebay structure and gatehouse, two penstocks and a powerhouse. The powerhouse contains three electric generators. The initial construction spanned 1929 to 1930. Several modifications have been made, the most recent in 2015.
Section of the intake canal investigated for seepage.
In 2016 Underwater Acoustics International, a division of C. H. Fenstermaker & Associates, L.L.C., was contracted to provide a solution to examine the canal bottom for indications of seepage and look for near bottom subterranean water flow indications in a defined reach of the 9.1 mile long intake canal. Also in the scope was the acoustic imaging of the submerged spillway components of the spillway designated as “Spill 1”.
In 2017 a subsequent effort in another reach of the intake canal was conducted. The intake canal configuration presents a challenging environment for acoustic sonar investigations as it is narrow – 20’ or less in width in most places and approximately 17’ deep at the deepest point in the center of the canal. With steep slopes along each bank, these conditions pose sonar data interpretation challenges. UAI, developed a data analysis solution that allowed for interpretation of the sonar and shallow seismic data to define areas that could correlate to seepage and also define areas that could correlate to subterranean water saturation or flow zones.
An underwater imaging investigation was performed to document the disposition of the spillway concrete structure
in relation to the surrounding embankment. A depiction of the results is shown below.
UAI’s investigation and subsequent report gave Centralia City Light scientific data and images to help identify anomalies and develop corrective actions. Micah Goo, Generation Manager of the Yelm Hydroelectric Project for Centralia City Light, was pleased with the investigation performed by Underwater Acoustic International’s UAI team. “Your data and images were a great help in us getting a better understanding of what we were seeing. During you 2017 survey of our Zone 15 and the area of Spill 1, you again helped us in identifying issues. Because of your detailed analysis, you were able to identify some anomalies and provide some insight to what we might be seeing.”