Frog Lake Central Dam Reconstruction
Frog Lake Dam was built in 1953 to provide additional storage capacity for the Oak Grove Hydroelectric Project near Estacada, Oregon. The reservoir would allow Portland General Electric to cycle water levels to produce electricity at peak times of usage. However, the reservoir leaked excessively and prevented the full use of off-peak storage. Sinkholes would form in the reservoir floor and redevelop shortly after remedial backfilling. PGE retained Cornforth Consultants, Inc. (CCI) to evaluate the cause of the sinkholes and develop repair options. Geologic reconnaissance by CCI revealed that the reservoir was built on a massive, 3-square mile ancient landslide. The sinkholes were developing as landslide movements stretched and cracked the reservoir. Leakage from the reservoir also raised groundwater levels within the slide mass, exacerbating the ground movement.
Portland General Electric considered abandoning the facility. CCI recommended an alternative plan of rebuilding a smaller reservoir to reduce embankment loads and avoid the area of major sinkhole formation. Following reservoir reconstruction, groundwater and slope stability conditions dramatically improved. The facility now allows PGE to cycle reservoir levels to meet peak power demand. The increased revenues generated by cycling allowed PGE to recoup the cost of reconstruction in just a few years.