Subsurface seepage flow at Coursier Lake Dam was identified by onshore and offshore self-potential surveys, and electrical resistivity profiles and soundings during a Deficiency Investigation by BC Hydro. For typical seepage investigations baseline geophysical data are collected at low-pool level and the measurements are repeated when high hydraulic gradient conditions exist. At Coursier Lake Dam a rather unanticipated outcome of the low-pool surveys was that significant seepage beneath the structure’was detected. The low-pool results were conclusive enough that, when combined with visual inspection and observation of sinkholes on the embankment, an immediate restriction was placed on the pool elevation. Thus, because of the identified potential hazard, the remaining geophysical investigations were conducted under a “minimum-pool” reservoir level in order to complete the comparative study. Therefore, the dam was studied under low- and minimum-pool reservoir conditions in the spring and fall of 1993, respectively.Low-pool data indicated very high resistivities (3000 to 5000 ohm-m) throughout the embankment indicating a coarse-average grain size, probably unsaturated sands and gravels. Higher resistivities (>5000 ohm-m) were obtained within the foundation deposits along the downstream toe indicating a combination of lower moisture content, coarser average grain size and higher porosity than the embankment. These electrical data indicate the subsurface conditions in the embankment and the foundation to be conducive to seepage.
Results from low-pool SP surveys, performed both on-shore and offshore, indicate a dispersed or sheet-flow seepage occurring nearly 1100 feet upstream of the intake. Therefore, apparently the seepage source begins far upstream of the embankment within the foundation deposits. Modeling the SP data indicated a number of anomalous areas interpreted as concentrated seepage flow paths within the foundation and lower portion of the embankment.