Seepage Investigation Using Geophysical Techniques At Coursier Lake Dam, B.C., Canada

Abstract

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 BChydro. For typical seepage investigations baseline geophysical data are collected at @llow-pool I1 level and the measurements are repeated when high hydraulic gradient conditions exist. At Coursier Lake Dam a rather unanticipated outcome of the low-no01 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 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 o'ffshore, 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. The minimum-pool data, acquired when only a stream entered the intake structure, confirmed the low-pool electrical and SP interpretations. Remediation of the seepage problems at Coursier Lake Dam was completed in 1996 utilizing a combination of an upstream cutoff trench to reduce the seepage flow through the foundation, and placement of a geomembrane (tied into the cutoff trench) over approximately twothirds of the core to reduce the flow gradients within the embankment.