Two locations outside the tunnel are used to monitor leakage. Surface leakage from the tunnel is measured at a weir located about 150 ft below the tunnel portal. Leakage also is monitored by measuring flows in two culverts that pass under a U.S. Forest Service road farther down the slope. Flows measured at the weir are principally due to direct leakage from the tunnel, while those observed at the road culverts farther down slope represent a combination of both tunnel leakage and the natural runoff from the area. It is likely that some of the leakage is lost to the groundwater in the rock mass below the tunnel and is not measured either at the weir or the culverts. Measurements of flow at the weir and at the road culverts were taken one day before the tunnel was refilled with water (November 16, 2001). At that time, the tunnel had been empty for about six weeks and the area had experienced several days of heavy rain. As expected, no flow was observed at the weir. The depth of flow through culvert 1 was approximately 2 inches (0.57 cfs) and at culvert 2 there was no flow (i.e., only dripping). These values can be taken to represent base flow conditions under wet, winter-like conditions. On December 3, 2001, sixteen days after the tunnel was refilled, the flows at the weir and at the culverts were again measured. At the weir there was still no flow, indicating that the leakage water that had formerly flowed directly from the tunnel portal area had ceased. Along the lower road, the depth of water in culvert 1 measured 1.75 inches (0.43 cfs) and culvert 2 was dry (0 inches). The slight decrease in this measurement corresponds to drier weather at the time of observation and the lack of tunnel leakage. In the past, tunnel leakage was easily observed as significantly increased flows at both the weir and the culverts within 24 hours of filling the tunnel with water.
展开▼
