In January 2009, a heavy rainstorm brought record-high inflow to the Eagle Gorge Reservoir in Washington State, southeast of Seattle. Impounding the reservoir is the Howard Hanson Dam, which controls the flow of the Green River to mitigate flooding in the Green River Valley (see figure 2). At the beginning of the storm, the dam was operated in minimum discharge mode; that is, it was set to store as much water as it could accommodate to limit the chance of flooding in downstream communities. Inside the reservoir, the water level quickly rose to 1,189 feet above sea level.
As the water accumulated, a high rate of sediment-laden seepage was observed in the drainage tunnel behind the right abutment. While it is common for a small amount of water to seep through earthen dams, an increased rate of flow or the presence of sediments is usually an indication of erosion caused by frictional forces1. The erosion can cause “piping”—a type of structural weakening that is responsible for a third of earthen dam failures in the United States. To avoid catastrophic failure, the operator of the dam, the United States Army Corps of Engineers (USACE) Seattle District, took immediate action to lower the reservoir level. Subsequent examination of the structure revealed two depressions in the right abutment (shown in Figure 1), which were believed to be the initial signs of piping.