Seismic Upgrade of the Roslyn Lake Embankment Dam

Roslyn Lake Dam is part of the Portland General Electric Company (PGE) Bull Run Hydroelectric Project, FERC No. 477, located approximately 20 miles east of Portland near the town of Sandy, Oregon. The dam impounds a 2,011-acre-foot reservoir that forms the headwater for hydroelectric power generation at the 21 MW powerhouse on the Bull Run River.
Roslyn Lake Dam consists of four earthfill dikes originally constructed in 1911. The dike addressed in this paper is referred to as the "west dike" and is shown in Figure 1. It is a homogeneous earthfill embankment with a maximum height of about 24 feet above original ground. The upstream slope is approximately 3H:1V, the downstream slope is approximately 1.5H: 1 V, and the crest is approximately 17 feet wide at El. 657.
Outflow from the reservoir is controlled by two penstocks located at the northeast corner of the lake which convey water to the Bull Run powerhouse. The reservoir is located on a plateau with no significant drainage area into the reservoir. Inflow into the reservoir is controlled by the diversion of up to 800 cubic feet per second from Little Sandy Dam and Marmot Dam. In addition, up to 170 cubic feet per second is sometimes diverted to Roslyn Lake from the City of Portland's two water supply pipelines (49 and 54 inches in diameter). There is no service spillway at Roslyn Lake Dam since it is an off­stream reservoir, however, there is a low area in the south dike that acts as a fuse plug spillway.