Safety of Dams Program

In Idaho, Safety of Dams modifications have been completed at Island Park , Deer Flat, Soldiers Meadow, Reservoir "A", and Deadwood dams.

Two phases of safety modifications were performed at Island Park Dam near Ashton on the Minidoka Project for a total cost of $6.3 million. The deteriorated concrete spillway was replaced in 1979. In 1983, modifications were made to the embankment to address seismic instability concerns. A major effort of this work included the excavation and replacement of a section of the right abutment and adjacent embankment.

Modifications were performed at Deer Flat Dam embankments near Nampa on the Boise Project from 1991 to 1993 for $20 million. Modifications consisted of construction of a filter and drain system in a deep interceptor trench and embankment berms along the downstream toe of the Upper and Lower Embankments. A soil-cement layer was also placed on the upstream face of the Lower Embankment as an erosion barrier. An emergency was declared on May 13, 2005 due to seepage and piping concerns at the Caldwell Canal outlet works conduit. An emergency berm was constructed around the Caldwell Canal gate structure and surrounding embankment. Monitoring continues as engineers and geologist evaluate alternatives for a permanent solution. Three other outlet works conduits are being evaluated as well to determine if similar concerns exist at these structures. Construction is scheduled for the fall of 2007.

Modifications at Soldiers Meadow Dam on the Lewiston Orchards Project were completed in 1986 at a cost of $1.7 million. Construction included upgrading the spillway crest and unlined channel, and seepage control measures on the embankment.

Modifications were completed at Reservoir "A" Dam also on the Lewiston Orchards Project in the spring of 1999 at a cost of $4.4 million. Modifications included excavating to bedrock along the downstream toe of the dam, placing a sand and gravel filer/drain on the exposed foundation, and constructing an embankment stability berm above the filter. The reservoir will continue to be managed with a water level restriction which was imposed in 1990.

Modifications were completed at Deadwood Dam on the Payette Division of the Boise Project in the fall of 2003 for $2.1 million. Approximately 60 rock bolts were installed in the downstream left abutment of the dam to stabilize the rock mass during a seismic event.

In Oregon, Safety of Dams modifications have been completed at Ochoco, Cold Springs, Crane Prairie, Fish Lake , Wasco, and McKay, Wickiup and Scoggins dams. Dam safety investigations are continuing at Arthur R. Bowman Dam.

Ochoco Dam, part of the Crooked River Project, was modified between 1994 and 1997 for a total cost of $35 million. Construction in 1994 and 1995 focused on embankment modifications, while work in 1996 and 1997 centered on modifications to the spillway and the addition of a spillway stilling basin.

Safety of Dams modifications at Cold Springs Dam on the Umatilla Project were performed during 1995 and 1996 for about $6.6 million. Modifications included removal and replacement of low strength materials in the foundation, installation of a filter/drainage system and stability berm along the downstream toe of the dam. The concrete spillway was removed and an enlarged RCC (roller compacted concrete) spillway constructed.

Minor modifications to the spillway at Crane Prairie Dam on the Deschutes Project were performed in 1992 at a cost of about $87,000. Purpose of the modifications was to upgrade the ability of the spillway to safely pass large floods. Modifications included raising the right spillway wall about five feet to prevent spillway discharges from potentially impacting the embankment.

Modifications at Fish Lake Dam on the Rogue River Project were performed in 1996 for $1.6 million. An auxiliary spillway was constructed and a rock buttress was added on the downstream face of the dam.

Construction at McKay Dam near Pendleton on the Umatilla Project commenced in September 1997 with modifications completed in December for $2.6 million. Modifications included excavation and replacement of low-strength foundation materials, construction of upstream and downstream berms along 700 feet of the left abutment, and seepage control and drainage system improvements.

Modifications to Wasco Dam near Maupin on the Wapinitia Project were completed in the fall of 1999 at a cost of about $1.2 million. Work included a filter trench at the downstream toe of the dam, drainage features, and a small stability berm. The upstream overflow weir in the gate structure was lowered 3.4 feet to control winter reservoir levels. The reservoir can be operated to full pool once access is established to the dam site in the spring so that regular monitoring and surveillance can be undertaken.

Modifications were completed in the fall 2003 at Wickiup Dam to upgrade the dam to meet current earthquake readiness standards. Seepage is a secondary concern being addressed by incorporating drainage features in remedial measures needed for seismic stability. Modifications include construction of embankment buttresses, filter and drain systems, and foundation densification treatment along the downstream toe of the dam using jet grouting methods. Estimated total project cost is $38.5 million.

Minor modifications to the arms of the radial gates at Scoggins Dam were completed in October 2003 for $65,000. Additional steel reinforcement on the arms was necessary to ensure the integrity of the radial gates under seismic loading conditions.

Investigations have identified a hydrologic safety deficiency in passing major flood events without overtopping the embankment at Arthur R. Bowman Dam, which impounds Prineville Reservoir, on the Crooked River Project. The project has been re-evaluated in recent years by applying new hydrologic techniques and a risk-based approach to overtopping. Modifications are scheduled for late 2007.

Safety of Dams modifications were performed at Como Dam near Darby on the Bitter Root Project from 1992 through 1995 at a cost of about $12.5 million. As part of this project, the State of Montana and local interests provided over $400,000 to raise the dam and reservoir by 3.3 feet to create an additional 3,000 acre-feet of storage for enhancement of instream flows in the Bitterroot River . Modifications included the construction of a deep seepage interceptor trench along the downstream toe of the dam. A sand and gravel filter/drain system was installed in the trench and an embankment berm constructed above the drain system. Major portions of the embankment were removed and reconstructed to a higher crest elevation. The concrete spillway structure was rebuilt with a new overflow ogee crest and a parapet wall added to the crest of the dam. The gatehouse and catwalk were replaced and new gate operators installed.

Safety of Dams projects in Washington include modifications at Bumping Lake Dam, Salmon Lake Dam , the breaching of Clear Creek Dam, and Keechelus Dam.

Modifications were performed at Bumping Lake Dam on the Yakima Project from 1994 through 1997 at a cost of about $7.5 million. Construction included a downstream stability berm with a filter and drain system, replacement of the concrete spillway, insertion of a steel pipe liner in the concrete outlet tunnel, and replacement of the concrete outlet chute.

Dam safety investigations revealed potential seismic instabilities in foundation materials at Salmon Lake Dam on the Okanogan Project. Construction began in July 2000, after a test section of the stone column foundation treatment in 1999 proved the method. Modifications included strengthening the foundation materials using stone columns and placing a buttress with filter and drain systems on the downstream face of the dam. Installation of steel pipe liner in the outlet conduit and construction of stilling basin at the base of the dam were completed in February 2003. Total project cost is estimated at $9.3 million.

An open void was discovered in June 1998 at Keechelus Dam near Easton on the Yakima Project. Subsequent investigations and explorations determined that widespread internal erosion of embankment materials was occurring at the dam. Modifications were performed on the dam between April 2002 and November 2003. Modifications included removing and rebuilding the dam cross-section to include embankment zones and drainage features, construction of a bentonite slurry cutoff wall in the right abutment, and construction of a downstream drain along the entire length of the dam. Estimated total project cost is about $36.5 million.

Clear Creek Dam on the Yakima Project was found to be unsafe for normal operations in December 1989, due to deterioration within the concrete arch dam. The reservoir was immediately drained and by April 1990, two openings were cut through the concrete, effectively breaching the dam and severely limiting the level of the reservoir pool behind the structure. Through the efforts of a public support coalition, a combination of Federal, State, and private funding was secured and the dam was rebuilt as a concrete gravity structure in 1993 at a cost of almost $5 million.

The Pacific Northwest Region includes the portion of Wyoming within the Snake River drainage including Jackson Lake and Grassy Lake Dams. Both dams have been the subject of Safety of Dams projects.

Jackson Lake Dam, located within Grand Teton National Park, is part of the Minidoka Project and underwent major Safety of Dams modifications during the period 1986 through 1988. Investigations had revealed that the dam was seismically unsafe. Modifications included removal of the embankment portion of the dam, densification of the foundation lakebed sediments using dynamic compaction and soil-mix wall techniques, rebuilding the embankment, replacing the upper portion of the concrete spillway/outlet section, and construction of bulkhead piers. Total project cost was $78 million.

Inspections in 1998 at Grassy Lake Dam discovered buckled floor slabs in the concrete spillway chute which would make the spillway unsafe to operate during flood events. Investigations have also determined that the drainage system at the downstream toe of the dam and abutments was not functioning properly. Construction is scheduled to be completed in the fall of 2005. Modifications completed in 2004 and 2005 included replacement of about 500 feet of the spillway chute, and construction of new toe drains and filter system on the left and right abutments. A third toe drain is scheduled for the summer of 2006. Total project cost to replace the spillway chute and to perform drain repairs is estimated at $6.3 million.