News Release
| Release Number: | 00-072 |
| Dated: | 5/4/2000 |
| Contact: | Matt Rabe, 503-808-4510 |
Portland, Ore. – Dam safety and geotechnical engineers from the U.S. Army Corps of Engineers say Cottage Grove Dam is safe, despite a newly-developed water seep coming from under the dam.
"Our findings tell us there is no danger," said Rich Hannan, geotechnical engineer with the Corps’ Portland District. "This dam has been here for 60 years, and it appears to be in as good a shape today as it was when constructed." Hannan said dams actually become stronger over time as the material used during construction settles and compacts.
"All dams experience some amount of seepage," he said. "These are not completely impervious structures and they are built with the appropriate drainage systems to collect and remove the water. We have found that this seep generally occurs after the reservoir fills above elevation 779.
"The good news is the water is flowing naturally under the dam, and not flowing through the structure, which could result in erosion of the dam." Hannan pointed out the water collected below the dam is clear, an indication no erosion of the earthen embankment is taking place.
Chemical analyses of the water confirm it is coming from the reservoir rather than some other underground source, and is concentrated near the west end of the dam.
Engineers have been monitoring the dam since last August – when the seepage first became apparent to workers. The Corps will continue to monitor the seep and will conduct additional studies of the ground under and near the dam this summer to better understand the geological forces at work in the area, Hannan said. This will help engineers determine why the seep developed now.
Hannan said steps could be taken to strengthen the structure if those studies determine strengthening is needed.
As with all dams the Corps has built, engineers studied the foundation materials and other design parameters before beginning construction on Cottage Grove Dam. They also develop and use hydrologic data – rainfall, snow pack and stream flow – under exaggerated conditions to produce worse-case water volume scenarios for design purposes.
Instruments built within the dams measure movement, water forces, internal strains and seepage. This information is gathered regularly throughout the year for engineers and scientists to use in assessing whether the dams are functioning properly.
The dams also are given a complete check-up every five year when inspectors from most every engineering discipline look at the structures. Because of this thorough inspection and maintenance program, any problems, flaws to the original design or operational abnormalities can be detected and corrected before they become serious.
Cottage Grove Dam was last inspected in 1997.
Cottage Grove Dam, located six miles south of Cottage Grove, Ore., on the Coast Fork Willamette River, was dedicated in 1942. It is an earthen embankment dam with flood damage reduction as the primary authorized purpose. It also is used for recreation, irrigation and improved downstream navigation.