Flower Creek Dam
Upper Flower Creek Dam is located about three miles south of Libby, Montana on Flower Creek, and is operated by the City of Libby, Montana as part of their water supply storage system. The 58-foot high concrete arch dam was completed in 1945 for Pacific Power and Light. The Upper Flower Creek reservoir has a normal capacity of 221 acre-feet. The Lower Flower Creek Dam is used as a diversion point for the raw water intake which feeds water by gravity to the water treatment plant.
The dam has been inspected by MT-DNRC about every five years since 1980 to comply with the dam safety operating permit requirements. The dam has historically leaked reservoir water through the constructions joints, resulting in a buildup of calcium carbonate on the downstream face. This dissolution causes the concrete to lose strength over time.
In 1966 various areas of the upstream and downstream face were repaired with higher quality concrete. By 1978 the dam began to again show signs of seepage through the construction joints. In 1995, a synthetic liner was installed on the upstream face of the dam to reduce the amount of seepage. Despite these steps, the dam still leaks and calcium carbonate continues to build up on the dam face.
In 2009 an inspection revealed that seepage was becoming more severe than in the past. The DNRC then recommended that the city conduct core drilling of the dam to determine the strength of the concrete. Core samples indicated freeze-related cracking that extended from about 6 inches in from the fact to as much as 18 inches in from the face of the dam.
Conclusions of the sampling are:
- The overall effective strength of the concrete is likely less than 1,000psi and the factor of safety is no longer believed to be 2:1.
- There is a risk of immediate failure if a significant seismic event were to occur.
- There is a risk of failure if the reservoir level is cycled and/or temporarily drained.
- The original concrete was likely substandard and had an excessive water to cement ratio, leaving it more susceptible to deterioration.
As a result of the inspection of Upper Flower Creek Dam, the City of Libby has begun the process to replace the dam. It is estimated that the cost will be $5 million-$6 million. Construction on a new access road began this autumn and will continue next year when conditions permit. Core drilling of the bedrock at a site 200 feet below the present dam will be completed in 2012. It is the intent of the city to replace Flower Creek Dam within about a 5 year time frame.