Dam Improvement Projects
In the fall of 1986, DSOD notified MWD that the seismic repairs to the dam must be completed by October 1988 or the Lake would be subject to a twenty foot drawdown. The project was well underway by the October deadline and the drawdown was not required. All work was completed in the summer of 1989 at a total cost of about $8.5 million.
Beginning in 1951, DSOD had required a three-foot drawdown from November to March each year because of spillway capacity. Following the completion of the 1988 modification project, DSOD rescinded the order as a result of improved dam safety. These seismic repairs were completed as Phase I of the total modification project.
Phase II work was originally scheduled to begin in 1990 and was dependent upon Caltrans’ relocation of the highway bridge to accommodate the installation of the spillway in bays 5, 6 and 8. However, the project was never implemented because the California Department of Transportation (Caltrans) had not yet constructed the new bridge. In 2002, the District’s Board of Directors determined that in the many years since the seismic retrofit was completed, the service spillways had deteriorated to the point that repairs were critical. Consequently, the Phase II work was divided into two separate projects, Phase IIA (Service Spillway Gate Repair) and Phase IIB (Installation of Auxiliary Spillways). The Caltrans highway bridge project was begun in 2009 and is scheduled to be completed sometime in 2011.
The original Bear Valley Dam service spillways consisted of five openings 4 feet wide by 6 feet high in bay 1 (north side of the Dam) and an identical arrangement in bay 10 (south side of the Dam). Each opening was closed by wooden bulkheads, which could be removed by hand-driven hoists as needed. In addition to reliability and safety, a major goal of Phase IIA was to maximize the release capacities of the spillway gates during a major storm event.
A Request for Proposal was prepared for the engineering services for Phase IIA and Christensen Associates Incorporated was awarded the project. In February 2003, construction bids were solicited and the project was awarded to John S. Meek Co. in the amount of $1,249,920. The final construction cost was $1,490,583, with most of the increase due to overages in excavation and concrete requirements.
The method selected for the spillway gate repairs made it possible for the work to be performed at any time regardless of Lake level, with all construction activity occurring downstream of the existing gates. The support structure was left in place until the new spillway system was completed. The project involved replacement of the existing wooden slide gates which were difficult to open and close. They were replaced with new stainless steel slide gates with remotely controlled motor operators in a configuration similar to the original gates. The timber operating deck and the deteriorated concrete piers and gate support structure were removed to below the gate sill level and new concrete piers and support structure were constructed. Due to the new location of the spillway gates, the highway precluded installation of a steel gate in the No. 10 gate position. This gate opening will contain wooden stoplogs until such time as the highway is removed by Caltrans and the new steel gate is installed. The combined release capacity of the new gates is estimated at approximately 3,690 cubic feet per second (cfs) as compared to the previous release capacity of approximately 1,300 cfs.
In addition to the gate and concrete repairs, the project involved significant erosion control work downstream of the dam. All costs for Phase IIA were paid from Assessment District 88-1 and Davis-Grunsky grant funds.
In February 2005, the District commissioned MWH Americas, Inc. to complete an assessment of the Bear Valley Dam. Issues included: repairs to the face of the dam, evaluation of the structural integrity of bays 5, 6 and 8, improvement to the dam outlet works, remote control of the spillway gates, cameras at the dam to verify releases and record downstream impacts, and evaluation of spillway capabilities. The report confirmed that bays 5, 6 and 8 are in need of repair as they constitute a “seismic risk”. When the 1988 seismic retrofit of the dam was completed, the top sixteen feet of these three bays were not filled with concrete. It was planned to install auxiliary spillway gates in each of these bays to accommodate Lake releases in major storm events. However, that project required the removal of the highway and this has not yet occurred. Consequently, the District determined it was critical that the three remaining bays be rehabilitated to eliminate the seismic risk, along with reducing the seepage that occurs in all three bays. The final engineering was completed and the bid process was initiated in June 2006. The project was awarded in July to John S. Meek Company at a construction cost of $614,555. Upon issuance of the permit to complete the project, the Division of Safety of Dams required that the District submit a work plan to address the following issues: completion of further hydrologic analyses for the 100-year storm and the PMP; protection of the cross-bracing bars in bays 5, 6 and 8 against corrosion; and repair and maintenance of the cracked and spalled concrete on the upstream face of the upper arches The work plan was submitted by the deadline and the work will be completed as financing, weather and lake level conditions allow.
In addition to the increased spillway capacity gained by rehabilitation of the service spillway gates, the District has improved its flood control capabilities with the adoption of a new Lake Release Policy. This policy allows the District to maintain the Lake at one foot below full between December 31 and March 1. This will improve the District’s ability to manage Lake releases in a major storm event.