The Metropolitan Water District of Southern California approved the repair of the forebay lining and outlet tower at Lake Mathews.
The Nov. 14 MWD board vote awarded a $3,248,000 contract to J.F. Shea Construction for the work while appropriating $4,570,000 for the project. The board action also found the project categorically exempt from California Environmental Quality Act review.
The Colorado River Aqueduct, which was built in the late 1930s, runs from Parker to Lake Mathews, and MWD’s supply from the Colorado River is distributed to Lake Mathews. Water stored in the Lake Mathews reservoir is withdrawn from the lake’s forebay and hydroelectric plant and conveyed through MWD’s Upper Feeder to the F.E. Weymouth Water Treatment Plant in Laverne and through the Lower Feeder to the Robert B. Diemer Water Treatment Plant in Yorba Linda. Lake Mathews was constructed in the 1930s as part of the Colorado River Aqueduct project, and the original facilities included the main dam embankment, the initial outlet tower and the forebay with a separate outlet tower.
The lake volume was increased to its current capacity of 182,000 acre-feet in 1961, when the main dam embankment was raised and two dikes were constructed. The Lake Mathews Hydroelectric Plant adjacent to the forebay was constructed in 1980 and can generate up to 4.9 megawatts of power, which allows for annual MWD revenues of $1.3 million. A second outlet tower for the lake was completed in 2004.
The Lake Mathews forebay is a reinforced concrete reservoir with a capacity of 31 acre-feet. The outlet tower is a rectangular concrete structure 60 feet in height with steel walkways and a steel control building at the top of the tower. Fixed-cone valves control the release of water into the lake, although that valve action produces moist conditions above the forebay’s water surface which creates a corrosive environment and has gradually damaged the concrete, reinforcing steel, steel walkways, control room and forebay lining. Although the structural integrity of those facilities is currently sound, the extent of deterioration has increased over the past decade, and repairs will be required to ensure reliable deliveries into the Central Pool. The California Division of Safety of Dams requires that the forebay and outlet tower remain operational at all times.
MWD’s March 14 meeting authorized the final design of the forebay repair, along with rehabilitating the electrical distribution system and repairing portions of the hydroelectric plant’s concrete structure. HDR Engineering, along with six consultants, performed the final design phase of the forebay tower repairs and cathodic protection system while MWD staff performed the final designs for the repairs of the forebay lining and the replacement of the steel walkways and control building.
The scope of work for the contract advertised for bid, Aug. 18, included removing the spalled and damaged concrete down to sound concrete, replacing severely corroded reinforcing steel, repairing less corroded reinforcing steel by sandblasting and applying a corrosion inhibitor, installing a cathodic protection system on the forebay tower, replacing damaged concrete on the tower and in the forebay and replacing the steel walkways and control building of the forebay’s outlet tower. Three bids were received by the Oct. 4 deadline. The $3,248,000 bid by J.F. Shea, which is headquartered in Walnut, California, was below the engineer’s estimate of $3,300,000 and complied with the requirements of the specifications. Kiewet Infrastructure West Co. of Santa Fe Springs, California, submitted a bid of $3,546,000, while Mladen Buntich Construction of Upland, California, sought a $6,817,500 proposal. The J.F. Shea proposal includes the use of eight subcontractors.
A shutdown of the Upper Feeder, Lower Feeder and forebay is scheduled for February 2018, so repairs will be performed during that time. The tower and lining repairs are expected to be complete by September 2018. MWD staff activity will include the dewatering of the forebay and the return of the facility to service, and MWD will also perform inspection of the construction. The $4,570,000 appropriation amount includes $400,500 for inspection, $132,000 for shutdown activities, $265,000 for submittal review and responding to contractor requests for information, $79,000 for project management and preparation of record drawings, $65,000 for technical support provided by HDR Engineering and $380,000 for contingency.