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MWD approves design of two Lake Mathews rehabilitation projects
Friday, February 21st, 2014
Issue 08, Volume 18.
The MWD board action February 11 authorized the preliminary design to perform repairs and replace the Howell-Bunger valves at the lake’s forebay facility and authorized the final design to repair the structure at the hydroelectric plant. The action also appropriated $1.94 million for the two design projects and adopted findings that the project is categorically exempt from California Environmental Quality Act review.
MWD constructed the Colorado River Aqueduct in the 1930s which allows water to be transported from the Colorado River into Lake Mathews, which is the western end of the aqueduct. Untreated water stored in Lake Mathews is withdrawn through the lake’s forebay and hydroelectric plant and then conveyed through the upper feeder to MWD’s Weymouth Treatment Plant in Laverne and through the lower feeder to MWD’s Diemer Treatment Plant in Yorba Linda.
The original Lake Mathews facilities included the main dam embankment, the main outlet tower, and the forebay with its own outlet tower. In 1961 the main dam embankment was raised and two dikes were constructed to increase the lake volume to its current storage capacity of 182,000 acre-feet. The hydroelectric plant adjacent to the facility was constructed in 1980 and can generate up to 4.9 megawatts of electricity each year, which equates to annual revenue of up to $1.3 million.
The forebay and the outlet tower have been in continuous use for 75 years. The forebay itself is a reinforced concrete reservoir with a storage capacity of 31 acre-feet. The outlet tower is a 60 foot tall concrete structure with steel walkways and a steel control building at the top of the tower. The discharge facility has ten Howell-Bunger valves, each 32 inches in diameter, which control flow from the lake into the forebay to dissipate the excess energy. The water exits the forebay through the outlet tower and enters the upper and lower feeders
The water exiting the discharge values into the forebay creates moist conditions above the forebay water surface and thus produces a corrosive environment. Over the years damage has occurred to the concrete, the reinforcing steel, the steel walkways, the control room, and the lining of the forebay. Although the structural integrity of the facilities is currently still sound, the rate and extent of concrete deterioration has increased during the past decade and repairs are needed to maintain reliable deliveries into the central pool.
The ten Howell-Bunger valves have also deteriorated gradually due to continuous use. A 2013 assessment concluded that a planned replacement program would be more cost-effective and less disruptive to water deliveries than replacing or refurbishing the valves as they fail. The forebay outlet tower also has five butterfly valves, each 54 inches in diameter, and four large slide gates which have been recommended for refurbishment or replacement.
Due to the long lead time to manufacture the Howell-Bunger valves, MWD staff recommended procurement prior to the actual rehabilitation so that the prepurchased valves could be provided to the repair contractor for installation during that future contract.
The planned scope of the repairs includes removing cracked, spalled, and damaged concrete down to sound concrete, replacing severely corroded reinforcing steel while repairing less corroded reinforcing steel through sandblasting and applying a corrosion inhibitor, installing a cathodic protection system to inhibit additional corrosion, replacing damaged concrete with new concrete, replacing the outlet tower’s steel
walkways, sandblasting and coating the steel control room, replacing the Howell-Bunger valves while installing valve discharge deflectors to reduce the fog-like conditions, and refurbishing or replacing the butterfly valves and slide gates. The estimated cost of the entire project, including the design funds requested and past expenditures, is between $5.5 and $7.5
The preliminary design phase activities include conducting a three-dimensional survey of the discharge facility, forebay lining, and tower to create record drawings, mapping the extent of surface-damaged concrete, shutting down the facility for field inspection to assess existing conditions of the piping, Howell-Bunger valves, butterfly valves, slide gates, and other ancillary equipment, determining the strength of the existing concrete through concrete coring and laboratory testing, testing for hazardous material, preparing conceptional layout drawings and environmental documentation, developing the final design criteria, developing valve procurement specifications and the receipt of bids for the new Howell-Bunger valves, and developing a more refined construction cost estimate.
The $1.75 million authorized for preliminary design covers $895,000 for the design work itself, $270,000 for a shutdown of the facility, field investigations, and a three-dimensional survey, $180,000 for preparation of design documents and receipt of bids for the procurement of the new valves, $142,000 for environmental documentation and project management, $100,000 for material testing services to conduct concrete coring, laboratory testing, and analyses, and $163,000 for contingency. MWD staff will perform the preliminary design activities while HDR Engineering will perform the material testing.
MWD staff expects to return to the board in mid-2015 to request the award of the valve procurement contracts and to authorize the final design.
The turbine at the Lake Mathews hydroelectric plant uses cooling water, and chlorine has been added to that water to prevent algae growth. That chlorine, however, has damaged portions of the building over the years. In an effort to reduce further damage, the chlorine injection system was removed in 2008 and a cooling water system using potable water now reduces the turbine temperature.
The scope of the hydroelectric plant repairs includes removing the damage concrete down to sound concrete, replacing reinforcing steel which has lost at least 20 percent of its cross-sectional area, repairing less-corroded reinforcing steel by sandblasting and applying a corrosion inhibitor, installing a cathodic protection system to inhibit additional corrosion of the steel reinforcement, and replacing damaged concrete with new concrete. The planned final design phase activities include conducting field investigations and material testing of the existing structure, preparing drawings and specifications, receiving competitive bids, and developing a construction cost
The total project cost, including the design funds requested and past expenditures, is estimated at $1.15 to $1.5 million. The $190,000 appropriated February 11 covers $124,000 for final design, $44,000 for bidding and project management, and $22,000 for contingency. MWD staff will perform all final design activities.
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