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With a series of historic storms having pounded the region since 2017 started, Santa Clara Valley Water District officials are bummed that their overall storage capacity is limited by various safety and environmental restrictions—allowing untold amounts of potential drinking water to wash into the bay shortly after it falls from the sky.
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On a more ominous note, SCVWD Board of Directors Chair John Varela said in a public letter Feb. 9 that Anderson Reservoir is on the verge of reaching its full capacity due to recent and coming storms—a condition that could pose severe flooding danger to the valley if a major earthquake hits near the compromised dam.
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SCVWD and state officials have cautioned that if a 7.25 magnitude earthquake hits within two kilometers of Anderson Dam when the reservoir is full, it could cause the dam to at least partially collapse. In such an event, the pressure of the water behind the failed dam could release a wall of water up to 35 feet high into the valley, potentially flooding Morgan Hill, Gilroy and San Martin within a couple hours.
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“What would happen if a massive quake did occur?” Varela’s letter states. “There’s a chance that the dam could be damaged, but the chance of immediate dam failure is exceedingly remote. It is possible that we would have several days or weeks to reduce the water level with emergency pumps before any further damage could compromise the integrity of the dam.”
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Because of these seismic safety issues with the dam that towers above northeast Morgan Hill, Anderson Reservoir—under drier conditions—cannot be full above 68 percent of its capacity until SCVWD completes a scheduled $400 million retrofit of the dam. The reservoir was 87 percent full Feb. 9, but the state Division of the Safety of Dams—the agency that placed the restriction in 2009—“understands that there are times when it’s just not possible to keep the reservoir down to that limit,” SCVWD spokesman Marty Grimes said.
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A giant outlet pipe at the bottom of Anderson Dam, which flows directly into Coyote Creek, has been “open 100 percent since Jan. 9,” Grimes said. “The rain—one storm after another—has outpaced the rate it can be released from the outlet.”
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That outlet—which shakes the ground above it when fully open—releases water at a maximum rate of 416 cubic feet per second (cfs), Grimes said. By contrast, on Feb. 7, for about an hour—also during a period of heavy rainfall—nearly 9,000 cfs was flowing into Coyote Reservoir due the rainfall.
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The SCVWD plans to keep the outlet pipe flowing at full blast until Anderson drops to the permitted level, Grimes added.
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Construction on the Anderson Dam retrofit project is expected to start in 2020 and take up to four years to complete.
Storage concerns
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Flushing all that water into Coyote Creek from the bottom of the reservoir is also an example of the loss of potentially valuable drinking water that cannot be stored for future use. Varela’s letter states that by the end of the current rainy season, the water district will have released up to 30,000 acre feet of water into the bay—”that’s enough water to supply 60,000 households for an entire year.”
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“Our project team and expert consultants are working diligently to complete the design of this major (Anderson Dam) project so that the reservoir can be fully utilized as soon as possible,” Varela’s letter states.
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Ongoing projects at two of the district’s other 10 reservoirs—Calero and Guadalupe—also limit the overall storage capacity, Grimes said.
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He added that after four years of a statewide drought, it has been “hard” during this year’s storms to watch rainwater rush over the spillway at reservoirs like Uvas and Chesbro, and continue downstream toward the Pajaro River, which flows into Monterey Bay.
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“We are very much looking forward to the day we can fill all those reservoirs again,” Grimes said.
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Until then, SCVWD—which is the wholesale drinking water provider for 1.8 million residents in Santa Clara County—has limited options to improve its overall storage capacity. Because of environmental restrictions, it’s almost impossible to build new reservoirs nowadays, Grimes said.
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“What’s more possible is raising existing dams,” but such efforts take years of planning, Grimes said. “We’re looking at some potential project partners to increase storage in other (reservoirs)” outside SCVWD.
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One of these is the Los Vaqueros Reservoir in Contra Costa County. Officials there are planning to raise that dam in order to increase the reservoir’s storage capacity. “That may be a benefit to Santa Clara County residents if we can use that space,” Grimes said.
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On the bright side, this year’s storms have contributed to a sunny outlook for the SCVWD’s groundwater storage for at least the next year, Grimes noted. So far this winter, Morgan Hill has received more than 30 inches of rain at the water district’s Edmundson Avenue rain gauge. The latest big storm Feb. 9 was projected to drop at least another inch on South County.
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Residents and businesses in South County do not rely on water stored in Anderson and other reservoirs. Instead, South County customers depend on wells and groundwater, which is supplied from various sources outside the county.
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Water stored in the reservoirs mostly serves North County customers.
Dam history
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Varela’s letter attempted to quell any fears of a potential failure of Anderson Dam. He noted that the likelihood of a 7.25-magnitude earthquake in the next few weeks—while the reservoir is full or nearly full—is “very remote.” Such a temblor would be larger than the 1989 Loma Prieta quake.
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Furthermore, he noted that Anderson Dam, which was built in 1950, has “performed well” in numerous earthquakes, including the 1989 shaker.
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Anderson Reservoir has reached its capacity 10 separate times since the dam was built, most recently in 2006, Varela added.
