Santa Clara county’s largest reservoir filling to brink


The largest reservoir in Santa Clara County is almost completely full and, as more rain continues to roll in over the weekend, managers think water could reach the dam’s spillway in the very near future.

The Anderson Reservoir near Morgan Hill can hold 89,491 acre-feet of water and is now 99 percent full, according to Santa Clara Valley Water District spokesman Jim McCann.

If the rains that are expected to hit the area over the next several days materialize, it is likely that the water level will rise to the point at which it reaches the spillway, sending water cascading down the large concrete and earthen ramp and into nearby Coyote Creek for the first time in about ten years.

Just because water hits the spillway, however, does not mean that there is an emergency situation at hand.

McCann said:

“It’s doing what it’s designed to do.”

The situation at the Anderson Reservoir should not be equated with what happened at Butte County’s Oroville Dam, where the possibility of a spillway collapse led to the evacuation of hundreds of thousands of people Sunday, McCann said.

The spillway at the Anderson Reservoir is not damaged, as was the case with the Oroville Dam.

Still, people downstream of Anderson should pay attention to the water level of Coyote Creek and other nearby streams, which could overflow their banks if the upcoming storms drop more rain than currently expected.

The storms that are hitting the region between today and Thursday are forecasted to bring up to 2.5 inches of rain to the area, according to National Weather Service forecaster Bob Benjamin.

Right now, the reservoir’s outlet, which is separate from the spillway, is pumping out 400 cubic feet of water per second in an effort to keep the water level as low as possible. The looming storms, however, mean the reservoir will fill up faster than it can be emptied via the outlet and, in that case, water will begin running down the spillway.

Emergency managers in Santa Clara County are watching the skies closely and announced they are ready to activate the county’s emergency operations center in order to respond “to any incident that has the potential to threaten life, property or the environment,” Dana Reed, director of the Office of Emergency Services, said in a statement Thursday.

So far, Coyote Creek is expected to remain below the flood stage, but that could change quickly depending on the weather.

The concerns about Anderson Reservoir’s water levels go beyond possible flooding, however. Because of “seismic vulnerabilities,” the reservoir has been kept to 45 feet below the dam crest, or 68 percent of capacity, since 2009.

Keeping the water level low would prevent a major flooding event should the top of the dam fail in a large earthquake, McCann said.

The dam is thought to be vulnerable to a 7.25-magnitude quake if it hits within two kilometers of the reservoir.

Currently, a $400 million seismic retrofit project is in the planning stage and construction is expected to start in 2020, McCann said.

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