The retrofitting of Anderson Dam in northeast Morgan Hill, which has been classified as seismically unsound, will close the reservoir to boating and fishing for at least two years, require explosions in the hills surrounding the body of water, kick up dust for several months and tear up a section of Cochrane Road which passes below the 60-year-old earthen dam, according to authorities.
The massive, $185-million upgrade project will last at least three years (starting in 2016), and the dam’s owner, the Santa Clara Valley Water District, is in the process of informing residents of the coming disturbance that some say will change the reservoir forever. Dozens of residents attended an informational meeting conducted by the SCVWD Tuesday night, which drew a standing-room-only crowd to the Morgan Hill Community and Cultural Center.
“During the summer we go up there all the time,” said a resident named Dennis, who declined to provide his last name at the meeting. “I’ll probably end up selling the boat. Everything is going to drastically change.”
Dennis, who lives off Mission View Drive in Morgan Hill, said he frequents Anderson reservoir with his grandchildren.
SCVWD staff presented six options to retrofit the dam in order to meet state and federal seismic safety guidelines at Tuesday’s meeting – all of which will require draining the 1,200 acre reservoir until it’s almost empty. The project will also require the blasting of the earth all around the reservoir in order to harvest materials to “buttress” the dam embankments.
“There are deer, coyotes and birds all over the place,” Dennis added. “If there’s no water, and they’re blasting, where are (the animals) going to go?”
A county park at the foot of the dam, which includes picnic areas and trails climbing up to the dam crest, will also be closed for at least two years during construction, district staff said. Plus, the section of Cochrane Road that passes below the dam will likely have to be torn apart and re-routed.
District staff said the biggest noticeable impact to nearby residents will be dust kicked up by trucks and earth-moving equipment during the summer months, and noise.
In the coming months, district staff and consultants will continue to evaluate the six options to repair the dam and choose one option to present to the SCVWD board of directors for approval. After that, the planning, design and environmental study process will begin, and then construction – which will consist of heavy, ongoing earth moving – in early 2016.
The district determined in 2011, after an independent study conducted two years earlier, that Anderson Dam would not withstand a major earthquake, and the crest of the dam could slump in such an event, leaving Morgan Hill under water within minutes if the reservoir was full. Flooding could even reach Gilroy and San Jose, according to the 2009 independent study.
The district’s study determined this damage could occur in the event of a 7.2-magnitude earthquake on the Calaveras fault, which is about 1.2 miles east of the dam. Since 2009, as a precaution, the reservoir level has been maintained at least 25.5 feet below the crest of the spillway, which corresponds to about two-thirds of the reservoir’s capacity. State and federal regulators have also restricted the reservoir’s level since 2009.
Also under seismic-related restrictions is Coyote reservoir, as that water body’s dam passes directly on top of another fault, district staff said. That reservoir, which is upstream from Anderson in Gilroy, has been restricted to a maximum level of 50 percent of its capacity for several years.
Consultants and district staff have spent the last six months evaluating the best way to bring Anderson Dam up to current safety standards. They came up with 19 different repair options, but narrowed those down to the six possibilities presented at Tuesday’s meeting, and at an SCVWD board meeting Tuesday morning.
Attendees at the meeting in Morgan Hill Tuesday night raised a variety of concerns, including the project’s potential impact on fish and wildlife and rare geological formations surrounding the reservoir, and the impact on the groundwater and drinking water storage.
District staff said they are still in the process of evaluating how the drainage of the reservoir would affect the fish that live there, and the project would likely have no effect on groundwater storage because there are other sources to replenish and maintain South County’s subterranean water basin.
In addition to buttressing both upstream and downstream embankments with more earth and rocks, the project will expand the capacity of the emergency spillway, raise the crest of the dam by seven feet, and replace an existing 49-inch outlet pipe at the bottom of the dam with a more stable conduit enclosed in an underground tunnel that could survive up to a four-foot shift of the earth’s crust in a major earthquake, district staff said.
The district will have to dismantle both dam embankments in order to remove alluvium deposits deep inside the bottom of the earthen dam, according to SCVWD deputy operating officer Katherine Oven. These deposits of clay, silt and sand – which formed the natural creek bed before the dam was constructed in 1950 – are the source of the dam’s current instability.
In the statistically remote occurrence of the largest predictable earthquake on the Calaveras fault, these materials could “turn to jelly” and cause a downward ripple effect up to the crest of the dam, Oven explained.
After removing as much of the alluvium as they can, the district will then have to replace the acceptable dirt and rocks that were temporarily removed to reach the offending materials, Oven said. They will likely blast areas surrounding the reservoir – primarily in the northwestern hills overlooking the water – in order to acquire more material to strengthen the dam embankments. The project is likely to enlarge the dam’s “footprint” up to 100 feet in both directions.
Blasting will take place maybe “once or twice a week,” Oven said, and district staff plan to notify residents when the blasts will occur.
The process will also require leasing private property to temporarily store the dirt and rocks that will be removed from the dam. Chris Borello, whose family owns about 120 acres just below the dam, said the district has already contacted him about using sections of their property for such storage. The Borellos are in the process of developing the San Sebastian residential project on part of the property, which also currently includes about 40 acres of cherry and apricot trees.
The dam project and dirt storage are unlikely to impact the Borellos’ development plans or the orchards even if the family agrees to lease the property to the water district, Borello said.
The district will also have to purchase at least one private property (not the Borellos’) to permanently house the expanded downstream embankment, Oven said.
Consultants considered a repair process that would not require draining the reservoir, but determined that for safety and quality reasons the work would be best achieved on a dry upstream embankment. How empty the reservoir will be during construction depends on which alternative the SCVWD board selects, but at minimum the level of the reservoir just upstream from the dam will drop from about 600 to 455 feet above sea level.
“There are some very high safety risks if we keep that reservoir full,” Oven said at the Morgan Hill meeting.
Chris Yacenda, an avid boater and Gilroy resident, said it’s “unfortunate” that they couldn’t fix the dam without draining the reservoir. During the summer, Yacenda brings his boat to Anderson at least once or twice a month, and has been doing so for about the last 20 years. Yacenda and his family and friends wakeboard, water ski, and “just enjoy the lake.”
Anderson is the largest of the water district’s reservoirs, and is larger than all nine other reservoirs combined. Without Anderson, South County boaters will likely crowd Coyote reservoir in Gilroy – or have to travel to the Sacramento Delta or some other large body outside the county – Yacenda said.
“It’s unfortunate because Coyote is not very accessible, and it’s not very big. Anderson is really nice,” Yacenda said. “I understand they’ve got to do what they’ve got to do, but I would think they could find a better way to do it.”
• 1950: Year the dam was built
• 89,073: Reservoir capacity, in acre-feet*
• 1,245: Reservoir surface acreage when full
• 7.8: Length of reservoir, in miles
• 3.3 million: Cubic yards of fill contained in dam
• 49: Outlet pipe diameter, in inches
Source: Santa Clara Valley Water District
*One acre-foot contains enough water to supply a family of five for two years
