The state Department of Water Resources lists 35 dams in Calaveras County and 30 dams in Tuolumne County, and several of the largest are at capacity or nearing capacity in the midst of one of the Central Sierra’s wettest winters to date.
No damage or failures have been reported at any Mother Lode dams, spillways and reservoirs so far this season.
But ongoing concerns at Oroville Dam on the Feather River, about 165 miles northwest of Sonora, and a North Fork Mokelumne River levee above Tyler Island in Sacramento County, have Mother Lode residents thinking about dams and dam safety with more rains and snow forecast to arrive later this week.
Major dams in the Mother Lode include Pardee and Camanche on the Mokelumne River, New Hogan on the Calaveras River, Donnells, Beardsley and New Melones on the Stanislaus River, O’Shaughnessy at Hetch Hetchy and Don Pedro on the Tuolumne River, and New Exchequer on the Merced River.
Pardee held 103 percent of capacity as of Friday, and New Hogan held 78 percent of capacity early Monday. Donnells and Beardsley held 81 and 89 percent of capacity, respectively. New Melones held 55 percent with surface level more than 60 feet higher than Jan. 15. Hetch Hetchy in Yosemite National Park held 92 percent and Don Pedro held 99 percent of capacity as of Monday. McClure on the Merced River held 91 percent of capacity with surface level more than 40 feet higher than Jan. 15.
None of these dams are as tall as Oroville, which at 770 feet is billed as the tallest dam in the U.S. None of the dams in Calaveras and Tuolumne counties impound as much water as Oroville, the second largest reservoir in the state. New Melones is 625 feet tall and impounds the fourth-largest reservoir in California when it’s full. Don Pedro is 584 feet tall and impounds the sixth-largest reservoir in the state.
Big dams in the Mother Lode are subject to the same scrutiny and inspections as Oroville, where recent events show that things can still go wrong when extra runoff arrives and structural limits get tested. A section of the June 2012 Emergency Operations Plan for Tuolumne County is devoted to flood/dam failure.
Last week, so much water was released from Oroville Dam on the Feather River in Butte County that a cavity opened on the lower reach of the primary spillway, and state Department of Water Resources staff shut down the spillway to investigate. They said the dam was sound, and no imminent threat to public safety existed.
Over the weekend, Oroville filled above capacity and water began moving down the emergency-auxiliary spillway, which also began eroding.
By Sunday afternoon, the National Weather Service and state Department of Water Resources were advising residents below the dam the auxiliary spillway could fail in the next 60 minutes.
More than 180,000 people were ordered to evacuate, and a flash flood warning for potential failure of a portion of the Oroville auxiliary spillway was extended through 4:15 p.m. today. The dam itself was still considered sound, but failure of the auxiliary spillway could result in uncontrolled release of flood waters from the giant reservoir, which remained 100 percent full Monday.
Oroville is one of about 1,250 dams statewide under Division of Dams Safety jurisdiction. State engineers generally inspect these dams once each fiscal year, state Department of Water Resources staff said Monday.
Before noon Monday, staff with the National Weather Service in Sacramento issued a flash flood warning for potential imminent failure of the North Fork Mokelumne River levee, which could mean Tyler Island will be flooded as well as parts of east Walnut Grove.
Sacramento County Office of Emergency Services staff called for evacuation of Tyler Island. If the levee were to break, Tyler Island would flood first. The East Walnut Grove area could be impacted at a later time.
The flash flood warning was through 11:30 a.m. today. Concerns on the North Fork Mokelumne in Sacramento County were unrelated to worries about Oroville on the Feather River in Butte County.
Twain Harte Lake
A recent, local example of how the California Division of Dams Safety responds to potential problems occurred in August 2014 when a granite footing cracked on the dam impounding Twain Harte Lake, a private recreational reservoir.
Param Dhillon, an engineer with the state Dams Safety Division, was among specialists who responded when a granite footing portion of the structure known as “The Rock” cracked, spurring concern and a flash flood warning downstream.
Division of Safety of Dams engineers and geologists responded to the incident at Twain Harte Dam on Aug. 3, 2014. The dam is a concrete multiple arch, 36 feet high, which impounds 143 acre-feet, Dhillon said. It is owned by Twain Harte Lake Association Inc. Twain Harte’s economy is dependent on dollars generated by members who visit the lake.
During inspections on Aug. 3 and 4, Dams Safety Division staff observed cracking and offsets in the left abutment rock dome and stress cracks in the left arch barrel of the dam, Dhillon said.
“The reservoir was not judged safe to store water, and the owner was ordered to open the outlet fully and dewater the reservoir,” Dhillon said.
The Dams Safety Division restricted the reservoir to remain empty until remedial work was completed at the dam. State engineers worked with the owner’s engineers to design appropriate repairs, Dhillon said. The design was approved, and Dams Safety Division staff oversaw construction to ensure compliance with approved plans and specifications.
Construction work was satisfactorily completed in April 2015, and the owner was subsequently authorized for full storage behind the dam, Dhillon said.
Dennis Wyckoff, general manager at Twain Harte Lake, has visited the reservoir two to three times a day this winter to monitor the vast amounts of water coming into the reservoir. In January, reservoir management kept a valve open to keep silt flushing out of the reservoir.
Don Pedro Dam
The New Year’s Day Flood of 1997 was one of the largest in a century of state records, and runoff exceeded the flood control capacity of Don Pedro Reservoir on the Tuolumne River.
Three days brought warm moist winds from the southwest blowing over the Sierra Nevada and poured more than 30 inches of rain into watersheds that were already saturated by one of the wettest Decembers on record.
The Tuolumne River flow peaked at more than 120,000 cfs, which was the largest flood since 1862. While flood releases from Don Pedro Dam peaked at less than half the peak inflow, nearly 60,000 cfs, it was more than six times the downstream channel design capacity at the time of 9,000 cfs.
The flood caused extensive damage in low-lying developed areas, including West Modesto and along the lower San Joaquin River. To date, the highest elevation at Don Pedro Reservoir was 831.2 feet on Jan. 2, 1997. As of Monday morning, Don Pedro was at 827.8 feet elevation.
Dam failure scenarios
The June 2012 Emergency Operations Plan for Tuolumne County has a section devoted to flood/dam failure, but it does not mention the New Year’s Day Flood of 1997, in part because flooding downstream of Don Pedro Dam occurred outside Tuolumne County.
A table in the flood/dam failure section lists consequences of dam failure for 14 major dams in Tuolumne County. Here are a few examples from the June 2012 Emergency Operations Plan for Tuolumne County:
• If Donnells Dam on the Stanislaus River failed, flood waters would overtop Beardsley Dam by more than 30 feet, likely causing Beardsley Dam to immediately fail. Beardsley would then empty into New Melones Reservoir.
• If New Melones Dam were to fail, water would flow downstream impacting people and property in the cities of Escalon, Oakdale and Riverbank.
• If O’Shaughnessy Dam failed and released the waters impounded in Hetch Hetchy, it would result in deep runoff in the narrow, steep canyon downstream, with rapid wave travel times. Depths as great as 325 feet would occur in some reaches of the Tuolumne River between the Hetch Hetchy and Don Pedro reservoirs. The peak of the flood wave would reach the headwaters of the downstream reservoir about 90 minutes after the dam failure.
• If Don Pedro Dam failed, water would flow down the Tuolumne River to its confluence with the San Joaquin River. Major flooding would occur along the entire Tuolumne River basin, including the towns of La Grange, Modesto and Waterford.
None of these scenarios is likely. Nevertheless, dam operators and state Division of Dams Safety staff are required to be aware of worst-case consequences in the event of specific dam failures.
Tuolumne Utilities District
Tuolumne Utilities District, the agency that provides most Tuolumne County residents with drinking water, has three dams inspected once every year with Division of Dams Safety staff, said Lisa Westbrook of TUD.
The three dams impound Phoenix Lake, Quartz Reservoir and San Diego Reservoir. Phoenix Lake was last inspected Oct. 21, 2016.
Two primary dams that provide water for TUD customers, Lyons and Strawberry at Pinecrest, are owned and operated by Pacific Gas & Electric. Paul Moreno with PG&E said they are each inspected twice a year.
Pacific Gas & Electric operates more than 90 dams, and in each case the utility has developed plans with local and state emergency agencies to warn the public in the unlikely event of a sudden dam failure, Moreno said. The utility regularly engages in drills and exercises with emergency agencies. During rain storms, PG&E increases patrols and monitors flows and runoff conditions to ensure operations of its hydro system.
Tri-Dam Project, a partnership of the Oakdale and South San Joaquin Irrigation District, operates Donnells, Beardsley and Tulloch, including dams, tunnels, penstocks, powerhouses and communications systems on the Middle Fork Stanislaus River in Tuolumne County.
Susan Larson, license compliance coordinator with Tri-Dam, said the agency maintains a 24- hour-a-day control room, with a total of 26 cameras. Control room operators are able to monitor facilities including dam surface, spillways, roadways and other facilities continually.
“These facilities are highly computerized, and many have sensors and other alarms allowing control-room operators to make changes remotely altering water flows, power generators, and to dispatch operations or maintenance crews to investigate any potential areas of concern,” Larson said.
Tri-Dam also employs field operations staff that visit facilities to perform visual inspections, “generally each day and more often, if needed to evaluate any condition as warranted,” Larson said.
Tri-Dam, like other dam operators, runs its facilities under licenses issued by the Federal Energy Regulatory Commission, also known as FERC. Other agencies also have jurisdiction over various aspects of Tri-Dam’s operations, including FERC Engineering and Safety Division, California Division of Safety of Dams, California Fish & Wildlife and the U.S. Army Corps of Engineers.
The two primary agencies responsible for ensuring structural safety and integrity of Tri-Dam’s dams and power facilities, beyond the staff at Tri-Dam, are FERC and the California Division of Safety of Dams, Larson said.
Turlock Irrigation District
Turlock Irrigation District and Modesto Irrigation District jointly hold the license for Don Pedro Dam. By virtue of their sharing agreement based on acreage served within each district, Turlock ID’s share and interest in the dam amounts to 68.46 percent while the Modesto ID’s share is 31.54 percent.
Don Pedro dam operators are required to conduct annual inspections with state Division of Dams Safety and the Federal Energy Regulatory Commission, said Calvin Curtin with Turlock Irrigation District.
“The last inspection was completed in January of 2016,” Curtin said. “Additionally, we are inspected every five years by an independent consulting firm, and we conduct quarterly inspections with TID staff. These are in addition to daily observations of dam conditions and the condition of the powerhouse by our power plant staff.”
The U.S. Army Corps of Engineers operates New Hogan Dam on the Calaveras River, as well as numerous dams in other California counties.
Corps dams are monitored daily, both visually and with cameras, said Paul Bruton, a public affairs specialist with the Army Corps of Engineers.
“We have cameras set up to monitor the dams 24 hours day,” Bruton said. “We inspect Corps owned and operated dams every year prior to flood season, with a more in-depth inspection every five years.”
Each year on National Dam Safety Awareness Day, May 31, dam owners, individuals living and working downstream, first responders, local and state officials are urged to know all risks and benefits of dams located in their respective communities.