Local officials look to Oroville for lessons on how to protect SLO County’s 5 dams

Mark Hutchinson, deputy director of San Luis Obispo County public works, stands below the Salinas Dam earlier this month when Santa Margarita Lake filled for the first time in years.
Mark Hutchinson, deputy director of San Luis Obispo County public works, stands below the Salinas Dam earlier this month when Santa Margarita Lake filled for the first time in years. dmiddlecamp@thetribunenews.com

In light of the crisis at Oroville Dam that forced 180,000 people to evacuate last week, operators of the five large-scale reservoirs in San Luis Obispo County are analyzing how information gleaned from that near-disaster could be applied locally to protect the thousands of residents living downstream from the county’s dams.

All five local dams have been inspected by regulatory agencies in the past four years and were found safe, records show. Safety reports, however, show that some spillways are due for repairs.

Last fall, repairs recommended by state inspectors were completed at the Lopez Lake Dam east of Arroyo Grande to prevent erosion behind the concrete spillway, which hasn’t been used in 19 years. Erosion around the Oroville Dam spillway in Northern California may have caused its collapse, according to an incident report prepared by state water officials.

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At the Salinas Dam on Santa Margarita Lake, a 2013 inspection found no “serious dam safety deficiency” but said maintenance needs to be done, including repairs to concrete damage, exposed threaded bars on the spillway wall and a larger crack at the crest of the spillway. That work hasn’t been performed.

As state and federal regulators review what happened at Oroville, local dam operators say they’re awaiting the results to see what they can learn.

“Obviously, if they inspect that dam and find the problem with the spillway — that could be transferable,” said Mark Hutchinson, deputy director of Public Works for San Luis Obispo County, which operates Lopez and Salinas dams. “We’ll look at everything to do with our plans, with our systems, our dams, our levees, in light of what comes out of the Oroville situation.”

“We’re anxious to see what the inspections are going to show us.”

Smaller dams

Heavy rains and melting snowpack led to water pouring over the spillway of the 770-foot Oroville Dam, the nation’s tallest dam, located in Butte County. When the concrete spillway developed a massive hole and a hillside used as a backup emergency spillway severely eroded, three downriver counties were evacuated last Sunday. Authorities feared the dam could fail, sending a 30-foot wall of water cascading down the river and through the Sacramento Valley.

Dammed reservoirs in San Luis Obispo County, which are regularly monitored and inspected, hold nowhere near the 3.5 million acre-feet of water that can be stored in Lake Oroville. The largest local reservoir, Lake Nacimiento, holds about one-tenth that amount.

Even with this season’s heavy rains, only one of the county’s dams has reached capacity, and no major damage or failures have been reported at any of the spillways or dams at the big five: Lopez Dam on Arroyo Grande Creek, Salinas Dam (Santa Margarita Lake) on the Salinas River, Nacimiento Dam on the Salinas River, Whale Rock Dam on Old Creek and Twitchell Dam on the Cuyama River.

Only the Salinas Reservoir has reached capacity and spilled so far this year. Lopez Lake has reached 47 percent capacity; Whale Rock Reservoir is filled to 65.6 percent capacity; and Nacimiento is at 86 percent capacity, as of Friday, according to online records.

Still, dam failure poses the potential for major life and property impacts to downstream populations within the county.

For the first time in nearly six years, Santa Margarita Lake is full and spilling over into the Salinas River. Mark Hutchinson, deputy director of SLO County Public Works, talks on Feb. 8, 2017, about the role of the lake, also known as the Salina

Spillway checks

County officials take operating dams seriously, Hutchinson said.

“It is not casual in the least; there are daily eyes on the dam and eyes on the instruments,” he added.

In addition, dams are inspected by state and federal engineers. Salinas Dam is inspected by the U.S. Army Corps of Engineers every five years. All other dams in the county are inspected annually by engineers from the California Department of Water Resources Division of Safety of Dams, with more thorough inspections every three years.

State inspectors review information gathered from visual inspections and instruments, including piezometers that measure water pressure.

The Tribune requested inspection records for the five large dams, and local agencies provided the most recent inspection reports for Lopez, Salinas and Whale Rock.

Salinas Dam was generally found to be in good condition when the Army Corps of Engineers last inspected it in December 2013. While repairs to the spillway were recommended, no problems were observed that would pose a threat to the safety, stability, integrity or operation of the dam or its structures.

The spillway repairs weren’t designated as urgent and probably won’t be addressed for a year or two because of the cost, Hutchinson said. The county works with the city of San Luis Obispo to schedule repairs in the annual budget and then “address them at a reasonable expenditure rate,” he said. Costs to operate Salinas fall on city ratepayers.

The two dams with potential to cause the most damage to downstream communities — Lopez Lake and Whale Rock — were both found by state inspectors to be safe for continued use, according to 2016 reports.

In May 2016, state inspectors directed the operators of Lopez Lake to “investigate and repair the spillway drain system that appears to be causing the reported seepage originating in the concrete V-ditch … near (a station) of the spillway.” The report said the concrete spillway showed no sign of instability or distress.

Small pipes beneath the spillway drain water away, to prevent erosion of the spillway’s foundation. Last fall, workers dug down and found that one of the pipes was disconnected. It was reconnected and pipes were cleaned to remove sediment to allow for steady drain flow, Hutchinson said.

In light of Oroville, “Most dam operators are anticipating a more intensive spillway inspection in the future,” said Noah Evans, supervisor of Whale Rock Reservoir.

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Like the San Luis Obispo County dams, the Oroville Dam is inspected by the state’s Division of Safety of Dams. The spillway at Oroville was checked “from some distance” and not closely inspected during the last check conducted in July 2015, according to its inspection report.

“That’s got our attention,” Hutchinson said. “The thing on our mind is we rely on DSOD. They’re experts on dams. They inspect dams throughout the state. They spot problems early on.”

Earlier this week, the Federal Energy Regulatory Commission ordered the state agency to perform a “forensic analysis aimed at determining the cause” of the failures at Oroville Dam.

“There are going to be questions about the mechanisms that led to the main spillway’s failure. Was it an issue with materials, was it something missed on the inspection report? Was there a design flaw?” Hutchinson said.

Information released as a result of that investigation could be helpful to improve dam safety, such as whether cracks in the spillway led to the failure. Repairing cracks in spillways is routine and expected in a long concrete structure with joints and seams that move, Hutchinson said.

“Is that really the cause of the failure of the (Oroville) spillway? We’re anxious to find out,” he said.

Emergency response

The county Office of Emergency Services has a Dam and Levee Failure Evacuation Plan that includes a hazard assessment outlining potential scenarios in the event of complete dam failure. The plan was developed in 2002 and was last updated in February 2016. That plan could be adjusted in light of the Oroville Dam evacuations, depending on what state investigations show.

“That would be a long time coming. We’ll definitely take a look at that. Anytime we can learn from something, we will,” said Ron Alsop, emergency services manager with the county Office of Emergency Services.

San Luis Obispo County already has extensive emergency preparedness and evacuation plans because of Diablo Canyon nuclear power plant. Those plans can be applied to a dam evacuation, Alsop said.

“I think this county, in general, is well-prepared because of Diablo Canyon,” Alsop said. “On the other hand, I don’t want to say everything is rosy and perfect. It can’t be. But we are going to do the very best we can do to ensure the safety of the people in this area.”

The county has sirens within the Diablo Canyon emergency zone, can use reverse 911 calls, and works closely with radio stations, allowing for warnings of evacuations. Multiple agencies frequently perform joint exercises, and the county has experience evacuating communities for hazardous materials and tsunamis.

“We do plan on worst-case scenarios. I don’t foresee something like that happening, but never say never,” Alsop said. “They’re called disasters for a reason.”

A Lopez Dam failure

A failure at Lopez Dam has the potential to cause the most property damage and threat to life because of the proximity to towns and the rapid rise of water that would be expected. If the reservoir were full and the dam failed, water would hit Highway 101 in 40 minutes.

In the event of an earthquake, operators of the Lopez Dam are at the top of the call list for county Office of Emergency Services staff.

“It’s No. 1 on the list because if it did have a lot of water and it failed, it would cause damage so fast. It’s not because we are concerned (it will fail),” Alsop said.

Lopez Dam is owned and operated by the county Flood Control and Water Conservation District. It’s an earthen dam with an outlet at the bottom that feeds to the creek or the wastewater treatment plant, as well as a concrete spillway on the side. Construction was completed in 1969, one year after the Oroville Dam went into operation.

It was expected to take years to fill the lake, but the reservoir reached capacity and water flowed over the spillway in March 1969. The spillway hasn’t been used since 1998, though in the past it was used on a regular basis.

“The spillway at Lopez is designed to carry what’s called the dam of record, or Noah’s flood,” Hutchinson said.

The dam was retrofitted to increase its strength to withstand an earthquake in 2001 and 2002. The work was performed at the recommendation of the state Division of Safety of Dams after specialists gained a better understanding of how material the dam sits on moves during an earthquake. The clay core of the dam goes down to bedrock. The upstream and downstream sides are layers of drainage rock, crushed rock and riprap.

“There was a lot of effort to stabilize that material. Basically, they drilled holes and installed stone columns under the downstream toe of the dam,” Hutchinson said. The dam was also made thicker.

That work was completed in August 2003, and the 6.6-magnitude San Simeon Earthquake hit four months later. The dam “came through with no damage,” Hutchinson said.



Capacity: 49,388 acre-feet

Type: Earth-filled dam with concrete spillway

Height: 165 feet

Built: 1969 (reinforced in 2002)

Outflow: West into Arroyo Grande Creek

Last inspected: April 2016


Capacity: 377,900 acre-feet

Type: Concrete dam with spillway

Height: 215 feet

Built: 1957

Outflow: North into the Salinas River


(Santa Margarita Lake)

Capacity: 23,843 acre-feet

Type: Concrete dam with ungated spillway

Height: 135 feet

Built: 1942

Outflow: North into the Salinas River

Last inspected: December 2013


Capacity: 224,300 acre-feet

Type: Earth-filled dam, no spillway

Height: 241 feet

Built: 1958

Outflow: South into Cuyama River to Santa Maria River

Last inspected: September 2015


Capacity: 38,967 acre-feet

Type: Earth-filled dam with concrete spillway

Height: 233 feet

Built: 1960

Outflow: Southwest into Old Creek

Last inspected: April 2016

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