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Spillway at Conn Creek Dam, Lake Hennessey

The Conn Creek Dam spillway at Lake Hennessey kicked up a giant plume from a major rainstorm in January. 

The fallout from February’s disaster at the massive Oroville Dam will include fresh inspections of the spillways at 93 other dams across California – including those serving Napa, St. Helena and Yountville.

Conn Creek, Bell Canyon and Rector Creek dams are among those for which the state Division of Safety of Dams has ordered re-evaluations of water-release channels, the agency announced last week.

Conn Creek Dam feeds Lake Hennessey, the larger of two reservoirs serving the city of Napa. Bell Canyon Dam is St. Helena’s municipal water source, and Rector Creek Dam, controlled by the state Department of Veterans Affairs, supplies Yountville and the CalVet-owned Veterans Home west of town.

California announced in June it would seek a closer look at dams across the state, after the erosion of two spillways at the 770-foot-tall Oroville Dam – triggered by exceptionally heavy water flows from winter storms – forced the snap evacuation of a reported 188,000 people across Butte, Yuba and Sutter counties on Feb. 12. Helicopter drops of boulders and sandbags finally stanched the overflow, allowing residents to start returning home two days later.

State regulators originally focused on more than 100 dams – less than a tenth of the roughly 1,300 in California – and then compiled their final list based on the age of spillways, the height and capacity of dams, and the potential downstream hazards at reservoirs. Dams also were evaluated for their design, construction, maintenance and geological conditions, the division’s supervising engineer Daniel Meyersohn said in a statement.

“These assessments may require acquiring additional information to adequately evaluate the spillways’ ability to perform satisfactorily during a flood event,” said Meyersohn. “It will not be known which spillways, if any, will need repairs until the comprehensive assessments are completed and reviewed” by the dam safety division.

Dam owners will be required to carry out any necessary spillway repairs before the start of the next flood season, the agency said.

Raising the urgency of dam inspections is their age, with those in California now 70 years old on average, according to Meyersohn. Locally, Conn Creek Dam dates to 1948, Rector Creek Dam to 1946 and Bell Canyon Dam to 1959.

Joy Eldredge, manager of the Napa city Water Division, said owners of the dams in question must file evaluation plans by Sept. 1. At Lake Hennessey, the city will review the dam’s design plans, study the surrounding soils, and finally use ground-penetrating radar to check for any weaknesses in the 800-foot-long spillway’s concrete structure.

“We’re learning from the experience they had in Oroville – to take anything related to inspections during construction, to go back in time and pretend you’re looking at it for the first time with fresh eyes,” she said Monday afternoon.

Conn Creek Dam’s most recent scheduled inspection in the spring revealed no exposed rebar or similar concrete damage in its structure, according to Eldredge. The most recent concrete repairs to the spillway took place in the early 2000s, she said.

The state also notified St. Helena and CalVet in early June of the inspection requirements for their respective dams, and both will file inspection plans before September, local officials said Monday.

An inspection of the 95-foot-tall Bell Canyon Dam and its 510-foot-long concrete spillway revealed no visible deficiencies, according to Erica Ahmann Smithies, St. Helena’s public works director and city engineer. However, the city in 2016 discovered deterioration in the dam’s water intake tower and has hired a consultant to design a replacement intake.

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City of Napa/Town of Yountville Reporter

Howard Yune covers the city of Napa and the town of Yountville. He has been a reporter and photographer for the Register since 2011, and previously wrote for the Marysville Appeal-Democrat, Anaheim Bulletin and Coos Bay (Oregon) World.