Napa County's Big Dams Pose Low Risks
Napa County’s biggest dam is Monticello Dam, a 300-foot concrete structure in the east county built in the 1950s. It forms massive Lake Berryessa, which is 23 miles long and three miles wide and can hold 1.55 million acre-feet of water.
A Monticello Dam failure would have little flooding effect on Napa County. But a Yolo County inundation map shows Winters and Davis would be affected.
A full analysis of statistical probability of flood or failure of Monticello Dam is available on the Lake Berryessa News website at:
Could Monticello Dam Fail? www.lakeberryessanews.com/lake-berryessa-technical/monticello-dam/monticello-dam-failure.html, and Can Lake Berryessa Flood Over the Dam? www.lakeberryessanews.com/lake-berryessa-technical/monticello-dam/flood-probability.html
Not to worry too much since worst-case scenarios for Monticello Dam had a VERY low probability – somewhere between 10,000 and 1,000,000 years. Even the probability of the lake reaching the top of the dam at 456 feet was very low with a 5% confidence limit of 417 years recurrence and 95% confidence limit of 16,667 years recurrence.
What is the probability of an earthquake at Monticello Dam? There is historic evidence that a serious earthquake can occur in this area, as it did in 1892 with a magnitude 6.4 quake that leveled downtown Winters. What would happen if the Great Valley fault, which basically runs under the dam, were to fail in an event such as the magnitude 6.4 Vacaville-Winters earthquake, which was located somewhere in that area? Very probably, this will not happen. But no one can say that it absolutely won’t happen.
Napa County has 58 dams from massive Monticello Dam to smaller Conn Creek Dam at Lake Hennessey, and none on a new Associated Press list of 2,200 dams nationwide that are in bad shape and threaten communities.
Several local dams — Conn Creek Dam at Lake Hennessey, Milliken Dam at Milliken Reservoir and Rector Creek Dam at Rector Reservoir — could cause big-time damage if they failed. Napa inspects the dam annually with the state Division of Dam Safety. It inspects the abutments and the valves and manages vegetation. It has a project out to bid to make minor concrete repairs to the dam spillway. Among other things, the city is looking at adding a secondary spillway.
A map tells the story. Conn, Rector and Milliken dams are in the mountains to the east of Napa Valley. If they failed, water from their reservoirs could swamp the Napa River and flood communities on the way to San Pablo Bay.
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