Weather Watch

100-year flood? History shows it’s happened in SLO County before

See SLO County’s first rainfall of the season

San Luis Obispo County residents broke out their umbrellas for the first rain storm of the season on Wednesday, Oct. 3, 2018.
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San Luis Obispo County residents broke out their umbrellas for the first rain storm of the season on Wednesday, Oct. 3, 2018.

A 100-year storm has a 1 percent chance of occurring in any given year. However, you can have several 100-year storm events that happen in one particular month or even a week. Likewise, there are similar classifications for 100-year flood events. A 50-year rainfall event has a 2 percent chance of happening in a year.

In other words, the 100-year storm is an attempt to simplify the definition of a storm or flood that statistically has a 1-in-100 chance of happening over a 1-year period.

Hydrologist would instead describe these events as having a 100-year recurrence interval. The San Luis Obispo County Department of Public Works and Transportation list the following rainfall intensity data as 100-year recurrence interval.

For the Central Coast, if you live an area that typically receives less than 14 inches a year, like Paso Robles or Santa Maria, a 100-year rainfall intensity event would be classified as 0.90 inches of rain in one hour, or 3.6 inches over a 10-hour period.

If you reside in locations that receive between 14 and 17 inches, like Lompoc or Arroyo Grande, it is 1.2 inches in a one-hour period or 4.9 inches over 10 hours. San Luis Obispo gets a little over 22 inches on average per year; consequently, areas that receive between 22 and 28 inches is 1.85 inches in one hour, or 7.6 inches over 10 hours.

Over the course of our history, these storms would probably be classified 100-year events.

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Toward the end of 1861, the “Noachian deluge of California Floods” occurred. A series of storms produced nearly continuous rain that lasted through February over most of California. Los Angeles recorded nearly 36 inches of rain, while the mountains surrounding the Los Angeles Basin may have received as much as 60 inches. Sonora in the Sierra Nevada foothills measured more than 100 inches.

The Santa Ana River in Southern California along with the Yuba River and Feather Rivers in Northern California all became a raging torrent, laying waste to farms along their banks. River settlements throughout California were either inundated or washed away.

Tragically, hundreds of Californians may have lost their lives. Ranchers lost hundreds of thousands of cattle.

By February 1862, the Sacramento and San Joaquin valleys became almost an inland sea stretching nearly 300 miles in length, forcing the state Legislature in Sacramento to convene in San Francisco. Also, the ordinarily salty waters in San Francisco and San Pablo Bay became nearly fresh with a continuous and almost unimaginably massive flow of silted water through the Golden Gate.

Of course, no one alive today witnessed this terrible series of storms, and weather data from so long ago is sparse. However, a few weather experts believe that atmospheric rivers stretching across the Pacific Ocean channel vast amounts of water vapor from near the equator into large storms coming out of the Gulf of Alaska.

In one weekend in January 1969, 10.5 inches of rain was recorded in San Luis Obispo. In fact, in one 25-minute period, one inch of rain fell as a cold front passed. Just four years later, a similar storm occurred in 1973.

Take a scenic drive up Highway 1 during a break in the storm on Friday, March 2, 2018. A strong storm swept through California on Thursday and Friday, dropping more than 1 inch of rain in several parts of San Luis Obispo County.

In March of 1995, the rain gauge at Diablo Canyon Power Plant recorded 8.5 inches in one 24-hour period. There were reports that locations in San Lucia mountains received more than 20 inches over this period. Communities throughout the Central Coast suffered flooding, but Cambria was probably the hardest hit as Santa Rosa Creek became a river.

In February 1998, the storm door swung wide open. The mid-latitude westerly winds at the surface and in the upper levels of the atmosphere dramatically increased and brought a series of storms that marched across the Pacific through California. Rain gauges throughout the Central Coast recorded rainfall nearly every day through Feb. 24. Overall, 15.6 inches of rain was recorded that month at Diablo Canyon.

At the end of that deluge, tragedy struck.

In the early morning hours of Feb. 24, the Cuyama River was roaring at 20,000 cubic feet per second, the highest flow ever recorded. The rock- and debris-infused water undercut and washed out a large section of State Route 166 northeast of Santa Maria.

California Highway Patrol officers Rick Stovall and Britt Irvine died when their patrol car plunged into the abyss carved out by the swollen river while responding to a report of a washout on the highway. Michael Tye also lost his life when his Chevrolet pickup went into the river.

A story written by Danna Dykstra of the Telegram-Tribune quoted Sisquoc Community Church Pastor Bill Klein, who was driving his Dodge Durango west with his daughter on Highway 166 when he noticed headlights from an eastbound vehicle coming toward him.

“We were about ready to pass each other when that car went straight down,” Klein said. “That caused me to slam on my brakes. If that Highway Patrol car had been a little sooner or a little later, my daughter and I would have gone over. The officers literally gave their lives for us. Had they not gone in ahead of us, we would not have survived; the others who came after them would not have survived.”

The safety of the communities we serve is PG&E’s top priority. Before floods and other emergencies occur, it’s a good idea to create an emergency supply kit. Stock up on enough supplies to last a week. Put the items in waterproof containers and store them in a place that’s easy to reach.

To learn more on how to prepare the kit, visit www.pge.com.

John Lindsey’s column is special to The Tribune. He is PG&E’s Diablo Canyon marine meteorologist and a media relations representative. Email him at pgeweather@pge.com or follow him on Twitter: @PGE_John.
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