San Francisco Chronicle - (Sunday)
Rivers rising to highest levels in years
Widespread flooding feared after next storm pounds swollen waters
Early this week, Northern California is expected to see the most widespread flooding yet from the extraordinary series of storms that has blasted the state — and perhaps some of the worst flooding in years — putting cities and counties on continued alert for disaster.
Rain falling over the next few days, adding to all that has already come down, will mean not just that rivers that normally flood, such as the Russian River, may overflow their banks. The risk will extend to waterways less prone to flooding, including the Napa River, the Guadalupe River in San Jose and the Carmel and Salinas rivers in the Monterey area.
The less frequently flooded rivers and creeks can be the most worrisome because residents in these areas don’t expect problems and the channels are generally far less fortified for surges.
“We really haven’t seen (the potential for flooding) on this broad of scale in a few years, and there are some rivers that haven’t seen flows forecast like this for an even longer time,” said Brett Whitin, service coordination hydrologist at the California Nevada River Forecast Center, a division of the National Oceanic and Atmospheric Administration that provides flooding guidance.
The center’s modeling shows that the bulk of waterways in the Bay Area and along the Central Coast are poised to swell to concerning levels Monday and Tuesday. Many are forecast to surpass their flood stage.
The sprawling Sacramento River, which receives inflow from many unregulated waterways, is also expected to exceed flooding thresholds in many places, but mostly in rural areas where the impacts are more easily absorbed.
The Cosumnes River, which flows to the Sacramento, is perhaps the most worrisome in the inland basin. The river’s levee’s breached last weekend, flood
ing huge amounts of farmland and a stretch of Highway 99 and killing at least three people. The local reclamation district has been fortifying the levees in anticipation of more flooding, forecast to begin Monday evening.
“We could see river levels there as high as they were” on New Year’s weekend, Whitin said.
The levee failures are illustrative of the ongoing challenge that California faces in safeguarding the vast majority of rivers and creeks in the state that have been engineered to flow where people want them to — not necessarily where Mother Nature intended them to go.
Emergency preparation
In the Bay Area, the Russian River is expected to reach flood stage Sunday evening, topping out Tuesday evening more than 6 feet above flood level. While water is likely to inundate roads and some homes and businesses west of Highway 101 in Sonoma County, it’s not expected to flood downtown Guerneville, which has often been the case during big storms in recent years.
In Napa County, the Napa River is forecast to crest early Tuesday, at about 2 feet below flood level. But county officials aren’t taking chances, saying Friday they plan to take the infrequent step of closing the river’s floodgates in downtown Napa on Saturday evening. This seeks to steer some of the big flows from the main stem to a protected flood plain.
Angel Hernandez, an emergency services officer for Napa County, said the county has been meeting regularly with representatives from the National Weather Service and public safety departments to monitor the forecast and plan for the possibility of flooding.
Such flood meetings between government agencies have been common across Northern California over the past week.
In Santa Cruz County, the city of Watsonville coordinated with other jurisdictions to issue a mandatory evacuation of several hundred homes along the Pajaro River. The river’s flows have since receded and the evacuation order has been lifted, but the Pajaro River along the Santa Cruz-Monterey county line is again expected to surge, reaching flood stage Tuesday morning.
Watsonville officials have not said whether they will reinstate the evacuation notice.
“We really haven’t seen (the potential for flooding) on this broad of scale in a few years, and there are some rivers that haven’t seen flows forecast like this for an even longer time.”
Brett Whitin, California Nevada River Forecast Center
Farther south, the Carmel River is forecast to hit flood stage Monday afternoon and the Salinas River to begin flooding Tuesday afternoon. The 175mile Salinas River, which runs from San Luis Obispo County to the Monterey Bay, has flooded only a handful of times in recent decades, according to the California Nevada River Forecast Center.
More evacuations possible
The Monterey County Sheriff ’s Office has issued a handful of evacuation orders over the past week for the region, most of which have been lifted, and it’s continuing to monitor to see whether they’ll again be needed.
Officials in Santa Clara County are watching San Jose’s Guadalupe River, which is forecast to begin flooding Monday afternoon.
Which of the rivers will pose problems and how bad they will be depend on where exactly flooding occurs, whether the area is adequately engineered to handle the excess water and, of course, what the weather brings.
The National Weather Service is expecting moderate rain over the weekend followed by a more potent atmospheric river early Monday through Tuesday. Rainfall totals early in the week could approach 10 inches in the North Bay and Santa Cruz Mountains. The systems follow several on-and-off storms over the past two weeks that have already raised rivers and left the ground saturated.
“There’s just a tremendous amount of moisture coming from the subtropics,” said David King, meteorologist with the National Weather Service.
A flood watch has been issued by the agency from early Saturday through Tuesday for the entire Bay Area and Monterey Bay regions.