"It's a low probability, high-impact event..."


Stockton Record
Harvey a sober reminder for Stockton

By Alex Breitler 
Record Staff Writer 
No living soul can testify of the winter of 1861-62, when 45 days of rain transformed the Central Valley into a 300-mile-long inland sea.
And only some Stocktonians are old enough to remember the last time the city itself flooded, in the 1950s.
The danger, then, is that we no longer comprehend the risk. Because catastrophic floods on the magnitude of Hurricane Katrina and Hurricane Harvey are not only possible here: They may be inevitable.
“It’s a low-probability, high impact event,” said state climatologist Michael Anderson. “But at some point, everything lines up.”

Stockton in 1861

One of the reasons many people don't equate catastrophic storms with other disasters such as earthquakes or hurricanes is because they haven't experienced one.
In a 1973 San Joaquin County Historical Society newsletter, Robert E. Bonta described what happened in Stockton during 45 days of rain in 1861-62:
• Dec. 24: After days of heavy rain, flooding is considered imminent as the Calaveras River spills its banks. The winds kick up and half of the tin roof at City Hall blows away.
• Dec. 28: Water covers Miner Avenue between Hunter and California streets, as well as low-lying lots all over the city. French Camp Road is closed to all traffic except boats.
• Dec. 29: The city is cut off from the outside world. Everything between French Camp and the Coast Range is a "solid sea of water."
• Jan. 8: After a brief dry spell, the rains resume, melting snow in the foothills and causing new flooding. The Center Street bridge washes away; a local newspaper reports the bridge is "somewhere between this (city) and Benicia by this time."
• Jan. 11: Stockton founder Charles Weber's gardens and vineyards are under water. The adobe portion of his house is in danger of disintegrating. Fremont Square is under 6 inches of water and city sidewalks (then built of wood planks) are floating.
• Jan. 16: More heavy rains. The water remains shallow enough that most people can stay in their homes, but whale boats are dispatched to check on rural families.
• Jan. 23-25: The peak of the flooding. Boats become the only sensible means of transportation. Church services are suspended for three weeks. The deceased are sent to San Francisco for burial. One farmer near Collegeville sails more than 10 miles to Stockton for critical supplies. "The county was literally one large sea, with only isolated high spots above water."
• Jan. 26: The waters begin receding.
• Jan. 27 and 29: Snow falls in Stockton, one final insult from Mother Nature.
Thanks to the U.S. Geological Survey, we have a pretty good idea what “everything lines up” might look like. In 2011, teams of scientists pieced together a scenario in which back-to-back atmospheric river storms pummel the state, a deluge roughly comparable to that landmark 1861 disaster.
Like Harvey in Texas, this theoretical storm is a roughly 500-year event in some areas, meaning that it has a .02 percent chance of happening any given year.
Despite the construction of reservoirs and levees over the past century and a half, the simulated storm overwhelms California’s infrastructure and once again forms an inland sea up to 20 miles wide. The cost totals $725 billion, exceeding that of a major earthquake in Southern California or on the Hayward fault. San Joaquin County alone suffers $22 billion in property damage, with most of Stockton dunked underwater, including neighborhoods that are not considered high-risk flood areas today.
Sewer and water services would be cut off, perhaps for weeks. As many as 1.5 million people would need to evacuate, which in Stockton would include many poorer, non-English-speaking residents who might be at greater risk.
The scariest part: This isn’t some freak storm. Scientists studying layers of sediment on the ocean floor have determined that other megastorms even larger than 1861 have occurred six times in less than 2,000 years.
“There is no scientific evidence to suggest that such extreme storms could not happen again,” the scientists concluded in their report.
Even some of those who labor to strengthen our levees acknowledge that the system will eventually be defeated by a catastrophic flood.
“I think it’s inevitable,” said Dave Peterson, a Folsom-based levee engineer with extensive experience in Stockton. “Eventually, there will be an event that exceeds the design (of the system) and causes a substantial amount of damage. My hope is that we have in place an evacuation plan that will prevent all deaths, so that we’re just limited to property damage.”
California doesn’t get hurricanes, of course, and it’s extremely unlikely that Houston’s 50 inches of rain could fall here in a few short days.
But take that same amount of rain and spread it over a longer period of time, and California could have equally serious problems — in some ways, even worse.
For starters, unlike a summer hurricane fueled by the 80-degree Gulf of Mexico, California is likely looking at frigid wintertime flooding from the Sierra Nevada, with water temperatures as low as 40 to 50 degrees. As is often the case, blasts of cold air could move in behind the storms, heightening the risk for anyone trapped in the floodwaters for extended periods of time.
“Hypothermia is certainly a possibility,” said Dale Cox, who led the USGS effort to develop the conceptual storm.
Stockton also faces a trifecta of risk related to climate change.
If snow turns to rain in the Sierra Nevada, floods on the San Joaquin River upstream of Stockton could be nearly eight times larger than they were at the peak of this past winter’s moderate flood, state officials say.
On top of that, sea level rise in the vast pool of the Delta could place added pressure on the levees that protect the west side of the city. And if the weaker levees that protect rural Delta islands fail, turning the estuary into an inland sea, large wind-whipped waves could smash against the urban levees and test them even more.
If the urban levees protecting Stockton failed, 10 feet or more of water could swallow up the lower western flank of the city, though the exact depth would depend on where the levees break. Shallower flooding would also extend east of Interstate 5 through much of central Stockton.
It’s scary, for sure. But the news is not all bad. Just as experts are able to track hurricanes and provide advance warning of where they will wander, water managers in California predict rainfall rates and river levels and should know early-on if downstream evacuations are warranted, Peterson said. It takes days for water from some upstream reservoirs to reach Stockton.
Peterson said communication among flood managers on the San Joaquin River has improved.
“They (Houston) saw that storm coming days and days in advance,” he said. “We would, too. If we understand the consequences of that coming storm, there should be no reason why evacuation can’t be done well in advance.”
Michael Mierzwa, a state flood management planner who is also from the Houston area, said California has done a better job than Texas in planning communities in a way that will protect them from floods, with stronger zoning rules and building restrictions in vulnerable areas.
The levees themselves are improved, as evidenced by the lack of any major Delta flooding this past winter. But that was nowhere near the kind of epic flood we’re talking about here. Many levees fall short of higher standards and there is much work to do.
The entire state spends about $30 million a year to maintain levees, substantially less than the Houston region alone, Mierzwa said. California really should be spending about $130 million a year, he said.
But maintaining the current system isn’t enough. The very day that Harvey struck, as it happens, California officials approved a new $21 billion plan to better protect the Central Valley, not only with better levees but also flood bypasses and easements that would allow tightly constricted rivers to spread out and recover some of their historic floodplain. That would alleviate some pressure on downstream communities like Stockton.
If the whole plan were to be implemented someday, the number of expected deaths from big floods along the San Joaquin River would decline from more than 400 to fewer than 10, the plan says.
Even then, the state is likely to be “humbled” by future floods, Mierzwa said. Asked if any level of improvements could prevent that from happening, he said, “We’ll never get there.”
“Even if we were fully funded and had $21 billion to invest in (flood projects),” he said, “50 years from now there is still going to be a risk for loss of life.”
Contact reporter Alex Breitler at (209) 546-8295 or abreitler@recordnet.com. Follow him at recordnet.com/breitlerblogand on Twitter @alexbreitler.