https://www.latimes.com/local/california/la-me-california-flood-20180325-
htmlstory.html

California�s drought-to-deluge cycle can mask the dangers Mother Nature
can have in store.

During one of the driest March-through-February time periods ever recorded
in Southern California, an intense storm dumped so much rain on Montecito
in January that mudflows slammed into entire rows of homes. Hundreds of
homes were damaged or destroyed, and at least 21 people died.

It was a grim reminder that in a place so dry, sudden flooding can bring
disaster.

Eighty years ago this month, epic storms over just six days caused
widespread destruction across Southern California.

Rain fell as fast as 2 inches for a one-hour period. Wide swaths of the
San Fernando Valley were inundated; floodwaters in the Los Angeles River
mowed down bridges and pulled apart railroads.

Government officials responded with a major flood control campaign,
building dams and deepening rivers and lining them with concrete to flush
water out to sea before floodwaters could rise.

But even those protections have limits. And history shows there is
precedent for even more devastation.

Several weeks of monumental storms would be all it would take to overwhelm
California�s flood control system and cause widespread flooding and
destruction.

Nightmare scenario

Overwhelming California�s flood system
Take the heavy rains from the winter of 2016-17 � which poured over a
period of 80 days � and compress them down to fall in 23 days: California
would experience a flood event so grave it would overwhelm our flood-
control systems, force 1.5 million people to evacuate, and cause economic
damage that could total $725 billion.

�Really, a major flood like this would dwarf a 7.8 earthquake in Southern
California� in terms of the financial toll, said Dale Cox, project manager
for the U.S. Geological Survey�s Science Application for Risk Reduction.
(A magnitude 7.8 earthquake is expected to cause $213 billion in economic
damage.)

�The storm is estimated to produce precipitation that in many places
exceeds levels only experienced on average once every 500 to 1,000 years,�
the report said.

And as the situation from Hurricane Harvey in Houston shows, once-in-a-
1,000-year storms can happen.

�Such an event could occur in any future winter,� the USGS said of a
modern-day scenario similar to the historic 1861-62 floods that
transformed much of the Central Valley and Los Angeles Basin into an
inland sea.

The scenario was dubbed ARkStorm � named for Atmospheric River 1,000 Storm
� and published in 2011 after input from more than 100 experts from public
and private sectors. The scenario envisions a pounding of California by a
series of �atmospheric rivers� � long plumes of water vapor that can pour
over the West Coast and hold as much as 15 times the liquid water flowing
out of the Mississippi River�s mouth.

Great flood of 1862

Plunging much of California under water
The Great Flood of 1862 occurred in a series of storms that lasted just 45
days, and plunged most of the Central Valley and Los Angeles Basin
underwater.

Modern-day flood-control systems built in the 20th century have recently
spared California from the kind of catastrophic flooding seen in the early
days of statehood.

For instance, officials launched an ambitious project to protect
California�s capital, which was built where the American and Sacramento
rivers met and struggled for decades with flooding since its founding. One
of the solutions was to set aside a massive area of farmland that
transforms into an inland lake during the wettest years, designed as a
relief valve to spare Sacramento from floodwaters.

But there is always the possibility that just the right set of
circumstances meet to cause a rare but devastatingly powerful series of
storms to head to California.

�It�s inevitable that all of our flood-control systems, at one point, will
be overwhelmed � because we only design for so much,� said Keith Porter, a
University of Colorado Boulder research professor and coauthor of the
scenario.

Officials in state and local agencies agree that the USGS extreme flood
scenario is one that could strike California.

�Yes, the ARkStorm is definitely a realistic scenario,� said Michael
Mierzwa, lead flood management planner for the California Department of
Water Resources.

Mark Pestrella, chief of the Los Angeles County Flood Control District,
agreed. �It�s plausible,� he said. �Many things would have to come
together for that to happen.�

On the worst-case scenario

Envisioning a realistic doomsday option
The ARkStorm is modeled on merging two recent extreme California storms
that could bring the kind of rainfall that struck in 1861-62. It combines
the great Southern California storm of Jan. 19-27, 1969, back-to-back with
the Northern California storm of Feb. 8-20, 1986, and what would happen if
the first of those storms stalled over a 24-hour period.

�While history will not repeat itself in exactly this way,� a summary of
the ARkStorm scenario said, �a story resembling [an uninterrupted
sequence] of these two events in terms of duration, windspeeds and
precipitation is entirely realistic.�

�Putting those two storms together was enough to exceed the flood capacity
of most of the state,� said scientist Lucy Jones, who coauthored the flood
scenario report for the U.S. Geological Survey.

The 1861-62 flood was a seminal moment in California history. That flood
was said to have forced the state�s eighth governor, Leland Stanford, to
row through floodwaters to his inauguration, and then back home to climb
in through the second story.

Floodwaters covered a region of the Central Valley 250 to 300 miles long
and about 20 to 60 miles wide, according to an account by geologist
William Brewer and reviewed by Jones for her book to be published in
April, �The Big Ones: How Natural Disasters Have Shaped Us and What We Can
Do About Them.�

In her book, Jones called the flood of 1861-62 �the most devastating event
in California history� � superseding even the great 1906 San Francisco
earthquake.

One-third of California�s taxable land was destroyed, Jones wrote, and the
state went bankrupt � the Legislature wasn�t paid for 18 months. Entire
industries were destroyed: mining equipment was swept from the mountains
and many miners died, a factor in the end of the Gold Rush, Jones wrote;
the ranching industry shrank as hundreds of thousands of heads of cattle,
sheep and lambs drowned, and California progressed from a ranching economy
to one focused on farming.

�Just trying to describe the extent of the damage is overwhelming,� Jones
wrote. �Yet, 150 years later, most Californians are unaware that it ever
happened.�

In Anaheim, Santa Ana River swelled to become four miles wide, �creating
an inland sea 4 feet deep that lasted for a month,� Jones said. The
overflowing river wiped out the town of Agua Mansa near where Fontana is
now � destroying what was once the largest settlement between L.A. and New
Mexico.

�In Los Angeles, the water was described as extending from mountain to
mountain, with no dry land between the Palos Verdes peninsula and the San
Gabriel Mountains,� Jones said. A rain gauge near where Marina del Rey is
today recorded 66 inches of rain falling in just 45 days, Jones said.
That�s more than quadruple the average rainfall downtown L.A. sees in an
entire year.

But those storms probably weren�t even the worst California has ever
faced. Geologic data points to six mega-storms striking California in the
last 1,800 years that were even worse than the 1861-62 storms, based on
geologic data collected in the San Francisco Bay and Santa Barbara areas,
the report said. And climate change could make such storms more likely in
the future � �as the oceans heat up, that means more fuel for an
ARkStorm.�

�California flood protection is not designed for an ARkStorm-like event,�
the report said. �Levees are not intended to prevent all flooding, such as
the 500-year streamflows that are deemed realistic throughout much of the
state in ARkStorm."

The ARkStorm could cause a flood 300 miles long and 20 or more miles wide
in the Central Valley, forcing 1.5 million people to flee their homes.
Sobering maps in the USGS report show much of Sacramento and Stockton
underwater, which are both located next to California�s largest rivers,
the Sacramento and San Joaquin.

The scenario forecasts 50 breaches of the levees as being realistic. The
scenario said officials� goals for the levees were to keep them intact
long enough to assure safe evacuation � not to protect property.

L.A. probably would not suffer as widespread inundation as it did in the
1861-62 flood, Cox said. �But there would be quite a bit of flooding,� he
said.

According to the report, the lower reaches of the Los Angeles and San
Gabriel rivers could spill their banks, turning sections of Long Beach,
Carson, Lakewood, Compton, Downey and West Covina into flood zones.
Coastal flooding could inundate areas such as Belmont Shore, Naples and
Seal Beach and the ports of Los Angeles and Long Beach.