It happens once every one to two hundred years. The last one hit in the winter of 1861/1862 and is considered by some scientists to be the greatest storm in the written history of California. Reports indicate that the storms lead to flooding across the State of California and the entire Sacramento and San Joaquin valleys were inundated with water for an extent of 300 miles. John Carr, a witness to the devastation which poured heavy rains on the state for a continuous four weeks and rapidly melted snow from the mountains, wrote of the event:

The winter of 1861–2 was a hard one…

…as the current became stronger amongst the trees, one after another began to fall, some floating off with the water, and others hanging by the roots. Trinity that morning was playing havoc with the settlers on its banks. It was dreadful to look upon. Standing on high ground, one could see property of all kinds on its way to the ocean. The river itself seemed like some mighty uncontrollable monster of destruction broken away from its bonds, rushing uncontrollably on, and everywhere carrying ruin and destruction in its course. When rising, the river seemed highest in the middle. When falling, it became lowest in the middle, and all the drift worked toward the center of the stream. When the river was at or near its highest, one could see floating down parts of mills, sluice-boxes, miners cabins, water-wheels, hen-coops, parts of bridges, bales of hay, household furniture, sawed lumber, old logs, huge spruce and pine trees that had withstood former storms for hundreds of years—all rushing down that mad stream on their way to the boundless ocean….Not a bridge was left, or a mining-wheel or a sluce-box. Parts of ranches and miners cabins met the same fate. The labor of hundreds of men, and their savings of years, invested in bridges, mines and ranches, were all swept away. In forty-eight hours the valley of the Trinity was left desolate.

The damage was estimated to have completely wiped out approximately one quarter of all taxable real estate in California.

The United States Geological Survey and the Multi-Hazards Demonstrations Project launched a research study into the possibility that such a storm could happen again. The research teams concluded that an atmospheric and river storm, dubbed ARk Storm, would cause more damage than a large scale ‘shakeout’ earthquake along the entire rift of the San Andreas fault line.

Some key findings from the research group:

• Another ARk Storm is Inevitable – Such storms have happened in the California historic record (1861-1862), but 1861-1862 is not a freak event, not the last time the state will experience such a severe storm, and not the worst case.
• Massive, State-wide Evacuations – Because the flood depths in some areas could realistically be on the order of 10-20 feet, without effective evacuation there could be substantial loss of life.
• Economic Catastrophe – A severe California winter storm could realistically flood thousands of square miles of urban and agricultural land, result in thousands of landslides, disrupt lifelines throughout the state for days or weeks, and cost on the order of $725 billion. This figure is roughly 3 times that estimated for the ShakeOut earthquake, another planning scenario reflecting an earthquake with roughly the same annual occurrence probability as an ARkStorm-like event. The $725 billion figure comprises about $400 billion in property damage and $325 billion in business-interruption losses.
• Wide-spread Flooding – Perhaps 25 percent of buildings in the state could experience some degree of flooding in a single severe storm.

Source: USGS Overview of the ARk Storm Scenario [pdf]

As we’ve seen in recent weeks, large scale flash floods or “instant Tsunamis” can cause catastrophic damage to communities, wiping out life and property in a matter of seconds.

The population of the Sacramento and San Joaquin valleys exceeds one million people, and because of how and where those valleys are situated, an ARkstorm similar to the one from 1861/62 would be absolutely devastating, likely displacing the majority of that population. This does not include other low lying areas of the state which would be affected. Such an event would affect not only California, but have disastrous effects across the entire nation, namely economically.

The USGS report also identifies various challenges faced by emergency response personnel:

There is a lack of policy and experience among state and local emergency responders and government managers in dealing with the complexity of mass evacuations, short- and long-term housing needs, and the restoration of communities statewide once the flood waters recede.

Translation: When it hits the fan, you’re on your own. Like any major natural (or man-made) disaster scenario from floods and earthquakes to hurricanes and tornadoes, expect that no one will be there to help, especially for the first 3 – 7 days. Federal, state and local response to Hurricane Katrina should be used as a guide.

For those who live in natural disaster prone regions, it is imperative that you have prepared food, water, shelter and personal security measures in advance. It will be too late after the fact.

For some preparedness guidance related to specific scenarios, consider the following from Tess Pennington of Ready Nutrition. The information provided by Tess is of benefit even to those not specifically located in natural disaster prone areas of the country:

Disaster Supplies for Surviving a Flood
Preparing for an Earthquake
Family Emergency Plan Check List
Are You Ready Series: 72 Hour Kits
Emergency Items: What Will Disappear First

Video: The ARkStorm Scenario:

What It Looks Like When The Rains Don’t Stop: Flash Flooding In Toowoomba, Australia [January 10, 2011]:

More Australia Flood Footage [January 11, 2011]:

USGS Shakeout Earthquake Scenario: