We may be heading into another drought, but must prepare for the alternative as well.
In November and December of 1861, heavy snowfall had covered the Sierra Nevada mountains of California. Beginning in late December, months of heavy warm rainfall, over 100″ in some areas, quickly melted the snow, flooding the entire Central Valley. A shallow inland sea formed, hundreds of miles long by dozens of miles wide, putting Sacramento under 10′ of water. During the 6 months it took for the water to subside, the capital moved to San Francisco, and California went bankrupt. Whites had never experienced this kind of flooding in California, but the native people moved out of the valley some weeks before the storms started.
In the last century, climate science developed an understanding of the long period cycles of California weather, which is shaped by our local geography interacting with two large Pacific Ocean patterns: the El Niño-Southern Oscillation (ENSO), and the Pacific Decadal Oscillation (PDO). ENSO arises from conditions in the tropical Pacific, with a period of 6-18 months, and PDO is tied to conditions in the northern Pacific, with a period spanning decades. These two oscillations shift the jet stream, and thus winter storms, now called “atmospheric rivers”.
California tends to be wetter than normal during the El Niño phase of ENSO, and dryer during the La Niña phase. The PDO cycle affects California rainfall similarly. Since the two cycles run at different rates, sometimes they compete and cancel each other out, but when they are in synch, we get severe drought or inundation.
Paleoclimatology is the study of climate from before modern record keeping began. Using sediment cores from river floodplains, estuaries, and lakes, the evidence of previous floods and droughts can be determined. We now know an inundation like 1861-2 happens every 150-200 years or so, and some of the previous events were several times larger.
As the details of this scientific investigation come clearer, the full impact is slowly growing among emergency planners. The U. S. Geological Survey Multi Hazards Demonstration Project pulls together scientists, emergency planners, businesses, and governmental agencies to investigate possible natural disaster impacts to better prepare our society. The first project, the ShakeOut Earthquake Scenario, published in 2008 focused on the impact of a 7.5 magnitude earthquake in southern California. In 2010, their second project, ARkStorm (Atmospheric River 1000 Storm), examined the impact of a storm similar to 1861-2, with the following conclusions.
“The Central Valley experiences hypothetical flooding 300 miles long and 20 or more miles wide. Hundreds of landslides damage roads, highways, and homes. Property damage exceeds $300 billion, most from flooding. Agricultural losses and other costs to repair lifelines, dewater (drain) flooded islands, and repair damage from landslides,…
Read more:: ARkStorm – The Ukiah Daily Journal