Saturday, October 17, 2009

American River Watershed & Floods

The most crucial bit of information in this news story from the Sacramento Bee is the reminder of the fact that the American River Watershed is one of the most flood-prone in the nation, due to the prevalence of rain storms that can flow in from the Pacific.

It is this propensity that created the importance for constructing the Auburn Dam, the only way to insure a 500 year level of flood protection for the Sacramento area, which now has a 100 year level, lower than that of New Orleans when it flooded.

As we saw just a little over 20 years ago in 1986, the potential for flooding in Sacramento from too much water in the American River Watershed and too little storage capability, is high.

An excerpt.

“The storm that soaked the state this week was the California equivalent of a hurricane, according to experts working to ready the public for the next one.

“Called an "atmospheric river," the storm pulled a geyser of moisture all the way across the Pacific Ocean, from the tropics near Indonesia. The narrow channel of intense rain scored a direct hit on California's Central Coast, then gushed like a fire hose all day.

“Near Big Sur, more than 20 inches of rain were recorded in a single day. Sacramento received 3 inches, enough to make this the city's fifth-wettest October in history.

“Had the storm hit farther south, it could have caused devastating mudslides on recently burned Southern California mountain slopes. Had it occurred later in winter, when the ground was already saturated, the storm might have caused dangerous floods in the Sacramento region.

"We just dodged a bullet," said Lucy Jones, chief scientist at the U.S. Geological Survey Multi-Hazards Demonstration Project, which is researching atmospheric rivers.

“Smaller versions, born near Hawaii, are popularly known as "Pineapple Express" storms.

"We've shown ... they are largely responsible for the big rains and big floods that occur," said Marty Ralph, chief of the water cycle branch at the National Oceanic and Atmospheric Administration in Boulder, Colo. "Those are the ones to watch out for."…

“Ralph's lab in 2005 began installing weather instruments in the canyons of the American River, making it the nation's most closely monitored watershed. The river was chosen for its potential to cause disastrous floods.”