Saturday, September 21, 2013

A 1,000 Year Event: Historic Rainfall and Floods in Colorado

by Michon Scott
Through the first week of September 2013, Colorado was exceptionally warm and dry. By September 12, everything had changed. Flood conditions stretched about 150 miles, from Colorado Springs north to Ft. Collins. Saturated soils left water with no place to go, and puddles turned to ponds throughout the densely populated Colorado Front Range. Rainwater swelled rivers and creeks, overtopped dams, flooded basements, and washed out roads. By September 16, authorities had confirmed six deaths, and more than 1,000 people remained missing.
Among the hardest-hit communities was Boulder, located on the northwestern end of the Denver metropolitan area. Boulder's Daily Camera reported that heavy rains started on the evening of September 11 and continued through the following morning. The National Weather Service recorded rainfall amounts exceeding 8 inches in Boulder on September 12, and amounts exceeding 4 inches the next day. Meteorologist Jeff Masters noted on his blog that the three-day rainfall recorded by the evening of September 12 exceeded the monthly total for any month since rainfall records began in 1897. Similarly high rainfall totals occurred in other spots along the Front Range. Masters exclaimed, "These are the sort of rains one expects on the coast in a tropical storm, not in the interior of North America!"
In an interview with KDVRDenver, Russ Schumacher of Colorado State University concluded that the precipitation in Boulder County and other parts of the state qualified as a 1,000-year event, meaning that any one year has just a 1-in-1,000 chance of experiencing such heavy precipitation. Ranking the actual flooding is more challenging than quantifying the precipitation because over time, people use the land differently. Bob Henson of the University Corporation for Atmospheric Research remarked in AtmosNews, "An identical weather event a century ago might produce a much different flood than the same event today."
On September 12, the Boulder Creek, which flows roughly eastward through town, crested in downtown Boulder at 7.78 feet—the highest water level observed at that location since 1894. The main highway running through Boulder was partially closed southeast of town, and partially destroyed northwest of town, isolating the nearby mountain community of Lyons. Thousands of residents faced power outages and evacuation orders in the Denver-Boulder area as officials called in the National Guard to assist rescue efforts. Schools, businesses, and government offices closed. Many roads remained closed and impassable, so multiple mountain communities remained isolated. And the rain kept falling. Read More