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199B - Colorado Flood Hazard Mitigation Plan (409) 2.2.1.1 General Rain Floods General rain floods can result from moderate to heavy rainfall occurring over a wide geographic area lasting several days. They are characterized by a slow steady rise in stream stage and a peak flood of long duration. As various minor streams empty into larger and larger channels, the peak discharge on the mainstream channel may progress upstream or downstream (or remain stationary) over a considerable length of river. General rain floods can result in considerably large volumes of water. The general rain flood season is historically from the beginning of May through Octo- ber. Because the rate of rise is slow and the time available for warning is great, few lives are usually lost, but millions of dollars in valuable public and private prop- erty are at risk. The October 5, 1911, floods in Pagosa Springs and Durango were a result of a general rain system over tributaries of the San Juan River Basin in southwest- ern Colorado. The June 3, 1921 flood in Pueblo was a result of a general rain system in the Upper Arkansas River Basin. The damaging floods of June 1965 in the Denver-metro area were a result of heavy to torrentiai rainfall over large portions of the South Platte River Basin, which lasted several days. 2.2.1.2 Thunderstorm Floods Damaging thunderstorm floods are caused by intense rain over basins of relatively small area. They are characterized by a sudden rise in stream level, short duration, and a relatively small volume of runoff. Because there is little or no warning time, the term 'flash flood" is often used to describe thunderstorm type floods. The average number of thunderstorm days per year In Colorado varies from less than 40 near the western boundary to over 70 in the mountains along the Front Range. The thunderstorm flood sea- son In Colorado is from the middle of July through October. The widely publicized Big Thompson Canyon fiood disaster of July 31, 1976, was a result of an intense thunderstorm cell, which dropped up to 10 inches of rain in a few hours over the basin. On May 15-16, 1993, a thunderstorm-induced flood event occurred at Rifle on Rifle and Government Creeks. As is usually the case, the highest flows in the shortest period of time occurred when an estimated 125-year flood discharge impacted Rifle. Structures and vehicles in harm's way suffered damages in ex- cess of $200,000. On June 17, 1993, a flash flood occurred on Shooks Run in Colorado Springs. Damages were confined to a F1le:FHMP-Ch8prer~2.pub Chapter 2 . 3 1/29/98 3:49 PM -.J.__