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Colorado Flood Hazard Mitigation Plan - 1 !199 ==::=-=- rain and snow storms are generally a result of moist air masses which originate in the cooler northern Pacific i Ocean and move inland across the Pacific Northwest. ! Floods caused by failure of man-made structures are a result of: . Hydrologic deficiencies . Structural deficiencies . Improper Operation or Sabotage (1 case in CO) Each of these causes results in floods that have dis.. tinct characteristics relative to rate of rise, volume, du- ration, and flood season_ 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 chan. : nel may progress upstream or downstmam (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 beflinning of May through October Because the rate of risl~ is slow and the time available for warning is great, few lives are usually lost but mil- lions of dollars in valuable public and private property are at risk. ThEl October 5, 19'11, floods in Pagosa Springs and Duran~lo were a result of a general rain system over tributaries of the San Juan River Basin in southwestern Colorado. The June 3, 1921, flood in Pueblo was a r4~sult of a general rain system in the Upper Arkansas ":iver Elasin The damaging floods of June 1965 in the Denver-metro area were a result of heavy to torrential rainfall OV4::c>r large portions of the South Platte River Easin that lasted several days. 2.2.1.:! 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 Be- cause there is :ittle or no warning time, the term "flash flood" is often used to describe thunderstorm floods The avera!~e number of thunderstorm da~'s per year in Colorado varies from less than 40 r<ear the western May 1999 Flooding in otero County, Colorado, La Junta Middle School, Built in 1901, on the State Historic Registry. ".."""'F-q~"",,~",_ .__I~''i!!o;~'',nl-I'''J,'~'n! -.'f*'"~;J>'~~'_';'''_. Chapter 2 - 3 - , r".';--I~'_";-"'I'Ct(,;,r,,,c; '1_".""." -1'.-. , ,,.,,, """ "._,' ,.-" ,"1' 1 :1123,00 10:()() PM