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v~' \~~ \~ \'" ~'!~~~~~ Floods caused by failure of man-made structures--dams and levees-- result from: 1) hydrologic deficiencies, 2) structural deficiencies, 3) improper operation or sabotage. Each of these causes results in floods which have distinct charac- teristics relative to rate of rise, volume, duration, and flood season. Precipitation Caused Floods.. <General Rain Floods General rain floods result from moderate to heavy rainfall occurring <lYer a wide geographic area and lasting several days. They are char- acterized by a slow steady rise in stream depth and a peak flood of ong duration. As various minor streams empty into larger and larger channels, the peak dischar!le on the mainstem cl1annel may progress ~pstream 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 usually from the beginning of May through October. Because the rate of rise is slow and the time available for warning is great, the pOSSibility of saving lives is increased but millions of dollars in valuable public and private property are put 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. The June 3.1921 flood in Pueblo was a result of a general raID system in the Upper Arkansas River Basin. The damaging floods of June 1965 in the Denver-Metro area were a result of heavy to torren- tial rainfall over large portions of the South Platte Fliver basin which nized as a potential source of structural problems due to liquefacfion of fine-grained material in the embankment of a dam.' Thunderstorm Floods Damaging thunderstorm floods are caused by intense rain over basins of relatively small areal extent. They are characterized by a sudden rise in stream level. short duration, and a mlatively small volume of runoff. Because there is little or no warning time, the term "flash flood" is otten IJsed to describe thunderstorm type floods. The thunderstorm flood season in Colorado is from the middle of July through October. The widely publiCized Big Thompson Canyon flood disaster of July 31,1976, was a msult of an intense thunderstorm cell which dropped up to 10 inches of rain in a few hours over the basin. Snowmelt Floods Snowmelt floods result from the melting of the winter snowpack in the high mountain areas. Snowmelt floods typically begin as spring runoff appears, atter the first spring warming trend. If the trend con- tinues up to 8-10 consecutive days, in a basin where the snowpack has a water content more than 150% of average, flooding can develop. The total duration of snowmelt floods is usually over a period of weeks rather than days. They yield a larger total volume of runoff in comparison to other varieties of flood. Peak flows. however, are generally not as high as flows for the others. A single cold day or cold front can interrupt a melting cycle. The rising water will then decline and stabilize until the cycle can b,~gin again, Once snowmelt floods have peaked, the daily decreases are moderate but constant Snowmelt flooding usually occurs in May. June, and early July. Floods in La Plata County in 1941 and 1949 and those in June, 1983, along the Cac~le la Poudre River in Fort Collins and Greeley, along Clear Creek and its tributaries in Silver Plume and Georgetown. and along the Arkansas River in Fmmont and Chaffee counties. were principally due to melting snow. Rain on Snow Floods During the months of May and June and the early part of July there is a potential for flooding due to rain falling atter spring snowmelt has already increased runoff. Sometimes such rain is over a small part of a basin and the resulting flood is of short duration and may often go