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Thunderstorm or Dam Failure 01 ILl <5 . . < ~ ~ - o o ~ " < 0. ~ Snowmelt General Rain / / / / ,I Ram on Snow Time In Day Comparison of Typical Flood Characteristics in Colorado. unnoticed in the lower reaches of a large drainage basin. In some cases. however, rainfall may' be heavy and widespread enough to noticeably affect peak flows throughout a basin. Flooding along the Colorado River in Grand Junction in July of 1884, flooding in June of 1965 at higher elevation communities including Georgetown and Frisco. and flooding in June of 1983 along the Gunnison River at Delta and Grand Junction and along the Colorado River at Grand Junction, are examples of flooding from rain on melting snow. The effect of the rainfall in the Colorado River basin in 1983 was felt as far downstream as Mexico, Ice Jam Floods Ice jam floods occur when the upper reaches of a stream that has been frozen abruptly begins to melt due to chinook winds. Mean- while downstream one of two conditions is occuring to promote an ice jam. Either the frozen stream partially melts or temperature inver- sions cause an unfrozen stream to partially freeze. The upper basin flows move the ice chunks downstream until they become lodged at some constriction and form a jam, The jam forces the water to be diverted from the stream channel causing a flood. The ice jams can also break up suddenly causing surges of water as the "reservoir" that was formed behind them is released. Ice jams occur in slow mov- ing streams where prolonged periods of cold weather are experien- ced. Sometimes the ice jams are dynamited, allowing the backed up water to flow downstream. In 1955, 1962, and 1983 flooding in Rangely resulted from ice jams, as did 1973 flooding in Meeker. Dam Failure Floods There are approximately 27,000 dams in the State of Colorado, This includes about 2,249 dams which fall under the review of the Colo- rado Division of Water Resources (State Engineer) dam safety pro- gram, 16,000 small dams for small capacity reservoirs known as "Livestock Water Tanks," and Erosion Control Dams (which are not normally inspected under the dam safety program), potential artificial impoundments created by highway embankments constructed across drainageways, and miscellaneous ponds. A dam has to be at least 10 feet high and the reservoir must have a capacity of at least 50 acre-feet or the reservoir must have a storage capacity of at least 1,000 acre-feet in order to fall under the review of the dam safety program. Dams and their related appurtenances are inspected to forestall failure due to lack of proper maintenance and repair. Few lives have been lost in floods caused by a dam failure, but property damage has been high. There have been at least 130 known dam failures in Colo- rado since 1890. Dam failure floods are primarily a result of hydro- logic or structural deficiencies. The operation of a reservoir can aiso influence the safety of the structure. ' "Hydrologic deficiency" means one or more of a dam's components do not have the capacity to contain or pass the large volumes of water flowing into the reservoir. The most significant hydrologic deficiency is inadequate spillway capacity causing overtopping of the dam during a large flood. Large waves generated from landslides into a reservoir or the sudden inflow from upstream dam failures are other causes of hydrologic dam failure. Overtopping is especially dangerous for an earthen dam (earthen dams are the most common type of structure in Coiorado) because the down rush of water over the crest will erode the dam face and, if it continues long enough, will breach the dam embankment and cause the rapid reiease of the stored water into the downstream floodplain. Structural deficiencies include seepage through the embankment. piping along internal conduits, erosion, cracking, sliding, overturning, or other structural weakness. Old age is often at the root of structural deficiencies. Seismic activity in Colorado has recently been recog- nized as a potential s()uroe of structural problems due to liquefaction of fine-grained mat~rial in the embankment of a dam. . '- 4