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Colorado Flood Hazard Mitigation Plan - 1999 ==:_ 1999 Flood Damages Figure 2.6 , ~ Public Assistance $ '>,345,302 Individual Assistance 892,366 Small Business Administration Home '5,726,200 Business 701,200 EIDL 161,500 Total Damage 1 :1,826,568 Source: Colorado Office of Emergencl' Management as of September 22, 1999 (note: the above damages repre.ent the federal programs as listed) Pueblo, and Weld Counties, These co,nties sustained damage to roads, bridges, culverts, homes, and busi. ness from overtopping, dike breaches, erosion, mud- slides, and rockslides, 2.2.1.3 Snowmelt Floods Snowmelt floods result from the me~il1g of the winter snowpack in the high mountain areas, Snowmelt floods typically begin as spring runoff appears, after the first spring warming trend, If the trend continues up to 8-10 consecutive days in a basin where the snowpack has a water content more than about 150% of average, serious flooding can develop, The total duration of snowmelt floods is usually over a period of weeks rather than days, They yield a larger total vol- ume in comparison to other varieties of floods in Colo- rado, Peak flows, however, are generally not as high as flows for the other types, A single Gold day or cold front can interrupt a me~ing cycle causing the rising water to decline and stabilize until the cycle can begin again, Once snowme~ floods have peaked, the daily decreases are moderate, but fairly constant. Snow- melt flooding usually occurs in May, ,June, and early July, Floods in June 1983, along the Cache 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 Fremont and Chaffee counties, were principally due to me~ing snow The 1984 floods on the western slope were primarily snowmelt flooding. L Chapter 2 - 7 2.2.1,.$ Rain on Snowmelt Floods Rain on snow flooding occurs most often in Colorado during the month of May, It is at this time of year that large gEmeral rainstorms occur over westl~rn Colorado. These rainstorms are most often caused when warm moist air from the GuN' of Mexico begins pushing far enough north that it begins to affect western weather, In combination with this movement of air mass is the continued possibil~y of cold fronts moving into Colo- rado from the Pacific Northwest. When tllese weather phenomena collide, long lasting general rainstorms can often occur. Rain on snowmelt exacerbates an already tenuous situation as snowmett waters rush down heavily incised slream channels, ~'ny abnormal increase in flow from other sources usually causes streams to leave their banks. During the spring months of May and June when rivers are running high, there is a potential for flooding due to rain falling on melting snow. Usually such rain is over a small part of a basin, and the resulting flood is of short duration and may often go unnoticed in the lower maches of a large drainage basin. To some extent, the cloud cover associated with the rain system can slow the me~ing cycle and offset the compound effect. In SOmE! cases, however, rainfall may be heavy and widespread enough to noticeably affect peak flows throughout the basin, Flooding along the Colorado River in Grand .Junction in July 1884, along Clear Creek at Georgetown in June 1965, and along the Gunnison and Colorado rivers at Grand Junction in June 1983, are examples of flooding from rain on melting snow. The effect of rain on me~- ing snow in the Colorado River Basin in '1983 was felt as far downstream as Mexico. In 1984, rain or melting snow caused severe flooding conditions al Pacnia, On May 28, 1993, rain on snowmelt floocling occurred at Paonia on the North Fork of the Gunnison River. The rainfall occurred over a five-hour period during the evenin~1 This caused the North Fork of the Gunnison River to reach its highest level since the 1984 flood season, Many miles of a9riculture land experienced severe bank erosion in unincorporated Delta County. 2.2.1.5 Ice Jam Floods Ice jam floods can occur by two phenomena In the mountain floodplains during extended cold periods of 20 to 40 c1egrees below zero, the streams ice over, The channels are frozen solid and overbank flow oc, curs, which results in iCI~ inundation in the floodplains. ice jam noods can occur when frozen wafer in the up' per reaches of a stream abruptly begins to me~ due to warm Chinook winds, Blocks of ice floating down- stream can become lodHed at constrictions and form a Jam, Th" jam can force water to be diverted from the 12/23/9910:00 PM