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I I I I I I I I I I I I I I I I I I I Colorado Flood Hazard Mitigation Plan -1999 1999 Flood Damages Hgure 2.6 ' I Public Assistance $ 6,345,302 892,366 Individual Assistance Small Business Administration Home 5,726,200 Business 701,200 161,500 $ 13,826,568 EIDL Total Damage Source: Colorado Office 01 Emergency Management as 01 September 22. 1999 (note: the .bon d.m.ges repr.sent the fed.,.,1 prog/'lms .s lisl.(1) Pueblo, and Weld Counties. These counties 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 Snowmeltlloods result lrom the mening a the winter snowpack in the high mountain areas. Snowmelt floods typically begin as spring runoff appears, alter the lirst spring warming trend, ~ 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 duratioo 01 snowmelt floods is usually over a period 01 weeks rather than days, They yield a larger total vol- ume in comparison to other varieties 01 floods in CoIc>- rado. Peak nows, however, are generally not as high as flows for the other types, A single cold day or cold front can interrupt a menlng cycle causing the rising water to decline and stabilize until the cycle can begin again, Once snowmen floods have peaked, the daily decreases are moderate, but lairly constant Snow- melt flooding usually occurs in May, June, and early July, Floods in June 1 983, along the Cache la Poudre River in Fort Collins and Greeley; along Clear Creek and rts tributaries in Silver Plume and Georgetown; and along the Arkansas River in Fremoot and Chaffee counties, were principally due to melting snow. The 1 984 floods on the western slope were primarily snowmell flooding, 2,2.1.4 Rain on Snowmelt Floods Rain on snow flooding occurs most often in Colorado during the month a May. It is at this time of year that large general rainstorms occur over western Colorado, These rainstorms are most often caused when warm moist air from the Gu~ a Mexico begins pushing far enough north that rt begins to affect western weather, In combination wrth this movement of air mass is the continued possibillly 01 cold Ironts moving into CoIc>- rado from the Pacific Northwest When these weather phenomena collide, long lasting general rainstorms can often occur. Rain 00 snowmelt exacerbates an already tenuous slluatioo as snowmell waters rush down heavily incised stream channels, Any 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 01 short duration and may often go unnoticed in the lower reaches of a large drainage basin, To some extent, the cloud cover associated with the rain system can slow the melting cycle and oIIset 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 Gunnisoo and Colorado rivers at Grand Junction in June 1 983, are examples a flooding from rain 00 melting snow, The effect 01 rain on melt- ing snow in the Colorado River Basin in 1983 was lelt as far downstream as Mexico, In 1984, rain or melting snow caused severe flooding conditions at Paonia, On May 28, 1993, rain on snowmelt flooding occurred at Paonia on the North Fork of the Gunnison River, The rainfall occurred over a five-hour period during the evening, This caused the North Fork 01 the Gunnison River to reach lis highest level since the 1984 flood season, Many miles 01 agricullure 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 degrees below zero, the streams ice over, The channels are frozen solid and overbank flow 0c- curs, which resulls in ice inundation in the floodplains, Ice jam floods can occur when frozen water in the up- per reaches of a stream abruptly begins to mell due to warm Chinook winds, Blocks 01 ice floating down- stream can become lodged at constrictions and form a jam, The jam can force water to be diverted from the Chapter 2 - 7 1/1.005:30 PM