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Colorado Flood Hazard Mitigation Plan - 1999 seen in the nght middle ground at some time in the past few thousand years; 2) Man has developed the valley floor with transportation and util~y facilities and town s~es; 3) Slide movement is reactivated as a re- su~ of disruptions by construction and increased soil moisture from "normal" climatic fluctuations; 4) Millions of cubic yards of slide materials move continuously into the valley despite efforts to stop or divert the flow; 5) Movement contnues untl ~ is stopped by piling up against the opposite valley side; 6) By this time the valley is blocked to a height of several hundred feet, all utility and transportation facil~ies are severed and the stream in the main valley is dammed; 7) Emergency response officials are faced with immediate decisions relating to backwater flooding, downstream flooding when the slide is overtopped, restoration of essential services, and evacuation of people from affected homes and townsites, 2.3 Historic Damages 2.3.1 Flood Damages Compilations of exact data on the history of floods in Colorado since settlement began are lacking. The ear- liest known floods are reported to have occurred in 1826 in the Arkansas River and Republican River ba- sins, Between 20 and 30 large magn~ude floods (in terms of peak discharge) occur somewhere in Colo- rado every year. The 22 most damaging floods in Colo- rado recorded history are listed in Fig- ure 2.4. The most lives lost due to a single flood event occurred in the Big Thompson canyon on July 31, 1 976, when 144 people were killed, 1965 The most damaging flood in Colorado occurred in June 1965 on the South 1969 Platte River when almost $2,2 billion in damages (1999 dollars) was sus- 1970 tained in the Denver-metro area, Since the turn of the century, 352 pee- 1973 pie have been killed and over $4.4 billion (1999 dollars) in properly dam- ages have resulted from the 22 most 1976 damaging floods in Colorado, A study completed In 1983 of the largest known floods in vanous front range 1982 drainage basins indicated over 352 people have died since the 1800s as the result of flooding, Using the con- 1984 sumer pnce index to adjust past flood damages at the time of each event to 1997 present worth, total flood damages are estimated at over $4.4 billion, ~ 1976 - BIG THOMPSON CANYON - LIVES LOST 144 ~ the potential to flood, In many parts of Colorado, spnng brings the greatest threat of flooding because of addi- tional water from me~ing snowpack, The average annual loss in Colorado due to floods is $16 million, Between 1965 and 1999 the president declared nine major disasters in Colorado as a result of floods in the past 30 years, Most of these disasters were caused by precipitation, but two were caused by dam failure, A summary of these Presidentially de- clared disasters is shown in Figure 2-8. 2.3.2 Mud and Debris Flow Damages Mud and debris flow damages have been common throughout the history of modern man in Colorado, Colorado's Recent Major Presidential Disaster Declarations Location Cause Front Range 33 Counties Sustained Rainfall Front Range 15 Counties Sustained Rainfall Southwest Sustained Rainfall (1) Kersey (2) Front Range 13 Counties (3) Southwest 13 Counties Big Thompson Front Range 2 Counties Lawn Lake Front Range 1 County (Larimer) Western Slope 15 Counties Front Range 13 Counties Dam Failure Sustained Rainfall Sustained Rainfall Flash Flooding, Heavy Rainfall Over Short Duration Dam Failure Snowmelt Floods and Mudslides Sustained Rainfall 1999 Sustained Rainfall Front Range 12 Counties All streams, regardless of size, have Figure 2.8 Chapter 2 - 10 12123/9910,00 PM