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Colorado Flood Hazard Mitigation Plan -1999 ~ I I I I I I I I I I I I I I I I I I I I seen in the right middle ground at some time in the past f_ thousand years; 2) Man has developed the valley floor with transportation and utility facil~ies 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 cI slide materials move continuously into the valley despite efforts to stop or divert the flOW; 5) Movement continues until ~ is stopped by piling up against the opposite valley side; 6) By this time the valley is blocked to a height cI several hundred feet, all utility and transportation facilities 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 cI people from affected homes and townsites, 2,3 Historic Damages 2,3,1 Flood Damages Compilations cI exact data on the histay 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 cI peak discharge) occur sam_here in Colo- rado every year. The 22 roost 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, 1976, 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-melro area, Since the turn of the century, 352 pee- 1973 pie have been killed and over $4.4 billion (1999 dollars) in property dam- ages have resu~ed from the 22 most 1976 damaging floods in Colorado, A study completed in 1983 01 the largest known floods in various front range 1982 drainage basins indicated over 352 people have died since the 1800s as the result 01 flooding, Using the con- 1984 sumer price 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 01 Colorado, spring brings the greatest threat 01 flooding because of addi- tional waler 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 resu~ of floods in the past 30 years, Most of these disasters were caused by precipitation, but two were caused by dam failure, A summary 01 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, Location Cause Colorado's Recent Major Presidential Disaster Declarations -- ~ Front Range 33 Counties Sustained Rainfall Front Range 15 Counties Sustained Rainfall Southwest (1) Kersey (2) Front Range 13 Counties (3) Southwea113 Counties Big Thompson Front Range 2 Counties Lawn Lake Front Range 1 County (Larimer) Western Slope 15 Counties Front Range 13 Counties Sustained Rainfall 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 1/1.005:30 PM