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199B - Colorado Flood Hazard Mitigation Plan (409/ and complex landslides. The most common charac- I teristics are the large volume of slide malerial and the ability to move considerable distances. An exception to this generalization is the rockslide, which is some- I times capable of blocking a canyon location without moving an unusual distance from its site of origin. I I I The most serious threat of large and catastrophic landslide events in Coiorado is probably from acceler- ated movement of marginally stable old slides. There are hundreds of large old slides in Colorado and 30 or more that show evidence of current activity. ~ 1976 - BIG THOMPSON CANYON - LIVES LOST 144 ~ A typical catastrophic landslide scenario for a reacti- vated old landslide is as follows: 1) A large landslide! earthflow complex occupied the wide tributary valley seen in the right middle ground at sometime in the past few thousand years; 2) Man has developed the valley floor with transportation and utility facilities and town sites. 3) Slide movement is reactivated as a result of disruptions by construction and increased soil moisture from "normal" climatic fluctuations. 4) Mil- lions of cubic yards of slide materials move continu- ously into the valley despite efforts to stop or divert the flow. 5) Movement continues until it 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 transporilation facilities are severed and the stream in the main valley is dammed. 7) Emergency response officials are faced with imme- diate decisions relating to backwater flooding, down- i stream flooding when the slide is overtopped, restore- tio of essential services, and evacuation of people from aflected homes and townsites. from the 17 most damaging floods in Colorado. A study completed In 1983 of the largest known floods in various front range drainage basins indicated over 350 people have died since the 1800s as the result of flooding. All streams, regardless of size, have the potential to flood. In many parts of Colorado, spring brings the greatest threat of flooding because of addi- tional water from melting snowpack. In addition, thunderstorms capable of producing flash floods are most common in May through September. Since the turn the tum of the century, damaging floods in the state have killed 337 people and caused prop- erty damage of $3.3 billion (1997 dollars). The average annual loss in Colorado due to floods is $16 million. Between 1965 and 1997 the president has declared eight major disasters in Colorado as a result of floods. The president has declared areas in Col- orado a major disaster during eight of the past 30 years. Most of these disasters were caused by precipi- tation, but two were caused by dam failure. A sum- I mary of these presidentially declared disasters is 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 earliest known floods are reported to have occurred in 1826 in the Arikansas River and Republican River basins. Between 20 and 30 large magnitude floods (in terms of peak discharge) occur somewhere in Colorado every year. The 17 most damaging floods in Colorado recorded history are listed in Figure 2--5. 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. The most damaging flood in Colorado occurred in June 1965 on the South Platte River when almost $500 million in damages were sustained in the Denver-metro area. Since the tum of the century, 331 people have been killed and over $3.3 billion in property damages have resuited Fi/e:FHMP-Chapter-2.pub "" ~~I~l!sidlintiaf.~!iH~imW~lu.lilolUii~~~ie'~"i'm Year I Location Cause '1965-[';:;;; R..;g;,33 c.;~~" . "s~~'R;;;~~n-MK" -------1-------------.--. .________ou_______ ------.-.- 1969 Front Range 15 Counties Sustained Rainfall ____~--_. ---e----- --- 1970 I Southwest Suslained Rainfall ..... - '.-" "...." '. 1973 (1) Kersey Dam Failure (2) Front Range 13 Counties Sustained Rainfall (3) Southwest 13 Counties Sustained Rainfall ..--------.-----------------.---1---------------.----------- 1976 I Big Thcxnpson Front Range Flash Flooding, Heavy Rain- 2 Counties fall Over Shat Duration 1-- ----. -----, ..-------.---------,----..- --.---- ------ ------.... -- 1982 I Lawn Lake Front Range Dam Failure 1 County (Larimer) '1984f w~si;,;;;.l!ic;,~~ti.;;--~=;~-Fi;;;;.;..;d M~';:: 1997' rF~;;;;;R.;;;g;;i3..c;,~~ij;.-..s~~-;,;;;U.- ...-. Chapter 2 .10 112911810:46 AM