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t99S. Colorado Rood Hazard Miti9ation Plan (409) e and complex landslides, The most common charac- teristics are the large volume of slide material and the abilrty to move considerable distances. An exception to this generalization is the rockslide, which is some- times capable of blocking a canyon location without moving an unusual distance from its site of origin. The most serious threat of large and catastrophic landslide events in Colorado 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. A typical catastrophic landslide scenario for a reacti- vated old landslide is as follows: 1) A large landslide! earthflow complex occupied the wide tnbutary 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 resu~ 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 opposrte valley side. 6) By this time the valley is blocked to a height of several hundred feet, all utility and transportation lacllrties 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- stream flooding when the slide is overtopped, restora- tio of essential services, and evacuation of people from affected homes and townsrtes. e 2.3 Historic Damages 2.3.1 Flood Damages e 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 Ar1<ansas River and Republican River basins. Between 20 and 30 large magnrtude 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 2oS. 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 turn of the century, 331 people have been killed and over $3,3 billion in property damages have resulted Fl/e:FHMP-chapuw-2.pub ~ 1976 - BIG THOMPSON CANYON - LIVES LOST 144 ~ 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 addrtion, thunderstorms capable of producing flash floods are most common in May through September. Since the turn the turn of the century, damaging floods in the state have killed 337 people and caused prop- erly 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- mary of these presidentially declared disasters is Recent Presidential Major Disaster Declarations - Figure 2.9 Year Location Cause 1965 FlOOt RMge 33 Counbes Sustained Rainfall 1969 FlOOt RMge 15 Counbes Sustained Rainfall - 1970 Soolllwesl Sustained Rainlall - - - - ~ ~ -- 1973 (1) Kersey DlOTl Fail.... (2) FlOOl RMge 13 Counties Sustained RlOnfall (3) Soolllwest 13 Counbes Sustained Rainfall ~ - - 1976 8~T~FIOOIRMge Flash Fkxxing. Heavy Rain- 2 Counbes fall ev... Short DlIation 1982 L_ Lake FlOOl RMge DlOTl Fail.... 1 County (llnnef) 1984 Weslim Slope 15 Counbes SI1O'Nll8Il Floods and Moo. slides - ---- 1997 FlOOt RMge 13 Counbes Sustained Rainfall Chapter 2 . 10 1J2I1JOIO:48AII