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1998 - Colorado Rood Hazard Miti9ation Plan (409J and complex landslides_ The most common charac- teristics are the large volume of slide material 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. 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 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 sttes_ 3) Slide movement is reactivated as a resutt of disruptions by construction and increased soil moisture from "normal" climatic f1uctuations_ 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 pilin9 up against the oppostte valley side_ 6) By this time the valley is blocked to a height of 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 imme- diate decisions relating to backwater flooding, down- stream flooding when the slide is overtopped, restora- I tio of essential services, and evacuation of people I from affected homes and townsites_ I I 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 fioods are reported to have occurred in 1826 in the Arkansas 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 resutted File:FHMP-chapter.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 f1ood_ In many parts of Colorado, spring brings the greatest threat of flooding because of addi- tional water from metting snowpack_ In addttion, thunderstorms capable of producing flash floods are most common in May through September. Since the tum 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- mary of these presidentially declared disasters is Recent Presidential Major Disaster Declarations. Figure 2-9 I --;:.1" Fr~;~~~~~~ti~~""-'S~id~~I-" "." ' 1969-I-F;;;nt Range 15 eo;,-;;-ti.;;;- -s~i;dRai~t.i-- - . =1970J-~~~---=-~ " ,SU~i~~nfall_ ",-- (1) Kersey Dam Failure (2) Front Range 13 Counties Sustained Rainfall (3) Sou1hwest 13 Counties Sustained Rainfall ;976-~I-Bi9 Th~pson F;;;nt R;,nge FI~h Floodin9.H~ ~in-:- 2 Counties fall Over Short Duration _Ud _ ___u____._u___ ____ ___u____.____ _____u._ 1982 I Lawn Lake Front Range Dam Failure 1 County (llrimer) '-1984-r-w;;;;si;;15-~~~ti;- --~~=;it-F;;';~;d-M~d:- , ---'------.-t-------'-----.-..--.- --------. -.--.-------.---------..---.-- 1997 I Front Range 13 Counties Sustained Rainfall 1973 ._ :i" "'. '__ ,~,.l-_,-, _ Chapter 2 - 10 1129198 f0:46 AM