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Colorado Flood Hazard Mitigation Plan -1999 ---- --- r This case was finally decided in 1972 by the Colorado Supreme Court, which recognized the Goncept of prob. able maximum flood as a predictable and foreseeable standard for spillway design purposes, The Lawn Lake Disaster of 1982 resutted form the fail- ure of a privately-owned dam on Fore:;t Service prop- erty, and $31 million of damage was sustained in Larimer County and Estes Park. A lawsuit awarded $480,000 to one of the four persons killed in the disas- ter, The most unusual flood from the failure of a man- made structure in Colorado is probably the complete draining of Lake Emma, a natural lake located high in the San Juan Mountains above Silverton, Colorado, On June 4, 1979, flood water flowed through a network of tunnels in an abandoned mine that extended under the lake, 2,2.2 Geologic Hazards Closel), Associated with Flooding Most geologic hazards are related one way or another to water, However, those selected for spec~ic atten- tion in the Flood Hazard Mnigation Plan for 1999 are only those most often or directly associated wnh flood- ing, heavy runoff, or dam failures (either as a cause or an effect), 2.2.2.1 Mud and Debris Flows Mudflows and debris flows, as defined in Colorado Statutes, are essentially synonymous with "mudslides" as used by FEMA and other federal ailencies, These are common events in mountainous anaas of Colorado where they most often occur in steep ravines of first or second order streams. Events occurring in larger stream basins are usually less frequer,t but can affeel much larger areas, Factors predispo"ing an area to mud and debris flow occurrences include basin size, geometry, and geology, combined wi':h high antece- dent soil moisture, The actual events are inniated by heavy runoff from either intense rainfall (cloud bursts) or sudden heavy snowmen, Rainstorm initiated events tend to involve smaller areas and sholter duration, as they are limned to the basin(s) affected by a single thunderstorm cell. Those caused by s 10wmelt can be more extensive in area and can conti 1ue as a threat for several weeks since they are influenced by more general snowpack conditions temperatllre fluctuations, Mud and debris flow events result in plugs of high ve- lOCity, high density mud, rock, and woody debris that scollr the middle and lower channel reaches and move considerable distances across the depositional area known as a "debris fan." i A debris fan is a sloping wedge-shaped heterogene- lous depOSit of rock, SOil, and woody debns at the Junc, tlon of a smaller stream wrth the valley of a larger one The fan IS created by penodlc hI9h-VE~OClty mud and Chapter 2 - 9 debris flows (mudslides) that come down the stream channel and are deposited on the fan, During a given event, one or several successive plugs can form and oescend upon the fan, Each pluil is fol- lowed by a pulse of heavily sediment-laden flood wa- ter. Any works of man encountered on thl:'l fan surface can be (jestroyed or seriously damaged, Wrthin the fan area thE~ plugs or resulting streams of mud, rock, and debris can shift position quite unpredictably during a single Hvent or from one event to the next. Colorado's vulnerability to the hazard results from the fact that our climate, geology, and terrain combine to make many areas of th" state sllbject to mud and de- b,.;s flow hazards, The high potentiat for damage is due to the fact that dozens of existing Colorado communi- ties are in hazard locations. In addrtion, numerous, at- tractive, blrt potentially hazardous, development sites remain throughout Colorado. 2.2,2.2~ Catastrophic Landslides Catastrophic landslides are herein defined as those landslides that have the potential to affect valley lands, populations, and facilitiE3S on a far greatm scale than the event ItseW, The mechanisms by which the wide- spread effects can OCCUI' include: . Dammlllg and backwater effects from th" landslide deposits . Breaching by erosion of such a lanclslide,formed dam with consequent downstream f1ooclin!~ . Massive landslide d€'posns that enter and displace water of an existing reservoir, thereby producing downstream flooding by dam failure or ov"rtopping A prototype incident of the catastrophiC landslide type was the Ttlistle, Utah, slide of 1983, Several different types of landslides common in Colorado l1ave demon- strated the potential for blocking of major streams wrth r"sulting backwater and other serious e1fects. These include large rock fall sheets, rock slides, Harthflows, and cornplE~x landslides. The most common character- 1stics are the large volume of slide material and the ability to move considerable distances. An exception to this generalization is the rock slide, which is some- times capable of blocking a canyon location without moving an unusual distance from its site of origin. Tile most serious threat of large and catastrophic land~ 'slide events in Colorado is probably from accelerated movem"nt of marginally stable old slides, There are hundreds of large old slides in Colorado and 30 or more that show evidence of curmnt 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 17J23/99 10:00 PM