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I I I I I I I I I I I I I I I I I I I Colorado Flood Hazard Mitigation Plan -1999 This case was finally decided in 1972 by the Colorado Supreme Court, which recognized the concept d prob- able maximum flood as a predictable and foreseeable standard for spillway design purposes, The Lawn Lake Disaster d 1982 resutted form the fail- ure d a privately-owned dam on Forest Service prop- erty, and $31 million of damage was sustained in Larimer County and Estes Park A lawsurt awarded $480,000 to one of the four persons killed in the disas- ter. The most unusual flood from the failure d a man- made structure in Colorado is probably the complete draining d 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 Closely Associated with Flooding Most geologic hazards are related one way or another to water. However, those selected for specific atten- tion in the Flood Hazard Mrtigation Plan for 1999 are only those most often or directly associated wrth flood- ing, heavy runoff, or dam failures (erther 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 agencies, These are common events in mountainous areas 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 frequent but can affect much larger areas, Factors predisposing an area to mud and debris flow occurrences include basin size, geometry, and geology, combined with high antece- dent soil moisture, The actual events are inrtiated by heavy runoff from either intense rainfall (cloud bursts) or sudden heavy snowmen, Rainstorm inrtiated events tend to involve smaller areas and shorter duration, as they are limrted to the basin(s) affected by a single thunderstorm cell, Those caused by snowmen can be more extensive in area and can continue as a threat for several weeks since they are influenced by more general snowpack condrtions temperature fluctuations, Mud and debris flow events resutt in plugs of high ve- locity, high density mud, rock, and woody debris that scour the middle and lower channel reaches and move considerable distances across the depositional area known as a "debris fan," A debris fan is a sloping wedge-shaped heterogene- ous deposrt of rock, soil, and woody debris at the junc- tion of a smaller stream wrth the valley of a larger one. The fan is created by periodic high-velocrty mud and debris flows (mudslides) that come down the stream channel and are deposrted on the fan, During a given event, one or several successive plugs can form and descend upon the fan, Each plug is fol- lowed by a pulse d heavily sediment-laden flood wa- ter. Any wor1<s of man encountered on the fan surface can be destroyed 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 event or from one event to the next, Colorado's vulnerabilrty to the hazard results 'rom the fact that our climate, geology, and terrain combine to make many areas of the state subject to mud and de- bris flow hazards. The high potential for damage is due to the fact that dozens of existing Colorado communi- ties are in hazard locations, In addrtion, numerous, at- tractive, but potentially hazardous, development srtes 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 facilities on a far greater scale than the event rtse~, The mechanisms by which the wide- spread effects can occur include: o Damming and backwater effects from the landslide deposits o Breaching by erosion of such a landslide-formed dam wrth consequent downstream flooding o Massive landslide deposrts that enter and displace water of an existing reservoir, thereby producing downstream flooding by dam failure or overtopping A prototype incident of the catastrophic landslide type was the Thistle, Utah, slide of 1983, Several different types of landslides common in Colorado have demon- strated the potential for blocking d major streams wrth resulting backwater and other serious effects, These include large rock fall sheets, rock slides, earthflows, and complex landslides, The most common character- istics are the large volume d slide material and the abilrty 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 rts srte of origin. The most serious threat d large and catastrophic land- slide events in Colorado is probably from accelerated 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 Chapter 2 - 9 1/1.00 5:30 PM