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t998. Colorado Rood Hazard Mitigation Plan (409) 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- . , , ~ I <<'t: ,- "",) .IFigUre2-Bll~ i tion in the Mitigation Plan for 1984 are only those most often or directly associated wtth flooding, heavy runoff - or dam failures (either as a cause or an effect)_ 2.2.2.1 Mud and Debris Flows Mudfiows 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 iarger areas_ Factors predisposing an area to mud and debris fiow occurrences include basin size. geometry, and geology, combined with high an- tecedent soil moistura_ The actual events are initiated by heavy runoff from etther intense rainfall ("cloud bursts") or sudden heavy snowmelt Rainstorm initi- ated events tend to involve smaller areas and shorter duration as they are limited to the basin(s) affected by a single thunderstorm cell. Those caused by snowmelt can be more extensive in area and can continue as a threat for several weeks_ They are Influenced by more general snowpack condition and temperature fluctuations_ Mud and debris flow events result in plugs of high veloctty, 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_ During a given event, one or several successive plugs can form and descend upon the fan_ A pulse of heavily sediment-laden floodwater follows each plug_ Any works of man encountered on the fan surface can be destroyed or seriously damaged_ Within 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 vulnerability to the hazard results from the fact that our climate, geology and terrain combine to make many areas of the state subject to mud and debris flow hazards_ The high potential for damage is due to the fact that dozens of Colorado communities are in hazard locations_ In addttion numerous, attrac- tive, but potentially hazardous, development sites re- main throughout ColoradO_ A debris fan is a sloping wedge-shaped heterogeneous deposit of rock, soil, and woody debris at the junction of a smaller stream with the valley of a larger one_ The fan is created by periodic high-velocity mud and debris fiews (mudslides) that come down the stream channel and are depostted on the fan_ Debris fans offer attractive but potentially hazardous development sites in many mountain valleys since they are well above the valley floor and the main stream floodplain (see Figure 2-8 Vail, CO)_ 2.2.2.2 Catastrophic LandslidllS 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 itself. The mechanisms by which the widespread effects can occur include: . damming and backwater effects from the land- slide deposits, . breaching by erosion of such a landslide formed dam with consequent downstream flooding, and . massive landslide deposits that enter and diS- place water of an existing reservoir thereby pro- ducing downstream flooding by dam failure or overtopping. A prototype incident of the catastrophic landslide type was the Thistle, Utah, slide of 1983_ Severai different types of landslides common in Colorado have demon- strated the potential for blocking of major streams with resulting backwater and other serious effects_ These include large rock fall sheets, rockslides, earthflows, File:FHMP.chapter-2.pub Chapmr 2 . 9 1/29198 1:35 PM