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199B - Colorado Flood Hazard Mitigation Plan (4091 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- k ~~>r' r~..' tion in the Mitigation Plan for 1984 are only those most often or directly associated with flooding, 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 w~h 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 iess 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 an- tecedent soil moisture. The actual events are initiated by heavy runoff from either intense rainfall ("cloud bursts") or sudden heavy snowmelt. Rainstorm initi- ated events tend to involve smaller areas and shorter duration as they are iimited to the basin(s) affected by a single thunderstorm cell. Those caused by snowmeJt can be more extensive in area and can continue as a threat for several weeks. They are influenced by more general snowpack condition and Rle:FHMP-ChBpter-2.pub -~ temperature fluctuations. Mud and debris flow events result in plugs of high velocity, 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 vuinerability 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 addition 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 flows (mudslides) that come down the stream channel and are depos~ed 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 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 itself. The mechanisms by which the widespread effects can occur include: . damming and backwater effects from the land- , slide deposits, I' . 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. Several different i 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, Chapter 2. 9 l1W981:35 PAl