FLOOD04156 - Laserfiche WebLink

Home Browse Search

1998. Colorado Flood Hazard Mitigation Plan (4091 shown in Figure 2-9. 2.3.2 Mud and Debris Flow Damages Mud and debris flow damages have been common throughout the history of modem man in Colorado. Many of the older mountain communities were built in part or entirely on the sides of major mountain valleys, which are the usual locations of the debris fans of ~maller tributary streams. A debris fan is the deposi- tional landform produced by successive mud and debris flow deposits. The towns of Glenwood Springs, Ouray, Telluride, and Idaho Springs have a long his- tory of damaging debris and mudflows. The town of marible in Gunnison County was nearly destroyed by severe flows in the 1930s and 19405, and the mining community of Brownville (near Silver Plume in Clear Creek County) was engulfed and destroyed by a series of flows in June of 1912. Much of the damage and loss of life during the Big Thompson storm and flood of 1976 were caused by multiple debris flows from smaller tributary streams. The 1965, 1969, and 1973 storm and flood events of the Front Range area produced extensive debris avalanching that originated on steep mesa side slopes of Douglas County. During the abnormally heavy spring snowmelt runoff of 1984 in Eagle County the. communities of Vail, Beaver Creek, and Redcliff were impacted by numerous debris flow events. In addition to threats to life and residential properties, the mud and debris flow events produce even more widespread effects on transportation and other public facilities requiring extensive and costly cleanup and repair an- nually throughout Colorado. Documentation and descriptive material regarding mud and debris flow phenomena in Colorado can be found in Colorado Geological Survey (CGS) publica- tions listed in Appendix C. Renewed development in mountainous areas of Col- orado has increased dramatically in the past thirty years, driven by the dernand for new resort communi- ties and second homes. This pressure has led to a tremendous increase in development of lands vulnera- ble to severe to moderate mud and debris flow haz- ards. Identification and mitigation of existing hazards and future recognition of hazards in advance of land use decisions could save many lives and millions of dollars in property losses in the years ahead. Damages in Colorado from debris flows and landsiides are known to have amounted to severai millions of dollars in 1984. More careful documentation of geo- logic damages and separation of these from floodwa- ter damage are listed in the Colorado Landslide Haz- ard Mitigation Plan, Bulletin 48, Colorado Geological Survey, 1988. 2.3.3 Catastrophic Landslide Damages Catastrophic landslides capable of damming major streams have been relatively rare in Colorado during the hlstonc penod. The most serious example is i probably the DeBeque Canyon slide of June 1924 which ternporarily blocked the Coiorado River and resuited in forced relocation of a small community, highway, and raiiroad. Several other slides have or are encroaching on a stream but have not as yet advanced rapidly enough to cause serious backwater effects. However, there are hundreds of somewhat older inactive or semf.active slides in many areas of the state that could be reactivated or accelerated by Increased ground moisture, stream erosion, man~ made excavations or neariby earthquakes. There is particular concern that continued increase in soil mois- ture and snowmeit runoff as experienced in 1983 and 1984 could lead to reactivation of some of these slides such as occurred at Thistie, Utah, in 1983 with serious consequences. 2.3.3.1 Buffalo Creek Flood Event (1996) I On the night of July 12, 1996, a thunderstorm oc- curred in the area of the community of Buffalo Creek, Colorado. The storm produced heavy precipitation over a short period of time. A flash flood occurred along Buffalo Creek, Sand Draw, Spring Gulch, the North Fork of the South Platte River (North Fork) beiow its confluence with Buffalo Creek, and several other tributary streams in the area. Two lives were lost as a direct resuit of the flooding. Roads, bridges, water lines, and other utility lines were damaged or de- stroyed. Numerous homes, outbuildings, and vehicles were damaged or destroyed as well. A large quantity of sediment and debris was carried from the water- shed and deposited along the affected stream reaches. Although the geographic area affected was smaller than in some other floods, the July 12 Buffalo Creek flood event was truly a disaster. Other smaller scale floods have occurred in Buffalo Creek between June and September of 1996 as well. In May of 1996, less than two months before the July 12 flood event, a wildland fire burned about 12,000 acres of forested area in the Buffalo Creek vicinity. The fire burned intensely and quickly, leaving behind charred timber and a barren landscape devoid of vegetation and ground cover. The bumed soils exhib- ited hydrophobic (water repelling) properties, and the burned area's natural erosion control and runoff in- hibiting characteristics were altered by the fire. Those conditions, in conjunction with a heavy rainstorm on July 12, were the recipe for disaster in Buffalo Creek. Peak discharges for the July 12 event for the North I Fork, Buffalo Creek, Sand Draw, and other tributaries Flle:1'HMP~-2.pub 1127m 6.14 All ChaptJer 2 -11