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t998. Colorado Flood Hazard Mitigation Plan (409) 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 smaller 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 marble in Gunnison County was nearly destroyed by severe f10ws 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 Iile during the Big Thompson storm and flood of 1976 were caused by multiple debris fIo>Ns from smaller tributary streams_ The 1965, 1969, and 1973 slorm and f100d events of ~he 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 Iile 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 throu9hout 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 AppenalX C. Renewed development in mountainous areas of Col- orado has increased dramatically in the past thirty years, driven by the demand 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 landslides are known to have amounted to several 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 period_ The most serious example is I probably the DeBeque Canyon slide of June 1924 which temporarily blocked the Colorado River and resuited in forced relocation of a small community, highway, and rallroad_ 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 semHilctive slides in many areas of the slate that could be reactivated or accelerated by increased ground moisture, stream erosion, man- made excavations or nearby 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 Thistle, Utah, in t983 with serious consequences. 2.3.3.1 Buffalo Creek Aood Event (1996) On the ni9ht of July 12, 1996, a thunderstorm oc- curred in the area of the community of Buffalo Creek, Colorado_ The slorm 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) below its confluence with Buffalo Creek, and several other tributary streams in the area Two lives were lost as a direct resuit of the f100ding_ 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 lire 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 (waler 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 Fork, Buffalo Creek, Sand Draw, and other tributaries FiIe;FHIIP.chapter-2.pub 1/27/98 6:14 AM Chapter 2. 11