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<br />Effects of rainstorms on water and sediment runoff following the 1996 wildfire, B1f~IP.. ~l)
<br />Creek, Colorado c. Cot"" . l'fo,.
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<br />R.D. Jarrett, U.S. Geological Survey, Lakewood, Colorado
<br />T.W. Browning, Colorado Water Conservation Board, Denver, Colorado
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<br />Abstract A basin-wide, monitoring approach provides important information to draw isohyetal
<br />maps of rainstorms, assess the effects of wildfire on water and sediment runoff, assess the
<br />effects of watershed-rehabilitation activities, determine watershed recovery time, and help
<br />manage forest ecosystems. The approach is flexible, requires minimal resources, and
<br />complements comprehensive instrumented monitoring in a short amount of time. The approach
<br />was applied to the mountain community of Buffalo Creek, Colorado, which had a catastrophic
<br />wildfire on May 18, 1996. Subsequent rainstorms produced 9 floods larger than a 1 OO-year (pre-
<br />fire conditions) flood as well as numerous smaller floods. Rates of flood runoff in 1997 were about
<br />the same as in 1996, which indicate persistent effects from the fire and minimal watershed
<br />recovery. The largest rainstorm, on July 12, 1996, of about 130 mm in an hour was about a
<br />1 ,OOO-year rainstorm. This extreme rainstorm produced a flood about 10 times larger than the 100-
<br />year (pre-fire) flood. On Sand Draw, and other small tributaries in burned basins near the center
<br />of the storm, peak discharges exceeded the 1,OOO-yr flood. Unbumedbasins in areas of
<br />maximum rainfall had minimal water and sediment runoff. Study results were used to help the
<br />National Weather Service determine threshold-rainfall amounts that could produce flash flooding in
<br />the Buffalo Creek area. Large quantities of sediment continue to be transported in bumed-area
<br />streams since the fire. Most of this sediment is deposited in Strontia Springs Reservoir, which is
<br />a major water supply for Denver located a few km downstream from the burned area.
<br />Investigations of alluvial sedirnents indicate at least 10 wildfire-flood sequences during about the
<br />past 2,500 years in the Buffalo Creek area. Study results indicate that prehistoric fires and
<br />subsequent increased runoff prior to fire suppression of the last century contribute to cyclical,
<br />geornorphic instability.
<br />
<br />Introduction
<br />
<br />"Wildfire is a naturai occurrence in this tinder-dry climate. But when humans put themselves in
<br />harms's way, a natural phenomenon can be transformed into a natural disaster." (Denver Post,
<br />Firefight Starts at Home, p. 4G, July 20,1997).
<br />
<br />Cornrnunities are encroaching into forested-rnountain areas in rnany parts of the United States,
<br />particulariy at the urban interface near metropolitan areas in the western United States. This
<br />growth has significantly increased the risk of natural hazards to people living in or visiting forested
<br />areas. Floods and debris flows, particularly in recently burned watersheds, pose a serious threat
<br />to hurnan life, property, and the environment. California suffers an average annual billion dollars
<br />in costs and loses to wildfires (Weise and Martin, 1995). Increased risks rernain for several years
<br />or decades until burned areas sufficiently recover to pre-burn conditions (Evenstad and Rasely,
<br />1995). Recent wildfires and associated flood and debris-flow hazards in southern California
<br />(Florsheim et aI., 1991; Weise and Martin, 1995); Helena, Montana (Parrett, 1987); Yellowstone
<br />National Park (Meyer et aI., 1995; Ewing, 1996); Storm King Mountain, Colorado (Cannon et aI.,
<br />1995); the Wasatch Mountains in Utah (Evenstad and Rasely, 1995); Boise, Idaho; Bandelier
<br />National Monument, New Mexico; and Mesa Verde National Park, Colorado (all in 1996)
<br />emphasize the potential risk for loss of life, property darnage, and costs associated with hazard
<br />mitigation and watershed rehabilitation. The effects of flood runoff, sediment delivery, geomorphic
<br />changes, watershed recovery time, and the linkages among them are poorly understood for
<br />burned watersheds. Watershed, sediment, and ecosystem models may not be applicable
<br />without rnodifications for assessing changes due to burned areas (Weise and Martin, 1995). The
<br />focus of this study was to develop a flexible, cost-efficient, rnonitoring and analysis approach
<br />that can be quickly implemented following wildfires wherever they may occur.
<br />
<br />Research was undertaken to rnonitor and determine the risk of hazards and to help mitigate, to the
<br />extent possible, loss of life and property damage from water and sediment runoff related to
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<br />Draft 3/30/98
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