FLOOD00083

• County: Jefferson
• Community: Buffalo Creek
• Title: Wildfire Hazard Mitigation and Watershed-Rehabilitation Monitoring
• Date: 12/17/1997
• Prepared For: CWCB
• Floodplain - Doc Type: Flood Mitigation/Flood Warning/Watershed Restoration

amounts of water were applied to burned areas (simple infiltration tests)
in 1996 and 1997. Generally, no water infiltrated and small droplets of
water formed indicating hydrophobic soils in 1996. During the wet spring
of 1997, applied water infiltrated rapidly, even on very steep hillslopes
(>30 %). However, after the soils dried, infiltration was low due to
reformed hydrophobic conditions or other factors. Data monitoring in this
study complements an instrumented, paired-basin analysis being conducted by
Casey Clapsaddle to assess the rehabilitation efforts used in Shinglemill
Creek and Morrison Creek (burned area was left untreated).

Despite extensive rehabilitation efforts in Buffalo Creek, smaller
rainstorms after July 12 in 1996 and 1997 produced similar rates of runoff
(fig. 3), which reflect persistent conditions that exacerbate flash-flood
potential. Natural debris (trees and sediment) present in many channels
(much removed during cleanup efforts) appeared to help slow water and
sediment runoff for many events. In addition, small runoff events (as much
as -3 m3/s) from burned hillslopes that reaches channels with thick (>3 m)
pea-gravel sediments such as in Sand Draw often infiltrated in a short
distance.

Because of the fire, numerous intermittent streams now are perennial. Base
flows as small as 0.2 m3/s after the July 12th flood were competent to
erode much of the new alluvial fans. Many fans had several
agradation-degradation cycles for small events (peak flows of -10-20 m3/s)
since the fire, which reflects channel instability. Lower tributaries
reaches that agraded on July 12, 1996, generally have degraded by about a
meter by the end of 1997, but hillslope erosion continues to contribute
sediment to channels. Sediment from the burned area has filled most
reaches of the NF and South Platte Rivers to Strontia Springs Reservoir
with about a meter of sediments. About 75 percent of Denve~s water comes
through the reservoir, which has a capacity of about 9.74 hm3. From May
1996 to October 1997, about 0.31 hm3 of sediment washed into the reservoir
compared to about 0.14 hm3 in the 13 years since the reservoir was built
(Denver Water Department, written commun., 1997). Snowmeit runoff in the
NF and South Platte Rivers likely will continue to move large amounts of
wildfire-produced sediments towards the reservoir.

Geomorphic investigations of alluvial sediments in the burned and unburned
(in 1996) areas indicate at least 10 fireslflood sequences have occurred in
the study area prior to 1996. At least one paleoflood was larger than the
July 12, 1996 flood. Radiocarbon dating of organic material in alluvial
sediments in a Buffalo Creek tributary indicated that these sequences span
about the last 2,500 years (John Elliott, USGS, written commun., 1997).
Additional investigations will help determine the long-term fire and flood
history in the region, which could help forest managers develop policy for
prescribed burns for watershed management.

Study results can be used to develop and verify hydrologic- and
sediment-budget models for burned watersheds. In conjunction with the
National Weather Service, preliminary rainfall-runoff modeling was
conducted to determine threshold-rainfall amounts of flash-flood
conditions. These results can be used to refine flash-flood warnings,
particularly to minimize the number of false alarms that can cause
complacency of those at risk. The Colorado Water Conservation Board (1997)
prepared an emergency response, hazard-mitigation, and awareness plan for
government officials, residents, and visitors in to the Buffalo Creek area.

Studying only the Buffalo Creek area can not provide answers to all
questions about wildfire hazards in other forest ecosystems. Thus,
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