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

Spring Creek (fig. 1). Before additional rainstorms, the extent of fresh
rill and gully erosion was compared to bucket-rainfall amounts. Hillslopes
(burned or unburned), which have sparse vegetation and less than about 25
mm rain had some sediment movement and minimal rill development.
Hillslopes in areas of about 50 mm of rain had rills about 75 mm deep and
50 mm wide. Hillslopes that had about 75 mm of rain had numerous gullies
up to 0.5 m deep and a meter wide. Numerous gullies up to a meter deep and
3 m wide about 5 km south of Buffalo Creek near the head of Sand Draw,
Shinglemill Creek, and Spring Creek were used to infer a maximum 1-hr
rainfall amount of at least 115 mm and then estimate an isohyetal map (fig.
1). Rainfall amounts decreased rapidly outside the burned area and the
storm covered about 50 to 75 km2. Henz (Henz Meteorological SeNices,
written commun., 1997) analyzed Doppler radar signatures and upper-air
obseNations for the July 12 storm. He estimated a maximum rainfall of
about 130 mm in about an hour with the cell located near the head of Sand
Draw with a similar isohyets, but oriented slightly different (fig. 1).
The 100-year, 1-hr rainfall is about 55 mm for the Buffalo Creek area
(Miller et aI., 1973). Thus, the July 12, 1996 maximum rainfall (130 mm)
for the Buffalo Creek storm was about 2.4 larger than a 100-year event.

Maximum water depths as much as 4 m occurred within 30 minutes of the
storm's onset (fig. 2). HWMs generally were good-to-excellent,
charcoal-debris lines and were used to estimate the water slope and flood
depth for each cross section. Peak discharge estimated with the
slope-conveyance (SIC) method usually is less accurate than estimates
using multiple cross sections. However, estimates provided here reflect
averages of several SIC estimates along a reach of channel, therefore,
probably are more accurate than when only one SIC estimate is used. SIC
uncertainty is shown in parentheses, which accounts for various factors
affecting discharge estimates (primarily n values, bulking of flow with
sediment and debris, and channel changes). The peak discharge was 450 m3/s
(+/-20%) for Buffalo Creek near the NF South Platte River (fig. 1, site 1;
fig. 2). This estimate reflects runoff from the burned area in Buffalo
Creek and its tributaries (notably Sand Draw, Spring Gulch, Shinglemill
Creek, and Morrison Creek). Sand Draw, about 3.6 km2 (fig. 1, site 2),
had a peak discharge of 200 m3/s (+/-25%). The peak discharge was 510 m3/s
(+/- 25%) for Spring Creek upstream from the South Platte River (fig. 1,
site 3). Maximum flooding in Spring Creek is attributed to severely burned
conditions, hydrophobic soils, basin slopes greater than about 30 percent,
extensive bedrock exposure, and because the storm moved down (easterly) the
basin. Flash-flood runoff at the South Platte River streamflow-gaging
station (06707500) measures the cumulative runoff from the 1996 burn area
(fig. 1, site 4), but principally from Buffalo and Spring Creeks. The peak
discharge was 325 m3/s (+/- 20 %), reflecting attenuation of flood peaks
from Buffalo and Spring Creeks. Maximum rainfall was about 100 mm in
unburned areas, however, streams in these areas had minimal runoff (fig.
3).

Large quantities of sediments were mobilized in the burned area during the
July 12th flood. Numerous estimates of sheetwash erosion (referenced to a
distinct black, burn boundary on rocks in pre-flood surfaces) on the burned
area suggest an average of about 10 mm of soil erosion. Locally, small
streams produced up to 6 m of scour, primarily by headcutting. Hundreds of
trees as large as a meter in diameter toppled into the floodwaters, which
exacerbated flood damages. Sediments moved on hillslopes ranged from silt
to cobble-sized material, and 2.5 m in diameter boulders were transported
in some channels. Many channels degraded during the July 12th flood,
although locally agradation (1-2 m) occurred particularfy along Buffalo,
Spring, and Shinglemill Creeks and on alluvial fans at tributary junctions
in burned areas. Many fans had dimensions of about 100 m x 30 m x 1.5 m.

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