FLOOD06861

• County: Jefferson
• Community: Buffalo Creek
• Basin: South Platte
• Title: Effects of Rainstorm on Water and Sediment Runoff Following the 1996 Wildfire, Buffalo Creek
• Date: 3/1/1998
• Prepared For: Buffalo Creek
• Prepared By: CWCB
• Floodplain - Doc Type: Flood Documentation Report

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The 100-year, 1-hr rainfall is about 55 mm for the Buffalo Creek area (Miller et aI., 1973), Diller
(1997) analyzed 24-hr, annual maximum rainfall values for 50 stations near Buffalo Creek having
a similar climate regime, He conducted a regional rainfall-frequency analysis using the method of
L-moments (Hosking and Wallis, 1997), The regional1-hr rainfall frequency curve developed by
Diller (fig, 5) is within 5 percent of the rainfall-frequency relation of Miller et al. (1973), The
recurrence interval was about 1,000 years for the July 12, 1996 maximum rainfall (130 mm) in
about 1-hr in Buffalo Creek,

Maximum water depths as much as 4 m occurred within 30 minutes of the stonn's onset in Buffalo
Creek (fig, 6), Spring Creek, and the NF and South Platte Rivers, High-water marks (HWMs)
generally were good-to-excellent and comprised of charcoal, leaf, and needle litter, silt, bent grass,
and wash lines, HWMs 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
an average of several SIC estimates along a reach of channel. These estimates probably are
more accurate than a single SIC estimate because of a good agreement of estimates along a
channel. The SIC uncertainty in discharge estimates is caused primarily by n values, bulking of
flow with sediment and debris, and channel changes, Estimates of uncertainties are shown in
parentheses, The peak discharge estimate was 450 m3/s (+1-20%) for Buffalo Creek near the NF
South Platte River (fig, 1, site 1; fig, 6), 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, with a drainage area of about 3,6 km2 (figs, 1 and 7, site 2), had
an estimated peak discharge of 200 m3/s (+1-25%), The estimated peak discharge was 510 m3/s
(+1- 25%) for Spring Creek upstream from its confluence with the South Platte River (fig, 1, site
3), Maximum flooding occurred in Spring Creek because the basin has slopes greater than about
30 percent, extensive bedrock exposure, hydrophobic soils, maximum rainfall occurred in its
headwaters, and the stonn moved from west to east down the basin,

The South Platte River streamflow-gaging station (06707500) measures the cumulative runoff
from the 1996 bum area (fig, 1, site 4), but principally from Buffalo and Spring Creeks, The SIC
estimated peak discharge was 325 m3/s (+1- 20 %), reflecting attenuation of flood peaks from
Buffalo and Spring Creeks, The July 12, 1996 peak stage in the South Platte River at South
Platte gage stilling well was 1,52 m lower than excellent high-water marks (HWMs), probably due
to an insufficient size intake to fill the large-volume, stilling well. The peak discharge using the
incorrect gage height (estimated during the stonn) from the rating curve was only 81 m3/s; it is
likely the entire flood hydrograph is suspect, Incorrect gage recordings are a serious concern for
flash-flood detection and issuing warnings to the public,

Large quantities of sediments were mobilized on hillslopes and in channels in the burned area
during the July 12th flood (eg" figs, 3c and 7), A distinct black, bum boundary on rocks in pre-
flood surfaces was used as a reference to estimate the general surface erosion from sheet wash,
Care was taken to estimate the general erosion rather than the local erosion around the rock, In
addition, pillars of soil were preserved under some surface rocks and metal objects on the burned
areas, About a hundred hillslope measurements throughout the study area suggest an average
of about 10 mm of erosion, The area of maximum sheet wash was limited to the headwaters of
Shinglemill Creek, Spring Gulch, Sand Draw, and the upper third of Spring Creek (e,g" figs, 3c
and 3d) and was used to help define the area of maximum rainfall (fig, 4),

Locally, small streams produced up to 6 m of scour, primarily by headcutting (fig, 3c), Hundreds
of trees, some as large as 0,75 m 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 diameter boulders were transported in some channels (fig, 8), The distinct black, bum
boundary on undisturbed trees and rocks in channels (fig, 9) were used to define pre-fire channel-
bed levels and estimate the amount of channel change, Many tributaries degraded during the
July 12th flood (fig, 9), although locally agradation (1-2 m) occurred along Buffalo, Spring, and
Shinglemill Creeks and on alluvial fans at tributary junctions in burned areas, Many new fans had
dimensions of about 100 m x 30 m x 1,5 m (fig, 7), September 1996 reservoir releases increased
flows in the NF South Platte River that were capable of transporting sediments from the Buffalo

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