FLOOD03485

• County: Denver
• Community: All
• Stream Name: All
• Basin: South Platte
• Title: Comparison of Estimated Flood Peaks with Historic Floods
• Date: 1/1/1986
• Prepared For: State of Colorado
• Prepared By: USDOI
• Floodplain - Doc Type: Educational/Technical/Reference Information

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, "~ :oucKll0rn DUH (dlmens,')nl'lss unit hydr09raph) was chosen for the PMF analysIs This 0Ur-i
'Y'-i~;I:-la18d froril a drainage ba~.(i that approximated the Eldorado drainage in size and veg€iat/Qf!
;\'p€S Computed lag times were compared with the observed intense rainfall perloo and time 01
arnval of the historic flood peak at the mouth of the canyon. A lag time of 0,75 h was chosen.
which corresponded to a C value of 052 in the theoretical lag equation, This C value. is low
compared with those in other studies in the region. This lower C value creates a higher unit
hydrograph peak and, ultimately, a higher PMF peak. Based on the computations the PMF peak
IS 110,700 ft3/s, and the corresponding 10-h PMF volume is 12,000 ft3/S. This computed PMF
hydrograph is displayed on Plate 2, The observed peak for the basin is 69 percent of the estimated
PMF peak,

The computed PMF volume is six times larger than the observed event; whereas, the computed
storm is four times larger than the natural event, The HMR 49 procedure is based on many
thunderstorm events in the region, which have been observed to have durations of up to 6 h. In
this study only the comparison of the PMF peak with the observed flood peaks is of interest. A
different historic event might be more appropriate for comparison of volumes, Plate 2 shows a
comparison of the computed PMF hydrograph peak with the historic flood peak for this basin,

2. Bronco Creek near Wickieup, Arizona

The discharge in the remote Bronco Creek drainage in northwestern Arizona was computed to
be 73,500 ft3/s from a drainage area of 19,0 mi' after an intense thunderstorm on July 18, 1971.
Because this area in so remote, little information is available for the actual storm and resulting
flood, The drainage basin for Bronco Creek consists of sparsely vegetated alluvial hills that slope
400 to 500 ft/mi, No rainfall data were available for this storm from within the basin, but 3 in of
rain in 45 min was measured just outside the basin. Because the discharge was so large and
aroused much controversy, several hydraulic methods were used to compute the magnitude [19J.
This discharge is one of the highest recorded in the Southwest for this size drainage.

The PMP values used in this study were computed using the thunderstorm criteria of HMR 49 [5],
Table 4 summarizes the results of the PMP portion of this study.

Duration,
h

Depth,
in

Table 4. - Basin averaged thunderstorm PMP for
Bronco Creek near Wickieup, AZ.

0,25
0,50
100
2,00
6.00

5,1
6.8
8,5
9,9
12.0

l

I

,j

The reported 3 inches in 45 min Just outside of this basin is the only historic rainfall data available
for comparison,

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