FLOOD10388

• Designation Number: 596
• County: Adams
• Stream Name: Comanche Creek, Wolf Creek
• Basin: South Platte
• Title: Comanche Creek and Wolf Creek Floodplain Study
• Date: 5/1/2002
• Floodplain - Doc Type: Floodplain Report/Masterplan

The paper, "Regional Flood Frequency Analysis in the Plains Region of Colorado and Kansas", was authored
by Wilbert O. Thomas, Jr., and Jeffrey N. King of Michael Baker, Jr., Inc. and Michael Grimm of FEMA, and
was presented at the 1999 Annual Conference of the Association of State Floodplain Managers. The paper
presents the analysis of records at 42 gaging stations in Colorado, Kansas, Nebraska, New Mexico, Oklahoma,
and Wyoming. An intent of the authors was to evaluate the effect of basin shape on flood discharges. One of
the products of the research was a regression equation that relates the 1-percent-annual-chance discharge to
watershed and climatic characteristics as follows:

EEMA

upstream to the Arapahoe-Elbert County line. According to UDFCD guidance, design points were established
so that sub-basins would be no larger than 130 acres, and the average size was about 75 acres. The watershed
of Box Elder Creek above Arapahoe County, measuring about 127 square miles, was also modeled to provide
representative inflow hydrographs at the upstream study limit. Its analysis was less detailed, with sub-basins
limited in area to 10 square miles (again according to UDFCD guidance) and averaging about 7 square.miles

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1)

(Equation

76

0.937 SF-{J.

/100

48DA-<l.lO

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Ql% = 218.8DA

The modeling results from the 1994 Box Elder Creek study contrasted sharply with the values developed and
used for the PISs, which were calculated, like those for Comanche and Little Comanche Creeks, using USGS
Water Supply Paper 1680. The 100-year flow from the Adams County PIS for Box Elder Creek at Interstate
70 is 26,300 cubic feet per second (cfs). However, for the same point on the creek, the CUHP-UDSWM2
modeling effort from the 1994 study produced 100-year flows of only 8,900 cfs under existing conditions of
development and just 12,000 cfs under future conditions

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the channel length squared divided by the drainage area.

Water Conservation Board produced a draft procedure for estimating 100-year flows in
This method is a regional regression equation solution, based on analysis of flow gaging
records on state streams. which uses drainage area as its sole independent variable and is stated as follows

in inches,
shape factor defined as

Where:
DA = drainage area in square miles,
1100 = 100-year 24-hour-duration rainfal
SF = a dimensionless basin

CWCB

In 1999, the Colorado
Colorado streams

FEMA Consultation

Since a primary goal of the Comanche and Wolf Creek study is to prepare a floodplain map and supporting
information that would satisfy FEMA's requirements for revising the County's Flood Insurance Rate Maps,
project staff met with the Region 8 Representative to review available information and establish a course of
action that FEMA would approve. The representative acknowledged that the flows in the PISs were suspect
and agreed that the modeling procedure used for the Box Elder Creek study would be appropriate to implement
for Comanche and Wolf Creeks. Even though these creeks lie outside the UDFCD service area, the similarity
between their watersheds and the Box Elder watershed was considered sufficient to warrant use of the UDFCD
procedure. However, FEMA set the condition that values be calculated by other regional regression
equations and compared to PIS flows and computer

flow
model results

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(Equation 2)

00 = 707.9 (DA) .654

Q

where DA = drainage area (sq mi)

should not be used in place of rigorous flow record analysis or
100-year flow, but rather to obtain a reasonable approximation

This procedure is still in draft form and
hydrologic modeling to determine a definitive
of the flow for general purposes

HYDROLOGY ANALYSIS

Estimates of 100-year flood flows were made using three regression equations and a computer model. A
comparison of the flows estimated by the four methods and the original PIS flows was made to confirm the
reasonableness of the computer modeling results. Additional flow estimates were made on Comanche Creek
for the 10, 50 and 500-year storms to update the detailed study reach in Section 34, T3S, R62W.

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USGS

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A study was begun in 1994 by the U.S. Geological Survey, in cooperation with the Colorado Department of
Transportation and the Bureau of Land Management, to include streamflow data collected since water year
1981 in the regionalized flood-frequency relations for Colorado. Longer periods of streamflow data and
improved statistical analysis methods were used to define regression relations for estimating peak discharges
having recurrence intervals of 2, 5, 10, 25, 50, 100, 200, and 500 years for unregulated streams in Colorado.
The regression relations can be applied to sites of interest on gaged and ungaged streams. Ordinary
least-squares regression was used to determine the best explanatory basin or climatic characteristic variables
for each peak-discharge characteristic, and generalized least-squares regression was used to determine the best
regression relation. Drainage-basin area, mean annual precipitation, and mean basin slope were determined to
be statistically significant explanatory variables in the regression relations. Separate regression relations were
developed for each of five distinct hydrologic regions in the State. The mean standard errors of estimate and
average standard error of prediction associated :with the regression relations generally ranged from 40 to
80 percent, except for one hydrologic region where the errors ranged from about 200 to 300 percent

Regression Equations

FEMA requested that the procedure outlined in the document "Regional Flood Frequency Analysis in the
Plains Region of Colorado and Kansas" (Reference No.4) be used to calculate 100-year flows for comparison
to the computer model results and the previously calculated flows from the PISs. The Colorado Water
Conservation Board (CWCB) has also developed a regression equation for estimating 100-year flows in
Colorado. The development and application of this equation is presented in a publication entitled, "Guidelines
for Determining 100-Year Flood Flows for Approximate Floodplains in Colorado, Version 4.0" (Reference
No.5). A third equation has been developed by the U.S. Geological Survey (USGS) and is documented in a
publication entitled "Analysis of the Magnitude and Frequency of Floods in Colorado (Reference No: 6). Each
of these equations was applied to portions of the watershed to calculate flows at key locations

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McLaughlin Water Engineers
P:\2000\A0-029\OO300\Final Aoodplain Reporl\Final Reporl.doc

2

Comanche Creek and Wolf Creek
Floodplain Study. May 2002

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