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

I

I

taken from the NOAA Atlas 2, Volume ill (Reference No.9). Although the study area
the rainfall depth was rather uniform within the watersheds. Storm frequency, durations
the Study Area are summarized in Table 1 below

Rainfall

Rainfall depths were
watersheds are large,
and rainfall depths for

of gagi ng
much as

coverage, number
were based on as
studies and include 64 gaging
area of a

t,"uUJ

flood-frequency studies
used. Results

of the previous
streamflow

study differs
used,

I

I

Frequency, Duration and Depth

Rainfall

Table 1

I

Depth (in.)
6-hour

Rainfall
I-hour

Storm

2.20
3.00
3.40
4.20

1.65
2.35
2.65
3.30

10-Year
50- Year
100- Year
500- Year

Region of Colorado was applied to portions of the

Based on the results of this work, the equation for the Plains
Study Area. This equation is stated as follows:

I

I

Rainfall distributions were created according to methods developed by the UDFCD for the 10, 50, 100 and
500-year storms. The design storm developed by the UDFCD is based on the one-hour rainfall depth
distributed over the initial 2 hours of the total storm duration. As the desired storm duration increases. the
additional rainfall is added to the end of the 2-hour distribution to lengthen the storm to the desired duration. It
is assumed that the storm event occurs over the entire watershed uniformly. Development of the design storm
is described in the UDFCD, Urban Storm Drainage Criteria Manual, Volume 1 (Reference No. 10)

method

this

Region of Colorado,

for the Plains

(Equation 3)

for error in predicting flood flows
reliable of the regional methods used.

where A = drainage area in square miles

1640 (A) .388

Due to the large potential
appears to be the least

00 =

Q

I

I

According to the UDFCD method, a 6-hour minimum storm duration is required for basins larger than
20 square miles. Design storm Types I and IT (24 hour duration) developed by the Soil Conservation Service
were also evaluated. It was determined that the UDFCD 6-hour storm generated the highest peak runoff in

the study area reaches

Due to the large size of the basins an areal adjustment was made to the rainfall distribution according to
methods defined by the UDFCD. The relationship between basin size and the adjusted rainfall distribution was
evaluated to determine whether smaller basins with higher amounts of rainfall could produce higher peak
runoff rates than larger basins with lower rainfalls. It was determined that the highest peak runoff occurs when
all of the upstream basin is contributing, even with the areally adjusted rainfall distribution

Two computer models, the Colorado Urban Hydrograph Procedure (CUHP) and the Urban Drainage Storm
Water Management Model (UDSWM2), were also used to develop flood flow values for the floodplain
analysis on Comanche and Wolf Creeks. The CUHP was used to generate hydrographs for sub-basins; then
UDSWM2 was used to combine and route the combined flows downstream. These models are the primary
models used within the UDFCD in the Denver metropolitan area and have been applied in basin studies for
nearly two decades. The UDFCD has also developed rainfall distributions for the region that are integrated
into the CUHP model. The UDFCD has recently been recognized as one of the first Cooperating Technical
Communities (CTCs) in the country and these procedures have been accepted by FEMA for application to
floodplain studies within the District.

Computer Models

I

I

I

I

are greater than 75 square miles
miles was applied to the entire
distribution for the study

ng from depressions and infiltration Were accounted for according to UDFCD guidance
subbasin. The great is undeveloped and used for agriculture
s 0.4 inches. This value was generally

Wolf Creeks
than 75 square
areal adjusted rainfall

basins larger
The 6-hour

The basins upstream of the Study Area on Comanche and
Therefore, the areal adjusted storm distribution for
watershed model according to UDFCD procedures
s shown 1

in Figure

Losses

Rainfal

CUHP

The CUHP is a unit hydrograph procedure based on the Snyder Unit Hydrograph procedure. The method was
developed in the 1970s by the UDFCD based on recorded rainfall and observed runoff hydrographs in the
Denver metropolitan area. Basin parameters including area (square miles), length, slope and infiltration are
input into the procedure and combined with the rainfall distribution to generate a storm hydrograph for each
subbasin. The procedure was coded into a computer program for use with PCs in the 1980s. Storm
hydrographs generated by the program are prepared for input into a modified version of the EPA's stormwater
model SWMM called UDSWM2. Detailed descriptions of the CUHP and UDSWM2 programs are provided in
their User Manuals (Reference Nos. 7 and 8)

I

I

I

majority of the basin area
the recommended depression storage

I

I

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

3

Comanche Creek and Wolf Creek
Floodplain Study. May 2002

I