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

Watershed Area Summary

84.0 sq mi
124.0 sq mi
245.2 sq mi

Table 3

Comanche Creek:
At Adams-Arapahoe County Line
Above Wolf Creek Confluence
Adams-Weld County Line

Soil maps of the watersheds published by the USDA-NRCS were used to assign a soil type to each subbasin
(Reference No. 11). The soils in the watersheds are primarily from the hydrologic soil groups Band C, with
some group A soils found close to the creek channels. The soil groups correspond to parameters used for
Horton's soil loss equation. This equation is integrated into the CUHP to calculate effective rainfall The
recommended infiltration rates for the NRCS Hydrologic Soil Groups are shown below

I

I

83.7 sq mi
100.3 sq mi

At Adams-Arapahoe County Line
Above Comanche Creek Confluence

Wolf Creek:

10)

Equation Parameters (Ref. No.

Decay Coefficient

sq mI

elevation model (DEM)
according to new UDFCD
to the centroid required for

272.0

Subbasins were delineated for the entire watershed of each creek using USGS digital
data. The area of the subbasins was limited to a maximum of 5 square miles
guidelines. Other dimensional parameters such as basin length, slope and distance
the CUHP were also measured from the USGS DEM data for each subbasin

Kiowa Creek below Comanche Creek Confluence

0.0007
0.0018
0.0018
0.0018

Recammended Horton's

on Rate (inches per hour)
Final Rate

1.0
0.6
0.5
0.5

Infiltrat
Initial Rate
5.0
4.5
3.0
3.0

NRCS Hydrologic
Soil Group
A
B
C
D

Table 2

I

I

I

The land use for the vast majority of the watershed areas, for both existing and future planned conditions, is
agriculture, mostly crops and some range. With a current population of approximately 1,400, Strasburg
constitutes the sole urban area in the watersheds. Other developments that are ongoing or planned include
subdivisions within a few miles to the west, north and south of Strasburg, each collection of areas totaling no
more than a couple of square miles. Additional widely-distributed, low-density development is occurring as
some agricultural land is being divided into 35-acre estate properties. No significant centers of highly
developed land are expected with the watersheds. Except for subbasins in the area of Strasburg an impervious
percentage of one (1 %) was used for all of the subbasins. The existing condition percent imperviousness for
the more developed areas near Strasburg was estimated from aerial photographs. Future condi tion
imperviousness was estimated by overlaying land use maps from the County's comprehensive master plan
(Reference No. 12) onto the subbasin boundaries. The percent imperviousness for each land use was applied
according to UDFCD guidelines The parameters for each subbasin are summarized in the Table 4, Subbasin
Parameters

The UDFCD method assumes that soil moisture conditions are low prior to the storm event and that initial
losses due to infiltration will be high. This is often the case when evaluating storms of short duration and does
not significantly effect the estimated runoff rates in highly urbanized basins. However, the watersheds in this
study have a very low percentage of impervious area and assumptions regarding the antecedent moisture have a
significant effect on the estimated rate of runoff. Also, in order for the design storm event to cover the entire
watersheds of Comanche Creek and Wolf Creek it is likely that it would be part of a larger weather system that
would occur over several days. Therefore, rainfall occurring in the days prior to the design storm event would
saturate basin soils. For these reasons, the UDFCD infiltration rates were adjusted to eliminate the higher rate
of infiItrat that occurs with unsaturated a constant rate of infiltration was assumed to occur
The constant was set to the final infiltration rate

equal

and
of infiltration

soils
rate

on
throughout the storm event.
recommended by the UDFCD

I

I

I

I

I

The watersheds of both creeks are long south to north and narrow
3-4 miles, but the total length of Comanche Creek from its
80 miles, while the length of Wolf Creek from headwaters to
The topography is generally rolling plains with modest
extremities of the watersheds.

UDSWM2

east to west. Their widths rarely exceed
headwaters to the Adams-Weld County line is
ts confluence with Comanche Creek is 52 miles.
steeper slopes are found in the southern

Basin Characteristics

I

The UDSWM2 program receives storm hydrographs generated from the CUHP program and combines and
routes them through elements representing basin features that may include detention facilities, pipe systems
and creek channels. A UDSWM2 model was created to represent both the subbasin relationships within
Comanche and Wolf Creeks and to combine the flow from Wolf Creek into Comanche Creek at their
confluence. The model also provides a flow at each element where subbasins are combined and at the end of
each routing element.

though

slopes

I

UDSWM2 model InClUde an identifier, channel bottom width,
connecting elements. Only trapezoidal channel
The characteristics of the channel elements

Channel Characteristics

parameters required for the
slope, side slopes, roughness and
were used to represent the basin conditions

Conveyance element
length, longitudinal

conveyance elements

The drainage areas of the creeks at key locations throughout the Study Area are given below. Comanche and
Little Comanche Creeks join immediately north of the Adams-Arapahoe County line, and Wolf Creek joins
Comanche Creek about 10 miles further north. Kiowa Creek, whose watershed is similar to those of
Comanche and Wolf Creeks and borders them on the west, has its confluence with Comanche Creek
immediately north of the Adams-Weld County line. Although Kiowa Creek is not included in the study area, it
was necessary to estimate its flow contribution to complete the floodplain analysis in the northern most reach
of Comanche Creek

I

I

I

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

4

Comanche Creek and Wolf Creek
Floodplain Study, May 2002

I

I