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

Cross sections were taken at key locations that were expected to cause changes in the water surface profile.
The creek cross-sections vary significantly through the Study Area. In many locations a well defined channel
carries the majority of flows with a small portion of the flow overtopping the main channel into a broad
overbank area. In several locations the channel is poorly defined and a large portion of the flow spills into
low-lying areas adjacent to the main channel or into a secondary channel. In some cases these spilled flows
develop their own floodplain before returning to the main stem of the creek. In some locations this required
that flow be diverted from the main channel into secondary channels, modeled as a tributaries and returned to
the main stem further downstram.

HYDRAULIC ANALYSIS

Results from the hydrologic analysis were combined with channel conveyance data to develop a hydraulic
model for determining water surface profiles. A water surface profile for the existing condition 100-year flow
was calculated for both Comanche Creek and Wolf Creek throughout Adams County. Water surface profiles
for the 10, 50, 100 and 500-year flows and floodway limits were calculated on Comanche Creek only in
Section 34, T3S, R62W, approximately one mile north of Colorado These calculated water
surface profiles will replace the approximate and detailed calculations in PIS

Highway 36
the previous

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Where culverts or bridges are placed across the creeks, these structures were coded into the appropriate entry
forms of HEC-RAS. Several structures were in a perched condition where the structure crosses the low-flow
portion of the channel, but high flows by-pass the structure and overtop the road at a low lying area adjacent to
the structure. This created complicated modeling configurations at several locations

The nature of the model configuration required some adjustment to the convergence parameters in some
portions of the Comanche Creek model. Settings that provided convergence of the model calculations in one
reach of the model caused other results to diverge from optimum solutions. Therefore, the Comanche Creek
was divided into a Lower, Middle and Upper reach model to increase the overall accuracy of the results.
were started with the results of the downstream water surface elevations as known

The drainage area of Comanche and Wolf Creeks totals approximately 70 square miles in Adams County.
However, the project boundary for photogrammetry and mapping was drawn to include, in its entirety, any
section that contained a portion of the watershed of either creek. In all, 93 sections in Adams County were
identified as being enclosed within this defined boundary. Two sections in Weld County and four sections in
Arapahoe County were added to the project area to assure continuity of representation of the creeks and their
watersheds. Thus the total area for project photogrammetry and mapping became 99 sections or
approximately 99 square miles. The aerial photography for the project was completed on April 28. 2000.

Topographic Mapping and Surveying

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HEC-RAS hydraulic model to
100-year flows used for hydraulic

two upper reaches
elevations

Flows from the CUHP-UDSWM2 hydrology model were coded
represent peak flood flows as they travel through the study reaches
analyses of Comanche and Wolf Creeks are shown in Figure 5, Discharge Profile.

into the
The

model
The

The mapping was based on a series of existing and new ground control points Existing control points
provided first order horizontal accuracy. Additional horizontal control points were provided with third order
accuracy. The ground control points were provided by Accurate Consultants, Inc. of Broomfield, Colorado. A
series of 200 scale maps were produced from this data showing planimetric features, control points and 2 foot
contours. This data was used to create stream cross sections for the hydraulic model and as a base for
delineation of the floodplain limits. The aerial photography and its conversion to topographic maps was
provided by Horizons, Inc. of Rapid City, South Dakota using LIDAR technology.

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Hydraulic conditions on Comanche Creek upstream of the Study Area have a significant effect on the analysis
within Adams County. Flows from Comanche and Little Comanche Creeks become combined where the
creeks cross 1-70 about one-half mile south of the county line. After crossing 1-70 the creeks continue as
combined flow due to further restrictions caused by limited channel capacity and bridge crossings at the Union
Pacific railroad and at Colorado Highway 36. These restrictions cause the water surface to rise above a small
ridge along the east bank of Little Comanche Creek allowing flood flows to leave the channel and flow east to
a low area. These diverted flows continue north across the railroad and Colorado Highway 36 and travel in a
to the main stem of Comanche Creek. These flows continue north through Sections 34
and eventually return to the main stem of Comanche Creek as they enter Section 22

The topographic mapping data was supplemented with a field survey of each of the structures constructed
across the creeks. Each structure was surveyed to identify sizes, invert elevations, low-chord elevations,
deck elevations and other information required to accurately represent their influence on the
profile calculations. Sections for each of the crossing locations are shown on Drawings S-l
detailed survey information and photos are provided in the Appendix

bridge
water surface
and S-2 and

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...

shallow swale parallel
and 27 in T3S, R62W

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of the 11,125 cfs of total flow to leave the
less than three feet deep and was delineated using
slopes along the reach

about 1,960 cfs
channel is

During the 100-year flood thIS condition causes
main channel. The floodplain in this secondary
normal depth calculations for representative cross-sections and

and
and large
"n" value

The roughness factor for the channels was selected based on site observatIOn, reference materials
engineering judgment. Vegetation in the creeks is characterized by native grasses and sparse shrubs
trees are rare. There is little evidence of large amounts debris traveling in storm flows. A Manning's
of 0.035 was typically used across the channel sections

WOUld have resulted in supercritical flow, critical depth was used for the water surface
assumed that no debris bloGkage of the crossing structures occurs during flooding.

ons
was

Where flow calculat
elevation. Also, it

The Corps of Engineers hydraulic model, HEC-RAS (Reference No. 13), was used to calculate water surface
elevations at selected locations along each creek. A reference line was established for the two creeks. The
alignment for the reference line mainly follows the low flow channel of the creeks, but due to severe
meandering in some areas, the alignment leaves the low-flow channel. This was done to maintain an
increasing location value that roughly coincides with the hydraulic distance from section to section. Stationing
for Comanche Creek was established relative to the Adams-Weld County line at the downstream end of the
Study Area. Stationing for Wolf Creek was established from its confluence with Comanche Creek. The
thalweg profile shown on the floodplain drawings represents the linear connection between low points at each
cross section location and not the ground profile along the reference line alignment

Computer Model

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McLaughlin Water Engineers
P:\20001A0.029\OO300\Final Floodplain Rc:pon\Final Report.doc

6

Comanche Creek and Wolf Creek
Floodplain Study, May 2002

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