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

also
large
on the

study area, is
model for
records

of the
hydrograph
stream gage

Another study, completed on Box Elder Creek in 1994, approximately 15 miles west
relevant to the current work. This study established the use of a synthetic unit
basins in the County in place of regression equations There are no long-term
creeks that could be used for flow frequency analysis.

INTRODUCTION

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Flood Insurance Studies

In July 2001, Adams County Public Works (ACPW) department initiated a project of surveying, topographic
mapping, floodplain mapping, and master planning for the unincorporated community of Strasburg and the
watersheds of Comanche Creek and Wolf Creek in the central portion of Adams County, Colorado. This report
includes a summary of the hydrologIc and hydraulic analyses leading to the proposed floodplain revisions. The
master planning effort is described in a separate study document.

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n these studies were calculated according to the regional approach outlined
Water Supply Paper 1680, which was issued in 1968. The 100-year flows

The PISs indicate that flows used
in US Geological Survey (USGS)
used in the PISs are as follows

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18,570 cfs
20,200 cfs

Comanche Creek above Little Comanche Creek
Comanche Creek below Little Comanche Creek

and tributaries in east central
entirely within
the watersheds

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Study Area

The study area for this work is the watersheds of Wolf Creek,
Adams County, Colorado and lies approximately 3 miles east of Denver. The area
unincorporated Adams the town of Strasburg being the only urbanized area
Drawing No.1 shows the extent of the watersheds and the Study Area.

Comanche Creek,
o

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County wi th
the project location

The PISs provided only approximate limits for the 100-year floodplain for Comanche Creek and Wolf Creek,
except for the most upstream mile of Comanche Creek. For approximately one mile north of Colorado
Highway 36 (Colfax St., Section 34, T3S, R62W) a detailed study of the floodplain was completed. The study
included estimates for the 10, 50, 100 and 500-year flow rates, water surface profiles for each of the storm
flows and calculations for the 100-year floodway within this reach. A similar level of detail was provided for
the creeks within Arapahoe County. The detailed study within Arapahoe County included Comanche Creek
and Little Comanche Creek for a distance of approximately one mile south of Colorado Highway 36 in Section
34, T3S, R62W. A single hydraulic model was used for both of the original Adams County and Arapahoe
County PISs. This model and the documentation supporting its development are not recoverable.

Both of the creeks originate far to the south in Elbert County. The headwaters of Comanche Creek reach
almost into EI Paso County. The creeks flow northward through Elbert and Arapahoe Counties, entering
Adams County in or near the community of Strasburg that lies between Bennett and Biers. Wolf Creek flows
into Comanche Creek approximately 10 miles north of the Arapahoe-Adams county line. Comanche Creek
continues northward and leaves the county approximately one-half mile before it flows into Kiowa Creek in
southern Weld County.

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Elevations across the watersheds vary from a high of about 7,200 feet at the headwaters of Comanche Creek to
a low of about 4,900 feet at the confluence of Comanche Creek with Kiowa Creek just downstream of the
northern Adams County line.

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The PISs indicate that flows in Comanche Creek and Little Comanche Creek combine into a common
floodplain at 1-70 due to restrictions created by bridges on 1-70. Downstream of 1-70 the two creeks continue
together due to limited channel capacities and additional restrictions at the Union Pacific railroad bridges and
at bridges on Colorado Highway 36. In addition, the PISs indicate that t:Iow is diverted upstream of the
railroad into a shallow channel east of the main channels where it creates a secondary floodplain and overtops
Colorado There is no indication in the PISs of the amount of flow expected to leave the main

are dotted with many
to travel unimpeded
leaving the watersheds.

No flood control facilities have been constructed on the creeks. However, the watersheds
small stock ponds. Very few road crossings intersect the creeks allowing flood flows
except where they cross Interstate 70. There are no out-of-basin flows entering or

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36.
of these conditions

Highway
channels as a result

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features, roughness factors, cross-section data and flow split
to establish an effective model for comparison with the analyses

survey datum information, structure
are not available, it is not possible
these studies

Box Elder Creek Study

Because
information
provided in

when a major
Flood
1965.

Flood History

Flooding has occasionally occurred on Comanche Creek. The most notable event was in 1965
regional storm caused the worst recorded flooding in the region's history. According to previous
Insurance Studies, residents have reported flooding on Comanche Creek in 1905, 1933, 1935 and
Discussions with current residents in Strasburg indicate that flood levels in 1965 were sufficient to overtop
Monroe St. which intersects Highway 36 about 500 feet west of the Comanche Creek bridge No record of
flooding on Wolf Creek has been identified.

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The 1994 study referenced above was an Outfal Systems Planning Study of Upper Box Elder Creek
(Reference No.3), including a Flood Hazard Area Delineation (FHAD) study for Coyote Run, which was done
by CH2M Hill for the Urban Drainage and Flood Control District (UDFCD), the City of Aurora, and Adams
and Arapahoe Counties. In this study, hydrologic analyses established peak discharges and volumes for six
storm frequencies under existing and future development conditions. Computer models developed by the
UDFCD (CUHP for subbasin runoff and UDSWM2 for hydrograph routing) were used. The project study area
of 75 square miles was the entire watershed of Coyote Run, from its headwaters in Arapahoe County to its
confluence with Box Elder Creek in Adams County, and the Box Elder Creek watershed from that confluence

Previous Studies

Prior analysis of the hydrologic and hydraulic characteristics within the Study Area is limited to the original
Flood Insurance Study (PIS, Reference No.1) completed in 1979. The PIS for the Arapahoe County portion of
the basins was completed in 1977 (Reference No.2). An update of the PISs for both Adams and Arapahoe
Counties was completed in August 1995 and supercedes all previous studies. These updated studies
ncorporated the results of the original studies, but did not revise the original studies within the Study Area.

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McLaughlin Water Engineers
P:\2000IAO-029\OO300\Final Aoodplain Repon\Final Report.doc

Comanche Creek and Wolf Creek
Floodplain Study. May 2002

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