FLOOD10171

• Designation Number: 218
• County: Boulder
• Community: Unincorporated Boulder County
• Stream Name: Lower Boulder Creek
• Basin: South Platte
• Title: Flood Hazard Area Delineation - Lower Boulder Creek - Valley View Road to Boulder Weld County Line
• Date: 3/1/1983
• Designation Date: 7/1/1983
• Floodplain - Doc Type: Floodplain Report/Masterplan

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The flood profiles and floodplain delineations presented in this report are
based on existing floodplain and channel conditions. Flood elevations and
the corresponding floodplain are often altered by road and bridge construction,
floodplain development, flood control improvements, or natural processes.
Prior to utilization of this report for planning or design purposes, the user
is advised to contact the Urban Drainage and Flood Control District or Boulder
County to determine if the information in this report has been amended.

- BASIN DESCRIPTION -

- ACKNOWLEDGEMENTS -

Boulder Creek and its tributaries drain a total of 440 square miles at its
confluence with St. Vrain Creek, approximately five miles east of Longmont
(Figure 1). The headwaters of Boulder Creek originate at the Continental
Divide at elevations as high as 13,500 feet (NGVD). The stream flows eastward
through steep, narrow mountain canyons about 22 miles before reaching the
canyon mouth near the western city limits of Boulder, at about elevation
5,400 feet (NGVD). Boulder Creek continues in an easterly direction through
the City of Boulder, for a distance of approximately 4.8 miles. After passing
through Boulder, the creek flows 18 additional miles in a northeastward
direction across the high plains to its confluence with St. Vrain Creek.
St. Vrain Creek is tributary to the South Platte River.

This report was prepared by Muller Engineering Company, Inc., Consulting
Engineers of Lakewood, Colorado, under contract with the Urban Drainage and
Flood Control District (Agreement No. 82-9.2).

The mapping was prepared according to the specifications of the Urban Drainage
and Flood Control District. Ground control surveys for the topographic mapping
were performed by Delta Aerial Surveys, Inc,

The study area for Lower Boulder Creek begins at the Boulder-Weld County Line
(Drainage area = 430 square miles) and ends at Valley View Road, a short
distance above the confluence of South Boulder Creek (Drainage area = 155 square
miles). Valley View Road is near the east edge of the City of Boulder. For
the purpose of this report, Boulder Creek is identified as Lower Boulder Creek
to avoid confusion with the Boulder Creek Flood Hazard Area Delineation Report
through the City of Boulder (Reference 2). Lower Boulder Creek represents the
reach of Boulder Creek below the confluence of South Boulder Creek.

Topographic mapping, digitized cross sections, and survey data for stream
crossings were provided by Delta Aerial Surveys, Inc. of Denver, Colorado.
Additional data for bridge crossings was supplied by Boulder County and the
COE. Original mapping was prepared with two-foot contour intervals at a
scale of I" = 200' (Reference I). The final mapping was reduced by 50-percent
for printing in this report.

The hydrology, hydraulics and floodplain delineations were completed by the
U.S. Army Corps of Engineers, Omaha District.

South Boulder Creek, Fourmile Canyon Creek, Dry Creek, Bullhead Gulch and
Coal Creek are the major tributaries to Lower Boulder Creek within the study
reach. The study reach length is approximately 13 miles with stream bed
elevations ranging from 5189 feet to 4940 feet NGVD. Stream bed slopes in the
study reach ranged from 18 to 21 feet/mile.

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