FLOOD05433

• Designation Number: 271
• County: Adams
• Community: Thornton
• Stream Name: Big Dry Creek
• Basin: South Platte
• Title: Flood Hazard Area Delineation - Big Dry Creek ‎(ADCO)‎, I-25 to Baseline Road
• Date: 11/1/1986
• Designation Date: 12/1/1987
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Flooded Areas

Encroachment into the floodplain along the Study Reach consists mainly of
various agricultural outbuildings, and a few complete farmsteads.
Occasional oil wells and associated storage facilities also encroach into
the floodplain. Industrial encroachment is limited to a greenhouse complex
near the Washington Street crossing.

This report contains the Floodplain Maps and Profile Sheets showing the
limits of the IOO-year floodplain and both the IO-year and the laO-year
water surface profiles. Table VII, "Floodplain and Floodway Reference
Data," presents the computed IOO-year flood elevations, along with other
pertinent floodplain information and descriptive data.

Development along the Study Reach of Big Dry Creek is light, but a gradual
transition from agricultural to residential land use is expected. Most of
the existing development is agricultural, composed of farmsteads with
assorted outbuildings. These farmsteads are widely spaced in the
downstream portion of the Study Reach. The upstream portion of the Study
Reach contains recent residential development and a single industrial
complex. The undeveloped land along the Study Reach is used mainly as
pastures and occasionally for cultivation.

Road crossings along the Study Reach include 136th Avenue, Washington
Street, 144th Avenue, York Street, 156th Avenue, 160th Avenue, Colorado
Boulevard, and 168th Avenue. In addition to these major crossings, three
private crossings offering flow obstruction also exist. One railroad
crossing spans the floodplain on a high embankment immediately upstream of
the Colorado Boulevard crossing.

Specific flooding problems which would be anticipated if a IOO-year flood
were to occur along the Study Reach of Big Dry Creek are described as
follows:

The Study Reach of Big Dry Creek covers approximately 9.1 stream miles from
the downstream study limit at 168th Avenue (Base Line Road) to the upstream
study limit at Interstate 25. Within the Study Reach, Big Dry Creek
consists of a small, entrenched low flow channel approximately 20 feet wide
which meanders through the wide, undeveloped floodplain, which is as much
as 1000 feet wide. Channel gradients vary from 0.1% at the upstream study
limit to 0.3% at the downstream study limit. Corresponding gradients for
the overbank areas are 0.2% and 0.4%, respectively.

1. All road crossings contain conveyance structures which are
inadequate to carry IO-year or larger floods, and the roads
would therefore be partially or completely submerged during
the 100-year event. Projected IOO-year flow depths over
these crossings are as follows: 136th Avenue, 3.2 feet;
Washington Street, 1.2 feet; 144th Avenue, 3.6 feet; York
Street, 3.9 feet; 156th Avenue, 1.4 feet; 160th Avenue, 8.6
feet; Colorado Boulevard, 3.2 feet; and 168th Avenue, 4.6
feet. Erosion and washout potential are particularly high
for the following road crossings which are unpaved:
Colorado Boulevard, 156th Avenue, York Street, and 144th
Avenue.

The Big Dry Creek floodplain is generally well defined with adequate
conveyance for the 100-year flood. However, problem areas do exist and are
created primarily by inadequate conveyance structures at road and railroad
crossings. The fOllowing pages of narrative contain a general description
of the projected IOO-year flooding for Big Dry Creek.

2. In general, the road crossings would offer minimal
ob st ruct i on to the IOO-year fl ow, si nce thei r conveyance
structures are small and the road surfaces are not
significantly elevated above the natural land surface. An
exception is the Washington Street crossing, which is
el evated two to four feet above the natural 1 and surface

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