FLOOD03592

• Designation Number: 328
• County: Adams
• Community: Commerce City
• Basin: South Platte
• Title: Flood Insurance Study - Commerce City, Colorado, Adams County
• Date: 9/1/1988
• Designation Date: 9/1/1992
• Floodplain - Doc Type: Floodplain Report/Masterplan

Table 1. Summary of Discharges

Flooding Source
and Location

Drainage
Area
(Square
Miles)

South Platte River
Franklin Street
Upstream of Conflu-
ence with Sand
Creek
Interstate 76

3,8201

3,8301
4,0251

Sand Creek
Quebec Street 189
Mouth at South Platte
Ri ver 196

Peak Discharges
(Cubic Feet per Second)

10-Year SO-Year 100-Year 500-Year

8,500 12,500 17 ,200 26,500
8,500 12,500 17,200 26,500
17,500 29,000 37,200 60,000
10,500 23,300 30,000 33,000
11 ,000 24,200 30,500 36,200

1 Includes Area Above Chatfield, Bear Creek, and Cherry Creek Dams.

3.2 Hydraulic Analyses

Analyses of the hydraulic characteristics of streams in the
community were carried out to provide estimates of the elevations
of floods of the selected recurrence intervals along each stream
studied in detail. Cross section data for the South Platte River
was obtained by field surveys and cross section data for Sand
Creek was taken from ael:"ial contour mapping (Reference 3). All
bridges and culverts were surveyed or the information was
provided by the Colorado Department of Highways to obtain
elevation data and structural geometry. Locations of selected
cross sections used in the hydraulic analyses are shown on the
Flood Profiles (Exhibit 1). For stream segments for which a
floodway is computed (Section 4.2), selected cross section
locations are also shown on the Floodway Map (Exhibit 2).

Roughness coefficients (Manning's "n") were estimated by field
inspection. The range of "n" values used for Sand Creek were
from 0.025 to 0.045 and those for the South Platte River were
from 0.035 to 0.070.

Water-surface profiles were developed using the HEC-2 step-
backwater computer program (Reference 4). Profiles were
determined for the 10-, 50-, 100- and 500-year floods. Flood
profiles were drawn showing computed water-surface elevations to
an accuracy of IO.5 foot. All starting water-surface elevations
for the South Platte River were taken from the Adams County,
Colorado, Flood Insurance Study (Reference 5). The starting
water-surface elevations for Sand Creek were those computed for
the South Platte River at the confluence. All elevations are

11