FLOOD00608

• Designation Number: 516
• County: Denver
• Community: Denver
• Title: Flood Insurance Study - Denver, CO, Volume I
• Date: 3/1/1996
• Designation Date: 5/1/2000
• Floodplain - Doc Type: Floodplain Report/Masterplan

Approximately three miles of the Sand Creek floodplain was
revised, beginning at the eastern corporate limits for the City
and County of Denver and extending downstream of the airport
boundary to Interstate 70 (1-70).

Meetings were held during the course of the study with the UDFCD
and local representatives to review the as-built drawings of the
bridges, the revised floodplain and floodway delineations, and
mapping provided by the City of Denver.

The original Flood Insurance Study was prepared using the 100-
year floodplain delineation and profile information from the Sand
Creek Flood Hazard Area Delineation Map, prepared for the UDFCD
by the COE in 1977. However, this report was not the source of
the 100-year floodway. As part of this revision, the 100-year
floodplain and floodway models were duplicated using the COE
HEC-2 Water-Surface Profile Program (Reference 46). The
hydraulic analyses for the revised conditions were also developed
from the HEC-2 model.

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The effective discharges were used for this reV1S10n. The 100-
year starting water-surface elevation for the revised reach of
Sand Creek was the computed lOO-year water-surface elevation at
the downstream tie-in section as shown in the Floodway Data Table
for Sand Creek.

Cross-section data were obtained from the topographic mapping.
Roughness values were estimated using the existing Flood
Insurance Study HEC-2 models, the Urban Storm Drainage Criteria
Manual, and field observations. Channel roughness values for the
channel ranged from .016 to .043. Overbank roughness values
ranged from .016 to .050. The bridge opening at the three new
bridges were field measured, Bridge routines modeled were
checked against current field conditions.

Floodway encroachments were set to define the effective flow
areas.

In response to an appeal from the UDFCD of the flood hazard
information presented in the preliminary revised Flood Insurance
Study report and on the Flood Insurance Rate Map for the City and
County of Denver, Colorado, dated November 30, .1994, FEMA
reevaluated the source of flooding in the low area located north
of Sand Creek and east of the Stapleton International Airport
runways. As a result of this reevaluation, FEMA determined that
flooding in this low area is due to overflow from Sand Creek
along the north overbank area in the vicinity of Cargo Bridge
Road, hereafter referred to as the Sand Creek Overflow. A total
of 640 cfs is lost from Sand Creek at this location. A portion
of this overflow, 39 cfs, is lost through the Union Pacific
Railroad (UPRR) culvert beneath the runway, and returns to Sand
Creek just downstream of the UPRR bridge, while the remaining
601 cfs flows into the Sand Creek Overflow and returns to Sand
Creek just.upstream of the 1-70 bridges. The Sand Creek Overflow

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