FLOOD04285

• Designation Number: 270
• County: Adams
• Community: Westminster
• Stream Name: Big Dry Creek & Tributaries
• Basin: South Platte
• Title: Flood Hazard Area Delineation - Big Dry Creek & Tributaries
• Date: 11/1/1986
• Designation Date: 12/1/1987
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Percent Impervious Values

the laO-year flows. Both reservoirs release a lOa-year outflow equal to
25-50% of the peak 100-year inflow. Refer to page 2 of this report for
further discussion concerning Standley Lake and Great Western Reservoir.

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The percentage of impervious area for each sub-basin was determined from
recent aerial photographs (Reference 12) for the existing conditions and
from the City of Westminster Comprehensive Plan (Reference 1) for ultimate
conditions. The typical percentage of impervious area for general land
use categories used in the study are shown in the following table.

Compu ted Resu 1 ts

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Single-Family Residential
Medium Density (4.4 du/acre)
Low Density (1.6 du/acre)
Multi-Family Residential
Commercial and Industrial
Open Space

40%
30%
60%
80%
5%

Based on the CUHP-D/HEC-l flood routing analyses, peak discharges for the
2-, 5-, 10- and laO-year floods assuming ul tima te deve loped condi tions
were computed for various design points within the basin. The locations
of the sub-basins are shown on the Drainage Basin Boundary Map (Sheets 3
and 4 of 83) located in the report Appendix. Flood Discharge/Probability
Profiles for each stream studied in this report are shown on Figures 1 to
19. It should be noted that hydrographs for the Nissen Reservoir and West
Lake Basins draining from the City of Broomfield were obtained directly
from Reference 13.

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TABLE 5
PERCENT IMPERVIOUS VALUES

Land Use

Percent Impervious

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All technica 1 documenta tion for this hydrologic study' is provided in a
separately published technical addendum (Reference 14) and is available at
the offices of the Urban Drainage and Flood Control District.

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For a sub-basin consisting of more than one land use designation, a compo-
site value was determined. Table 6 lists data developed for all sub-
basins in the hydrologic analyses.

Unit Hydrograph Coefficients

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Snyder Unit Hydrograph coefficients'were developed using the UD&FCD Cri-
teria (Reference 10). The' unit hydrograph coefficients are based on basin
slope, basin area, basin length, centroidal length and percent of impervi-
ous area. Table 6 lists data developed for all sub-basins in the hydro-
logic analyses.

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Rou tings

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For the stream routing anlayses, a typical cross-section for' each sub-
basin was determined for use in the HEC-l Modified PuIs, normal depth,
Storage-Outflow channel routing method. Mannings roughness values of
0.035 to 0.055 for channel areas and 0.05 to 0.075 were used for overbank
areas to produce stable channel routings and Froude Numbers less than 0.9.

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For Standley Lake and Great Western Reservoirs, reservoir routings were
performed using a Modified PuIs routing based on elevation/storage/dis-
charge relationships developed for proposed reservoir conditions (Refer-
ences 6 and 7). Both reservoirs were assumed to be full to the spillway
crest as the initial condition. All other reservoirs were assumed inef-
fective for flood control purposes. The same assumption was applied to
undersized road crossings considering the uncertainty of future develop-
ment and crossing improvements.

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The effects of the channel routings on peak flows (channel and overbank
storage) resul ted in reduc tions of 0 to 30 percent. Both Standley Lake
and Great Western Reservoirs have a significant impact on the reduction of

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