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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Outfall Systems study, Big Dry Creek (ADCO), Westminster and Vicinity, avail-
able at the offices of the UD&FCD. The following paragraphs provide a de-
tailed description of the input parameters used for the hydrologic analysis by
Greiner Engineering:

TABLE 3
100-YEAR POINT RAINFALL DISTRIBUTION
WESTMINSTER AREA LESS THAN 5 SQUARE MILES

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Precipi ta tien

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Rainfall depth isopluvials were determined for the entire study area for
the 2-, 5-, 10-, 50- and 100-year recurrence intervals. The 2-, 3- and
6-hour design storms for each sub~basin were based on the point rainfall
depths with areal adjustments for basin area. For drainage areas up to 5
square miles, no areal adjustments were necessary. The 2-hour rainfall
distribution was used for areas of less than 10 square miles, the 3-hour
distribution was used for areas of 10 to 20 square miles and the 6-hour
distribution was used for areas greater than 20 square miles. Since both
Standley Lake and Great Western Reservoir substantially attenuate the
upper basin flows, areal adjustments were restarted for basins downs team
of the reservoirs.

Time (Min.) Incremental Rainfall Time (Min.) Incremental Rainfall
Depth in Inches Depth in Inches
0 0 65 0.11
5 0.03 70 0.05
10 0.08 75 0.05
15 0.12 80 0.03
20 0.21 85 0.03
25 0.38 90 0.03
30 0.67 95 0.03
35 0.38 100 0.03
40 0.21 105 0.03
45 0.17 110 0.03
50 0.13 115 0.03
55 0.11 120 0.03
60 0.11
TOTAL 3.08 inches

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For the precipitation analysis, rainfall data were obtained from the
UD&FCD's isopluvial maps for the I-hour and 6-hour precipitation depths
(Reference 10). These precipitation data are based on the procedures and
data developed by the U.S. Department of COlIllIlerce, National Oceanic and
Atmospheric Administration (NOAA) (Reference 11).

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TABLE 2
POINT RAINFALL DEPTHS

Loss Rates and Depression Storage

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0-5
5-10
10-20
20-30
30-50
50-75

2
2
3
6
6
6

Rainfall Depth Years)
(Recurrence Interval in
2 5 10 50 100
1.15 1.60 1.89 2.75 3.08
1.12 1.58 1.85 2.71 3.01
1.20 1.71 2.05 2.83 3.17
1.39 1.86 2.25 3.06 3.43
1.36 1.83 2.21 3.02 3.41
3.39

The loss rates calculated by the CUHP-D program were based on Horton's
variable loss rate equation (Reference 10). The initial loss rate (fi),
final loss rate (fo), and decay rate (x) are based on the Soil Conserva-
tion Services (SCS) Hydrologic Soils Groups for each sub-basin. The SCS
soils groups were identified from information supplied by the SCS (Refer-
ence 3-5). The loss rate values used in the hydrologic study are listed
in Table 4, below.

Basin Size
(Sq. Mi.)

Storm Dura tion
(hrs)

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TABLE 4
INFILTRATION LOSS RATE DATA

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The following table lists the typical lower basin 100-year, I-hour design
storm distribution used for the hydrologic determinations.

Hydrologic fi fo x
Soil Group in/hr in/hr per second
A 4.5 1.1 0.0007
B 4.0 0.7 0.0018
C&D 3.0 0.5 0.0018
For sub-basins cons is ting of 1 or more soil groups~ a comp os i te loss ra te
was de termined by interpola tion.

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Values for both pervious and impervious depression storage were obtained
from the UD&FCD criteria and were identified as 0.3-0.4 and 0.1 respec-
tively for the sub-basin areas. Table 6 lists data developed for all
sub-basins in the hydrologic analyses.

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