FLOOD07750

• Designation Number: 448
• County: Arapahoe
• Community: Unincorporated Arapahoe County
• Stream Name: Big Dry Creek
• Basin: South Platte
• Title: Flood Hazard Area Delineation - Big Dry Creek & Tributaries
• Date: 11/1/1996
• Designation Date: 3/1/1997
• Floodplain - Doc Type: Floodplain Report/Masterplan

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TABLE III-I

enhanced information design for perviouslimpervious area connections and water quality capture volume were

not used for the study areas and the program defaulted to the analysis method used for traditional drainage

practices. Sub-watershed geometries were measured directly from the mapping prepared for this study. The

impervious percentages used in this study were derived from City and County land use maps and development

plans and aerial photography. The percent imperviousness for each sub-watershed was estimated as the

composite value resulting from a weighted area average computation for the various imperviousness zones.

Several areas within the study limits were examined in detail in order to more accurately determine the

appropriate composite imperviousness values.

TWO-HOUR DESIGN STORM FOR BASINS BETWEEN FIVE AND TEN SQUARE MILES

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INCREMENTAL RAINFALL DEPTH/RETURN PERIOD
ZONE I
TIME 2-YR 5-YR IO-YR 50-YR 100- YR
(MIN) (IN) (IN) (IN) (IN) (IN)
5 .02 .03 .03 .03 .03
10 .04 .05 .06 .08 .08
15 .09 .12 .14 .11 .12
20 .16 .21 .24 .18 .21
25 .26 .35 .40 .33 .35
30 .14 .18 .19 .55 .62
35 .07 .08 .09 .26 .35
40 .05 .06 .07 .18 .21
45 .03 .05 .06 .11 .16
50 .03 .05 .05 .11 .13
55 .03 .04 .05 .07 .10
60 .03 .04 .05 .07 .10
65 .03 .04 .05 .07 .10
70 .02 .04 .05 .05 .05
75 .02 .04 .05 .05 .05
80 .02 .03 .04 .04 .03
85 .02 .03 .03 .04 .03
90 .02 .03 .03 .03 .03
95 .02 .03 .03 .03 .03
100 .02 .02 .03 .03 .03
105 .02 .02 .03 .03 .03
110 .02 .02 .03 .03 .03
115 .01 .02 .03 .03 .03
120 .01 .02 .02 .03 .03
TOTAL 1.18 1.60 1.85 2.54 2.93

Detention, retention and infiltration parameters were assigned to each hydrologic soil group as

recommended in Tables 2-1 and 2-2 of tbe Runoff Chapter of the USDCM. Parameters were then determined

for each sub-watershed using the predominate soil zones. A summary of the COOP parameters used for the

analysis are presented in Table lII-2.

D. STORM WATER MANAGEMENT MODEL

The personal computer version of the UD&FCD SWMM was used to route the COOP-generated
runoff hydrographs. The version of the model used was UDSWMM3. The routing element parameters
required for SWMM are: element number, type (e.g. channel, pipe, etc.), length, slope, and Manning's "n"
value. The type of SWMM routing elements used were based on the existing drainageway configurations and
shapes.

For streets and storm sewers, a Manning's "n" value of 0.020 was used. For existing channels, the

Manning's "n" value was estimated using Equation 2 recommended in the SWMM users manual (Reference

2). The SWMM routing element parameters used for this study are presented in Table III-3.

Existing systems for the baseline flow rouling modeling included all structures that are currently in
place. Structures that are expected to be constructed in the near or distant future as part of the Big Dry Creek
Master Plan are not included in the baseline run. These future facilities (i.e. alternative detention ponds) have
been modelled as part of the alternatives evaluation Dhase of this project. None of these future facilities are
used for this FHAD study. The SWMM routing elements are shown on Figures III-2A, B and C.

Only detention ponds which are publicly owned and maintained were included in the SWMM routing.
Storage-discharge relationships were taken from drainage studies approved by Douglas County (References
4, 5, 6, 7, 8, 9) and are tabulated in Appendix A.

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