FLOOD10256

• County: Douglas
• Community: Douglas County
• Stream Name: Spring Creek
• Basin: South Platte
• Title: FHAD - Spring Gulch
• Date: 12/1/1986
• Prepared For: Douglas County
• Prepared By: UDFCD
• Floodplain - Doc Type: Floodplain Report/Masterplan

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watersheds in the Denver area, The particular relationships utilized in

this study were based on the information presented in the UDFCD Urban Storm

Drainage Criteria Manual (USDCM).

The percent imperviousness was estimated using a plot of housing density

versus percent irnperviousnes for various densities in Arapahoe County.

This is shown in Figure 2 (page 5 of Appendix) on a semi-log scale, The

housing density was determined from the proposed Development Plan for

Highlands Ranch, which includes the Spring Gulch drainage area, This

closely approximates the housing density vs. percent imperviousness for

developed areas of Arapahoe County, An impervious factor of 2% was used

for nonurban areas. For tributary areas lying upstream and outside the

Highlands Ranch boundary, a developed impervious factor was used for both

the major and initial storm, These factors are consistent with Douglas

County and UDFCD recommendations,

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The rainfall used in the CUHP method was based on a storm duration of two
hours, The design point rainfall (1 hr) value used in the computer
applications of CUHP is shown on Table I, below. This point value was
recommended by the Douglas County Storm Drainage Design and Technical
Criteria Manual (SDDTCM).

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No areal rainfall adjustment factors were used as recommended in Table 4-1

of the USDCM, Surface retention losses were taken as 0,1 inch for

impervious surfaces and 0.4 inch for pervious areas. Infiltration rates

were determined from Horton's equation with an initial rate of 3.0 in/hr,

a final rate of 0,5 in/hr and decay rate of 0,0018 sec -1

Table 2 lists the parameters used for each CUHP Basin for the developed

condition (also see Exhibit 3 - Spring Gulch Master Plan of Drainage

Detention) ,

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B, DESCRIPTION AND APPLICATION OF SWMM

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The measured slope as shown in Figure 1 (page 4 of Appendix) was found
utilizing the following slope weighting equation:

4.17

The SWMM Model, as suggested by UDFCD, is a suitable means of routing flood

hydrographs developed by the CUHP, The Model, originally developed by the

Environmental Protection Agency (Reference D), assumes a watershed to

consist of a set of sub-basins connected by a system of pipes or open

channels, The Model can be used in 2 ways: (1) by developing its own

hydrographs for the sub-basins and then routing them through the connecting

systems; or (2) by utilizing hydrographs already developed from the CUHP

program and routing them through the connecting system, The latter method

was used to develop discharges on Spring Gulch,

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0,24 0,24
L1 Sl +L2S2

0.24

+ ,... .+LnSn

S

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L1+L2+L3 "'" ,Ln

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The values L, Land S were determined from a topographic map for each of
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the subareas used in the study. The CUHPE/PC version of the program was
used for all computations, This version is designed to compute the peaking
coefficient (Cp) and time to peak coefficient (Ct), The Model will also
calculate hydrographs for drainage areas less than 90 acres to fit peak
flows computed using the Rational Method.

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TABLE 1
DESIGN POINT RAINFALL VALUE

The Kinematic Flow-Routing Technique of the SWMM program takes into account

pipe/channel storage, and maintains a constant volume of runoff throughout

the storm, Reduction of peak discharges by detention basins at

roadway-culvert crossings can also be computed by the SW~n1 model, For each

basin, a storage volume and total outflow relationship is computed and

provided as input to the computer program,

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ONE HOUR POINT VALUE
COUNTY ZONE 100-YEAR
Douglas County 2,56
Zone I II

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