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FLOOD01081
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Last modified
11/23/2009 12:58:25 PM
Creation date
10/4/2006 9:46:51 PM
Metadata
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Template:
Floodplain Documents
Designation Number
286
County
Douglas
Community
Unincorporated Douglas County
Title
Flood Hazard Area Delineation - Spring Gulch
Date
12/1/1986
Floodplain - Doc Type
Floodplain Report/Masterplan
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<br />I <br /> <br />I <br /> <br />watersheds in the Denver area. The particular relationships utilized in <br /> <br />this study were based on the information presented in the UDFCD Urban Storm <br /> <br />Drainage Criteria Manual (USDCM). <br /> <br />The percent imperviousness was estimated using a plot of housing density <br /> <br />versus percent irnperviousnes for various densities in Arapahoe County. <br /> <br />This is shown in Figure 2 (page 5 of Appendix) on a semi,log scale. The <br /> <br />housing density was determined from the proposed Development Plan for <br /> <br />Highlands Ranch, which includes the Spring Gulch drainage area. This <br /> <br />closely approximates the housing density vs. percent imperviousness for <br /> <br />developed areas of Arapahoe County. An impervious factor of 2% was used <br /> <br /> <br />for nonurban areas. For tributary areas lying upstream and outside the <br /> <br />Highlands Ranch boundary, a developed impervious factor was used for both <br /> <br />the major and initial storm. These factors are consistent with Douglas <br /> <br />County and UDFCD recommendations. <br /> <br />I <br /> <br />I <br /> <br />The rainfall used in the CUHP method was based on a storm duration of two <br />hours. The design point rainfall (1 hr) value used in the computer <br />applications of CUHP is shown on Table I, below. This point value was <br />recommended by the Douglas County Storm Drainage Design and Technical <br />Criteria Manual (SDDTCM). <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />No areal rainfall adjustment factors were used as recommended in Table 4,1 <br /> <br /> <br />of the USDCM. Surface retention losses were taken as 0.1 inch for <br /> <br /> <br />impervious surfaces and 0.4 inch for pervious areas. Infiltration rates <br /> <br />were determined from Horton's equation with an initial rate of 3.0 in/hr, <br /> <br />a final rate of 0.5 in/hr and decay rate of 0.0018 sec -1 <br /> <br />Table 2 lists the parameters used for each CUHP Basin for the developed <br /> <br /> <br />condition (also see Exhibit 3 ' Spring Gulch Master Plan of Drainage <br /> <br />Detention) . <br /> <br />I <br /> <br />I <br /> <br />B. DESCRIPTION AND APPLICATION OF SWMM <br /> <br />I <br /> <br />The measured slope as shown in Figure 1 (page 4 of Appendix) was found <br />utilizing the following slope weighting equation: <br /> <br />4.17 <br /> <br />The SWMM Model, as suggested by UDFCD, is a suitable means of routing flood <br /> <br /> <br />hydrographs developed by the CUHP. The Model, originally developed by the <br /> <br /> <br />Environmental Protection Agency (Reference D), assumes a watershed to <br /> <br />consist of a set of sub,basins connected by a system of pipes or open <br /> <br /> <br />channels. The Model can be used in 2 ways: (1) by developing its own <br /> <br /> <br />hydrographs for the sub, basins and then routing them through the connecting <br /> <br />systems; or (2) by utilizing hydrographs already developed from the CUHP <br /> <br /> <br />program and routing them through the connecting system. The latter method <br /> <br /> <br />was used to develop discharges on Spring Gulch. <br /> <br />I <br /> <br />0.24 0.24 <br />L1 Sl +L2S2 <br /> <br />0.24 <br /> <br />+ .... .+LnSn <br /> <br />S <br /> <br />I <br /> <br />L1+L2+L3 ..... .Ln <br /> <br />I <br /> <br />The values L, Land S were determined from a topographic map for each of <br />ca 0 <br />the subareas used in the study. The CUHPE/PC version of the program was <br />used for all computations. This version is designed to compute the peaking <br />coefficient (Cp) and time to peak coefficient (Ct). The Model will also <br />calculate hydrographs for drainage areas less than 90 acres to fit peak <br />flows computed using the Rational Method. <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />TABLE 1 <br />DESIGN POINT RAINFALL VALUE <br /> <br />The Kinematic Flow'Routing Technique of the SWMM program takes into account <br /> <br /> <br />pipe/channel storage, and maintains a constant volume of runoff throughout <br /> <br /> <br />the storm. Reduction of peak discharges by detention basins at <br /> <br /> <br />roadway,cu1vert crossings can also be computed by the SW~n1 model. For each <br /> <br /> <br />basin, a storage volume and total outflow relationship is computed and <br /> <br /> <br />provided as input to the computer program. <br /> <br />I <br /> <br /> ONE HOUR POINT VALUE <br />COUNTY ZONE 100 ,YEAR <br />Douglas County 2.56 <br />Zone I II <br /> <br />4 <br /> <br />I <br /> <br />I <br />
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