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I I I I The weighted channel slopes were adjusted using Fif!ure 4-/ of the Runoff Chapter of the USDCM for input into CUHP. Sub-watershed geometries were measured directly from the mapping prepared for this study using ARC/Info. Maps were created delineating the existing and future imperviousness percentage within the study area and in the off-site area based on existing zoning, the Douglas County Master Plan (Reference-! 0), aerial photography, USGS quadrangle maps, USGS County Maps, and site visits. Future imperviousness is the imperviousness that is expected at ultimate build- out of the watershed. The future percent imperviousness of the watershed (study area and off-site) is relatively low because the development that may occur in this watershed is limited due to the lack of available water to support development and because a large portion of the off-site watershed is National Forest land (and therefore is unlikely to be developed). Figure 1ll-8 shows the existing imperviousness in the study area. Figure 1ll-9 provides the future imperviousness in the study area. Figure Ill-I 0 shows the existing imperviousness in the off-site watersheds and Figure III-II contains the projected imperviousness in the off-site watersheds. For each sub-watershed, a composite value of the percent imperviousness was calculated using ARC/Info. I I I I I I I I I A soil map of the NRCS hydrologic soil groups was created for the study area from information supplied by Douglas County. Figure IlI-I2 is the soil map for the study area. Figure II1-13 is a soil map for the off-site watersheds which was created from the NRCS Soil Survey (Reference-I 4). This map is based on the NRCS soil association group. Each soil association group is comprised of several different soil types that may have different hydrologic soil group classifications. Composite infiltration parameters were then estimated based on hydrologic soil groups in each soil association group. Table Ill-II summarizes this analysis. Detention, retention and infiltration parameters were assigned to each hydrologic soil group as recommended in Tables 2-1 and 2-2 of the Runoff Chapter ofthe USDCM. For each sub-watershed, a composite value of the detention, retention, and infiltration was calculated using ARC/Info. The off-site sub-watersheds are included in the study area models. D. STORM WATER MANAGEMENT MODEL I I The personal computer version of the UDFCD SWMM was used to route the CUHP-generated runoff hydrographs. The version of the model used was UDSWM98. The routing element parameters required for SWMM are: element number, type (e.g. channel, pipe, etc.), length, slope, and Manning's "n" value. A summary of the SWMM input parameters used for the analysis are presented in Tables Ill-I 2 through II1-15. Table II1-12 is the SWMM input parameters for the west section ofthe study area. Table Ill-I 3 is the SWMM input parameters for the southeast section of the study area. Table II1-14 is the SWMM input parameters for the east section ofthe study area. Table Ill-I 5 is the SWMM input parameters for the off-site area. I I I I The types 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 the equation that is recommended in the SWMM user's manual. The hydraulic radius was computed for typical cross-sections of the drainageway and used in said equation to determine Manning's "n". Existing systems for the baseline flow routing modeling included all structures that are currently in place. Structures that are expected to be constructed in the future as part of the Plum Creek Watershed Outfall Systems Plan are not included in the baseline run. These future facilities will be modeled as part ofthe alternatives evaluation phase of the project The SWMM routing elements for the study area are shown on Figure III-I 4 through Figure III-19. The SWMM routing elements for the off-site watersheds are shown on Figure III-20. E. MODELING RESULTS The output from SWMM is a hydrograph at each design point and routing element Table III-I 6, III-I 7, and III-I 8 provide the peak runoff at each design point and routing element in the study area's west section, southeast section, and the east section, respectively. Typical Hydrographs and Peak Flow Diagrams for each of the modelled drainageways are included in Appendix A. The unit runoff (cfs/acre) ofthe I OO-year flow for each sub-watershed was compared to the imperviousness of the future land use conditions to check for inconsistencies in the runoff calculations. Figures III-21, II1-22, and 1ll-23 provide this comparison for the west section of the study area, the southeast section ofthe study area, and the east section of the study area, respectively. In general, it would be expected that the unit runoff would increase proportionally with an increase in the percent of impervious area within a sub-watershed. The sub-watersheds with substantially higher or lower unit runoff rates were checked to ensure that the CUHP sub-watershed parameters justified the runoff values. The peak I OO-year runoff values forthe future land use conditions will be used as the baseline model for master planning and floodplain delineation. It is important to note that these flows assume that sediment is not suspended in the flow. Suspended sediment can significantly increase the flow volume and thus the flow rate. During the 1965 flood on Plum Creek, the bed was scoured several feet, which would probably have created a high concentration of suspended sediments during the flood event. The 500-year storm event was not formally hydrologically modeled. A regression analysis of peak flows at several locations in the Denver Metropolitan area indicates that the 500-year flood event typically exceeds the 100-year flood event by a factor of 1.7 to 1.9 (Reference 1-5). Therefore, forthis study a 1.8 factor was used to obtain the 500-year flood peak flow estimate. 1I1-25