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I I I I I I I I I I I I I I I I I I I E. D. STORMW A TER MANAGEMENT MODELING Delineation of Subwatersheds TC&B created three hydrologic scenarios: the Base Model, Futurc Model, and Existing Model. Each 0 f thcsc scenarios is described below. A total of 168 subwatersheds were delineated for the First Creek and DFA 0055 study area. The sub watershed identification and locations arc displayed in Appendix C. The Base Model is the master planned drainage system, with master.planned detention facilities, and the future land.use condition. It incorporates the MartinIMartin model with minor modifications, noted earlier. It represents the master plan upstream from the RMA. The results from this modeling scenario identify thc peak flows and detention volumes along the First Creek mainstem and tributaries when the watershed is fully developed and the master planned upstream drainage system is in place. The Base Model flow rates that are shown in this report represent the effects of the drainage system plan that the sponsors selected for implementation within the study area. The two. foot contour mapping, provided by Adams County and RMA, was uscd to verify subwatershed boundaries. Field inspections were made to verify flow paths and boundaries. The subwatersheds rangc in sizc from 6.4 acres to 717 acres. Minor changes were made to the watershed delineation used in the WWE study to conform to the 2. foot contour mapping provided. Imperviousness The Future Model is the existing drainage system, existing detention facilities, and the future land- use condition. The results from this modeling scenario identify the peak flows and detention volumes along the First Creek mainstem and tributaries when the watershed is fully developed and the drainage system is in its current configuration. The watershed imperviousness was determined for the futurc land use and the existing land use conditions. Zoning information, provided by Commerce City (Reference 15 & 16) and Adams County (Reference 17 & 18), was used to identify levels of imperviousness. Landiscor Aerial Photographs 1998/1999 were used to identify the existing land use for the First Creek watershed (Reference 2). Changes were made to reflect the existing percent imperviousness from the aerial photographs. The range of the imperviousness was betwcen 2 and 90 percent. The Existing Model is the existing drainage system, existing detention facilities, and the existing land-use condition. The results from this modeling scenario identify the peak flows and detention volumes along the First Creek mainstem and tributaries with the watershed at the current level of urban development and the drainage system in its current configuration. For the existing drainagc system, TC&B revised the future drainage system file to reflect the existing condition throughout the watershed. All proposed detention elemcnts, except those that have been constructed, were removed from the model. The existing condition channels from the WWE UDSWM model wcre used as the basis for these revisions. For the future imperviousness within the study area, there werc thrce locations that had been rczoned from commercial to residential, which reduced the previous percent impervious from 85 to 50 percent. Within the RMA, two sites had been designated as commercial/industrial. One site has been removed and restored to open space, and the other site's area has been reduced by approximately onc-half. The UDFCD's Colorado Urban Hydrograph Procedure (CUHP F95) model was used to develop runoff hydro graphs for each subwatershed. The runoff hydrographs were routed through the existing stream nctwork using UDFCD's Storm Water Management Model (UDSWM 95). A map illustrating imperviousness for future and existing land use conditions is shown in Appendix C. Slope CUHP MODELING The slopes for each subwatershed were estimated according to the weighted slope method documented in the USDCM. The length of each subwatershed was broken up into small segments. The slope of each of the segments was computed and correctcd according to Figure 4-1 of the Runoff chapter of the USDCM (Referencc 4). The segment lengths and corrected slopes were entered into Equation 4.5 of the Runoff chapter of the USDCM (Referencc 4) to determine the weighted subwatershed slope. CUHP models were developed for each subwatershcd to estimate the existing and future hydrology for Lower First Creek and its tributaries. The design storms described in Section C were modeled in each CUHP model. The subwatershed characteristics required for the CUHP models included the identification number, area, length, centroid length, slope, impervious percentage, retention storage losses, and infiltration losses. The methods for estimating the subwatershed characteristics are described below. Table 111-3 and Table 111-4 present the CUHP input data for existing and future land use conditions, respectively. - 14-