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I I I I I I I I I I I I I I I I I I I All values for the 10-year storm agree within 19.8% and for the 100-year storm, all values agree within 17.5S. The lake is owned by the Federal Correctional Institute and receives moderate use as a recreational and irrigation facility. The lake can be filled by the Fort Logan Lateral, but is primarily filled from natural drainage. The lake is considered to be a permanent facility. (4) - Storm Runoff and Hydrographs Having been successfully calibrated to CUHP, SWMM was used for the hydrologic analysis in this study. The dynamic hydrologic behavior of the area was simulated by calculating the hydrographs at several design points within the study area, using a step-by-step accounting of rainfall, infiltration, detention, overland flow, and gutter flow. The SWMM program routes and combines the individual sub-basin storm hydrographs to determine peak flows and runoff volumes at selected hydrologic design points. (2) - RID Site The effects of detention at the RTD Park-n-Ride facility were not evaluated in determining the baseline hydrology ("existing drainage facilities") for this study. There are no formal agreements with RTD to maintain the existing detention pond as a regional detention facility. Future expansion of the parking area could alter the present detention facility and any future detention is reported by RTD to be solely for onsite requirements. A summary Table 111-2. facil ities. of the input for each subcatchment, channel, and pipe is listed on The gutter segment data is presented for the existing drainage (3) - Academy Park The small detention area immediately upstream of the culvert beneath Hampden Avenue was considered in the hydrologic analysis. However, the 3.0 acre-feet flood detention volume has virtually no effect on reducing the flood peaks. A sample output from the SWMM program is shown on Table 111-3. The output is arranged to give a depth (feet) and discharge (cfs) for each hydrologic design point at 5 minute intervals. Flood hydrographs for five hydrologic design locations were plotted and are presented on Figures 111-3 through 111-7. The peak flow diagram for Henry's Lake Basin drainageway is presented on Figure 111-10. D. Flow Splits C. Flood Routing A flow split occurs when a portion of the flood flows completely separate from the main channel, due to Inadequate drainage facilities. If the flow does not return to the main channel, is a significant portion of the flood peaks, and/or is a potential flod hazard, then the condition requires an independent analysis. (1) - Henry's Lake For thi s study, the water level in Henry's Lake was assumed to be at the service spillway crest ("normal maximum water surface"). According to drawings on file at the State Engineer's Office (SEO), the crest is 0.7-feet lower than the emergency spillway crest. This results in a flood storage of approximately 18 acre-feet, and affords considerable downstream protection and flood peak reduction as shown on Figure 111-10. The greatest reduction in peak flows occurs for the more frequent storms (2- to 10-years) and becomes less effective for the rare events (50- to 100-years). A major flow split occurs along the drainageway in the Henry's Lake Basin. The first flow split occurs at Wadsworth Boulevard. The capacity of the existing 72" x 44" CMPA culvert beneath Wadsworth Boulevard is approximately 75 cfs which is considerably less than the 2-year peak runoff of 135 cfs at the culvert. When the capacity of the culvert is exceeded, the rising floodwaters spill out of the channel onto Wadsworth Boulevard. For both the 2-year and 5-year storm, all the flow onto -8-