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I I I I I I I I I I I I I I I I I I I Values used for this study are indicated in brackets. storage retention loss was estimated to be 0.4 inches for and 0.1 inches for impervious surfaces in the basin. In addition, the pervious surfaces The attenuating effect of the proposed 5-year detention facility is graphic- ally displayed in Figure III-3 and III-4. Typical hydrographs are shown for the 2, 10 and 100-year events at several locations in the basin. In addi- tion, the 100-year hydrograph with detention is plotted to show the effect of flow routing through the detention facility. DETERMINATION OF DESIGN FLOWS The Urban Drainage and Flood Control District used a computer model of the 3207 bas in which is a vers ion of the Environmental Protect ion Agency Storm- water Management Model (SWMM) modified by the Missouri Division of the Army Corps of Engineers. Peak flows were calculated for the 2, 5, 10 and 100-year flood events, based on present and future basin development. These flows are listed in Table 111-2 and depicted graphically in Figure 111-2. TABLE II 1-2 PEAK RUNOFF FLOWS PRESENT AND FUTURE CONDITIONS (All Values in Cubic Feet per Second) Peak Flow for Various Frequencies 2-Year 5-Year 10- Year 100-Year Location Present Future Present Future Present Future Present Future Daphne Street 10 90 130 40 210 130 400 Miramonte Boulevard 80 130 230 170 310 420 670 Main Street 90 150 370 330 400 420 670 E. 10th Avenue 120 220 370 330 570 730 1,090 Ash Street 120 250 370 340 570 730 1,250 (20) (20) (580) Birch Street 170 290 450 450 700 970 1,410 (140) (210) (700) Sheridan Boulevard 180 370 580 520 900 1,200 1,850 (240) (360) (1,100) Lowell Boulevard* 780 1,210 1,860 3,400 (670) (1,020) (2,650) NORTH TRIBUTARY** At Nissen Reservoir 3 35 20 60 80 105 At Main St./Confluence 3 20 10 40 40 80 SWMM (Storm Water Management Model) is a digital computer model which calcu- lates storm runoff from an input rainfall event. The drainage area is model- ed by a series of catchment (overland flow) elements and stream (gutter, pipe, and channel) elements. The catchments and streams are described by physical parameters which can be obtained from maps, photos, soil surveys, and field investigations. The main parameters which describe a watershed area are: drainage area, flow length, slope, infiltration characteristics, sur- face flow resistance characteristics, and channel or pipe sizes. Additional details re9arding the computer modeling of the basin may be found in Appendix B of the Phase A Report for the 3207 Basin. Reduced peak runoff flows were developed utilizing the proposed 5-year deten- tion facility upstream of Ash Street. The stage-storage and stage-discharge relationships were input into the SWMM Model, and runoff hydrographs were generated for the 2-year, 5-year and 100-year flood events. The revised peak flows are shown in parentheses in Table III-2. Comparison of flows with and without detention demonstrates the effectiveness of the 5-year detention fac- il ity. For example, at Birch Street, the 100-year future development peak flow is reduced from 1,410 cfs to 700 cfs. *Includes flow from City Park Basin ** Future development flow values are those resulting from routing through Nissen Reservoir. The proposed 5-year detention facility will serve to elongate the runoff per- iod; not to decrease the runoff vo 1 ume. Duri ng a 100-year event, the pond would fill, the overflow spillway would operate, and the total peak flow at 7th Avenue would be approximately 640 cfs. The pond would retain approximate- ly 34-acre-feet, which would discharge through the 24-inch pipe spillway. It will take approximately 32 hours to completely drain the pond. Note: Flows in parentheses are those resulting from installation of a 5-year detention facility upstream of Ash Street and represent residual storm flows after detention. The 10-year event was not modelled with detention. 11