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i I 1-3 TABLE 111--] 100-Year Peak F10l; Summary (Cubic Ft. ,. ) per .>ec:. Present F u t u re Basin CUHP M !TCA T CUHP ~II TCAT U~;ACE -'-- ---- -- I 1060 1450 1470 1810 1770 2 1370 1540 2020 2200 3 1640 1620 2470 2500 3,4,5 2410 2710 3480 3750 .~800 6 2970 2720 44LfO 3830 ',200 Maple Grove 860 8]0 2600 1490 3800 8 2320 2180 3160 2450 9 2850 2170 3270 2450 1300 The USACE was contacted in order to determine a reasonable explanation for the substantial variation of predicted inf'low to Maple Grove Reservoi r from Basin 6. This occurred becaus" of two distinct variations in the bas i c data wh i ch are understood as follow.: I. The enti re basin other than Apex Gu]ch wa'i assumed to be 50 percent urbanized whereas the information used by this office indicated that Green Mountain, South Table Mountain and other areas were not to be developed, based on verbal discussion with Mr. Juane Johnson and Mr. George Patenode, U.S. Army Corps of Engineers, District Office, Omaha, Nebraska; and 2. The entire basin was assumed to have an infi ltration rate of one inch per hour. Extensive field testing denonstrated that higher rates were generally more prominent resul ting in hi gher flow estimates by the USACE. It is ou r op in i on that if the bas i n doe~ deve lop as assumed by t~~JSACE that their estimates are quite valid an~1 \"ou~2.ersede the estimates given. For the purpose of this study, theMITCI\T values wi 11 be used as that method can demonstrate the various sensitivities of design alternates and develop- ment patterns. Figure I I 1-1 presents typical hydrographs for the 100-year return period at U.S. 6 and Lena Gu]ch for present and future conditions with existing thalweg, future conditions with hardlined channel, and future conditions with on-strean detention reservoirs. Similarly, Figures II ]-2 and 3 present the same hydro- graphs flowing into Maple Grove Reservoir and into Basin 9. The somewhat dil.. ferently shaped hydrographs of Figure 1 I 1-3 are caused by the temporal rela- tionship of the peak discharges between Basin 7, Basin 8 and Maple Grove Reservoir. Figure 1 I 1-4 presents the peak discharge at vario~s points along the gulch for various future conditions and flood frequencies. It is interesting to note that if a cleared grass-lined channel were to be constructed, the design flow based on a 25-year event under future basin conditions would have to be nearly the same or larger than the 100-'year flood under present channel and basin conditions. Figure 111-5 presents in more clarity the lOO-year flood for existing channel conditions an~ for master plan channel conditions.