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I I I I I I I I I I I I I I I I I I I Flood hydrographs and associated peak flows for 10-year and 100-year design storms were developed for the governing condition of uniform rainstorms over the entire 65.4 square mile watershed. A total base flow of 480 cfs, discharging from Standley lake and Great Western Reservoir, was included in the 100-year flood hydrographs. Hydrographs were generated at six "Hydrologic Design Points," labeled "A" through "F," at the locations on Figure 1. fABLE V FLOOD HYDROGRAPHS ON BIG DRY CREEK Af INTERSTATE 25 An investigation was made of a smaller design storm, located only over the 26.2 square mile drainage basin directly tributary to the Study Reach downstream from Interstate 25. Such a smaller storm would be more intense, si nce it woul d not have as 1 arge of an Area Adjustment as a 1 arger storm over the ent ire watershed. It was cons i dered poss i b 1 e that thi s small er, more intense storm might create higher peak flows than runoff from the entire 65.4 square mile watershed. lO-Year Flow 3 100-Year Flow At Point F1 Adj. Hydrograph At Point F1 Adj. Hyd rog raph Time From Ref. 3 for po~nts A, B, From Ref. 3 for po~nts A, B, (min) (cfs) C, D (efs) (cfs) C, D (cfs) 0 0 0 480 480 20 80 80 570 570 40 1430 1380 2730 2660 60 2730 2640 6400 6230 80 3040 2950 7190 6990 100 3620 3510 7460 7250 120 4300 4180 8380 8140 140 4920 4780 9480 9210 160 5360 5200 10340 10040 180 5600 5430 10940 10620 200 5620 5450 11190 10870 220 5430 5270 11020 10710 240 5180 5020 10480 10180 270 4730 4590 9220 8950 300 4260 4130 7750 7530 330 3790 3670 6450 6270 360 3270 3170 5420 5270 390 2930 2840 4730 4610 420 2530 2450 4090 3980 450 2220 2150 3520 3430 480 1940 1880 3110 3030 510 1700 1650 2750 2680 540 1550 1500 2490 2430 570 1440 1400 2340 2280 600 1350 1310 2260 2200 1 Includes Area Adj ustment for a basin area of 30 to 50 square mi 1 es 2 Includes Area Adjustment for a basin area of 50 to 75 square mil es 3 base f1 ow of 480 cfs from Standl ey lake and Great I ncl udes a total Western Reservoir Hydrologic Design Point F, at the upstream end of the present Study Reach, coincides with the most downstream point of hydrologic calculation in Reference 3. The 10-year and 100-year hydrographs at this point in Reference 3 were adoptea as the hydrographs at Hydrologic Design Point F, and are tabulated in Table V. The Reference 3 hydrographs were compiled into the SWMM hydrologic calculations for this study, as shown in Figure 2. However, as the tributary areas along Big Dry Creek increased, from 39.2 square miles at Point F to 65.4 sqvare miles at Point A, a change in Area Adjustment to the design storm required that similar adjustments be made to the hydrographs from Reference 3. This adjustment was accomplished by modifying the flow rates of the hydrographs generated in Reference 3 by the 97% ratio of the volumes of runoff from a basin with Area Adjustments of 50 to 75 square miles and 30 to 50 square mil es. The" Adj usted Hydrographs" were used in the SWMM hydrology calculations at Hydrologic Design Points A through D and are listed in Table V. The design storfTlS for the watershed directly tributary to the Study Reach (Sub-Basins 1 through 9) were also adjusted for a basin si ze of 50 to 75 square mil es. Peak f1 ows computed in thi s manner are illustrated in Figure 3 and tabulated in Table VI. 8