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Tnis Study's Flood Frequency Anaiysis Adjusted from Pikeview Gage to Fountain Creck 30.ooocfs 4.300cfs Table 12 shows that the lOO.year discharge of 39,400 cIs projected by this study's HEC-I model is 23 percent higher than the 32,000 cfs calculated by the COE in 1971. and 31 percent higher than the 30,000 cis determined from this study's flood frequency analysis. Table 12 also shows that the 10'year discharge of 3,230 cfs projected by this study's HEC-I model is 25 percent lower than the 4,300 cfs determined from this study's flood frequency analy.~is. The lOO-year hydrographs in Figure 26 have the typical shape associated with flood hydrographs. The existing peak flows are less than the future peak flows as would be expected. The future condition peak flow occurs quicker than the existing peak flow. This would also be expected due to the increased development and its greater amount of impervious surface area causing quicker travel times for the storm runoff. The ascending and descending leg of the hydrographs are generally the same at each particular location. The lO,yearhydrographs in Figure 27 are comprised of combinations of hydro graphs . In reviewing the hydrographs associated with the Air Force Academy boundary, a small peak discharge is evident at 12 hours and a higher. more sustained peak flow is shown at around 14 hours. The second peak is probably due to the runoff from the upper reaches of the basin wh ich reaches the south boundary of the Academy after the initial peak has begun to recede. In moving from the south boundary of the Academy to the Fountain Creek confluence, the initial peak becomes more pronounced. The second peak becomes less man the initial peak and occurs at around 15 hours as compared to 14 hours at the Academy boundary. The differences between the shapes of the hydrographs at these two locations reflects the impact of the developed areas within the City upon the nature of the peak flows. The developed area within the Monument C'rl'e-k hasin he,tween the Academy boundary and the confluen~-e with Founl3.i.n creek causes a relatively large and fast responding hydrograph at the confluence. This becomes even more pronounced in the future condition hydrograph when the peak doubles and occur .I approximately one hour earlier. It appears that thcre is a signilicant flow, around2.(I()(l cfs, that flows from the upper reaches of the basin. This peak flow appears to attenuate as it travels the length of the basin to the outfall. While this peake tntvels through the City of Colorado Springs, the effect of the attenuation becomes greater due to the increased runoff volume in the vicin ity of the City of CoJoradoSprings The difference between the 10- and loo-year hydrographs is apparent when examining Figures 26 and 27. The reason for the difference in hydroj,.'Taphs pertains to the amoums and locations of runoff volume produced by each storm. Obviously. the runoff from the lOO-year storm will be: grearcr than the lO-year srorm due to the difference in the rainfall amounts, The lO.year hydrograph indicates that there are twO significant peaks in runoff for the basin, One peak is caused by the accumulation offiow ov('r the emire ba,in as the peak travels frnmthc upperreaehestotheoutfalJ. The otherpc:ak is associated with the impervious areas of the basin Shown on Table II is a comparison of 100- and lO-year discharges at various design points along Monument Creek for both the existing and future condition for the Hydromet52 storm. Figures 20 and 21 show the flood discharge profiles for the selected design stonns. Figures 22 through 25 show the existing and future condition hydrographs for both the lOO-and IO-year storms at the southern boundary of the Air Force Academy and at the confluence with Monument Creek. Figure 26 shows both the Air Force Academy Boundary and the Fountain Creek confluence JOO-)'car hydrographs. The same hydrographs are shown for the lO.year stonnooFigure27. Comoarison of BEC-l Results to Other Methods Table 12 shows the peak discharges of Monument Creek at the confluence of Fountain Creek developed in the 1971 COE study, the flood of record, and results of this hydrolo!l;ic analysis" Table 12 Monument Creek Di.~charge Comparison at Fountain Creek Description lOO-Year Discharge lQ-Year Discharge Intenncdiare Regional Rood (from regional analysis) COE Floodplain Infonnation Repon, 1971 32.ooocfs '_~JXl'._,""".l('>Y'~J '~""J,T""''''--'.'o-',., Hood of Record May30-31,1935 This Study's HEC-1 Model (Hydromet 51. 24 Hour, Existing Conditions) 5O.000cfs 28,500cfs 2,060cfs 33.4oocfs 4,050cfs 39,400ds 3,230cfs 46,700cfs 6,570cfs l"hlsStudy'sHEC-l Model (Hydromet 51. 24 Hour. Future Conditions) This Study's HEC-I Mode! (Hydromet 52, 24 Hour, ExlSting Conditions) This Stud/s HEC-I Model (lJydromel 52. 24 Hour, Future Conditions) IV.31 IV-32