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We have completed a review of the Army Corps of Engineers hydrologic analysis of the South Shooks Run Basin in Colorado Springs, Colorado. analysis is contained in the 1976 Report on Hydrologic Investigations Flood Insurance Studies. This for ~ ~v CDM CAMP DRESSER & McKEE INC, environmental engineers, scientists, planners. & management consultants Writer's Tower. Suite 1100 1660 South Albion Street Denver, Colorado 80222 303758-4351 June 12, 1981 l~t~~~)~I~;~~ COLORADO WATFR CO'!S':r:\.li\TiO~j BOARD Dr. John Liou Federal Emergency Management Agency Denver Federal Center, Bldg. 710 Lakewood, CO 80225 !,~ If, Dear John: rii, ~" ~ The analysis consisted of a synthetic unit hydrograph technique in order to estimate the flows for the 10-, 25- and 100-year recurrence intervals. Rainfall patterns were obtained from Technical Paper 40, and the rainfall losses (initial and infiltration), were applied to the rainfall data. The loss rates used are representative of loss rate values for fully urbanized areas throughout the Denver Metropolitan Area. The unit hydrograph calculated at the outfall of the basin was applied to the effective rainfall and the peak flow rates were obtained. The peak 10-, 25- and 100-year flows were then plotted on logarithmic-probability paper to obtain the 50- and 500-year frequencies. The peak flow rates are as follows: South Shooks Run Hydrology Location Area (sq.mL) Peak Discharges (cfs) 10 50 100 500 at Fountain Creek Confluence 9.43 4,900 6,700 7,800 11 ,500 Basin parameters such as slope, area, length and lag times were checked and found to be accurately calculated. The existing land development in the South Shooks Run basin consists of single family, downtown business areas, industrial and some open space (golf course). The development within the basin has not changed in either type or in extent of impervious areas since the 1976 Corp of Engineers report, and therefore no change in flow rates would be expected due to development changes in the basin since 1976. The flow rate per unit area for this basin is 827 cfs/sq.mi. using the COE hydrologic results. This unit flow rate was checked against unit flow rates and derived in studies done by CDM and others in similarly developed basins for the Colorado Springs and Denver area. It was found that the unit flow rates estimated for the South Shooks Run basin compared favorably with peak unit flow rates estimated for other urbanized basins which utilized methods such as the Colorado Urban Hydrograph Procedure (CUHP), the Stormwater Management Model (SWMM) and the SCS method.