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. . ,. I. . ,. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . '. I . . . I: . . . . . . . . . . !. , . appropriate equation from these guidelines for flooding sources studied in La Plata County is as follows: Q = 589.2(A)0512 A = Drainage area in square miles (l<A<760 sq mi) Q = 100-year peak discharge in cfs A 100-year discharge rate for the Florida River downstream of Lemon Reservoir was computed by three different methodologies. Details of these three approaches are provided below along with a recommendation of which to utilize. 1. Gage Analvsis. A flow frequency analysis of USGS Gage No. 09363000 (Florida River Near Durango) has been carried out. This gage has 46 years of record between 1901 and 1960 which is a period before the completion of Lemon Dam in 1963. The gage location is approximately 5.5 miles below Lemon dam downstream of Red Creek and has a drainage area of 97.4 sq mi. Flood frequency analysis of the gage record was carried out using the HEC-FFA (U.S. Army Corps of Engineers, Hydrologic Engineering Center, May 1992) software package which utilizes the procedures of United States Water Resources Council Bulletin #17B (1981). The computed 100-year discharge rate at the gage using this approach is 3,130 cfs. 2. Regional Regression Equation Analvsis. Application of the previously presented CWCB regional regression equations to the 97.4 sq mi drainage area to the USGS gage location yietds a discharge rate of approximately 6,140 cfs. This value is substantially higher than the 100-year discharge rate determined by gage analysis. A serious shortcoming of the regional regression approach is that it does not account for any attenuation of peak flows by Lemon Reservoir. Though it has no dedicated flood storage pool, the reservoir could provide some flood control benefit depending on storage levels at the time of the event. Because of this possibility, the regional regression approach likely overestimates peak flows below the dam and is not recommended. 3. Dam Outlet Capacitv + Regional Regression Equation Analvsis. The approach adopted by the US ACE in its analysis of the Los Pinos River below Vallecito Reservoir was to assume a discharge rate from the dam equivalent to its outlet works capacity (3,100 cfs) and add to this peak flow rates determined by hydrologic modeling (HEC-I) of the portion of the watershed located below the dam. A similar approach could be adopted for the Florida River below Lemon Dam. The outlet works capacity for Lemon Dam is 900 cfs. CWCB regional regression equations could then be applied to drainage areas located below the dam and computed peak flows added to the assumed dam discharge. For the location of the USGS Florida River Near Durango gage, this drainage area is 28.4 sq mi and would generate a peak discharge of 3,270 cfs. Including dam releases of 900 cfs, the 100-year peak discharge rate computed by this methodology for the USGS gage location would be 4,170 cfs. The recommended methodology for obtaining lOO-year discharge rates for the Florida River below Lemon Dam is the third methodology described above. The method consists of adding a discharge rate from the dam of 900 cfs to a peak flow computed for the portion of the basin 6