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. . . . . . . . . . . . . !. . . . . . . I. '. . . I. I. . '. . . I. . . . . . . . . . . . . . . . . . . . I. . . . As noted in the table, because McPhee Reservoir does not provide flood control storage and because the spillway capacity of 33,000 cfs is greater than the estimated 100-year inflow of 12,000 cfs, it has been assumed that McPhee Reservoir does not affect l00-year flood flows. In actuality, there will be some flood flow attenuation due to surcharge capacity in the reservoir and travel times from the reservoir inflow points to the spillway. Peak flow records are available at the Dolores River Below Rico gage (USGS Gage No. 09165000) from 1952 through 1996. Using the gage records, USGS Bulletin 17B procedures were used to calculate discharges for specific return intervals at the gage. These discharges were then used to estimate peak flows for the Dolores River at the eastern Montezuma County line using CWCB Technical Memorandum No. I procedures. The estimated 100-year discharge using these procedures is 2,600 cfs. This is 25 percent less than the estimate of 3,500 cfs using the regression equations. Due to the relatively short period-of- record, it is recommended that 3,500 cfs be used for planning purposes. Dolores County should regulate the approximate floodplains on a case-by-case basis. FEMA requires that a detailed floodplain analysis must be performed by a professional engineer and submitted by the developer or development proponent for development projects involving more 5 acres or more than 50 units (whichever is more restrictive). Hvdraulic Analvsis Hydraulic methods used for the approximate floodplain delineations shown on the Dolores County FHBM panels are not available or have not been documented by FEMA. For future studies, an approximate 100-year flood elevation at a particular location on the river may be obtained by using the 100-year peak flow value (as provided in this report) in conjunction with an acceptable hydraulic analysis. Hydraulic analyses for determining 100-year water surface elevations for approximate floodplains may be performed using FEMA's Quick-2 computer program, the Corps of Engineers' HEC-2 computer program, Flowmaster, or other acceptable hydraulic methods. Channel and floodplain cross-sections may be obtained from detailed topographic mapping or from field surveys. Cross-sections obtained from USGS quadrangle maps are not advised. Floodolain Maooine The existing FHBM for Dolores County depicts areas subject to 100-year flooding based on approximate technical methods. The map does not show flood profiles, base flood elevations, or sufficient detail in order for County officials, engineers, developers, and land owners to make floodplain determinations with any certainty. The Colorado Water Conservation Board can be contacted to request technical assistance for approximate floodplain analyses as needed. 6