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<br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br /> <br />discharges estimated above for Leopard Creek and Fall Creek, Because the PIS hydrology <br />calculations were more rigorous than the hydrology calculations calculated in this report, it is <br />recommended that the PIS hydrology calculations be used for planning purposes, <br /> <br />Table 3 shows the USGS gaging stations that contain an adequate record length for computation <br />of peak discharges, Using the gage records, USGS Bulletin 17B procedures were used to <br />calculate discharges for specific return intervals at the gage and are shown in Table 3, <br /> <br /> a e . ornpu e ea ISC arges a agml! Ions <br /> Computed <br />Station Total 100-yr Peak <br />10 Station Name Period-of-Record Years Discharge (cis) <br />09172500 San Miguel River Near Placerville. Co, 1909,1911,1930-1934.1942.1999 65 5,290 <br />09173000 Beaver Creek Near Norwood. Co, 1942-1961,1963-1967,1975-1981 32 1,040 <br /> <br />T bl 3 C <br /> <br />t d P k D. h <br /> <br />t USGS G . Star <br /> <br />The Flood Insurance Study developed a more intense estimation technique for the gage data that <br />differentiated between rainfall and snowmelt caused events and adjusted estimated peak flows <br />accordingly, The F1S estimated peak discharges at the Placerville gage of 6,210 cfs. Again, <br />because of the improved technique used in the PIS and because detailed floodplains in the PIS <br />were based on this value, it is recommended that 6,210 cfs be used for planning purposes, <br /> <br />San Miguel County should regulate the approximate floodplains on a case-by-case basis, FEMA <br />requires that a detailed floodplain analysis must be performed by a professional engineer and <br />submitted by the developer or development proponent for development projects involving more 5 <br />acres or more than 50 units (whichever is more restrictive), <br /> <br />Hvdraulic Analvsis <br />Hydraulic methods used for the approximate floodplain delineations shown on the San Miguel <br />County FIRM panels are not available or have not been documented by FEMA, For future <br />studies, an approximate loo-year flood elevation at a particular location on the river may be <br />obtained by using the loo-year peak flow value (as provided in this report) in conjunction with <br />an acceptable hydraulic analysis, Hydraulic analyses for determining loo-year water surface <br />elevations for approximate floodplains may be performed using FEMA's Quick-2 computer <br />program, the Corps of Engineers' HEC-2 computer program, Flowmaster, or other acceptable <br />hydraulic methods, Channel and floodplain cross-sections may be obtained from detailed <br />topographic mapping or from field surveys, Cross-sections obtained from USGS quadrangle <br />maps are not advised, <br /> <br />Floodplain Mapping <br />The existing FIRM for San Miguel County depicts areas subject to loo-year flooding based on <br />approximate technical methods, The map does not show flood profiles, base flood elevations, or <br />sufficient detail in order for County officials, engineers, developers, and land owners to make <br />floodplain determinations with any certainty. The Colorado Water Conservation Board can be <br />contacted to request technical assistance for approximate floodplain analyses as needed, <br /> <br />8 <br />