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<br />Discharges calculated in the PIS for determination of detailed floodplains are shown in the
<br />Appendix, These calculations include discharges along the Dolores River, West Dolores River,
<br />Mancos River, Lost Canyon Creek and Chicken Creek, The discharges calculated in the PIS
<br />analysis are slightly higher than the discharges estimated above for the Mancos River, and are
<br />slightly lower than the discharges estimated above for Chicken Creek, The PIS discharge is
<br />significantly lower for Lost Canyon Creek, Because the PIS hydrology calculations were more
<br />rigorous than the hydrology calculations calculated in this report, it is recommended that the PIS
<br />hydrology calculations be used for planning purposes,
<br />
<br />Table 2 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 . omvu ea ISC al'2es at a21n lations
<br /> Computed
<br />Station Total 100-yr Peak
<br />10 Station Name Perlod-of-Record Years Discharge lefs)
<br />09165000 Dolores River Below Rico, Co, 1952-1996 45 2,740
<br />09166500 Dolores River At Dolores, Co, 1896-1898,1901-1903,1911,1922- 85 9,870
<br /> 1999
<br />09372000 McElmo Creek Near Colorado- 1951-1998 4B 3,810
<br /> Utah State Line
<br />09371000 Mancos River Near Towaoc, Co, 1921,1927-1929,1931-1943,1951- 66 5,710
<br /> 1999
<br />
<br />T bl 2 C
<br />
<br />led P k D' h
<br />
<br />USGSG ' S
<br />
<br />The McElmo Creek near the State Line gage analysis calculated a peak discharge of 3,810 cfs,
<br />while the regional regression equation calculated a peak discharge of 7,700 cfs, Because the
<br />regression equation is a more generalized equation that does not account for localized
<br />characteristics associated with individual drainages, such as infiltration rates, surface storage and
<br />climate, the method is considered more approximate than an actual gage analysis, Therefore it is
<br />recommended that the gage analysis estimate of 3,810 cfs be used for planning purposes,
<br />
<br />The Mancos River gaging station is significantly downstream of the study area, However,
<br />comparing the unit discharges (discharge per area), the regional regression analysis calculated a
<br />unit discharge at the Mancos River immediately upstream of Weber Draw of 27.3 cfslmi2, while
<br />the gage analysis shows a unit discharge of 10,9 cfslmi2, Based on the comparisons at both
<br />McElmo Creek and the Mancos River, it appears that the regional regression equations may be
<br />overestimating peak discharges in general, However, because the equations are the best available
<br />information, it is recommended that the values calculated by the regional regression equations be
<br />used until more advanced analyses are performed.
<br />
<br />As previously stated, because McPhee Reservoir does not provide flood control storage and
<br />because the spillway capacity of 33,000 cfs is greater than the estimated loo-year inflow of
<br />12,000 cfs, it has been assumed that McPhee Reservoir does not affect loo-year flood flows. In
<br />actuality, there will be some flood flow attenuation due to surcharge capacity in the reservoir and
<br />travel times from the reservoir inflow points to the spillway,
<br />
<br />3
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