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<br />
<br />@
<br />
<br />LEGEND
<br />
<br />@ u.s. Highway
<br />
<br />e Slate Highway
<br />
<br />I DEPARTMENT OF THE ARMY
<br />SACRAMENTO DISTRICT. CORPS OF ENGINEERS
<br />SACRAMENTO, CALtFDRNIA
<br />
<br />A
<br />
<br />Index Point
<br />(see T.'l)
<br />
<br />FLOOD HAZARD INFORMATiON
<br />NORTH FORK GUNNISON RIVER
<br />HOTCHKISS TO.SOMERSET
<br />
<br />COLORAOO
<br />
<br />DRAINAGE BASIN
<br />
<br />3
<br />
<br />Paonia Dam and Reservoir, a multiple-
<br />purpose project built by the u.s. Water and
<br />Power Resources Service on Muddy Creek just
<br />upstream from the junction of Muddy and
<br />Anthracite Creeks, became operational in 1961.
<br />
<br />FUTURE FLOODS
<br />
<br />Although floods with magnitudes and fre-
<br />quencies similar to those of past floods could
<br />recur, discussion of future floods in this report
<br />is limited primarily to one designated as the 100-
<br />year flood. A 100-year flood has a peak flow
<br />magnitude with a 1 percent chance of being
<br />equalled or exceeded in any given year, or a
<br />frequency of occurrence of about once in 100
<br />years on the long-term average. It must be
<br />understood that the term "100-year flood"
<br />relates to a flood magnitude and does not mean
<br />that the flood will occur one time in a 100-year
<br />period. Essentially, "probability of occurrence"
<br />is implied. Thus, during the term of an average
<br />mortgage (30 years), the chance of a 100-year
<br />flood occurring is approximately 1 in 4; during
<br />an average lifetime (70 years), the chance is
<br />approximately 1 in 2. In a 100-year period, the
<br />chance of a 100-year flood is approximately 3 in
<br />5.
<br />
<br />To determine flows of the 100-year and other
<br />frequency floods for this report, a flow.
<br />frequency analysis was made for five index
<br />points on the North Fork Gunnison River. The
<br />annual peak flows for rain and snowmelt were
<br />analyzed independently and frequency ~urves
<br />were developed for each. All-event peak flow-
<br />frequency curves were developed by combin-
<br />ing the rainfall and snowmelt flow-frequency
<br />curves for each index point. The upstream
<br />index point was the u.S. Geological Survey
<br />gaging station near Somerset. Those down-
<br />stream along the study reach were at points that
<br />reflect significant inflow. Criteria outlined in
<br />the u.s. Water Resources Council Bulletin 17a
<br />were used in the analysis.
<br />
<br />in general, the flow-frequency analysis
<br />involved developing peak flow frequency
<br />curves reflecting natural (unregulated) flow
<br />prior to construction of Paonia Reservoir, and
<br />adjusting these curves to account for the
<br />regulatory effect of the reservoir. Specifically,
<br />
<br />Affording 21,000 acre-feet of storage capacity,
<br />the facility is operated by the North Fork Water
<br />Conservancy District to provide flood control
<br />during periods of high snowmelt runoff.
<br />
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<br />the unregulated peak flow frequency curve for
<br />the Somerset index point was established from
<br />analysis of unregulated flow at the gage:
<br />
<br />1. for the period of record, 1934-1960,
<br />and
<br />
<br />2. for the years 1897-1933, as estimated
<br />from correlations with the North Fork
<br />Gunnison River near Paonia gage (1922-
<br />1932), the Gunnison River below Gunni-
<br />son Tunnel gage (1906-1960), and the
<br />Gunnison River near Grand Junction gage
<br />(1897-1960).
<br />
<br />Unregulated peak flow frequency curves for the
<br />four index points downstream from Somerset
<br />were developed from generalized frequency
<br />relationships estimated from a regional fre-
<br />quency analysis of gaging stations in the
<br />Gunnison River Basin. Records used in this
<br />analysis covered at least 10 years of unregulated
<br />flow.
<br />
<br />A regulated peak flow frequency curve for
<br />present conditions at the Somerset index point
<br />was developed from analysis of flows recorded
<br />after construction of Paonia Dam, and from
<br />reservoir routing studies previously made by
<br />the Corps of Engineers and the Water and.
<br />Power Resources Service. Regulated peak fiow
<br />curves for the downstream index points were
<br />developed by:
<br />
<br />1. computing the difference between
<br />the curves for natural (unregulated) and
<br />present (regulated) conditions at
<br />Somerset, and
<br />
<br />2. subtracting that difference from the
<br />curves developed for natural conditions at
<br />the four downstream index points.
<br />
<br />Peak flows (for the 10-, 50-, 100-, and SOO-year
<br />floods, existing stream regimen) resulting from
<br />the foregoing hydrologic analysis are shown on
<br />the opposite page.
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