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<br />00;12'14
<br />
<br />For streams that cross regional boundaries,
<br />peak-discharge estimates for a given recurrence
<br />interval can be quite different depending on the
<br />regional equation used, The following equation for
<br />weighting estimates from two regional equations can
<br />be used:
<br />
<br />)
<br />
<br />QT(W) = QT(a) Area(a)
<br />+ QT(bl Area(b/ Area(t)
<br />
<br />)
<br />
<br />)
<br />,
<br />,
<br />,
<br />,
<br />,
<br />,
<br />,
<br />,
<br />,
<br />,
<br />,
<br />,
<br />,
<br />,
<br />,
<br />j
<br />j
<br />j
<br />,
<br />,
<br />,
<br />,
<br />,
<br />,
<br />,
<br />,
<br />,
<br />,
<br />
<br />where
<br />QTIW)
<br />
<br />is the weighted discharge, in cubic feet per
<br />second, for T-year recurrence interval;
<br />is the discharge for region (a), in cubic feet
<br />per second. for T-year recurrence interval;
<br />Areala) is the drainage area in region (a), in square
<br />miles;
<br />is the discharge for region (b), in cubic feet
<br />per second, for T-year recurrence interval;
<br />Area(b) is the drainage area in region (b), in square
<br />miles; and
<br />Arealr) is the total drainage area in both regions, in
<br />square miles.
<br />An example calculation to determine the
<br />50-year peak discharge for an ungaged site with
<br />a drainage area in the northwest regIOn and m the.
<br />southwest region follows. The total dramage area IS
<br />composed of 280 mi2 in the northwest region and .
<br />55 mi in the southwest region. Mean annual preCIpIta-
<br />tion is given as 25 inches for the site.
<br />Calculate the 50-year peak discharge using the
<br />appropriate equation for each region from tahle 1 and
<br />the drainage area in each regIOn:
<br />. Southwest region drainage area
<br />
<br />QT(a)
<br />
<br />QT(bJ
<br />
<br />0.709
<br />QlO = 102.0 (A)
<br />
<br />where
<br />A
<br />
<br />. .,
<br />is the drainage area, In nu-
<br />
<br />QlO = 102.0 (55)0709 = I. 750 ft3/s.
<br />
<br />Northwest region drainage area
<br />
<br />Q;n = 3S.S(A)0667 (p)OcIO
<br />
<br />.,
<br />,
<br />
<br />.,
<br />
<br />.
<br />I
<br />
<br />where
<br />A is the drainage area, in square miles; alld
<br />p is the mean annual precipitation. ill illches.
<br />
<br />Q50 = 38.8(280)066\25)0210 = 3.270 r["1/s.
<br />
<br />. Total drainage area using equation (4)
<br />
<br />(4)
<br />
<br />QT(W) = QT(a) Area(a) + QT(I,) Area(l,/ Are"rt)
<br />
<br />Q50(W) = 1,750(55) + 3, 270(280)/(2KO + 55)
<br />= 3,020 ft3/s.
<br />
<br />When a site is on a stream that crosses a State
<br />boundary, peak discharge can be calculated by aVer-
<br />aging estimates from relations for both States. For
<br />example, to determine the lO-year recurrence-interval
<br />peak discharge at a site near the Colorado- Wyoming
<br />State line, the 10-year peak discharge needs to be
<br />calculated as the average of the estimates obtainetl
<br />using both the equation for Colorado and the equation
<br />for Wyoming (Low ham. \988). Regional regression
<br />equations for Utah, Arizona, and parts of Wyoming
<br />and New Mexico are presented by Thomas and
<br />others (\ 994). Regression equations are presentetl
<br />for Kansas by Clement (\ 987) and for Oklahoma by
<br />Tortorelli and Bergman (\ 985). Regional equations are
<br />presented for Texas by Asquith and Slade (1997) and
<br />for Nebraska by Soenksen and others (1999).
<br />
<br />:/
<br />I
<br />
<br />SUMMARY
<br />
<br />Various Federal, State, and local governments in
<br />Colorado use hydrologic data collected and published
<br />by the USGS in making decisions about the cost-
<br />effective planning and design of highway bridges and
<br />culverts, flood-plain management. reservoir manage-
<br />ment, and other water issues. Part of that data is
<br />used to develop regression equations for determining
<br />the magnitude and frequency of floods in Colorado.
<br />Regression equations and the methods for determining
<br />the magnitude and frequency of floods on unregulatetl
<br />streams were developed for recurrence intervals
<br />ranging from 2 to 500 years. The methods for deter-
<br />mining peak discharges depended on whether the site
<br />was gaged, was on a stream near a gaged site. Or was
<br />
<br />I
<br />I
<br />
<br />SUMMARY 11
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