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<br />1018
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<br />the sediment transport rate at a given discharge
<br />should be increasing with time. The comparison
<br />of sediment transport rates as a function of water
<br />discharge at the Green River, Utah, gage for the
<br />pre- and post-reservoir periods, however, found
<br />no evidence of an appreciable increase in sedi-
<br />ment transport rate for a given flow after the
<br />completion of Flaming Gorge Dam in 1962.
<br />
<br />Change in Effective Discharge
<br />
<br />One of the principal downstream effects of
<br />Flaming Gorge Reservoir is to decrease the
<br />range of daily mean flows. As noted previously,
<br />the mean annual discharges of the Green River
<br />during the pre- and post-reservoir periods are
<br />virtually identical at both the Jensen and the
<br />Green River, Utah, gages. The percentage of
<br />time that various daily mean discharges are
<br />equaled or exceeded, however, is substantially
<br />different for most flows. The durations for daily
<br />mean discharge for the pre- and post-reservoir
<br />periods are compared in Figure 5 for the Green-
<br />dale gage, Figure 6 for the Jensen gage, and
<br />Figure 7 for the Green River, Utah, gage. The
<br />magnitude of 110ws that occur less than 10% of
<br />the time has been significantly decreased since
<br />regulation of the Green River by Flaming Gorge
<br />Reservoir began. For a given duration within
<br />this range, the decrease in discharge is about the
<br />same at aU three gages. Maximum daily dis-
<br />charge, for example, has decreased by 14,000
<br />ft3/s, whereas the 2% exceedence discharge has
<br />decreased by 7,000 ft3/s. Discharges equaled or
<br />exceeded <5% of the time at the Jensen and
<br />Green River, Utah, gages have decreased by
<br />-25%. Thus, although the drainage area is 2.7
<br />times greater at the Green River, Utah, gage
<br />than it is at Flaming Gorge Reservoir, the effect
<br />of the reservoir is still substantial 290 river miles
<br />downstream.
<br />Discharges equaled or exceeded < 10% of the
<br />time usually transport a majority of the annual
<br />sediment load (Wolman and Miller, 1960).
<br />Furthermore, the increment of discharge that
<br />transports the largest quantity of sediment over a
<br />period of years, called the effective discharge,
<br />typically is equaled or exceeded <5% of the
<br />time, but it determines the bankfull-channel di-
<br />mensions and pattern. Andrews (l9RO) com-
<br />puted the effective discharge for 15 gages in the
<br />Vampa River basin and showed that the effec-
<br />tive discharge was nearly identical to the bank-
<br />full discharge at all sites. The bankfull-channel
<br />dimensions thus appeared to be adjusted to the
<br />effective discharge.
<br />In order to determine the effect of flow regu-
<br />lation on the effective discharge in the Green
<br />River, the mean annual quantity of sediment
<br />transported during the pre- and post-reservoir
<br />periods by various increments of discharge was
<br />
<br />E. D. ANDREWS
<br />
<br />TABLE 1. COMPARISON OF REGRESSION EQU"TIONS RELATING SUSPENDE.I).SE.DIMENT TRANSPORT RAltS TO WATER
<br />DISCHARGE DURING THE PRE. AND POST.RESERVOIR PERIODS FOR THE GREEN RIVER NEAR JENSEN. UTAH
<br />
<br />Sediment Period Relation belweon oediment CorTdalion Lnd or
<br />siu or di.o:h.I'I"(I,)1IKl wirer lIlClliaClII ...,ifianoe
<br />lraction """rd di.o:h.I'I"(Q)
<br /><0.004 .1962 I, . 1.08QI.OO 0.31 F"
<br /> >196J I, . 0.3J6Q 1.11 .28
<br />0.004-0.016 .1962 1,' O.0821QIlO .42 F97J
<br /> >196J I, . 0.0108QI.36 .42
<br />0.0 16-0.06~ .1962 1,.2.16.10-"QI.1I .77 F99
<br /> >1963 I, . 5.0J . I 0~Q2.26 .79
<br />0.0625~.m .1962 1,' 5.04' 10.'~49 .88 F75
<br /> >1963 I, . 2.26 . 10~Q2-JO .84
<br />0.1~~.2S0 .1962 I, . 5.59 . 1O'8Q2-7J .88 F99
<br /> >196J I, . 1.67 . 10.502.09 .72
<br />0.2>>-0.500 .1962 1,.1.24' 10-7Q2-5l .82 F!IO
<br /> >1963 1,.5.04' lo-7Ql-l4 .69
<br />Sand .1962 I, . 3.38 . 10.1Q2-62 .87 F!IO
<br /> >1963 I, . 2.04 . 10-5Q2.16 .79
<br />All !ius .1962 1,' 6.16' 10-lQI.70 .62 F"
<br /> >1963 1,' 1.72' 1O'2QI.56 .62
<br />
<br />T"BLE 3. COMPARISON OF REGRESSION EQUATIONS RELATING SUSPENDE.I).SEDIMENT T1l.ANSPORT RAltS TO WATER
<br />DISCHARGE DURING TIlE PRE. AND POST.RESERVOIR PERIODS FOR THE GREEN RIVER NEAR GREEN RIVER. UTAH
<br />
<br />Sedimenl hood Rel.lion between sedimenl Comlalion L<vcl or
<br />siu or dU<hlrg< (1,) IIKl "I'" meffiaenl Jianific:ance
<br />rrulion """rd discha'l" (Q)
<br />.0.004 .1962 I, . 1.4901.05 0.35 F97.S
<br /> >196J I, . 00693~IJ4 .40
<br />0.004-0016 .1962 I, . O.13IQ .19 .40 F99.5
<br /> >196J I, . 0.271 . 10-29160 .44
<br />0.016-00625 .,962 I, . 1.99 . IO"Q .95 .74 F99
<br /> ~196J I, . 4.54 . 10.5Ql.I16 .67
<br />O.06~-O.I~ .1%1 I, . 1.02' 1O-'Q268 .86 F97.5
<br /> >1963 I, . 3.07 . 1O.9Ql04 .84
<br />0.125~.250 <1962 1,.1.24' 10-7Q26O .89 F!IO
<br /> >1963 I, . 3.14 . 10"02.47 .15
<br />0.250-0.500 .1962 I, . 7.52' 10-'0223 .87 F!IO
<br /> >196J I, . 2.04' 1O-5QI.87 .41
<br />Saod .1962 1,.1.24' 1O-'QI.71 .88 F!IO
<br /> >196J I, . 2.06 . 10-8Q2.!IO .80
<br />All !ius .1962 I, . 0.050JQI5l ." F95
<br /> ~196J I, . 0.J6J . 10-2Q1.1I .61
<br />
<br />computed for three reaches. These reaches are
<br />representative of conditions in alluvial sections
<br />of channel in each of the downstream zones-
<br />degradation, equilibrium, and aggradation-
<br />defined by the sediment-budget analysis.
<br />Selected reaches are (1) through Browns Park
<br />(degrading); (2) immediately downstream from
<br />the Jensen gage (equilibrium); and (3) in the
<br />vicinity of the Green River, Utah, gage (aggrad-
<br />ing). The duration of discharges and the relation
<br />of sediment transport rate to discharge need to
<br />be known to compute the effective discharge.
<br />This information is provided at the latter two
<br />reaches by the records of the nearby gages.
<br />There are no comparable gage records of dis-
<br />charge and sediment transport in the Browns
<br />Park reach. The duration of discharges recorded
<br />at the Greendale gage therefore were assumed to
<br />be representative, because the contribution by
<br />intervening tributaries is rarely significant. The
<br />quantity of bed-material transport by various
<br />discharges was computed, using the Engelund-
<br />Hansen relation (Engelund and Hansen, 1967),
<br />with measured values for slope, width, and bed.
<br />material size distribution.
<br />The object of the analysis is to assess the po-
<br />tential for channel changes, and so it is prefer-
<br />
<br />able to consider only those sediment sizes that
<br />are present in the channel bed and banks in
<br />quantities of more than a few percent. The
<br />transport rate of those particle sizes that are
<br />present in the channel perimeter tends to be
<br />more closely correlated with flow, because the
<br />quantity of material available does not vary
<br />greatly over a period of years. Analysis of the
<br />relations between sediment transport rate and
<br />discharge for various sediment-size fractions
<br />found that the coefficient of determination (r2)
<br />increased significantly for particles larger than
<br />0.016 mm in diameter (see Tables 2 and 3).
<br />Consequently, particles with a fall diameter of
<br />0.016 mm were assumed to be the smallest par-
<br />ticles present to an appreciable degree in the
<br />channel bed and banks in the vicinity of the
<br />Jensen and Green River, Utah, gages.
<br />Sediment-load duration relations for each size
<br />fraction ;0,0.016 mm were computed from the
<br />transport rate versus discharge (Figs. 3 and 4)
<br />and the Dow duration relations. The ranges of
<br />discharge were divided into -30 equal incre-
<br />ments. The sediment-discharge duration rela-
<br />tions then were integrated between the limits of
<br />each increment and the result multiplied by 365
<br />days. The mean annual quantity of sediment
<br />
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