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Measured and Predicted Velocity and Longitudinal Dispersion at Steady and Unsteady Flow, Colorado River, Glen Canyon Dam to Lake Mead In the present study, dye-sampling sites were selected to define subreaches of major differences in geology that control channel slope, width, and depth (Figure 1), The 11 subreaches defined by Schmidt and Graf (1990, Table 2, p, 55) were the basis for site selection, but some of the shorter subreaches with small differences in geometry were combined into subreaches that were long enough to make sampling possible with the constraints of available personnel and boat travel time on the river, Geometry of the channel in measured subreaches was defined from sonic depth-sounder records of a longitudinal profile and 199 cross sections measured in 1984 (Wilson, 1986), Widths, depths, and areas determined for mea- sured subreaches from the cross, sections (Table 1) were computed by averaging values for cross sections in the specified subreach, In the study by Wilson (1986), cross sections were measured at locations where it was feasible to maneuver a motorized raft across the channel; therefore, cross sections were not surveyed at sections where rapids occurred, Rapids, however, account for only about 10 percent of the river length, Data on velocity and dispersion during flows at and above the range of powerplant releases that are potential management options are required, For the Glen Canyon reach, two measurements made in May 1991 and one in October 1989 provide information on velocity and dispersion at steady flow over much of the powerplant operating range of about 30 to 850 m3/s, Velocity was measured at the peak discharge of the unsteady flow (651 m3/s on May 8, 1991) because the dye was expected to travel through the reach dur- ing the passage of on ly a small part of the daily hydrograph, Measurements also were made in the reach during the steady flow of 425 m3/s in May 1991 and in October 1989 at 139 m3/s, The 1991 measure- ments on the Grand Canyon reach were scheduled to evaluate the effect of unsteady flow on velocity and dispersion for a mid-range release from the power- plant. Discharge at Lees Ferry during the steady-flow measurement, May 20-25, 1991, was 425 m3/s; during the unsteady-flow measurement, May 6-11, 1991, dis- charge ranged from 92 to 754 m3/s with a daily mean discharge of 428 m3/s (Figure 2), The study plan includes two measurements that have not yet been made - the Grand Canyon reach at a steady discharge of about 140 m3/s and both reaches at about 1300 m3/s, The highest discharge is about equal to the sum of the maximum releases possible through the power- plant and releases through the river outlet works, Data from the velocity and dispersion measure- ments were used to calibrate a one-dimensional flow model, DAFLOW, and a solute-transport model, BLTM, developed by Jobson (1989) and Jobson and Schoellhamer (1987), respectively, The calibrated models were then used to predict velocity and disper- sion for two of the dam-release patterns under evalua- tion as a part of the Glen Canyon Dam EIS, MEASURED VELOCITY AND DISPERSION Methods Tracer Dasage and Injectian. The measurement in October 1989 consisted of one injection at Glen Canyon Dam and sampling at two gaging stations downstream (Figure 1), Each 1991 measurement con- sisted of two injections - an injection of dye from a point just downstream from the dam and sampling at TABLE 1. Characteristics orSubreaches Defined by Dye-Sampling Sites. [Determined rrom surveyed era!! eections at about 1.6-kilometer intervale at a discharge or about 680 cubic meten per second. Subreach 1- Glen Canyon Dam to Lees Ferry; 2 - Lees FelT)" to Nautiloid Canyon; 3 - Nautiloid to gage above the Little Colorado River; 4 - Little Colorado gage to Nevill'. Rapid; 5 - Nevill'. Rapid to Mile 118 Camp; 6 - Mile 118 Camp to National Canyon; 7 - National Canyon to Pumpkin Springs; 8 - Pumpkin Springs to Gneiss Canyon. Subreach 1 is the Glen Canyon reach and subreaches 2..s make up the Grand Canyon reach.] Ratio of Area Length Bed Width Depth Width to (oquare Subreach (kilometers) Slope (meters) (meters) Depth meters) 1 24.5 0,00038 99,1 2 57,7 ,00141 71.6 8.2 8,7 573 3 40,6 ,00126 106,1 6,1 17,4 642 4 24,9 ,00274 119,2 5,2 22,9 613 5 66,1 ,00195 59,1 8,8 6,7 517 6 78,6 .00151 63.4 7,6 8,3 468 7 75,7 .00134 94.2 6,7 14,1 609 8 36,9 ,00161 71.6 9.1 7,9 661 269 WATER RESOURCES BULLETIN .' ~ . .,,;,. ' ~ ol<'~' :,' ;'1 '"I ~ ~ .' .;~tS1 :i;....:... :~:: . .-;,'; ,. '~". "'. ~:i~~ .~::,...._; , , " ~'. .";.' '., ,. ;'-.,: .~. '~'. ~l