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A r 1-} mrcuJs t- J\je/son ,q?q 'brezN L It/:~:I ) r- I !J.y.U I , . r Published in 1989 by the American Geophysical Union. Topographic Response of a Bar in the Green River, Utah to Variation in Discharge E. D. Andrews and J. M. Nelson L4kewood, Colorado Abstract A fully nonlinear numerical model was used to analyze the characteristics of flow Gand sediment transport in a reach of the Green River near Ouray, Utah. The study reach has gentle, but complicated. curvature and contains a prominent mid-channel bar compOsed of medium to fine sand. Bankfull channel width and area vary significantly through the study reach. Results of the model calculations were compared with the observed lateral distribution of unit discharge and mean annual bed-material transport and were found to be in excellent agreement. ~ Adjustment of channel topography over a period of time at a constant discharge was calculated at three discharges, 50 m3/s (approximately 40 percent of the mean annual flow), 275 m3/s, and 475 m3/s (approximately the bankfull discharge). Each simulation be1!:an with the same topography, which was surveyed at a discharge of ...275 m:J/s on July 15-16, 1986,5 weeks after a peak discharge of ...620 m3/s. Gradually decreasing flows following the flood peak and significant bed-material transport rates tended to maintain a channel topography, in equilibrium with the discharge. As expected, computed changes in bed elevation were quite small everywhere during a discharge of 275 m3/s for 2 days. Conversely, channel topography evolved rapidly over a period of 2 days at simulated discharges r of 50 m3/s and 475 m3/s. Bar topography was enhanced greatly at a discharge of L 475 m3/s. Sediment accumulates on the bar surface and is eroded from the side \" channels except in the downstream part of the secondary channel where some I, sediment is deposited. At a discharge of 50 m3/s, the bar crest stands nearly 1 m ,. above the water surface, and there was no flow through the secondary channel. ,. More than 1 m of material accumulates in the primary channel along most of the ! length of the bar, over a period of two days. I.~ A comparison of aerial photographs taken of the Ouray reach between 1963 and 1987 shows that the bar we investigated has changed very little during this period, in spite of the fact that bar topography tends to adjust relatively quickly to .changes in river discharge. Neither established vegetation nor debris appear to have a stabilizing effect upon the bar. Long-term stability of the bar appears to be due to a constriction in channel width and area located slightly downstream of the bar apex. Introduction Most of the science of river mechanics has been discovered from studying rivers carefully selected to ensure equilibrium between channel configuration and the water and sediment discharge. Existing analytical approaches and methods typically are 463 Cl/05L