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. 764 HYDRAULIC ENGINEERING '94 RauIIa The spatial pallem of flow shows a strong response to the controlling influeoce of the bedrock constriction in the bend near the upstream end of the reach, Supcrclevation is observed in the bend, with maximum cross-stream differences in water-surface elevation up to I m (fig,2), Maximum velocities in excess of 5 mls are attained in and just downstream of the constriction, The main thread of flow apears to shift from the inner toward the outer bank as flow moves through the bend, but predicted velocities through the constriction are oniformly high over much of the width of the flow freld. Row vectors in the constriction and further downstream (not shown here) are gencrally oriented parallel to the valley ontlines and do not appear to be guided by the underlying topography; presumably the sensitivity of flow direction to topographic steering is greater al lower discharges and shallower depths. The spatial distribution of resultant velocity shows some differences in detail when prc- and post-flood conditions are compared, but no strong trend is observed, Several cross.sections and one longitudinal profile were constructed to illustrate the comparison in water-surface elevations for pre- and post-flood conditions in the reach upstream of the debris fan, For the four sections shown in fig,2, the post-flood buundary condition yields water-surface elevations 20 10 60 cm lower than the pre-flood condition, This difference disappears along the downstream part of the study reach where flood-induced topographic changes are less significant The differences illustrated in the cross sections suggest that the flow field al a discharge of 1415 ems may be relatively insensitive to observed changes in the underlying topography, We expect a stronger response once the spatial pattern of roughness is incorporated in our modeling, as the debris fan and islands removed by the 1985 flood were covered by trees and would have offered more resistance to the flow than an unvegetated surface. The longitudinal profile indicates that the water-surface profile steepens and flow accelerates over the upstream end of the fan. The plot of predicted shear- stress indicates a local peak 30-40% higher in the presence of the fan than in the scoured channel after removal of the fan, However, shear stress and velocity decline rapidly downstream from this point in the pre-flood simulation, This suggests that erosion is focused most sharply at steps and obstructions and that the stress peak should move progressively downstream as erosion proceeds in that direction, Continuing work at this site will consider the sensitivity of the flow pattern to spatial inhomogeities in boundary roughness and local flow obstructions, The analysis will be expanded to consider a range of steady-state discharges as well .s transient solutions to the flow model. ~ . . 765 BOUNDARY CONDITIONS & FLOW PATTERNS Figure I Conloor maps of sIudy silo showing topography before (left) and after (right) 1985 flood, Row din:c1ed from bottom to upper right. ROOman Run is not shown bullhe lIebris fan at its mouth is immediately downstream of channel bend almg the left valley walt Reach shown is about 1400 m long; constriction has a minimum width of about tOO m. R..rermftlS Cited Bos!l COI'l*ation and Brigham Young University, 1993. FastrABS User's Manual. Miller. AJ..in press. 'Debris-Fan Constrictions and Flood 'Hydraulics in River Canyons: Some Implications from 2-d Flow ModeUng: Earth SurfdCe Processes and Landforms. Miller. AJ. and l'arkinson,. DJ.. "Flood Hydrology and Geomorphic Effects on River Channels and Rood Plains: the Flood or November 4-5, 19~. in the South Branch Potomac River Basin of West Virginia.' U.s, Geological Survey Bulleti./98/-F., 96 pp, Thomas. W.A. and McAnally. W.K., Jr. 1990. 'User', Manual/or the Gentralized Computer Program System: Open-Channel Flow and Sedimentadon, TABS-2', U.s. Anny Engineer Waterways Experiment Station. Vicksburg. Mississippi. ~ ..