Effects of bendway weir size on flow and bank stability in erodible channel bends

Randy S Cunningham, Purdue University

Abstract

Laboratory experiments were conducted to study the effects of bendway weirs on bank stability and the mean velocity field in a 90° live-bed channel bend. Previous (physical rather than numerical) research has largely been restricted either to fixed-bed experiments or to specific field applications. In this study, nine different experimental cases with and without weirs, but limited to the same flow depth, bend radius of curvature to top width ratio (3.33), and uniform bed sediment (median diameter, 0.024 in.), were examined. Three different weir configurations, differing primarily in crest height (50%, 80%, and 100% of the flow depth), were considered; the other characteristics of the bendway weir field were based on FHWA HEC-23 guidelines. Most experiments were conducted with the same discharge for various durations up to 48 hours, but the effect of a higher discharge was studied in limited experiments. At selected cross-sections, bed elevations were measured with an acoustic sensor, and mean point velocities were obtained with a side-looking Acoustic Doppler Velocimeter. Comparisons of bed elevations at the end of an experiment with those at the beginning or with those of the baseline experiment show that the upper regions of the outer bank remain susceptible to bank erosion. Even for the longer duration experiments, equilibrium in the sense of stable banks was not necessarily achieved. In contrast to the mean velocity field in the absence of weirs, where the largest velocities in the downstream half of the channel were typically found near the free surface region of the outer bank, the velocity field with the weirs present exhibited extensive low-velocity regions near the outer bank and peak velocities in the main channel near the weir toes.

Degree

M.S.C.E.

Advisors

Lyn, Purdue University.

Subject Area

Geomorphology|Civil engineering

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