Bendway weirs are being considered by INDOT as a potential alternative countermeasure for bank erosion at channel bends that might be more environmentally sensitive than the traditional riprap. These are linear structures extending riverwards from the bank to be protected, but unlike the more familiar spur structure, they are intended to be overtopped by the design flow. The flow over the weir crest is supposed to be directed perpendicular to the plane of the weir, and so by appropriate placement of the weir(s), the flow can be directed away from the bank, thus protecting it.

Design guidelines for such structures are available in HEC-23 (Lagasse et al., 2009), but these have not received much detailed scrutiny regarding their performance. Also, the HEC-23 design is independent of approach velocity. A laboratory study was con-ducted to examine the effectiveness of bendway weirs based on the HEC-23 guidelines in protecting the outer bank of 90° bend, characterized by a single ratio of radius of curvature to top width of 3.3. The laboratory model had both erodible bed and banks. Experiments were conducted with and without weirs, with three different weir crest heights (including one that was essentially not overtopped), and two approach velocities. Measurements of erodible boundary elevations as well as point velocities were made. Effectiveness was assessed by comparison with the corresponding no-weir case, and with the initial ‘artificial’ channel geometry.

Compared to the initial geometry, the HEC-23-based weir protected the toe of the outer bank, but, under design conditions, still allowed significant erosion in the upper part of the outer bank. This remained the case even when the weir crest height was increased above the level recommended in HEC-23, and only the case where the weir crest was above the water surface was there any significant improvement in protection of the upper outer bank. Higher approach velocities were found associated with an increased rate of erosion. The point velocity measurements did not give strong evidence that the overtopping flow had substantial erosion potential. They did suggest that erosion could occur even where the local velocities were markedly below the critical velocity associated with equilibrium straight-channel flows, even where slope effects were included. Mass failure or slumping rather than direct shear erosion seems a more plausible mechanism for much of the observed bank retreat.

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Bendway weirs, bank erosion countermeasure, riprap, spur structure, SPR-3011

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Performing Organization

Joint Transportation Research Program

Publisher Place

West Lafayette, Indiana

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