Live-bed failure modes of bendway weirs and rock vanes in alluvial channels
Date
2022
Authors
Wittmershaus, Alex, author
Ettema, Robert, advisor
Thornton, Christopher, advisor
Kampf, Stephanie, committee member
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Abstract
Bendway weirs and rock vanes are instream rock structures primarily used for managing the alignment of a channel's thalweg. Built from rock, bendway weirs and rock vanes are intended to function by directing flow away from a channel's outer bank and thereby reducing flow velocity along the outer bank. The present study investigated how bendway weirs and rock vanes placed in curved, alluvial channels subject to live-bed flow conditions (active bed-sediment transport) may fail. Further, the experiments then sought to recommend design dimensions so that bendway weirs and rock vanes accommodate failure (and loss of rock), thereby enabling them to continue performing as intended. A curved flume was constructed in Colorado State University's Hydraulics Laboratory to conduct experiments that illuminated the failure modes and to confirm (or modify) preliminary design recommendations obtained from experiments using a straight flume fitted with three bendway weirs or rock vanes. The curved flume experiments involved a series of six bendway weirs or rock vanes and used a hydrograph procedure to simulate the rising limb of a hydrograph of flow along a medium sized river like the Middle Rio Grande; the proportions of the flume were like selected bends in that river. Six bendway weirs or rock vanes were needed to direct flow around the curved flume, as opposed to the need for three bendway weirs or rock vanes in the experiments in the straight flume. Two sizes of non-uniform bed sediment also were used (a medium sand and very coarse sand) for the experiments. The two sands were used to see if bed sediment size affected the failure modes. The experimental results showed that bendway weirs and rock vanes experienced rock dislodgement primarily via contraction scour, which undermines the end, or tip, of these instream structures. Destabilized rock then tumbles into the scour zone along the channel's shifted thalweg, armoring the bed. This observation was observed for both the beds comprised of medium sand and very coarse sand. As flow depth increased above the mean elevation of the bendway weirs or rock vanes, contraction of flow reduced as more flow passed over the structures. The flow field at each bendway weir or rock vane changed. The hydrograph procedure yielded similar changes in bed bathymetry for beds of medium sand and very coarse sand over the rising limb of the hydrograph. When (Δy+H)/H = 0.75, a deep scour hole formed in between the first two structures in the configuration within about 15 minutes. Then, when (Δy+H)/H = 1.25, the scour hole was partially filled with sediment and extended downstream largely along the series of bendway weirs or rock vanes. Further, when (Δy+H)/H = 2.0, the scour hole was again partially filled with sediment, but scour extended along the entire configuration of bendway weirs or rock vanes, thereby delineating a defined thalweg. As the flow depth increased, the maximum scour depth along the thalweg decreased for the experiments. The bendway weirs and rock vanes experienced structural deformation due to rock dislodgement primarily from contraction scour. Less rock dislodgement occurred for these instream structures placed on the medium sand than when on the very coarse sand. Also, the rock vanes experienced less rock dislodgement than did the bendway weirs in general. This finding is attributed to upwards slope of the crest of rock vanes; the sloped crest directed more flow around each rock vane and over the already armored bed. The results from using the hydrograph procedure in a curved flume confirmed the preliminary design recommendations from the straight flume. The design recommendations required that bendway weirs or rock vanes be lengthened by 2d100 and their crests be widened by d100; here d100 is the diameter of the largest rock used to build bendway weirs or rock vanes. This lengthening and widening accounts for the shortening and narrowing of bendway weirs or rock vanes subject to scour. A prior study recommended the size of rock chosen in design to form bendway weirs or rock vanes.
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Subject
bendway weirs
scour
rock vanes
alluvial channels