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Structures that modify velocity distribution such as deflectors, bank barbs,spur dikes and other <br /> similar designs often accompany"bank hardening." The US Army Corps of Engineers recently <br /> developed bendway rock weirs(Derrick, et al, 1994). This structure points slightly upstream at a <br /> departure angle of 60-70 degrees from a tangent line from the bank. The intent of the bendway <br /> weir was initially to"scalp"point bars and re-locate the thalweg to the inside of the bend for <br /> navigation. In addition,this structure subsequently induced sediment deposition in the near-bank <br /> region. There is generally re-circulation, back eddy erosion on the upstream side of the structure <br /> due to the abrupt nature of re-directing the angle of attack of the near-bank velocity vectors. <br /> This structure has been installed on large rivers and has been effective in meeting its design <br /> objectives in many instances(Derrick, 1996). <br /> Spur dikes and bank barb structures are common bank protection structures but generally <br /> produce an upstream and downstream re-circulation eddy that often increases bank erosion. This <br /> tends to occur when the thalweg is forced too far across the channel and/or the structures are <br /> oriented 45 to 90 degrees upstream from a tangent line to the bank.Bank barbs create a vertical <br /> vortices due to their abrupt angle to the bank that is often responsible for bank erosion and <br /> accelerated scour at the"point"of the barb. Rock and/or log deflector structures, pointing <br /> downstream, often direct the velocity vectors into the bank when flows overtop the structure <br /> increasing near-bank velocity gradients and causing accelerated bank erosion. Subsequently, <br /> some of these structures have created unexpected adverse adjustments in the channel. <br /> Vortex Rock Weirs and Root Wads were installed in the 1980s for grade control, fish habitat and <br /> streambank erosion protection(Rosgen, 1996). After monitoring for approximately 15 years,the <br /> author determined that these structures produced back-eddy erosion during major floods <br /> resulting in streambank erosion and the loss of some structures. The problems of the Root Wad <br /> and Vortex Rock Weir structures were documented which subsequently led to major changes in <br /> their design. <br /> As additional objectives of river engineering have evolved there has arisen a need for a <br /> "softer" substitute for streambank stabilization. The departure from traditional"hard" <br /> procedures has been slow but steady as the use of natural materials and methods have grown in <br /> popularity. This has, in turn,encouraged the pursuit of additional techniques to offset existing <br /> problems of various structures observed in the field. A properly designed river structure should <br /> meet more than one specific objective (such as grade control). <br /> Structures should also: <br /> 1. Maintain the stable width/depth ratio of the channel; <br /> 2. Maintain the shear stress to move the largest size particle to maintain stability <br /> (competence); <br /> 3. Decrease near-bank velocity, shear stress or stream power; <br /> 4. Maintain channel capacity; <br /> 5. Ensure stability of structure during major floods; <br /> 6. Maintain fish passage at all flows; <br /> 7. Provide safe passage or enhance recreational boating; <br /> 8. Improve fish habitat; <br /> 9. Be visually compatible with natural channels; <br /> 10. Be less costly than traditional structures; <br /> 4 <br />