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<br />234 <br /> <br />C. H. THEILING <br /> <br /> <br />Navigation Channel <br /> <br />I!!!I Land <br /> <br />o Sand Bar <br /> <br />o Water <br /> <br />. Rock <br /> <br />Figure 7. Schematic illustration of bendway weirs. Stone weirs are> 3 m below the low water reference line <br /> <br />(Chen and Simons, 1986). They have an adverse effect of isolating non-channel aquatic habitats at low river <br />stages and increasing side-channel sedimentation by slowing current velocity and altering side-channel <br />hydraulics at high river stages. . <br />Chevron dikes (Figure 6) represent an alternative to side-channel closures while achieving the same <br />maintenance objective. 'V'-shaped rock structures (chevron dikes) are placed in a downstream staggered <br />arrangement (Figure 6) that gives the hydraulic appearance of a solid object (Strauser, US Army Corps of <br />Engineers, pers. comm.). In addition to diverting flow, the structures are also designed to allow for dredge <br />disposal between the arms of the 'V' (Figure 6). They are designed to allow hydraulic sculpturing of the <br />dredge spoil into topographically diverse aquatic habitat when they are overtopped during high river stages. <br />These are relatively new structures (2 years old); physical and environmental monitoring is underway. Initial <br />observations reveal that dredge spoil placement may be an important determinant in their success at <br />providing slackwater habitat. The side-channel has remained open between the dikes. <br /> <br />Bendway weirs <br />River bends are notorious problem spots for engineers charged with maintaining the navigation channel. <br />Lateral erosion on the outer bend is controlled with revetments, but downcutting (river bed erosion) can <br />create a very deep channel (l 0- 30 m). Dredging on the inner bend is necessary to control point bar expansion <br />into the navigation channel. Extreme main channel water depths are thought to be of little fisheries value <br />(Baker et aI., 1991), but the sand bar created on the inner bend may benefit the endangered least tern (Sterna <br />antillarum) and other sand bar dependent fauna. <br />Bendway weirs were designed to reduce dredging requirements in river bends by controlling point bar <br />development (Davinroy, 1990). They consist of a series of submerged dikes (> 3 m below the low water <br />reference plane) constructed around the outer edge of a river bend (Figure 7). Each dike is angled 300 <br />