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r <br />Crest---? <br />Direction af Flow <br />??' . . • ' . . • . '. <br />. ? ,y' . . . , <br />; • . tis???`?r ? ?--?_? '' ' . <br />' . . ??????'= • . . -Iiasz Itcx:k <br />t:i% c:Yhcd l?iT d':md cAIhl6) . . ' . . : • '. .• • • ' • <br />: . . : . ? . ,• . .. . . • <br />? . , - ' . • . . ' <br />_. . _ . " . . . . . .' : • , <br />Figure 3. A section view of the apex of the U-structure. <br />Figure 3 shows a section view of the apex of the arch. This Figure shows that the arch is • <br />sunk into the riverbed such that the crest of the structure, which is the highest point on the <br />upstream side, is even or below the existing grade of the riverbed. Setting the structure at or <br />below grade has two effects. First it ensures that the unconstricted river level is no higher than <br />the existing river level and secondly it ensures that there are no dynamic fluid forces on the <br />structure due to current flow. Sinking the arch in the riverbed also supports the structure <br />laterally and prevents torsion on the structure. <br />A base rock, also shown in Figure 3, is shown at the downstream end of the structure. <br />The base rock is sunk into the cobble and is keyed into the rock that forms the structure in order <br />to reinforce the structure and to protect the structure from erosion caused by eddy currents. <br />These structures are designed such that the river is usable at a variety of water levels. <br />Figure 4 shows a profile view of the U-structure looking upstream. This view highlights the <br />versatile design of these structures. The low flow channel can be designed to provide deep <br />passage and a usable hydraulic feature during typical low flows. During the mid-range flows the <br />low flow channel and part of the high flow would be submerged. During run-off the high flow <br />channel would create a larger, more usable hydraulic feature (a hydraulic jump or wave train), <br />and at extreme flood the river will overflow the shoulders of the structure. The downstream side <br />of these shoulders are stepped to provide seating during normal flows and to prevent the <br />formation of dangerous hydraulics during extreme flooding. <br />• <br />-7-