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A <br />Figure 4. A section view of the apex of the U- structure. <br />Figure 4 shows a section view of the apex of the arch. This Figure shows that the arch <br />is sunk into the riverbed such that the crest of the structure, which is the highest point on the <br />upstream side, is even with the existing riverbed at the center of the low flow channel. Setting <br />the low flow point of the structure even with the existing riverbed has two effects. First it <br />ensures that the unconstricted river level is no higher than the existing river level and secondly <br />it ensures that there are no dynamic fluid forces on the structure due to current flow. Sinking <br />the arch in the riverbed also supports the structure laterally and prevents torsion on the <br />structure. <br />A base rock, also shown in Figure 4, 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 />12 <br />