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to external loading on the top face are compressive upon the rock and grout material. Both of <br />these materials are very strong in compression. <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 />-10- <br />