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<br />e <br /> <br />.. <br /> <br />. <br /> <br />e <br /> <br />,. <br /> <br />-;: <br /> <br />e <br /> <br />II. Cu1vertHydraulics <br /> <br />Conventional Culverts <br /> <br />A culvert operates in either inlet or outlet control. Under <br />outlet control, headwater depth, tai1water depth, entrance configura- <br />tion, and barrel characteristics all influence a culvert's capacity. <br />The'entrance configuration is defined by the barrel cross sectional <br />area, shape, and edge condition, while the barrel characteristics <br />are area, shape, slope, length, and roughness. As shawn in Figure 1, <br />the flow condition for outlet control may be full or partly full <br />for all or part of the culvert length. The design discharge usually <br />results in full flow. Inlet improvements in these culverts reduce <br />the entrance losses, which are only a small portion of the total <br />headwater requirements. Therefore, only minor modifications of the <br />inlet geometry which result in little additional cost are justified. <br /> <br />In inlet control, only entrance configuration and headwater <br />depth determine the culvert's hydraulic capacity. Barrel character- <br />istics and tai1water depth are of no consequence. These culverts <br />usually lie on relatively steep slopes and flow only partly full, <br />as shown- in Figure 2. Entrance improvements can result in full, <br />or ~ear1y full flow, thereby increasing culvert capacity significantly. <br /> <br />Figure 3 illustrates the performance of a 30-inch circular <br />conduit in inlet control with three commonly used entrances: thin- <br />edged projecting, square-edged, and groove-edged. It is clear that <br />inlet type and headwater depth determine the capacities of these <br />culverts. For a given headwater, a groove-edged inlet has a greater <br />capacity than a square-edged inlet, which in turn outperforms a <br />thin-edged projecting inlet. The performance of each inlet type <br />is related to the degree of flow contraction. A high degree of <br />contraction requires more energy, or headwater, to convey a given <br />discharge than a low degree of contraction. Figure 4 shows <br />schematically the flow contractions of the three inlet types <br />noted in Figure 3. <br /> <br />Improved Inlets <br /> <br />The improvements presented in this Circular are inlet geometry <br />refinements beyond those normally used in conventional culvert <br />design practice, such as those discussed above. Several degrees <br />of improvements are presented, including bevel-edged, side-tapered, <br />and slope-tapered inlets. <br /> <br />13-5 <br />