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3 e IMPROVED INLETS FOR HIGHWAY CULVERTS \ A recent field survey (1) conducted by the Federal Highway Administration indicated that considerable savings could be realized in highway culvert construction by improving the entrance flow conditions. The use of improved inlets on conventional culverts has resulted in reported savings of over $2,000,000 on 66 installations across the United States. If a full accounting of all improved inlets had been possible, the savings would likely have been many times greater. A single installation in Tennessee resulted in a cost reduction of $482,000, or 39 percent of the culvert cost. . Conventional Culverts A conventional highway culvert is generally defined as one with its invert and soffit (roof) on a constant slope, and with the same cross sectional area throughout the culvert length. No significant effort is directed at improving the flow conditions at the entranc es of these culverts. . AJ3 noted in Hydraulic Engineering Circular (HEC) Number 5 (2), culverts may operate in either inlet control or outlet control. When a culvert operates in inlet control, the entrance restricts the flow, and the barrel could permit the passage of more water. In outlet control, the culvert barrel is acting as the flow restraint, and the inlet is capable of more efficient hydraulic operation. As shown in Table 1, the factors influencing outlet control culvert capacity include headwater depth, tailwater depth, entrance configuration, and barrel characteristics. In inlet control, only conditions at, and upstream of, the culvert face have an effect on the culvert's hydraulic capacity. These conditions include headwater depth and entrance configuration. . TABLE 1. Factors Influencing Culvert Hydraulic Capacity ~~ Outlet Control Inlet Control Headwater depth Tailwater depth Headwater depth Edge condition Cross sectional area Shape Edge condition Cross sectional area Shape . Total culvert fall Barrel length Barrel roughness