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4 e In outlet control, the culvert barrel is usually flowing nearly full at the design discharge, and inlet improvements would only reduce the entrance losses, which are a small portion of the total headwater requirements. Therefore, only minor modifications of the inlet geometry which result in little additional cost are justified on culverts operating in outlet control. I The capacity of culverts operating in inlet control is greatly influenced by the culvert entrance conditions. These culverts generally lie on steep slopes and always flow partly full. Entrance improvements can result in nearly full flow at the design discharge, thereby increasing culvert capacity and efficiency significantly. . The effect of edge condition can be illustrated by some common types of culvert entrances. The following entrances are in order of increasing hydraulic efficiency. First, the thin-edged projecting inlet, such as a corrugated metal pipe (c.m.p.) extending out from the fill slope is the least efficient inlet type (Figure 1). This inlet causes an extreme flow contraction at the inlet; that is, the flow separates from the barrel walls at the inlet and much of the available barrel area is wasted (Figure 2). More headwater is required to pass a given flow rate using this type of inlet. e The mitered inlet, such as a c.m.p. cut off flush with the embankment slope represents a slight hydraulic improvement, but not as much as might be expected (Figures 3 and 4). A thiCk-edged projecting inlet, which could be represented by a square-cut end on a concrete pipe, is about as efficient as a c.m.p. in a headwall with square edges (Figure 5). Wingwalls have no significant effect on the inlet capacity of a circular pipe in a headwall, but they do increase the efficiency of box culverts (Figure 6). By simply leaving the socket end on a concrete pipe, its inlet efficiency can be greatly increased in either a projecting condition (Figure 7) or in a headwall (Figure 8). The grooved end increases the effective entrance area of the culvert presented to the approaching flow. The grooved end (projecting or in headwall) and the box culvert with wingwalls at 300 to 750 angles with the barrel represent the most efficient conventional culvert inlet types. Notice that to this point, no construction modifications have been directed at improving the inlet edge conditions of any of the preceding culverts. ~ e