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e ; , ,e e extended as a straight line will cut the plane of the outlet cross sec- tion at a point above critical depth (water surface). This point is at a height approximately eg).lll.l to one half the distance between critical depth and the crown of the culvert. The elevation of this point can be used as an equivalent hydraulic grade line and H, as determined by equa- tion 2 or the nornographs, can be added to this elevation to find the water surface elevation of' the headwater pool. The f'ull flow condition for part of the barrel length, figure 2C, will exist when the headwater depth RW, as computed from the above head- water pool elevation, is. equal to or greater than the quantity y2 D + (1 + ke) 2g where Y is the mean velocity for the f'ull cross section of the barrel; ke, the entrance loss coefficient; and D, the inside height of the cul- vert. Ii' the headwater is less than the above value, a free water sur- face, figure 2D, will extend through the culvert barrel. The part f'ull flow condition of figure 2D must be solved by a backwater computation if accurate headwater depths are desired. De- tails for making this computation are not given in this circular. In- stead the solution used is the same as that given for the flow condi- tion of' figure 2C, with the reservation that headwater depths become less accurate as the discharge for a particular culvert decreases. Generally,for (iesignpurposes, this method is satisfactory for head- water depths above 0.75D, where. D is the height of the culvert barrel. Culvert capacity charts found in Hydraulic Engineering Circular No. 10 give a more accurate and easy solution for this free surface flow con- dition. Headwater depth RW can be expressed by a common equation for all outlet-control conditions, including all depths of tailwater. This is accomplished by designating the vertical dimension from the cul- vert invert at the outlet to the elevation from which H is measured as 110. The headwater depth RW equation is RW=H+ho-LSo (3) All the terms in this equation are in feet. H is computed by equation 2 or found from the full-flow nomographs. L is the length of eulvert in feet and So the barrel slope in ft. per ft. The distance he is discussed in the following paragraphs for the various conditions of outlet-control flow. HeadMater RW is the distance in feet from the invert of the culvert at the inlet to the water surface of the head- water pool. When the elevation of the water surface in the outlet channel is equal to or above the elevation of the top of the culvert opening at the outlet, figure 2A, ho is equal to the tailwater depth. Tailwater 5-9