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~ Equation 2 can be solved for H readily by the use of the full-flow nomographs, Charts 8 through 14. Each nomograph is drawn for a partic- ular barrel shape and material and a single value of n as noted on the ',respective charts. These nomographs can be used for other values of n by modifying the culvert length as directed in the instructions (p. 5-29) for the use of the full-flow nomographs. ., In culvert design the depth of headwater HW or the elevation of the ponded water surface is usually desired. Finding the value of It from the nomographs or by equation 2 is only part of the soluti.on for this headWater depth or elevation. In the case of figure 2A or figure 3, where the outlet is totally submerged, the headwater pool elevation (as- sumed to be the same elevation as the energy line) is found by adding H to the elevation of the tailwater. The headwater depth is the difference in elevations of the pool surface and the culvert invert at the entrance. When the tailwater is below the crown of the culvert, the submerged condition discussed above no longer exists and the determination of headwater is somewhat more difficult. In discussing outlet-control flow for this condition, tailwater will be assumed to be so low that it has no effect on the culvert flow. (The effect of tailwater will be discussed later.) The common types of flow for the low tailwater con- dition are shown in figures 2B, 2C and 2D. Each of these flow condi- tions are dependent on the amount of discharge and the shape of the culvert cross section. Each condition will be discussed separately. Full flow at the outlet, figure 2B, will occur only with the higher rates of discharge. Charts 15 through 20 are prOVided to aid in deter- mining this full flow condition. The curves shown on these charts give the depth of flow at the outlet for a given discharge when a culvert is flowing with outlet control. This depth is called critical depth dc. When the discharge is sufficient to give a critical depth equal to the crown of the culvert barrel, :full flow exIsts at the outlet as In :fIg- ure 2B. The hydraulic grade lIne will pass through the crown of the culvert at the outlet for all dIscharges greater than the discharge causing critical depth to reach the crown of the culvert. Head H can be measured from the crown of the culvert in computing the water sur- face elevation of the headwater pool. When critIcal depth falls below the crown of the culvert at the outlet, the water surface drops as shown in either figures 2C or 2D, depending again on the discharge. To accurately determine headwater for these conditions, computations for locating a backwater curve are usually required. These backwater computations are tedious and time consuming and they should be avoided if possible. Fortunately, head- water for the flow condition shown in figure 2C can be solved by using the nomographs and the Instructions given in this circular. For the condition shown in fIgure 2C, the culvert must flow full for part of Its length. The hydraulic grade line for the portion of the length in full flow will pass through a point where the water breaks with the top of the culvert as represented by point A in figure 2C. Backwater computations show that the hydraulic grade line If 5-8 e . " e . e