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. ~ ~ ~ . ~ .. ~ ~ ~ . . t ~ ~ f ~. ~ , ~ WATER SURFACE - -~ A. SUBMERGED _I HW - - -- .....-..- B. UNSUBMERGED HW- HEADWATER TW - TAl LWATER '((S.- WATER SURFACE H - LOSSES THROUGH CULVERT H t- TW W.s. - CONTROL SECTION DOWNSTREAM ----L- H ...---~. c (CONTROL SECTION) Figure I-I4--Typical outlet control flow conditions. culvert play a role in determining its capacity. These characteristics include all of the factors governing inlet control, the water surface elevation at the outlet, and the slope, length, and hydraulic roughness of the culvert barrel. (table 1) 3. Headwater. Energy is required to force flow through a culvert. This energy takes the form of an increased water surface elevation on the upstream side of the culvert. The depth of the upstream water surface measured from the invert at the culvert entrance is generally referred to as headwater depth. (figures 1-13 and 1-14) A considerable volume of water may be ponded upstream of a culvert installation under high fills or in areas with flat ground slopes, The pond which is created may attenuate flood peaks under such conditions. This peak discharge attenuation may justify r a reduction in the required culvert size. 4. Tailwater. Tailwater is defined as the depth of water downstream of the culvert measured from the outlet invert. (figure 1-14) It is an important factor in determining culvert capacity under outlet control conditions. Tailwater may be caused by an obstruction in the downstream channel or by the hydraulic resistance of the channel. In either case, backwater calculations from the downstream control point are required to precisely define tail water , When appro- priate, normal depth approximations may be used instead of backwater calcula- tions. 5. Outlet Velocity. Since a culvert usually constricts the available channel area, flow velocities in the culvert are likely to be higher than in the channel. These increased velocities can cause 9