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Ie ;; e . e Computing Depth of Tail.water In culverts flowing with outlet control, teil.water can be an im- portant factor in computing both the headwater depth and the hydraulic capacity of a culvert. Thus, in many culvert designs, it becomes nec- essary to determine teilwater depth in the outlet channeL Much engineering judgment and experience is needed to evaluate possible tail.water conditions during floods. A field inspection should be made to check on downstream controls and to determine water stages. Oftentimes tail.water is controlled by a downstream obstruction or by water stages in another stream. Fortunately, most natural channels are wide compared to the culvert and the depth of water in the natural chan- nel is considerably less than critical depth, thus the tailwater is in- effective and channel depth computations are not always warranted. An approximation of the depth of flow in a natural stream (outlet channel) can be made by using Manning's equation (see page 5-12) if the channel is reasonably uniform in cross section, slope and roughness. Values of n for natural streams for use in Manning's equation may be found in Table 2, appendix B, p. 5-50. If the water surface in the outlet channel is established by downstream controls, other means must. be found to determine the tailwater elevation. Sometimes this neces- si tates a study of the stage-discharge relationship of another stream into which the stream in question flows or the securing of data on res- ervoir elevations if a storage dam is involved. Velocity of Culvert Flow A culvert, because of its hydraulic characteristics, increases the velocity of flow over that in the natural channel. High velocities are most ~ing just downstream from the culvert outlet and the ero- sion potential at this point is a feature to be considered in culvert design . Energy dissipators for channel flow have been investigated in the laboratory and many have been constructed, especially in irrigation channels. Designs for highway use have been developed and constructed at culvert outlets. All energy dissipators add to the cost of a cul- vert, therefore, they should be used only to prevent or to correct a serious erosion problem. (See references 4 and 5.) The judgment of engineers working in a particular area is re- quired to determine the need for energy dissipators at culvert out- lets. As an aid in evaluating this need, culvert outlet velocities should be computed. These computed velocities can be compared with outlet velocities of alternate culvert designs, existing culverts in the area, or the natural stream velocities. In many streams the max- imum velocity in the main channel is considerably higMr than the mean velocity for the whole channel cross-section. Culvert outlet veloci- ties should be compared with maximum stream velocities in determining 5-ll