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<br />The only safe procedure is to design on the basis that <br />erosion at a culvert outlet and downstream channel is to <br />be expected. A reasonable procedure is to provide at least <br />minimum protection, and then inspect the outlet channel <br />after major storms to determine if the protection must be <br />increased or extended. Under this procedure, the initial <br />protection against channel erosion should be sufficient <br />to provide some assurance that extensive damage could not <br />result from one runoff event. <br /> <br />e <br /> <br />TYPES OF SCOUR <br /> <br />Two types of scour can occur in the vicinity of culvert <br />outlets--Iocal scour and general channel degradation. <br />Culverts are generally constructed at crossings of small <br />streams, and the majority of these streams are eroding to <br />reduce their slopes. Channel degradation may proceed in <br />a fairly uniform manner over a long length, or may be evident <br />in one or more abrupt drops progressing upstream with every <br />runoff event. The latter type, referred to as headcutting, <br />can be detected by location surveys or by periOdic maintenance <br />inspections following construction. Information regarding <br />the degree of instability of the outlet channel is an essential <br />part of the culvert site investigation. If any substantial <br />doubt exists as to long-term stability of the channel, measures <br />for protection should be included in the initial construction. <br /> <br />e~ <br /> <br />Long term lowering of the stream channel through natural <br />processes and local erosion at the culvert outlet may occur <br />simultaneously. Local scour is the result of high-velocity <br />flow at the culvert outlet, but its effect extends only <br />a limited distance downstream. Natural channel velocities <br />are almost universally less than culvert outlet velocities, <br />because the channel cross section, including its flood plain, <br />is generally larger than the culvert flow area. Thus, the <br />flow rapidly adjusts to a pattern controlled by the channel <br />characteristics. <br /> <br />, <br /> <br />The highest velocities will be produced by long, smooth- <br />barrel culverts on steep slopes. These cases will no doubt <br />require protection of the outlet channel at most sites. <br />However, protection is also often required for culverts <br />on mild slopes. For these culverts flowing full, the outlet <br />velocity will be critical velocity with low tailwater and <br />the full barrel velocity for high tailwater. Where the <br />discharge leaves the barrel at critical depth, the velocity <br />will usually be in the range of 10 to 20 feet per second. <br /> <br />, <br /> <br />e <br /> <br />11-4 <br />