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,e ; .. e . e U. S. DEPARTMENT OF TRANSPORTATION Federal Highway Administration HYDRAULIC CHARTS FOR THE SELECTION OF HIGHWAY CULVERTS Prepared by Lester A. Herr Chief, Hydraulics Branch, Bridge Division In Collaboration with Herbert G. Bossy Highway Research Engineer, Hydraulic Research Division Introduction Designing highway culverts involves many factors including esti- mating flood peaks, hydraulic performance, structural adequacy, and overall construction and maintenance costs. This circular contains a brief discussion of the hydraulics of conventional culverts and charts for selecting a culvert size for a given set of conditions. Instruc- tions for using the charts are provided. No attempt is made to cover all phases of culvert design. Subsequent circulars will cover culverts with modified inlets and outlets designed to increase performance or to apply to a particular location. Some approximations are made in the hydraulic design procedure for simplicity. These approximations are discussed at appropriate points throughout the circular. For this discussion, conventional culverts include those commonly installed, such as circular, arch and oval pipes, both metal and con- crete, and concrete box culverts. All such conventional culverts have a uniform barrel cross section throughout. The culvert inlet may con- sist of the culvert barrel projected from the roadway fill or mitered to the embankment slope. Sometimes inlets have headwalls, wingwalls and apron slabs, or standard end sections of concrete or metal. The more common types of conventional culverts are. considered in this cir- cular. Culvert Hydraulics Laboratory tests and field observations show two major types of culvert flow: (1) flow with inlet control and (2) flow with outlet control. For each type of control, different factors and formulas are used to compute the hydraulic capacity of a culvert. Under inlet con- trol, the cross-sectional area of the culvert barrel, the inlet geom- etry and the amount of headwater or ponding at the entrance are of pri- mary importance. Outlet control involves the additional consideration 6f the elevation of the tailwater in the outlet channel and the slope, rouglmess and length of the culvert barrel. It is possible by involved hydraulic computations to' determine the probable type of flow under which a culvert will operate for a 5-1