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e . . e .,. ~ e U.S. DEPARTMENT OF TRANSPORTATION Federal Highway Administration HYDRAULIC DESIGN OF IMPROVED INLETS FOR CULVERTS Prepared as a cooperative effort of the Hydraulics Branch, Bridge Division, Office of Engineering and the Demonstration Projects Division, Office of Development, Region 15 Principal Authors L. J. Harrison, J. L. Morris J. M. Normann and F. L. Johnson I. Introduction The passage of water through highway culverts involves complex hydraulic phenomena, some of which are not yet thoroughly under- stood. A variety of fluid dynamic and pneumatic Situations may occur, making it extremely difficult to exactly define culvert flow characteristics at a given time under a specified set of conditions. Recognizing the potential for substantial savings which would result from improved knowledge and design techniques in the field of culvert hydraulics, the Federal Highway Administra- tion (FHWA, then the Bureau of Public Roads) initiated research in 1954 to obtain hydraulic information from a series of model tests. The research was performed by the National Bureau of Standards (NBS) and resulted in seven progress reports (5, 6, 7, 8, 9, 10, 11) covering conventional culverts with a constant slope and cross section as well as inlet modifications to improve flow characteristics at the culvert entrance. Culvert flow capacity was found to be limited either by the culvert entrance conditions or by barrel resistance. The former was designated "inlet control" and the latter "outlet control." When a culvert operates in inlet control, the barrel will permit the passage of more flow than the inlet, and in outlet control the reverse is true. Hydraulic Engineering Circular No.5 (HEC No.5), "Hydraulic Charts for the Selection of Highway Culverts," (12) and HEC No. 10, "Capacity Charts for the Hydraulic Design of Highway Culverts," (13) incorporate results of the conventional culvert research and present design methods for these culverts in both inlet and outlet control. These Circulars are in common use throughout the United States and HEC No. 5 has been translated into several foreign languages, including Spanish, French, and Norwegian. Design methods presented herein are an extension of methods and information presented in HEC No.5. A thorough ,understanding of culvert design principles contained in that Circular is necessary to an understanding of methods presented in this Circular. 13-1