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~ .. , ~ ~ ~ J ~ ~ ... 1 minimum performance are ease of design and assurance of adequate performance under the least favorable hydraulic con- ditions. ... j 1 1 j . j j 1 ~ .~ 1 ~ 1 ~ ~ Figure I-6--Corrugated metal arch culvert (ARMCO). Research by the National Bureau of Standards (NBS) sponsored and supported by the Federal Highway Administration (FHWA), formerly the Bureau of Public Roads (BPR), began in the early 19S0's and resulted in a series of seven reports. (4 to 10) These reports provided a com- prehensive analysis of culvert hydraulics under various flow conditions. These data were used by the BPR staff to develop culvert design aids, called nomographs. These nomographs are the basis of the culvert design procedures in HEe No. S, HEC No. 13, and this publication. The approach presented in HEC No. S is to analyze a culvert for various types of flow control and then design for the control which produces the minimum per- . formance. Designing for minimum perfor- mance ignores transient conditions which might result in periods of better per- formance. The benefits of designing for 1. Flow Conditions. A culvert barrel may flow full over all of its length or partly full. Full flow in a culvert barrel is rare. Generally, at least part of the barrel flows partly full. A water surface profile calculation is the only way to accurately determine how much of the barrel flows full. ~ ~ a. Full Flow. The hydraulic con- dition in a culvert flowing full is called pressure flow. If the cross-sectional area of the culvert in pressure flow were increased, the flow area would expand. One condition which can create pressure flow in a culvert is the back- pressure caused by a high downstream water surface elevation. A high upstream 1 ~ j 4