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\. \ , f II PROPOSED I I ' CULVERT / \ / (.,~-,~ V UPSTREAM INVERT -.. . j 1 J j J I , ~ . j ~ ELEVATION . J PAVEMENT ELEVATION ., -SHOULDER ELEVATION , I Sel 1 . . J j CULVERT . J ~ I . < J SECTION A-A . 1 . j Figure II-7.--Roadway cross section and culvert length. c. Lonl!itudinal Roadwav Profile, The roadway profile represents the ob- struction encountered by the flowing stream. The embankment containing the culvert acts much like a dam. The culvert is similar to the normal release struc- ture, and the roadway crest acts as an emergency spillway in the event that the upstream pool (headwater) attains a sufficient elevation. The location of initial overtopping is dependent upon the roadway geometry. (figure II-S) The profile contained in highway plans generally represents the roadway center- line profile, These elevations may not represent the high point in the highway cross section. The culvert designer should extract the profile which estab- lishes roadway flooding and roadway over- flow elevations from the highway plans available. The low point of the profile is of critical importance, since this is the point at which roadway overtopping will first occur. S. DesignHeadwater. Themosteconom- ical culvert is one which would utilize all of the available headwater to pass the design discharge. Since the discharge . . capacity increases with increasing head, the available headwater elevation must be determined. This design headwater elevation generally hinges on one of three factors; economic considerations, regulatory constraints, or arbitrary constraints. An increase in available headwater can be obtained at some sites by depress- ing (burying) the culvert inlet. This procedure is advantageous for steep cul- verts which operate under inlet control. Additional information on this procedure is contained in chapter III. a. Economic Considerations. As ponding elevations increase upstream from a culvert, detrimental economic consequences can occur. Although for major structures it may be advantageous to perform a flood risk analysis (chapter VI), site-specific constraints are some- times adopted in lieu of a full risk analysis. Such constraints are based on some designated elevation that is not to be exceeded within a specified return period. This elevation may correspond to some critical point on the Toadway such as the roadway shoulder of the road- 22