Cross Section Locations for Culverts

The culvert routines in HEC-RAS require the same cross sections as the bridge routines. Four cross sections are required for a complete culvert model. This total includes one cross section sufficiently downstream from the culvert such that flow is not affected by the culvert, one at the downstream end of the culvert, one at the upstream end of the culvert, and one cross section located far enough upstream that the culvert again has no effect on the flow. Note, the cross sections at the two ends of the culvert represent the channel outside of the culvert. Separate culvert data will be used to create cross sections inside of the culvert. The figure below illustrates the cross sections required for a culvert model. The cross sections are labeled 1, 2, 3, and 4 for the purpose of discussion within this chapter. Whenever the user is computing a water surface profile through a culvert (or any other hydraulic structure), additional cross sections should always be included both upstream and downstream of the structure. This will prevent any user-entered boundary conditions from affecting the hydraulic results through the culvert.

Cross Section 1 of Culvert Model. Cross Section 1 for a culvert model should be located at a point where flow has fully expanded from its constricted top width caused by the culvert constriction. The cross section spacing downstream of the culvert can be based on the criterion stated under the bridge modeling chapter (See "Modeling Bridges" for a more complete discussion of cross section locations). The entire area of Cross Section 1 is usually considered to be effective in conveying flow.

Cross Section 2 of Culvert Model. Cross Section 2 of a culvert model is located a short distance downstream from the culvert exit. This distance should represent the short distance that is required for the abrupt transition of the flow from the culvert to the channel. Cross section 2 does not include any of the culvert structure or embankments, but represents the physical shape of the channel just downstream of the culvert. The shape and location of this cross section is entered separately from the Bridge and Culvert editor in the user interface (cross section editor).

The HECRAS ineffective area option is used to restrict the effective flow area of Cross Section 2 to the flow area around or near the edges of the culverts, until flow overtops the roadway. The ineffective flow areas are used to represent the correct amount of active flow area just downstream of the culvert. Because the flow will begin to expand as it exits the culvert, the active flow area at Section 2 is generally wider than the width of the culvert opening. The width of the active flow area will depend upon how far downstream Cross Section 2 is from the culvert exit. In general, a reasonable assumption would be to assume a 1.5:1 expansion rate over this short distance. With this assumption, if Cross Section 2 were 6 feet from the culvert exit, then the active flow area at Section 2 should be 8 feet wider than the culvert opening (4 feet on each side of the culvert) Figure 6-4 illustrates Cross Section 2 of a typical culvert model with a box culvert. As indicated, the cross section data does not define the culvert shape for the culvert model. On the figure below, the channel bank locations are indicated by small circles, and the stations and elevations of the ineffective flow areas are indicated by triangles.

Cross Sections 1 and 2 are located so as to create a channel reach downstream of the culvert in which the HECRAS program can accurately compute the friction losses and expansion losses downstream of the culvert.

Cross Section 3 of Culvert Model. Cross Section 3 of a culvert model is located a short distance upstream of the culvert entrance, and represents the physical configuration of the upstream channel. This cross section should be far enough upstream from the culvert face, such that the abrupt contraction of flow has room to occur. Also, the culvert routines take into account an entrance loss in all of the calculations. This entrance loss requires some distance to occur over. The culvert method uses a combination of a bridge deck, Cross Sections 2 and 3, and culvert data, to describe the culvert or culverts and the roadway embankment. The culvert data, which is used to describe the roadway embankment and culvert openings, is located at a river station between Cross Sections 2 and 3.

The HECRAS ineffective area option is used to restrict the effective flow area of Cross Section 3 until the flow overtops the roadway. The ineffective flow area is used to represent the correct amount of active flow area just upstream of the culvert. Because the flow is contracting rapidly as it enters the culvert, the active flow area at Section 3 is generally wider than the width of the culvert opening. The width of the active flow area will depend upon how far upstream Cross Section 3 is placed from the culvert entrance. In general, a reasonable assumption would be to assume a 1:1 contraction rate over this short distance. With this assumption, if Cross Section 3 were 5 feet from the culvert entrance, then the active flow area at Section 3 should be 10 feet wider than the culvert opening (5 feet on each side of the culvert). The figure below illustrates Cross Section 3 of a typical culvert model for a box culvert, including the roadway profile defined by the bridge deck/roadway editor, and the culvert shape defined in the culvert editor. As indicated, the ground profile does not define the culvert shape for the culvert model. On the figure below, the channel bank locations are indicated by small circles and the stations and elevations of ineffective area control are indicated by triangles.

Cross Section 4 of Culvert Model. The final cross section in the culvert model is located at a point where flow has not yet begun to contract from its unrestrained top width upstream of the culvert to its constricted top width near the culvert. This distance is normally determined assuming a one to one contraction of flow. In other words, the average rate at which flow can contract to pass through the culvert opening is assumed to be one foot laterally for every one foot traveled in the downstream direction. More detailed information on the placement of cross sections can be found in "Modeling Bridges". The entire area of Cross Section 4 is usually considered to be effective in conveying flow.