7.7 Culvert Modeling Using HEC-RAS

Data for the culvert structure is entered on two templates in HEC-RAS: the Deck/Roadway Editor for roadway information and the Culvert Editor for the physical data defining the culvert.

Roadway Geometry

Roadway geometry (station and roadway surface elevation) is defined in the same manner as in bridge modeling. Figure 7.14 shows the Deck/Roadway Data Editor with sample data used to model a culvert and Figure 7.15 shows the plot of the data in the Bridge/Culvert Editor.

Figure 7.14 Entering deck and roadway data for a culvert in HEC-RAS.

Figure 7.15 Plot of culvert cross sections in HEC-RAS.

No low-chord data appear in the Deck/Roadway Editor shown in Figure 7.14 because the culvert data and the opening through the embankment are entered into the Culvert Data Editor, shown in Figure 7.16. The width of the roadway and the distance to the upstream cross section that must be supplied on the Deck/Roadway Editor can be different for culverts than for bridges. The length of the culvert can be entered in the Width field on the Deck/Roadway Editor or the actual width of the roadway over the culvert can be used. Similarly, the Distance (distance between upstream cross section, section 3, and deck/roadway) on the Deck/Roadway Editor can be entered as the distance from the upstream face of the culvert (BU) to section 3 or as the distance from the upstream edge of roadway to section 3.

Figure 7.16 Culvert data editor.

HEC-RAS computes the distance from the downstream roadway edge to section 2 by summing the Width and Distance values and subtracting the result from the distance between sections 2 and 3, part of the data entered for the geometry of section 3. This calculation must result in a positive distance for the program to run.

Although not required for culvert computations, the modeler may also choose to enter embankment side slopes for the upstream and downstream embankment faces, U.S. Embankment SS and D.S. Embankment SS, respectively, on the Deck/Roadway Data Editor. The sloping embankment is used for graphical purposes on the cross-section plots. In Figure 7.14, a value of 2 has been entered for both embankments, specifying a 1:2 (vertical to horizontal) embankment side slope.

On Figure 7.16, displaying the Culvert Data Editor, the Culvert Length is shown as 80 ft and the Distance (from BU) to Section 3 is shown as 5 ft. HEC-RAS computes the distance from the culvert exit to section 2 by subtracting the sum of the culvert length and the distance to the upstream cross section (80 + 5 = 85 ft) from the distance between cross section 2 and 3 taken from section 3 input data (90 ft). The difference (5 ft) is the distance from the downstream culvert face (section BD) to section 2. Again, this computation must result in a positive value for the program to run.

Inlet Control Data

The primary information specific to inlet control analysis is the inlet geometry, with the corresponding chart and scale numbers. When a specific culvert shape is selected in HEC-RAS, the program automatically chooses and displays the first set of FHWA chart and scale numbers that correspond to that shape, shown in Figure 7.16 as Chart 61 and Scale 3 for the Conspan Arch shape. The modeler may accept these numbers or select different values from the list provided (under the drop-down menu adjacent to the chart and scale number fields). Culvert slope, cross-sectional area, and height are also needed for an inlet control analysis, but these data are generated from other general information (diameter or rise and span) entered into the Culvert Data Editor. Culvert slope is not directly entered but is computed based on the upstream and downstream invert elevations and the culvert length previously specified.

Outlet Control Data

The entrance loss coefficient is required along with the Manning's n value for the culvert material for an outlet control analysis. HEC-RAS allows different Manning's n values for different portions of the culvert cross section. A table similar to Table 7.3 is present within the program (click on the question mark button next to the Entrance Loss Coeff. box) to assist the modeler's selection of the appropriate entrance loss coefficient. The exit loss coefficient defaults to 1.0, but the modeler has the option to adjust this parameter. However, there is no reliable information at present to indicate that a value other than 1.0 is appropriate. A tailwater elevation is not required; this is computed by HEC-RAS as part of the downstream water surface profile calculations. However, a tailwater elevation must be estimated if hand computations are to be performed, or if a culvert design program is used that does not compute a downstream water surface profile.