The culvert routines in HEC-RAS are similar to the bridge routines, except that the Federal Highway Administration's (FHWA, 1985) standard equations for culvert hydraulics are used to compute inlet control losses at the structure. Outlet control is computed by balancing the energy equation from downstream of the culvert to upstream of the culvert. The HEC-RAS culvert routines are also capable of reproducing all 6 of the USGS flow classifications for culverts, outlined in their report "Measurements of Peak Discharge at Culverts by Indirect Methods", (USGS, 1976). The figure below illustrates a typical box culvert road crossing. As shown, the culvert is similar to a bridge in many ways. The walls and roof of the culvert correspond to the abutments and low chord of the bridge, respectively.
Typical Culvert Road Crossing

Because of the similarities between culverts and other types of bridges, culverts are modeled in a similar manner to bridges. The layout of cross sections, the use of the ineffective areas, the selection of loss coefficients, and most other aspects of bridge analysis apply to culverts as well.