Culverts are common hydraulic elements in a sewer system. There can be stand-alone culverts under highway embankments or conduit vaults in detention pond outlet structures. In Bentley StormCAD , a culvert can be a conduit specified as a culvert or one of controlling elements in a composite control structure. Since a culvert is a type of hydraulic structure that transports water as full or partially full, culvert hydraulics is more complicated than other control structures.

Conduits can be treated as culverts if the "Is culvert?" property is set to True. This can only be done for box and circular conduits.

Hydraulically a culvert can be under inlet control or outlet control conditions. The computational procedures for these conditions are very different:

• Inlet control - A culvert is under inlet control if the culvert barrel hydraulic capacity is higher than that of the inlet (entrance) and there is no backwater from downstream. In this condition, the relationship of flow and headwater is mainly dependent on the inlet configurations.
• Outlet control - A culvert is under outlet control when the culvert barrel is not capable of conveying as much flow as the inlet opening will accept. When the culvert is under outlet control, the flow will depend not only on the headwater but also the tailwater.
• EQT curves - Dynamic culvert conditions are complicated in that the flow can change from inlet control to outlet control or vice versa. As a result of this complexity, the computation of culverts can be tedious. In Bentley StormCAD , a sophisticated procedure has been developed to build up a comprehensive EQT data set for any culvert configuration. The EQT represents the headwater (E), flow (Q), and tailwater (T) tabular curves in the way it covers all possible operating ranges of the headwater and tailwater so that any hydraulic conditions are accounted for by the EQT. The Bentley StormCAD dynamic engine builds the EQT for every culvert and uses the EQT for culvert computation dynamically at any time step.
• Culvert calculations for DW solver - Dynamic culvert conditions are complicated in that the flow can change from inlet control to outlet control or vice versa. As a result of this complexity, the computation of culverts can be tedious. In Bentley StormCAD DW solver , a sophisticated procedure has been developed to build up a comprehensive EQT data set for any culvert configuration. The EQT represents the headwater (E), flow (Q), and tailwater (T) tabular curves in the way it covers all possible operating ranges of the headwater and tailwater so that any hydraulic conditions are accounted for by the EQT. The Bentley StormCAD DW dynamic engine builds the EQT for every culvert and uses the EQT for culvert computation dynamically at any time step.
• Culvert calculations for SWMM solver - The explicit solver in Bentley StormCAD (SWMM) uses culvert code developed in EPA SWMM to carry out the culvert calculations.
• Culvert calculations for GVF solver - The GVF solvers in Bentley StormCAD (GVF-Rational, GVF-Convex) use procedures that are similar to HDS-5 methods, an energy-equation-based backwater calculation is carried out from the downstream end-wall-node up to the upstream inlet node and the depth from the backwater profile calculation is compared with the culvert inlet control depth and the largest depth is used, and the control status (inlet control or outlet control) is determined accordingly based on the depth being used at the inlet-head-wall-node.
• Broken-Back-Culvert: this is a case that a culvert is composed of a series of culvert conduits between a head-wall and end-wall. In this case the conduits are connected by transition nodes and only the most-upstream culvert conduit is specified as a culvert. The culvert data can be defined either from the head-wall and end-wall data or by user-defined culvert data in the conduit.

A culvert-conduit can have different section shapes, the following table lists sections that are supported by solvers: