Water flows through culverts in two basic states known as subcritical and supercritical flow, These states are controlled by the constructed slope of the channel and geometry of the culvert and fall into the category of Inlet Control and Outlet Control.

Under outlet control conditions, the roughness of the culvert will have a significant effect on the flow through the culvert. The energy loss due to the hydraulic resistance or roughness of a culvert is part of the "hydraulic price" of flow through a pipe. Two commonly used equations to describe the flow as a function roughness in a culvert are the Darcy equation and the Manning equation.

See also: Manning's n, Darcy Friction Factor