Culverts are water conveyance structures that usually allow for continuous flow under roads and highways. Culvert design theory is largely based on the principle of conservation of energy, where discharge is a function of the flow resistance along the pipe length and the difference in tailwater and headwater levels. A pipe that is flowing full is considered “hydraulically efficient” since it is pressurized and conveying the maximum amount of water possible. For this condition to occur the upstream end of the pipe will be submerged and the downstream velocities will be higher than naturally found in the stream channel at that point.
Hydraulic efficiency has been the general goal in the design of culverts for many years among highway agencies and municipalities. This allows designers to maximize conveyance while minimizing pipe size and cost. As result the impacts of culverts on aquatic species and to the geomorphic conditions of stream channels have been largely overlooked.