A perched culvert is one with an outlet elevated above the downstream water surface, allowing a freefall condition (also referred to as a hanging or shotgun outlet). This condition requires migrating fish to leap into the culvert from the downstream pool.
The leap height at a perched culvert is defined as the difference between the water surface elevation of the water leaving the culvert and the water surface elevation of the pool. This height will change as the flow changes due to back water effects and depth of flow in the culvert. The depth of the outlet pool is also an important parameter that will be used to determine if the pool depth is adequate for fish leaping.
Perched culverts often result from the erosion that occurs at the outlet of an undersized culvert. Water exiting the culvert at high velocities has a high erosive potential that results in downstream scour of the channel bed and banks. The incision that occurs as a result of this scouring can be limited to the locality of the culvert and is known as local incision or can be propagated upstream due to changes in the watershed and is known as global incision. In the case of global incision the culvert may be acting as a knickpoint that can stop global incision from migrating further upstream, causing a perched condition to result.
Photo of Perched Culvert
Additional Reference: Geomorphic impacts of Culvert Replacement and Removal, J. Castro, USFWS