When one or more changes of grade occur within the culvert profile, it is called a broken-back culvert. Broken-back culverts are usually placed in areas where laying a straight culvert would require large excavations or where other site conditions dictate a break should occur. Broken-back culverts may also be intended to reduce outlet velocities when normal outlet velocities are greater than desired. A single broken-back culvert consists of only a steeply sloped section and an outlet section. As water flows through the steeply sloped section, it becomes supercritical or rapid.
Supercritical flow occurs when inertial forces dominate the gravita- tional forces.
If the supercritical flow continues through the outlet section, there will likely be problems with scour at the outlet and erosion in the downstream channel. Therefore, the goal of broken-back culvert design is to produce subcritical or tranquil flow within the outlet section. Subcritical flow occurs when the gravitational forces dominate the inertial forces. If a hydraulic jump occurs or is forced to occur inside the culvert barrel, then the flow at the outlet will be subcritical. It is important for the designer to know the water surface profile throughout the broken-back culvert and to be able to predict th e occurrence of a hydraulic jump.