Energy Dissipation in Six-Foot Drop Broken Back Culverts under Open Channel Flow Conditions

This research investigates the reduction in scour downstream of a broken-back culvert by forming a hydraulic jump inside the culvert. A broken-back culvert is used in areas of high relief and steep topography because it has one or more breaks in profile slope. A broken-back culvert represents a 1 (vertical) to 2 (horizontal) slope after the upstream inlet and then continues at a 1% slope to the downstream outlet. The prototype for these experiments was a two-barrel 10-ft by 10-ft reinforced concrete culvert. Three flow conditions were simulated, consisting of 0.8, 1.0, and 1.2 times the culvert depth. The Froude number of this hydraulic jump ranges between 1.8 and 2.3. This range of Froude number values is indicative of a weak type of hydraulic jump. In a weak jump a series of small rollers develops on the surface of the jump, but the downstream water surface remains smooth. The jump was initiated near the toe by systematically positioning sills in the culvert. The sills contained two small orifices at the bottom to allow the culvert to completely drain. For new culvert construction, the best option to maximize energy dissipation under open channel flow condition is to use one 3.0 ft high sill located 69 ft from the outlet. The maximum length of the culvert, 150 ft, can then be reduced between 42 to 56 ft. Such a scenario is important where right-of-way problems exist for culvert construction.