Laboratory Modeling of Energy Dissipation in Broken-back Culverts

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 as it has one or more breaks in profile slope. A broken-back culvert in the laboratory represents a 1 (vertical) to 2 (horizontal) slope after the upstream inlet and then continuing 100 feet at a 1 percent slope in the flat part of the culvert to the downstream outlet. The pro-type for these experiments were either a two barrel 10-foot by 10-foot, or a two barrel 10-foot by 20-foot reinforced concrete. The drop between inlet and outlet is selected as 24 feet. Three flow conditions were simulated, consisting of 0.8, 1.0 and 1.2 times the culvert depth. The Froude number of the hydraulic jump created in the flat part of the culvert ranges between 2.7 and 3.6. This Fr classifies the jump as an “oscillating jump”. Such a jump moves up and down in the barrel, its location changing over time. The jump in experiments began nearly at the toe by placing sills in the flat part. The optimal location was determined at a distance of 45 and 25 feet from the outlet face of the culvert in pressure flow conditions. The sills contain two small orifices at the bottom to allow the culvert to completely drain. The impact of friction blocks was found to be minimal. No friction blocks were used to further dissipate. The length of the culvert cannot be reduced as the pressure flow fills up the culvert barrels nearly completely. For new culvert construction, the best option to maximize energy dissipation under open channel flow condition is to use one sill located 40 feet from the outlet. Again, frictional blocks were not effective in further reduction of energy. The maximum length of the culvert can be reduced by 15 feet to 44 feet. Such a scenario is important where right-of-way problems exist for culvert construction.