Energy dissipation in eighteen-foot broken-back culverts using laboratory models (FHWA-OK-12-05)

Abstract

This report represents Phase III of broken-back culverts with a drop of 18 feet. The first phase of this research was performed for a drop of 24 feet and the second phase of this research was carried out for a drop of 6 feet. 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 114 feet at a 1 percent slope in the flat part of the culvert to the downstream outlet. Also, in this project, two other slopes, 0.6 and 0.3 percent, were simulated. The prototypes 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 18 feet. Three flow conditions were simulated, consisting of 0.8, 1.0 and 1.2 times the culvert depth. The Froude number (Fr1) of the hydraulic jump created in the flat part of the culvert ranges between 2.5 and 4.12. This Fr1 classifies the jump as an oscillating jump. The jump in experiments began nearly at the toe by placing sills in the flat part. For new culvert construction, the best option to maximize energy dissipation under open channel flow conditions is to use one 5-foot sill located 43.3 feet from the outlet. The maximum length of the culvert can be reduced by 30 feet to 43.3 feet. In pressure flow conditions, The optimal location was determined at a distance of 62 feet from the outlet for 2.5-foot sill and at distance of 45 feet from the outlet of face of the culvert for 3.3-foot sill. The length of the culvert can be reduced by 40 feet to 45 feet. Such a scenario is important where right-of-way problems exist for culvert construction. 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 energy. The change of slope in the Broken-Back culvert does not significantly affect to the efficiency of the hydraulic jump.