ENERGY DISSIPATION IN THIRTY-FOOT BROKEN-BACK CULVERTS USING LABORATORY MODELS

Abstract

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 90 feet at a 1 percent slope in the flat part of the culvert to the downstream outlet. 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 culvert. The drop between inlet and outlet was selected as 30 feet. Three flow conditions were simulated, consisting of 0.8, 1.0 and 1.2 times the culvert depth. This report represents Phase V of broken-back culverts with a drop of 30 feet. The first phase of this research was performed with a drop of 24 feet, the second phase of this research was carried with for a drop of 6 feet, the third phase of this research, performed was a drop of 18 feet, and the fourth phase of this research was performed with a drop of 12 feet.

The Froude number (Fr1) of the hydraulic jump created in the flat part of the culvert ranged between 2.53 and 5.66. This Fr1 classifies the jump as an oscillating to steady jump. The jump began nearly at the toe by placing sills and friction blocks of different sizes in the flat part. For new culvert construction, the best option to maximize energy dissipation under open channel flow conditions is to use one 5.83-foot sill located 33.33 feet from the outlet. The maximum length of the culvert can be reduced from 25 feet to 30 feet that was for middle sill. For existing and new culverts, the best option is to use one end sill with height of 6.67-foot. In pressure flow conditions, the optimal location was determined at a distance of 31.67 feet from the outlet for 4.17-foot sill. The length of the culvert can be reduced by 25 feet to 30 feet. Also, for new and existing culverts, the optimal end sill is to use one 4.17-foot sill height under pressure flow conditions. Such a scenario is important where right-of-way problems exist for culvert construction. Also examined was a slotted sill which has a cut in the middle for cleanup purposes. In open channel flow conditions, the best option to maximize energy dissipation is to use one 5.83-foot slotted sill located 33.33 feet from the outlet, and for end slotted sill is to use 7.5-foot height. In the pressure flow conditions, the optimal slotted sill was 5-foot at a distance of 41.67 feet from the outlet, and for end slotted sill is to use 4.17-foot height. The regular and slotted 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.