Energy Dissipation in Twelve-Foot Drop Broken-Back Culverts under Pressure Flow Conditions

Hydraulic jump formed in broken-back culverts was investigated experimentally by using energy dissipation devices. This paper investigates the reduction in scour downstream of a broken-back culvert by forming a hydraulic jump inside the culvert. A broken-back culvert in the laboratory is modeled to represent 150 foot prototype. The drop between inlet and outlet was 12 feet with a 1 (vertical) to 2 (horizontal) slope after the upstream inlet and then continuing 126 feet at a one percent slope. Three flow conditions were simulated, consisting of 0.8, 1.0 and 1.2 times the culvert depth. The results were analyzed in terms of the inlet Froude number. All experiments were conducted under pressure flow which is defined by the fluid exerting pressure against the top of the model. The Froude number of the hydraulic jump created in the mild part of the culvert ranges between 1.80 and 2.50 which indicates a “Weak jump”. To locate the jump near the toe, different sill and friction block arrangements were tested. For new culvert construction, the best option to maximize energy dissipation under pressure flow conditions is to use two sills, one 2.5-foot sill at 62 feet from the end, and one 3.33-foot sill at 45 feet from the end of the culvert. Friction blocks had minimal impact on energy dissipation in the culvert. The length of the prototype culvert can be reduced by 40 to 45 feet. Such a scenario is important where right-of-way problems exist for culvert construction.