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

Scour reduction downstream of a broken-back culvert is studied here by forming a hydraulic jump inside a culvert. A broken-back culvert has one or more breaks in profile slope, and is used in areas of high relief and steep topography. The purpose of a culvert is to safely pass water underneath the roadways constructed in hilly topography or on the side of a relatively steep hill. A broken-back culvert model in the laboratory represents a drop between inlet and outlet of 6 feet with a slope of 1 (vertical) to 2 (horizontal) after the upstream inlet, and then continues 138 feet at a 1 percent slope in the mild part of the culvert to the downstream outlet. The prototype for these experiments was a two barrel, 10-foot by 10-foot reinforced concrete culvert. The simulated flow conditions are 0.8, 1.0, and 1.2 times the culvert depth. Pressure flow is defined by the fluid excreting pressure against the top of the model. The Froude number of the hydraulic jump created in the mild part of the culvert ranges between 1.7 and 2.3. This Froude number classifies the jump as a weak jump. In these experiments, the jump was made to begin nearly at the toe by placing sills in the flat part. For new culvert construction, the best option to maximize energy dissipation under pressure flow conditions is to use one 2-foot sill. Also, the optimal location was determined to be a distance of 42 feet from the outlet face of the culvert. Friction blocks had minimal impact on energy dissipation in the broken-back culvert. No friction blocks were recommended to further energy dissipation. The length of the culvert cannot be reduced as the pressure flow fills up the culvert barrels completely.