Steep stream – Steady/ Mixed

[Kechha Shrestha]

I am modeling a stream that around 3% slope. Is this considered steep stream? When I run the model, Froude number is >1 at most location. In order to run a mixed flow, i don’t have any upstream boundary condition. Can the same normal depth as BC used for both U/S and D/s to perform mixed flow? Also, at an existing culvert xing, the culvert is in inlet control with only half full and overtopping. Can you all share what can be done to make culvert flow full and then overtop?

[Scott Miller]

Three percent is a moderate slope on a natural stream, however, you could say a channel is steep based on dominantly supercritical flow. There needs to be an upstream boundary condition, as well as downstream, in order to model a mixed flow regime. Normal depth is usually a good choice. Just make sure your boundary conditions are far enough away from the reach you’re studying. (Calculate backwater distance.) An inlet controlled culvert will not flow full. If you have water surface elevation for a known flow rate, you can calibrate the inlet coefficient, but don’t expect a full culvert unless something on the downstream end make it outlet controlled.

[Kechha Shrestha]

Thanks scott. At the culvert, no known WSE vs Q data is available. I get message as the program could not balance the culvert/weir flow. The reported outlet energy grade may not be valid.I know that the inlet geometry affect culvert capacity under inlet control. Will U/S stream modification help in taking more Q inside culvert?

[Scott Miller]

If you’re looking at design alternatives, start with the culvert inlet. Wingwalls and a headwall would get more flow into the culvert. Another remedy would be to upsize the culvert. Whatever is going on at that constriction will affect how the bed adjust to stream flows.

If you’re asking about changing the modeled bed to reduce modeled overtopping, it is possible to design a transition that minimizes head loss at the inlet. But that is design for concrete form work that might be applied to a low loss aqueduct ($$), not natural bed and bank materials.

[Author]

Posts