Hec-RAS Culvert Dam Breach Analysis

[Capnrav]

I am designing a culvert dam for a client and have a need to conduct a breach analysis. I prepared an unsteady model using Hec-Ras version 3. My first attept at the breach analysis I converted the culverts into an inline gated structure of same dimensions and adjusted the gate opening such that the maximum water surfaces for the culvert run and inline gate run are similar.

Now I am looking at Hec-RAS version 4 and noticed a new feature that allows entering a user defined gate. I utilized the upstream rating curve from the culvert model converting it to a user defined inline gated structure and ran the model.

My first question is which approach would better represents my scenario?

And secondly, could anyone possible recommend a better methodology to accomplish a dam breach analysis of a culvert dam?

Thanks in advance

 

[damsafeengineer]

I can't answer your first question. How did each analysis run? Did you run both in steady flow (no breach) first and what error messages and warnings did you get? Have you tried a breach analysis run and did the model crash?

Regarding the second question, it might be better to ignore the culvert altogether. The flow that you would get from the culvert would likely be inconsequential compared to the flow coming from the breach and shouldn't add any noticeable difference to the downstream maximum water surface elevation. If you need a minimum flow to keep the downstream channel wet (so the model can do the calculations without crashing) while the breach is first developing, then add an initial flow in the unsteady flow editor, initial conditions tab. The point of a breach analysis is to get the maximum water surface elevation downstream, so if the culvert flow doesn't add to that, why add the complication? You could also create a spillway that has dimensions so that the max flow is close to what you calculated the max culvert flow to be, but unless the spillway flow adds to the flood elevation I would consider ignoring it.

I did a breach analysis similar to what you have described, only the culvert was through a highway embankment downstream. After I ran HEC-RAS and determined that the highway would be overtopped and possibly breached, I remodeled the highway as an inline structure (so that I could breach it as well). I modeled the culvert as a gate. It ran fine for steady flow, but the model kept crashing during unsteady flow after the flood overtopped the gate. So I had to change it to a spillway that mimiced the stage/discharge of the culvert and then the breach analysis would run. The gate editor is really to analyze the flow through true gate structures where the gates are large and the flow through them is substantial.