Hec-Ras Culvert modeling
Hec-Ras Culvert modeling
(OP)
Hello,
I am currently using hec ras 4.1.0 and have a situation where i have the stormwater from a 300 acre drainage area traveling down a stream and the stream becomes a 30" pipe that is 200 feet long, and then the stream picks up again on the other side. The pipe is way under capacity obviously, and what i think happens is that the pipe passes X amount of water and the rest is overland flow (above the pipe) for the 200 feet until you pick up the stream on the other side.
Is there a way to model the culvert and overland flow all in RAS?
This isn't exactly a bridge/culvert scenario because the grading for the 200' of overland flow is not consistent. I couldn't for example just treat this as a 200' wide bridge deck with a culvert because i need to describe how the grade changes at every 25' which is what our state environmental dept. reviewer is requesting.
If i were to model the pipe OUTSIDE of RAS, what would be the best program? I tried to just model the pipe in STORMCAD, and by trial and error kept increasing flow until the hydraulic grade was above the upstream pipe headwall (overland flow). but i don't know if there is a better way???
thanks in advance
andy
I am currently using hec ras 4.1.0 and have a situation where i have the stormwater from a 300 acre drainage area traveling down a stream and the stream becomes a 30" pipe that is 200 feet long, and then the stream picks up again on the other side. The pipe is way under capacity obviously, and what i think happens is that the pipe passes X amount of water and the rest is overland flow (above the pipe) for the 200 feet until you pick up the stream on the other side.
Is there a way to model the culvert and overland flow all in RAS?
This isn't exactly a bridge/culvert scenario because the grading for the 200' of overland flow is not consistent. I couldn't for example just treat this as a 200' wide bridge deck with a culvert because i need to describe how the grade changes at every 25' which is what our state environmental dept. reviewer is requesting.
If i were to model the pipe OUTSIDE of RAS, what would be the best program? I tried to just model the pipe in STORMCAD, and by trial and error kept increasing flow until the hydraulic grade was above the upstream pipe headwall (overland flow). but i don't know if there is a better way???
thanks in advance
andy





RE: Hec-Ras Culvert modeling
You can use lidded sections in HEC-RAS which will allow you to do this. I've seen varying success with this method...it is very finicky based on whether you are using steady/unsteady flow modeling and the ratio of overland to pipe flow. In instances, I've seen very crazy results where several profiles (from 2- to 500-year) cross and zigzag in and out of the lidded sections. But it may be worth a shot if you want everything in one model.
Unless have a more robust model (such as SWMM), your HEC-RAS/StormCAD combo is a good bet! Just reduce your flow in HEC-RAS by your pipe capacities.
Good luck!
RE: Hec-Ras Culvert modeling
RE: Hec-Ras Culvert modeling
thanks again.
andy
RE: Hec-Ras Culvert modeling
i was able to generate a ratings table from Hydrocad. Basically i modeled a small basin of insignificant storage, and can get the pipe flow at each elevation of headwater. HOWEVER! i need to put something as my downstream TAILWATER elevation which would significantly reduce the flow. I'd obviously like to use FREE DISCHARGE, but i know that there will be a flood elevation....
This is a chicken and egg situation. I want Hec Ras to tell me the Flood elevation for the entire stream, but in order to get the proper ratings curve for the pipe, i NEED the flood elevation. i guess i could use the top of bank elevation downstream as the tailwater elevation, assuming that the stream is totally flowing full - that would be super conservative.... i just hope there is a better way
RE: Hec-Ras Culvert modeling
RE: Hec-Ras Culvert modeling
RE: Hec-Ras Culvert modeling
i actually ended up adding the culverts to the overland cross sections, and added lids above the culverts. The cross section looks complicated, but at least it's all in ONE program. It gives me a result that is similar to what i would have expected. i now need to see how it compares to the method of doing the pipe calculation in another program. i want to see how close they are.