Encroachments in Hec-Ras

[TheEnginerd]

Hello All,

I seem to be having trouble performing the encroachment calculation for my model.
I created a second profile (PF2) with the same flows and cross sections as the original (PF1) and ran the encroachment option using method 4 (with all cross-sections designed to have a 0.2' rise), which is as per my state's DEP requirements. The results are bizarre!
Instead of showing a 0.2' rise, many of the water depths for PF2 remain unchanged, and some when compared to PF1 show a rise in water depth up to 2 feet! Does this sound like something anyone has encountered? I could provide any information that you think would be helpful to diagnose the problem.
FYI, I have used this process successfully, years ago - probably an earlier version of hec-ras. So i feel that the error may have to do with this particular model.

Andy

 

[froude]

Unfortunately I encounter this all the time. The HEC-RAS floodway engine is very sensitive and finicky. Even though you set a target rise using Method 4, I don't believe the program iterates enough to relegate encroachments to your specified "maximum."

I've noticed that I get very high delta WSELs in areas near bridges, culverts, and reaches with many sections in close proximity. You may try to import all values to Method 1, and then manually alter your encroachments until you get a working analysis. This will give you an idea of where you can encroach more and where you have encroached too much. It's tedious, but, to me, the only way to be sure you don't have any crazy encroachment patterns. Also be sure to take into account any existing regulatory floodway encroachments, if they exist.

Good luck!

 

[TheEnginerd]

thanks Froude,
Glad to know that i'm not alone, if nothing else.... but no matter what i try, i cannot seem to "encroach" portions of my model. i tried the encroachment procedure on a file that i had from a little while back and it works fine, it's just this file. I'm out of options, except adding cross-sections to make eliminate all of the warning messages, etc.
andy

 

[TheEnginerd]

froude,
i painstakingly am doing the encroachments using method 1, one by one, moving from upstream to downstream, careful so that i do not upset the water surface for the upstream cross-sections. Thanks, this seems like the only way. what a pain!
andy

 

[froude]

If you're doing a subcritical analysis, you may want to work downstream to upstream. Some of your downstream encroachment changes could very well affect upstream deltas.

 

[Crystalct1]

Yup. The manual makes it seem like the program can do this efficiently, but I found that I had to manually set a lot of my encroachment stations. Even found that bringing the stations in closer to one another, made the resulting elevation get lower! I would move on to the next one, set that one and come back to the first one and it usually worked out after many trials. Nope. You are not alone.

 

[TheEnginerd]

ha, i also had cross-sections where bringing the stations closer to one another made the WSE get lower... bizarre! Since my last post, i have submitted our model for permitting. AsiIt turns out our state DEP reviewer does not accept the fact that HEC RAS might not be perfect, and is giving me a really hard time... i'm very over budget on this particular project just trying to make the model show the "expected" results. All these thousands of dollars wasted, and the ironic part is that we're making the situation in this stream 1000% percent better as a result of the proposed improvements.