"Contrary" Results with HEC-RAS on a No Rise Project 1

[ajh123]

First time HEC RAS user here. How do you establish the floodway widths with this model?

Also, this model gives us contrary results that involve the specific energy curve and sub- versus super-critical flows; i.e., decrease the n and wsel increases; remove soil and wsel increases.

What's the best approach to take here to overcome these effects?

Thanks.

 

[froude]

Chapter 10 of the users manual (

http://www.hec.usace.army.mil/software/hec-ras/documents/userman/ch10.pdf)

will walk you through exactly how to do your floodway model.

How you establish your widths are dependent largely on the floodplain regulations/desires of your community's floodplain official. You may be able to match existing floodway encroachments that are shown on the FIRM maps or in the FIS, or you may have to develop entirely different widths for your reach.

Typically, I like to use Floodway Encroachment Method 4 (see the manual linked above) and set the target WSEL change to 0.8 to 0.9 feet). If the resulting resulting WSEL increase is 1.0 feet (+/-), I mport the encroachments into Method 1 (again, see manual). Then I fine tune the encroachments to ensure that there is no negative surcharge and no increase above 1-foot in WSEL. If the change in WSEL far above 1.0 feet, then you may have to start with a lower target WSEL.

Regarding the relationships in n-value, cut/fill, and WSEL...welcome to HEC-RAS...sometimes if you hunt you can find the rhyme and reason. Other times you can't. However, there usually would not be a change in 'n' or cut/fill between the Post-Project FP and Post-Project FW model...the geometries should be the same. That being said, the FW model can be very sensitive to changes in X-Section geometry and n-values. So if you make changes to bank stations or anything else in your FW model, make sure you change it in your Post-Project.

Hope this helps. Good luck.

 

[ajh123]

Questions Concerning "Continued HEC-RAS Contrariness"

Well, I found I do not need to determine floodway widths or the super elaborate FEMA requirements for a "full-blown" approach to HEC-RAS use (such as given in that 7-page monster put out by FEMA). I simply need to show that WSE is not increased for this no-rise investigation.

I've gotten very close to showing no-rise, within a rise of 0.01' in 4-5 cross sections, but the model is so extremely sensitive that it's still ellusive. I've tried removing soil and reducing n-values, to no avail. But we have quite a few trees on the site, it's a lot on the river, and I would like to enter these trees as areas of ineffective flow in the existing conditions model run with my XSs added, then perhaps reduce the 0.08 n-value used by FEMA in this residential area to offset tree removal, and then remove the trees for the final conditions model run. Is this a no-no? It seems logical to me, since this model is so sensitive. What do you say? And what is the best text book on all this?

I've tried repeatedly to remove soil from the site but the model won't tolerate it, no matter how slight, showing WSE rises each time.

Also, I can't find any information on the appropriate use of Obstructions and/or Areas of Ineffective Flow to depict the addition of an elevated house in the HEC-RAS model. The User's Manual describes briefly how to employ both of these tools, but in no way discusses the conditions of their most appropriate use; i.e., the Manual tells us how to use the model, but never tells us "how and when" to use this model. Am I alone in this conclusion? What other texts are commonly used to teach this model's use? (I've already ordered the Haestad text on it.)

Thanks for any reply - help you can give.

ajh123 in Texas

 

[froude]

As far as the 0.01' rise goes, this will just take some patience on your end to work out. Usually when you get it down to 0.01' rise it means that your improvements probably work in theory, but that you need to fine tune your model.

You may consider adding more sections to see if you achieve no rise. Sometimes you may have to re-align sections. Other methods may include moving your bank stations around (within reason) or using horizontal variation in n values.

Regarding the conditions and appropriate uses, the only thing I can add is that there are several ways to skin a cat. If what you do is logical and within reason and you can get your reviewer to buy off on it, then you can use it. I would say that if you just want to show your house, then use blocked obstructions and put in your left and right limits using the "multiple block" option.