How to model a tight s curve in HEC-RAS 3

[MNcivil1]

Hello,

I am trying to model an "s curve" in HEC-RAS, the channel width is about 30 feet and there are 2 "s curves" in about 500 feet. After about the 25 year event the stream comes out of the channel into the overbank. How should I do my cross sections? Does HEC-RAS effectively model this situation?

Thanks!

 

[Ryb01]

Your channel cross sections should be cut perpendicular to the direction of flow, within the floodplain. If the flow depth is deep enough where the "S" shape in the channel has little to no impact on the flow direction, then your cross sections should reflect this. It would all depend on what return period you are modelling.

Hope this helps.

 

[francesca]

Your cross-sections should bend to always be perpendicular to the flow. I would wager than it's a smaller storm event that sees the majority of the flow in the overbanks. A good rule of thumb is that bank-full is a 1.5 year storm.

Nevertheless, you will still station your river according to the length on the channel, but your left and right downstream reach length will be substantially shorter, representing the distance along the overbanks in a straight line. You will need to raise your channel Mannings value. For meandering streams with woods on the banks we use n=0.05 to n=0.06 for streams that otherwise (straighter) would be 0.04 to 0.045.

 

[beej67]

I ran into this on a LOMR down in Florida. There's actually a really slick way to handle this, by adjusting your Manning's coefficient for the relative meander of the S curve, based on some Chow work in the late 50s.

Look for a pdf document online called:

Guide for Selecting Manning's Roughness Coefficients for Natural Channels and Flood Plains -
United States Geological Survey Water-supply Paper 2339

This link currently works:

http://www.fhwa.dot.gov/bridge/wsp2339.pdf

It's substantially the same document as FHWA-TS-84-204, same authors / etc. That version is referenced in my grad open channel flow text (Sturm) which I also highly recommend.

Download that, and fish down for the Mannings adjustment due to 'meandering.' In a nutshell, you calculate a ratio of straight line distance vs centerline distance along your 'S Curve,' and use that ratio in their method of generating a composite n value to account for the meander.

The whole document is a great read. They have photos of flood plains with n values assigned to them also, which I've found quite helpful.


Hope this helps!

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[MNcivil1]

Thank you all for your help, however I don't think I did a good job in explaining my problem... I have attached a picture for your analysis. The issue as I see it arises at the beginning of the "s curve" this cross section would cut across the entire upland and through the channel twice. If the upland in the middle floods out in the event that I need to model (Q100). I have labeled this cross section "problem cross section" in my picture. I came up with a solution and labeled it "solution cross section?". Let me know if this is the proper way to solve this problem.

Thank you for your help!!

 

[MNcivil1]

Also, if that is correct, how often should I do a new cross section?

Thanks!

 

[Ryb01]

Provided that your flows for the return event you are looking at are contained within your "solution cross sections", then the way you have them cut seems reasonable. You may want to cut more sections in the vacinity of where more confined section of the channel opens up into the broader flooplain area (start of the "S" curve.

Generally cross sections should be cut at locations where it is aniticipated that there will be a significant change in the flow profile, (i.e bridges, culverts, expansions and contractions in the floodplain, etc). Beyond the general recommendations placing cross sections associated with the above, the user, you, will have to decide an acceptable spacing between sections to provide the level of detail you are looking for. HEC-Ras interpolates between sections. The spacing would depend on what the flow is seeing. If you have a straight reach with minmial changes between cross sections, cross sections every couple hundred feet may be appropriate. However, if there is chanses within the sections, as shown in your pdf, then the cross section spacing would decrease.

Hope this helps.

 

[SMIAH]

Is it fully sub-critical in this reach?

 

[beej67]

The thing to understand here is that you've got a complicated system, that's only really model-able "correctly" with some kind of finite element analysis of the navier stokes equations in 3d, which is more of a doctoral thesis than an engineering project. You want to make assumptions to simplify the system enough to where RAS can give you an answer that's right "enough" for your problem.

As a simple check, try a model with a section upstream of the S, a section downstream of the S, and no sections between the two. Then apply the (probably fairly severe) meandering adjustment to the mannings coefficient for that stretch of the river, and see what it gives you at those two sections. Compare it to what you're working toward now. It should be fairly similar, and if not, then you might want to rejigger your intermediary 'S' sections in such a way that the model gives you results similar to that 'check' model.

The thinking being that the meandering coefficients are in there for a reason, you might as well try to match their results.

Another approach you might consider, is making a conservative engineering assumption that the midpoint of the "problem cross section" doesn't really pass a lot of flow across it, and if that's the case then the flow follows the channel centerline around in a big semicircle. You could cut several sections around the semicircle, and have them all terminate at the center point of the circle. That point lays right on your "problem cross section," near where you have those three inflection points shown. If you end the section there, RAS will assume there's a bit of a vertical wall there, and pass water through the rest of the section, which matches your assumption.

I attached a guess markup. What do you think about that Ryb01? You're sharp, and I value your opinions quite a bit. As I understand it, that would get RAS to properly evaluate the assumption of no short circuit flow through the floodplain. The question then would be how valid is that assumption? It certainly seems conservative.

The next thing to ask, is what regulatory hoops you're facing MNcivil1. If you're just trying to get this right on your own, that's one thing, but if you're submitting it to FEMA then all the brain power of ENG-TIPS can't trump whatever your reviewer wants to see. It's good to have an approach in mind, and it's even better to be able to defend it, but your reviewer might tell you to scrap it and do it a different way, and you're honestly basically stuck with doing whatever they tell you. You might save yourself a lot of headache by asking your reviewer how he wants it done in advance in addition to asking other professionals for their opinions.



Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[francesca]

What are the numbers on those contours? Unless they are multiples of 10 or 20 (in which case your dashed lines are correct), you need a lot more topo. Anything greater than a 1-year storm will probably blow the banks. I would call the line you labeled as "problem cross-section" the average flow direction in a flood.

I recently had to completely do over a model that someone else had had three stabs at. The first modeler was looking narrowly at the channel, whereas I looked at the whole floodplain. Focusing on the channel it was complicated, focusing on the flood plain it was simple, even though the channel paralleled the road at one point and was perpendicular to the overall flow direction at many others.

 

[Ryb01]

I'd think that you're on track with your assumption beej67.
If the geometry of channel would allow for this assumption very time, it'd be a good way to cut cross sections for most projects.

I haven't yet to date come across a short circuit in the model when modelling channel sections. However, have come accross some interesting scenarios. One being, I've received modelling which has had x-sections cut in a way that I could only describe by shaking my head. Recut the cross sections to what I had perceived as being correct, to find only a marginal difference in flood elevations. Mind you the flows were relatively large.

As francesca has pointed out, depending on flows, and contours of the map, the cross sections as provided may not be sufficient to contain the flows. Accordingly, the cross sections would have to be recut. The level of detail required and bugetary constraints will determine the approach for most projects.

I often think that the required complexity of the engineering work we complete. The interesting thing about stormwater management, hydrology and hydraulics, is the user variance in which a project is completed. Five people could do the same work, on the same project and all come up with different answers. Which one is right?

beej67 also hits on another key point with regards to your local reviiewing agency. I've always found it to be helpful to have a discussion with the reviewer before you get too deep in a project(if possible) to discuss general approach and expectations. They may be able to provide guidance as well if you have a unique project that they may have had experience with before. Two heads are better than one, most times.

Hope this helps.

 

[mherr78]

Are you utilizing levees and ineffective flow areas? You could place a levee to the right of the channel, at the highest point before the second crossing of the channel. This would prevent the calculations from using the second channel crossing's section properties in the modeling. It would only use that geometry if the flow breached the top of the levee.

I hope that I was clear, or that I understood your problem. I've had this problem on an inundation study, and the levees allowed me to contain the flow in the primary channel.