HEC RAS - Modeling winding river in high flow conditions 4

[Jarvdog]

This is my first post to eng-tips, so if you have any tips about forum posting, I will take them into consideration next post. I would like to state that I am relatively inexperienced in the civil engineering world. I just received my B.S. in CE in May of 2011 and have been soaking in as much knowledge and experience I can at work. I have modeled a few bridges and culverts in hec-ras over the last year and a half, however I know I really haven't even scratched the surface. Please keep this in mind. Any recommendations or suggestions would be helpful.

I am currently modeling an existing private bridge on a windy stream in a flat, wide, flood plain with a low flow of 400cfs. A few hundred feet upstream of the structure, the stream turns and runs parallel with the road until it gets to the bridge, where it turns again to flow underneath it. I created the geometry in microstation and imported it into hec-ras. I created cross sections perpendicular to the stream. Once I ran the model with a Q100 of 1750cfs, it quickly became apparent that it the stream over tops the road for a few hundred feet and the original winding low flow stream alignment is no longer applicable.

Assuming that the flood plain is flooded and is now using the road as a weir, in addition to the bridge, my flow path goes from parallel to perpendicular to the road. So now I need to create a new stream alignment, bank/over bank location, and cross section locations. I'm assuming my cross sections will need to be parallel with the road and bridge. Is there a way to make nonlinear x-secs in hec? Another issue is placing my channel bank locations and over bank locations on the new cross sections. The low flow situation locations of the banks is obvious, however once the channel width becomes the width of the entire flood plain, it becomes unclear. The same goes with the stream alignment location for the very same reasons. I'm hoping this is a common situation.

I hope I was able to describe the issues and circumstances well enough here. The hydraulic engineer on staff is currently out of office and cannot be reached, so I would greatly appreciate any recommendations.

Thanks,
Ryan

 

[gbam]

Jarvdog - (keeping your current alignment) you can recut the sections in INROADS/MicroStation using a Selection Set. Import the geometry into HECRAS then manually adjust main channel and overbank lengths to reflect the flood condition. Remeber the "flow path" is alogn the centroid of flow. You can then model the roadway as a perpendicular weir with the bridge as a multiple opening if my understanding is correct. A plan view of the xsections would be helpful.

 

[Jarvdog]

Gbam - Thank you for responding! I understand that I can recut the sections in new locations, although am I uncertain how to lay them out. As you can see from the attached picture (hopefully it shows up!) the road has a slight horizontal curve to it. Would I place the x-sections completely parallel to the road as I would do at the bridge opening? Or would making them linear be just as good?

If my understanding is correct, I would extend the embankment the length of the road is parallel to the river to make a weir out of it. And I can keep my current flow alignment and just change the x-section orientation? If the flow path is the centroid of the flow, I would assume I would have to change the alignment so it's perpendicular to the road. Attached is a .jpg of the drawing. Hopefully it shows up well enough on here.

-Ryan

 

[gbam]

First off the horizontal curve is very flat and could probably be treated as straight. Your horseshoe bend is always troublesom. If you can terminate your study before the bend that would be great. I would run your initial model and smooth out the floodplain then cut cross sections perp to flow. Also, I would maybe treat the roadways on both sides of the bridge as lateral weirs (perp to xsecs). Remember to incorporate the expansion and contraction boundaries (ineffective flow) at least to the upstream bend. These tight 90^ bend are dificult to model with a one dimensional model like HECRAS. Have you considered FLO-2D?

 

[cvg]

whoa, you are going to throw the newbie into flo2d land?!

 

[Jarvdog]

Alright, I believe I have the right idea now. Since I last posted, I ran a pond volume analysis to make sure check to see if I had enough storage to lower the water elevation. Turns out it fills up in little over 3 minutes, so that's a non-factor. I will definitely use your suggestion of stopping the study before the bend. I'm about to re-run the model and see what happens with the new x-sections. I doubt the roadway to the right will account for much flow as a weir because that is higher ground to the east.

Hopefully this goes well! Also, sorry for the late response. I was expecting that I would receive an email for every reply, but I did not. I'm glad I checked the site again! To answer your FLO2D question, No, I have not. And from what CVG says, this might not be the right time to learn haha!

 

[francesca]

No! You can't stop the study before the bend because energy losses in the bend will control your WSE! This is not really that complicated of a situation. The road should be a lateral weir, not a perpendicular weir. Your cross-sections as you have them are only relevant for channel flow only (<2 yr storm). You need to cut your cross-sections perpendicular to the center of mass of the water, which will be mainly flowing in the overbanks. Consider all areas of the overbanks deeper than 3 ft to be active flow. In a meander under overbank flow large areas of the channel are ineffective because they are opposed to the flow direction. You can handle all of this in your reach length adjustments. By setting your ineffective flow areas carefully you can handle the fact that only channel flow will pass through your existing bridge. I strongly recommend an early conversation with your reviewers to agree the modeling approach. This will prevent a lot of anguish and do-over.

 

[gbam]

Francesca the horseshoe bend is upstream of the area of interest. How will it impact the downstream watersurface profile? near the bridge?

 

[francesca]

Okay, I didn't realize the horseshoe bend was upstream. However, it is very close to the area of influence and I wouldn't leave it out. Eventually the river is going to do a cut-through at the apex of the bend, cutting off most of the meander that's to the right where the topo runs out. Most of that meander will be ineffective under flood conditions. (Assuming there aren't bluffs in between.) The main water flow direction is pretty much straight, at a small angle to the road. It's not that challenging of a situation if you recognize the ineffective channel flow and concentrate on the overbanks where the majority of the flow is. You still need to set your expansion and contraction coefficients to correctly handle the large energy losses. Something closer to bridge section values may be appropriate through the meander. What is the purpose of the study? There need to be a few more sections downstream of the bridge to let the WSE calculation settle before getting into the bridge expansion/contraction reaches, but I would wager that the bridge is largely underwater with pressure flow at flood stages, so flow may be fully expanded anyway.

 

[Jarvdog]

Thank you for responding Francesca! The study is for a private bridge replacement. In the state of Wisconsin, a replacement bridge cannot raise the upstream water elevation which the existing bridge gives us during a 100 year storm. So, I analyze the existing bridge, find the upstream water elevation profile. Then I analyze different bridge designs and find designs that replicate the existing bridge's upstream water profile (w/out raising it).

I understand that once the stream floods, the meanders become obsolete and the stream starts to straighten. I did not completely straighten the stream, but I pretty much bypassed the large bend upstream. Since my new stream has a gradual bend, would I still need to adjust the contraction and expansion values upstream? I kept the original bank and OB locations the same as the low flow channel because those determine the locations to the changes in Mannings coefficients. I also adjusted my x-sections to meet the new stream alignment, and extended them along the road, as it acts as a weir. Since I imported the geometry for microstation, hec-ras automatically calculates the reach lengths. Attaches is both the microstation drawing of the geometry and a screen shot of my hec-ras geometry (minus the ineffective flow areas).

Based off of a preliminary run of my model, the flooded event is going to overtop the bridge by about a foot. I am a little confused about setting the overbank regions to be active when it's deeper than 3', because the majority of my flow will be in the overbank. As far as the ineffective areas go, I was thinking of I would start by assuming the river is straight and do the 1:1 upstream and 1.5:1 downstream from the bridge face, setting their elevations to the low point of road. For there, see where I am at and then expand the ineffective area and see the impact. The ineffective flow areas and the contraction and expansion coefficients are really the tricky part on this now.

 

[Jarvdog]

Oops, Apparently I don't know how to attache multiple files at once. The one above was the hec-ras screen shot and this one is the microstation screen shot.

Also, as far as more x-sections below the bridge, I took it as far as we had survey data for the site. So I can't really go any further unless I start interpolating elevations from the topo quad.

 

[cvg]

set the overbank areas to be ineffective for depths less than 3 feet...
that might be an oversimplification, use your judgement on what is effective or not and it could be deeper than 3 feet.
if its overtopping the bridge, hardly any need to extend sections downstream. the bridge is the controlling structure, unless you have a very restricted channel downstream drowning out your weir

 

[Jarvdog]

Okay, it took me a while to understand why I would do that, but it makes sense to me now. Although the water will be in the overbank regions, it's still going to be somewhat stagnant due to wetland grass and brush, until the water gets high enough to start actively flowing. I would have never thought of that, but it makes a ton of sense! Thanks!

 

[LincolnPE]

"Once I ran the model with a Q100 of 1750cfs, it quickly became apparent that it the stream over tops the road for a few hundred feet and the original winding low flow stream alignment is no longer applicable."

There is no way you could determine that with those little sections you show. You are too focused on the channel and not the floodplain. Your sections do not have to be straight. They should be perpindicular to contours, but also generalized across the floodplain at the same time.

That road is a potential Levee and you need to do a Levee analysis first, seeing if the flow stays on the channel side with the sections I generally show attached. No lateral weirs here.

 

[LincolnPE]

Attachment:

 

[gbam]

Jarvdog - I question your last set of Xsecs. Those do not look correct. My first trial for this model would look like this, see attached. It is okay to "bend" your xsecs. Also, your xsecs may need to be revised the more runs that you make. Everyone has to tweak their model before they call it good.

 

[Jarvdog]

Okay, I see what your saying. LincolnPE, I agree with you that I was too focused on the channel, rather than the plain itself. It was my thought that the xsecs had to be perpendicular to the flow. Either way, I don't believe the xsecs would cross the road because the river downstream runs parallel with the way you oriented the xsecs. Gbam - I like the look of how you positioned the xsecs, however once the plain floods and it over tops the road, the flow is now perpendicular to the road. Wouldn't that change the orientation to be more parallel with the road? Also, the road to the right is high ground with a house, so I shouldn't have to take that into consideration with the model. Thoughts?

I really do appreciate everybody's input and effort here. Thank you very much!

 

[francesca]

I agree with gbam's cross-sections and I would include the road as a lateral weir for the upstream cross-sections. You don't have to worry with expansion/contraction reaches because the flow is fully expanded due to the overtopping. The one thing I would note is that I would consider the bridge to be skew maybe as much as 45 deg. It looks like you have undercutting on the upstream west bank and deposition on the downstream west bank.

 

[gbam]

jarvdog - no, I would not parallel the roadway with XSECs. The embankment is sqeezing the flow towards the bridge. Flow can overtop the road as a lateral weir as depicted in my XSECs. francesca brings up a good point of skew on the the bridge. you are being baptised on good one!

 

[LincolnPE]

The one thing I would note is that I would consider the bridge to be skew maybe as much as 45 deg.

This is not true. The skew angle is measured relative to the upstream bridge cross section, not the upstream flow in river. Flow usually makes the turn and heads through the brdiges. This is a little used option because the modeler's sections usually address the situation.