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HEC RAS - Modeling winding river in high flow conditions
4

HEC RAS - Modeling winding river in high flow conditions

HEC RAS - Modeling winding river in high flow conditions

(OP)
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that it the stream over tops the road for a few hundred feet and the original winding low flow stream alignment is no longer applicable.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

RE: HEC RAS - Modeling winding river in high flow conditions

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.

RE: HEC RAS - Modeling winding river in high flow conditions

(OP)
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

RE: HEC RAS - Modeling winding river in high flow conditions

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?

RE: HEC RAS - Modeling winding river in high flow conditions

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

RE: HEC RAS - Modeling winding river in high flow conditions

(OP)
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!

RE: HEC RAS - Modeling winding river in high flow conditions

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.

RE: HEC RAS - Modeling winding river in high flow conditions

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

RE: HEC RAS - Modeling winding river in high flow conditions

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.

RE: HEC RAS - Modeling winding river in high flow conditions

(OP)
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.

RE: HEC RAS - Modeling winding river in high flow conditions

(OP)
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.

RE: HEC RAS - Modeling winding river in high flow conditions

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

RE: HEC RAS - Modeling winding river in high flow conditions

(OP)
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!

RE: HEC RAS - Modeling winding river in high flow conditions

"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.

RE: HEC RAS - Modeling winding river in high flow conditions

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.

RE: HEC RAS - Modeling winding river in high flow conditions

(OP)
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!

RE: HEC RAS - Modeling winding river in high flow conditions

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.

RE: HEC RAS - Modeling winding river in high flow conditions

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!

RE: HEC RAS - Modeling winding river in high flow conditions

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.

RE: HEC RAS - Modeling winding river in high flow conditions

The cross sections shown by gbam show too much concavity in their in shape. I suggest straightening those out a bit more across the floodplain.

Too visualize better, draw two or three longitudinal streamlines (remember fluid dynamics) down the floodplain for the Major event, then draw sections generally perpindicular to those streamlines across the floodplain.

RE: HEC RAS - Modeling winding river in high flow conditions

Er, LincolnPE, the bridge is skew. All the skew function does in HEC-RAS is reduces the x-ordinate distance by the cosine of the skew angle. It accounts for the fact that the flow area in the direction of the flow is reduced. This bridge should have abutments that are at about a 45-deg angle to the road so that the open area is not reduced. Personally, I don't use the skew function in HEC-RAS much. It's pretty straightforward to measure the effective opening width (between west upstream abutment and east downstream abutment). The new bridge should have abutments that are as close as possible to in-line to the flow direction.

RE: HEC RAS - Modeling winding river in high flow conditions

That's not correct Francesca.

The skew angle is measured from a line perpendicular to the bridge's bounding sections. If he is going to use those sections gbam provided, there is no skew angle.

The skew should not be based on the direction of flow upstream of the bridge.

RE: HEC RAS - Modeling winding river in high flow conditions

Show me a reference page in the Hydraulic Reference Manual and I'll show you mine.

RE: HEC RAS - Modeling winding river in high flow conditions

the sketch I saw showed perhaps 5 degrees of skew.

this is not a hecras definition, but a structural / USDOT / AASHTO definition

Skew
When the superstructure is not perpendicular to the substructure, a skew angle is created. The skew angle is the acute angle between the alignment of the superstructure and the alignment of the substructure.
http://www.dot.state.oh.us/Divisions/Communication...

RE: HEC RAS - Modeling winding river in high flow conditions

(OP)
Before this gets out of hand, Yes, I realize the bridge is slightly skewed to the stream. However in the Hecras Bridge/Culvert Skew input, it says "In model testing, skewed crossings with angle up to 20 degrees showed no objectionable flow patterns. For increasing angles, flow efficiency decreased.". Between that statement, and the fact that I placed my bounding xsecs parallel with the bridge face, I am assuming the effect of the bridge skew is negligible.

RE: HEC RAS - Modeling winding river in high flow conditions

The HEC-RAS definition is what's important to the modeling. The bridge is perpendicular to the road (or maybe 5 deg skew), which is almost parallel to the stream. Hydraulically, the bridge is substantially skew, and the flow area needs to be reduced accordingly. The cross-sections are correctly placed to use the HEC-RAS skew function, which will reduce the flow area by the cosine of the skew angle entered. I would use 45 degrees in this case -- even though it's closer to 90 deg, the channel does bend and you can tell from the undercutting that the water is turning like a meander. I would make the new abutments skew at 45 deg (like this: http://www.fhwa.dot.gov/pavement/concrete/pubs/if0...) with lots of scour protection on the upstream side. I would also make the bridge asymmetrical about the channel, with more overbank on the west side through the opening, but there may be channel migration if there are large storm events, which needs considering if the replacement bridge is not a clear span.

RE: HEC RAS - Modeling winding river in high flow conditions

Francesca:

The bridge skew is not relative to the upstream flow direction. The flow makes the turn.

Here is a more clear example:

Say we had a completely straight stream section, no meandering.
We have a bridge that is skew to the straight stream sections.
Then, we would use the bridge skew to show a reduced projected area, or we would not use the HEC RAS bridge skew function, and calc our own reduced projected area.



RE: HEC RAS - Modeling winding river in high flow conditions

There are head losses in the flow making the turn, which raise the water surface elevation. There are also significant scour effects, particularly if the replacemen bridge has piers. The HEC-RAS Hydraulic Reference Manual does not refer to skews less than 20 degrees meaning how the engineer has cut the sections, it means bridges skew to the flow direction. The manual assumes that the engineers will cut the sections as they recommend, which is parallel to the base of the abutments. Skews greater than 20 degrees, like in this example, do have hydraulic effects. You can't ignore them.

RE: HEC RAS - Modeling winding river in high flow conditions

Francesca:

You are not grasping this conceptually.

The only "flow direction" HEC RAS knows is based on the sections. How could it know any different?

Yes, go ahead and add more loss coefficients to the bridge approach sections as you see fit.

Yes, go ahead and add abutment scour protection as you see fit.

RE: HEC RAS - Modeling winding river in high flow conditions

LincolnPE, I think you are only looking at the channel. In a 100-yr flood event, the majority of the water is flowing on the overbanks and not in the channel. In low flow, the bridge is not skew and the water turns in the meander. In high flow, the water comes barreling almost parallel to the bridge and has to make the turn, resulting in the undercutting you can see on the contours on the upstream inside bank and the deposition you can clearly see in the wake area (expansion reach in HEC-RAS terms) on the downstream bank.

RE: HEC RAS - Modeling winding river in high flow conditions

Yes, we are just going to see how this Major event fits and looks after this first run. Should be a little bit of an iterative process.

For example, how much flow goes Left Overbank in the open area? That would align the general flow direction more with the bridge. If majority of the flow distribuiton comes barreling down Right Overbank like you suggest then skew would be weighed more in consideration.

It also depends on what sections Dog ends up going with. It's a little hard to get more topo detail from the picture.

RE: HEC RAS - Modeling winding river in high flow conditions

Considering that a typical channel floods its banks in a 2-yr recurrence storm event, there is enough flow on the right overbank for the skew to be considerable. There is also the constriction on the left bank at the bridge which will shift flow across and cause even more head loss on the upstream end. I would lay money on it that the entire intersection is underwater and you'll have to rethink your cross-section lines entirely.

Is this model being worked on by both Jarvdog and LincolnPE?

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