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that it the stream over tops the road for a few hundred feet and the original winding low flow stream alignment is no longer applicable.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
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
RE: HEC RAS - Modeling winding river in high flow conditions
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
RE: HEC RAS - Modeling winding river in high flow conditions
RE: HEC RAS - Modeling winding river in high flow conditions
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
RE: HEC RAS - Modeling winding river in high flow conditions
RE: HEC RAS - Modeling winding river in high flow conditions
RE: HEC RAS - Modeling winding river in high flow conditions
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
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
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
RE: HEC RAS - Modeling winding river in high flow conditions
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
RE: HEC RAS - Modeling winding river in high flow conditions
RE: HEC RAS - Modeling winding river in high flow conditions
I really do appreciate everybody's input and effort here. Thank you very much!
RE: HEC RAS - Modeling winding river in high flow conditions
RE: HEC RAS - Modeling winding river in high flow conditions
RE: HEC RAS - Modeling winding river in high flow conditions
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
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
RE: HEC RAS - Modeling winding river in high flow conditions
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
RE: HEC RAS - Modeling winding river in high flow conditions
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
RE: HEC RAS - Modeling winding river in high flow conditions
RE: HEC RAS - Modeling winding river in high flow conditions
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
RE: HEC RAS - Modeling winding river in high flow conditions
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
RE: HEC RAS - Modeling winding river in high flow conditions
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
Is this model being worked on by both Jarvdog and LincolnPE?