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that it the stream over tops the road for a few hundred feet and the original winding low flow stream alignment is no longer applicable.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

 

[LincolnPE]

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.

 

[francesca]

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.

 

[LincolnPE]

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.

 

[francesca]

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

 

[cvg]

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/Communications/BridgingtheGap/Pages/BridgeTermDefinitions.aspx

 

[Jarvdog]

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.

 

[francesca]

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/if07034/ch3.cfm)

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.

 

[LincolnPE]

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.

 

[francesca]

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.

 

[LincolnPE]

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.

 

[francesca]

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.

 

[LincolnPE]

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.

 

[francesca]

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?