Bridge at large skew to Flow 2

[jdp2248]

See attached pdf. I have a new railroad bridge with a 50deg skew to the flow lines. I assume the skew option in the Bridge editor only modifies your geometry. With this in mind I input my geometry as is (projected onto a plane perpendicular to flow), since the skew option will only let me go up to 45deg. My problem now is how to deal with the abutments. I was thinking i should use obstructions in the cross sections US and DS. Also, i'm concerned the calculations will not be accurate since i'm not able to cut sections 1,2,3&4 (per manual) correctly.

Would it be more accurate to just use blocked obstructions to model the piers and normal obstructions to model the abutments and not model with a bridge at all.

Thanks,
Josh

 

[jdp2248]

I should have mentioned. I'm modelling with the latest version of HEC-RAS.

Thanks for the help.

 

[francesca]

I have just completed several models of road bridges some with skews up to 45 degrees and some with variable skews. I've had some fun times with bridges that have roadway overflow where the road runs parallel to the river in places. These models have been internally and externally reviewed and approved so more than one engineer has signed off on the methodology. The most complicated one we got consensus in-house and then went to the external reviewer so we could discuss and agree on the methodology before I even cut the cross-sections. I really recommend you do the same.

The way you have your cross-sections looks good to me. Generally for skew bridges that were not simple enough for the skew function (which indeed just reduces the horizontal ordinate by the cosine of the skew angle) I cut my cross-sections perpendicular to the stream and determined a "perpendicular clear span" for the bridge. Calculating the bridge length then was up to the bridge engineer. (The HEC-RAS span really just determined a clear area under the 100-year elevation that the bridge engineer needed to provide, rather than specifying a span.)

I agree that there are some issues that this approach does not adequately consider:
- angle of attack on abutments/piers: does the flow "bend" or continue perpendicular through the opening?
- do the 1:1 contraction/2:1 expansion ratios apply in this case? (With a 45 degree skew, the abutment is entirely within the contraction reach so no additional obstruction needs specifying.)
- do the 0.3 expansion / 0.5 contraction coefficients hold in this case? Should lower values be used to be conservative? Or maybe higher because you're effectively only constraining one bank at a time?

I am most concerned with the question of angle of attack on piers. The abutment scour equations are so grossly over-conservative and abutment scour protection is fairly trivial. However, if the skew angle induces even a 15 degree meander through the bridge section, that significantly increases your pier scour depth. Think about the center of gravity of the water and how its inclined to respond. Draw in your contraction and expansion reaches and look for areas where eddies will form. Determine your skew angle (for in-line piers) based on the center of gravity of approach, but check your pier scour for a lower flow condition with an angle of attack based on the angle of the channel to the angle of the piers.

I can't see much of the channel from the image you shared, but it looks like the channel varies quite substantially in width, the river is on a slight meander and there may be some anabranching. These are indications of instability. You might want to check historical flow data and see when last they had a big flood, to see if the mature-looking trees on the banks give a false impression of channel stability.

 

[jdp2248]

Thanks so much for your insight. I've uploaded some pics that will paint the whole picture more clearly. You'll notice that there are several bridges very close to each other.

I wanted to make sure i was correct. The skew angle in my case is 51deg (to be exact, 90-39), per my first exhibit. Its the angle the bridge makes with a line perpendicular to the flow.

I gather from your comments that i'm heading in the right direction.
-Cut sections perpendicular to flow as normal
-Add bridge deck and piers
-Use obstructions to model the abutments. (i'll have a right obstruction in my US x-sect and a left obstruction in my DS x-sect).

I'm unfamiliar with scour analysis so I didn't quite understand your following statements: "However, if the skew angle induces even a 15 degree meander through the bridge section, that significantly increases your pier scour depth. Think about the center of gravity of the water and how its inclined to respond. Draw in your contraction and expansion reaches and look for areas where eddies will form." My next step was to read up on scour.

I have asked the bridge designer to make the piers in-line with the flow and they believe this is feasible. Our beam span though is 70' so we will have piers in the floodway and floodplain. I'm intending to do some compensatory grading and concrete riprap to mitigate. I'm assuming you used HEC-RAS to come up with a design span needed but how does that account for the piers or is that a minor effect. For large floodplains isn't typical to have abutments in floodplain and span the floodway?

Also, do you mean i should meet to discuss methodolgy with a FEMA reviewer, the Floodplain Admin, or 3rd party engineer or all of the above?

How does existing channel stability affect my analysis or approach?

Thanks for your help and expertise! Hopefully the zip file works.

 

[gbam]

zip worked well. Might I suggest that you model the watercourse using either the blocked flow or piers defined using the gemetry editor. If the water surface for your maximum flood does not over top or pressurize your bridge then you ar home free. If it does then you need to address the modeling approach as franseca mentioned above. This is a difficult condition to model using 1-dimensional method.

 

[francesca]

You appear to have already used the pier function, so that area is removed by HEC-RAS from the perpendicular flow area that it reports on the Bridge tab. When you put your piers in, remember to reduce the span by the skew angle. Typically bridge engineers will place a longer span over the main channel. Personally, I place as many piers in the flood plain as needed in HEC-RAS and let the bridge engineer determine the configuration.

When you pass the perpendicular flow area to the bridge engineer, you have to add back in the area removed by piers (# piers x pier width x depth of water). Around here, the bridge designer has to provide the perpendicular clear area under the "natural" 100-year elevation (i.e. no bridges). We therefore add back in/remove the area of water above/below that elevation under the bridge (top width x elevation difference). We compare the elevation on the Bridge Table to the natural elevation linearly interpolated at the downstream face of bridge station.

You appear to be using the upstream and downstream cross-sections for your internal bridge x-sections. (This is the HEC-RAS default.) I will cut a cross-section under the bridge at the exact location the bridge engineer will be using. If you look at your exhibit, the cross-sections are quite different, and different again to what the bridge engineer will use. Using the actual bridge location internal x-section and properly adding back in pier and water surface elevation difference areas means that your bridge engineer shouldn't have to provide compensatory grading.

I wouldn't necessarily bother to put obstruction in the cross-sections because the abutments are within the expansion and contraction reaches and will be inside ineffective flow areas, and therefore already blocked out from the flow area by HEC-RAS. The bridge engineer should design the deck to be a trapezoid rather than a rectangle, with end bents and abutments parallel to the flow direction. (Like this.)

You absolutely MUST leave scour protection up to someone who understands it and is experienced with it. Grouted riprap is very BAD for scour protection. There are a number of NCHRP documents on bridge scour (prompted by bridge failures with loss of life) and some outstanding classes. This book will be a useful start. This one explains scour countermeasures.

Here regardless of apparent channel stability we provide pier scour protection to the depth required if the pier were in the channel. Channel migration does occur, usually when you least expect it. In a live bed situation (where scour holes are filled on the down leg of the flood hydrograph) visual scour monitoring after a flood is useless.

WRT who to discuss your modeling approach with, I'd pick whichever of the FEMA person and the floodplain admin you have the better relationship with/who will sign-off on the model. Hydraulics is usually on the critical path, and nobody likes it when the reviewer makes you redo your model. (Fortunately I was the one redoing other people's models.)

 

[jdp2248]

Here is an answer from HEC for why the bridge modeling skew option only goes up to 45degrees:

"Water will end up "turning" to go through openings, so HEC-RAS contends that even if a bridge is skewed more that 45degrees to the river, the opening water sees will an opening no less that if the bridge was skewed at 45 degrees. If you talk to Gary about it and look at some video, you will see water will turn even more rapidly. "

So if you want to skew the bridge, use 45 degrees as the max.