Copyright © RASModel.com. 2013. All rights reserved.

FIGURE 1

FIGURE 1

If a bridge
is not oriented perpendicular to the flow lines going through the bridge,
HEC-RAS has an option for skewing the bridge deck and/or piers. (A user can also skew cross sections in a
similar manner) When skewing a bridge,
the user reduces the bridge’s deck/roadway stationing by multiplying the values
by cos Î˜ *b. The new bridge opening
width becomes the equivalent length that is perpendicular to the flow lines
passing through the bridge. Figure 1
shows an example of a bridge to be skewed.

The HEC-RAS
Reference manual makes a note that the skew angle (Î˜) should be based on the
angle of the flow path as it goes through the bridge compared with a line
perpendicular to the cross sections bounding the bridge. It should not be based on angle upstream of
the bridge as shown in Figure 1 with the gray lines, which would overestimate
the skew angle. As the water approaches
a bridge that is highly skewed, it’s common that the flow lines will turn
before going through the bridge. A
field visit is very important to visualize the flow pattern at the bridge and
to help estimate the skew angle.

In
determining a bridge skew angle, a modeler should also consider the question,
“Will I be modeling large flows or small flows and will the flow paths through
the bridge vary between the two?” If you
are modeling a dam breach, it’s more likely that the flow path during the peak
of the flood won’t turn as much through the bridge, and therefore the hydraulic
skew should match the geometric skew of the bridge. If you are only modeling lower flows, the
HEC-RAS Reference Manual suggests that skew angles below 20Âº to 30Âº do not
appreciably affect the flow patterns through a bridge – the reason being that
during lower flows the water/flow lines will be able to turn or meander more
easily through the bridge opening than during large flows.

Users should
also remember to apply the same skew angle to the bounding bridge cross
sections. If for some reason you don’t
want to skew the bounding cross sections, you may have to manually alter either
the cross section or bridge deck stationing to ensure that the bridge opening correctly
aligns with the cross section.

Do the cross-sections have to be perpendicular to the flow for Computation. My Surveyor got the XS data skewed at an angle ranging from 0-30. Entering the data does not represent a true picture of the river. What should be the corrective action?

ReplyDeleteIf the skew is significant, you can provide a skew angle to any cross section. I would certainly do that for skew angles greater than 10 degrees, plus or minus.

DeleteWill my geometry file show the angle change? I entered the skew for the bridge and the geo file did not change, despite recognizing that I had entered a skew.

ReplyDeleteIt will not look any different in the geometry schematic. You'll notice that x scaling change in the cross section plot though.

DeleteChris

HEC-RAS allows for skew angles between 0 and 45 degrees. What if the skew angle is 60 degrees? I have a case where flow is parallel to the roadway upstream and downstream of the bridge and passes beneath the bridge at a severe 60 degree skew. The length of the bridge along the centerline is 86 ft and the normal bridge opening (perpendicular to flow) is 43 ft.

ReplyDeleteOnce you get skews that high, it is common that the flow lines will bend and the actual skew is still less than 45 degree. IN other words, from an aerial photo the skew may look like 60 degrees, but if you measure it off of the flow lines it is much less.

DeleteHave you ever come across needing to set a skew angle for just an entrance or exit? See W Calaveras Blvd Bridge, Milpitas, CA. Lat: 37°25'48.18"N, Long: 121°54'33.27"W.

ReplyDeleteWith respect to the X axis scaling change after applying a skew, would cuts and fills to the skewed XS be representative of actual real world cuts and fills given that the X stations are now shorter in the skewed XS

ReplyDeleteThey should be. Yes. But double check it.

ReplyDeleteChris, should circular piers be skewed? I guess if you look at a circular pier from all angles, the width (Diameter) doesnt change and thus should not require a skew in HEC-RAS. What do you think.

ReplyDeleteGood question. I suppose if it is just a single circular pier (not a column of piers) then I'd say you are correct.

DeleteIf a small stream bends differently u/s from d/s of the bridge, would one skew the u/s and d/s internal sections differently? Should it use the angle of the flow d/s determined as described for u/s or just use the u/s skew for both as would normally be done?

ReplyDeleteI usually pick an average skew angle and use it for both. However, it might not be a bad idea to try a few different ways and use the more conservative result.

DeleteChris, what about the case where the flow through the bridge is not skewed, but the bridge structure itself is skewed above the channel? See the image hosted at https://drive.google.com/open?id=0BzWjEONrpQsEWEVSa0ItX1ZvSms. Thick red lines show the bridge, flow is from right to left. It is also worth noting that the bridge needs to be modeled under pressure flow. How would you suggested orienting cross sections here? Should the skew option you discuss here be used at all? It seems like this option is for when flow lines skew through the structure, which isn't the case with the project from my image. Any help would be appreciated! Thank you.

ReplyDeleteIn this case, put the bounding cross sections (2 and 3) parallel to the bridge. Then skew the bridge deck and the bounding cross sections, but not the pier. You should skew any time flow lines are NOT perpendicular to the bridge deck or any cross section.

DeleteThank you, Chris. From the Hydraulic Reference manual and the dialogue box for bridge skew it looks like it's not generally recommended to go above 30 degrees for the skew. Measuring mine how you've explained in this post shows a skew of 44 degrees. Do you think I would be better off changing cross sections 2 and 3 to be at 30 degrees, or leaving them parallel and using 44 degrees in the skew dialogue window?

DeleteI'd still leave them parallel. The 30 degree limit is just a guideline and is based on the idea that much more than 30 degrees skew, and the flowlines will typically bend to get through the opening or over the deck. It's very subjective. I recommend trying a range of skews and build that into your sensitivity analysis.

Delete