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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 Θ *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 (Θ) 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 20º to 30º 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.