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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 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.
