The first thing I would say is that velocities are determined via a calculated force and since it is not a tabular output currently, we have to use RAS Mapper to determine velocities. If I was modeling this and had this issue, I would adjust the internal XS manning’s n values to ridiculously high (0.5) and see if that stops it. This would then tell us that the velocity calculated is not out of a general range of expectations.
Is the bridge overtopping? In RAS versions 6.4 and earlier pressure flow was not capable of being calculated and that also may be the issue as those versions could only simulate low flow conditions. I would look into that by upgrading the model to V6.4.1.
There are so many things to look at here and hard to debug without the model in front of me. Here are some additional things to check
-Adding near repeats to your SAConn is also recommended by HEC.
-Make sure the “distance to the US XS” in the bridge editor falls within the first cell parallel to the bridge.
-Make sure the bridge is enforced correctly because that will cause odd results, i.e. NO crossing breaklines.
-Make sure the cells on the SAConn for your bridge are not drastically smaller than the global cell spacing
Hope any of this helps!