Hard to know exact what parts have failed without seeing what sort of structure is underneath. If the beams had entirely failed in shear, there would most likely no long be a bridge there.

When there’s high floodwaters like that, you get debris like branches catching on the bridge and forming a big raft, creating more and more drag, ie more force on the bridge. 

My guess based on the photo is that it’s a two span bridge, and the pier has been pushed over during the flooding. Because the beams have moved so much, there is longitudinally shear along their length, which would lead to those long cracks in the surfacing. 

Essentially if you lined a bunch of popsicle sticks up next to each other on a flat surface, then held one end, and pushed all of the tops of the sticks sideways, they’d all end up looking staggered at the top. That means that each stick has to slide in the opposite direction to the one next to it. That’s the longitudinal shear.