Drift can entangle in a parapet but apertures are typically small, and entangling is probably less severe than on a truss. Some arrangements of pier caps, beams, and deck create apertures that carry flow and trap drift. Flow may pass over beams and under the deck. Some arrangements of diagonal bracing create triangular apertures through which flow passes. Some arch bridges include openings between the deck and arches that are too small for logs to pass through.
Many bridges lack small apertures conveying flow.
Single-column piers, whether walls, cylinders, or hammerheads, deflect all flow through wide adjacent gaps. A deck resting directly on solid beams, with a solid parapet, is another example. The apertures in a single row of columns carry no flow and do not contribute to drift trapping if the row is aligned to approaching flow.