Two thoughts on the question posed in this image. The protrusions from the bridge might i)only have had a function when the bridge was being constructed and may indeed have supported planks on which the craftsmen worked to complete the parapet or ii) they could conceivably be to hold in place a watergate of some sort which would prevent stock from straying under the bridge. This appears to be the upstream side of the bridge - are there also some on the other side?
V8784 : Bridging the Gap