I’ve talked before about figure-ground issues. I happen to find that particular piece of epistemology and perception to be a useful way to organize some topics. For any given object of interest, what’s the object and what’s the background? As the cliché goes, the answer may surprise you.
The picture above is the Otter Creek Cove Bridge and Causeway in Acadia National Park in Maine. (Let me put in a touristy plug here for Acadia – it is a truly beautiful place.) The photograph was taken standing on the causeway, looking toward the triple-arched bridge. The arches are rather short spans – one 15 feet long and the other two 12 feet long – in a road that is 24 feet wide. To give a sense of the arches, here’s a view from the protected side of this shore causeway:
So, is this a bridge? Or does the causeway have three tunnels running under it? Or, if I wanted to be really insulting, does the causeway have masonry culverts below? This is a classic figure-ground question: is the thing of interest the hole or the road above the hole? The details of the structure actually make the answer less clear rather than more so. The causeway follows a natural sandbar that served as a natural weir between the cove (open to the ocean) and the creek behind. Building a causeway obviously interferes with the natural flow of water over the bar, so the engineers studied several options:
You could build an earth causeway that focussed flow at one locations (as was built), you could build a dam similar to this design but limiting the flow further, you could build an earth causeway with a large opening requiring a “real” bridge, or you could build an elevated causeway on piers that left the natural bar mostly intact. The details on the lower right showed a proposal to raise the weir to create a larger body of water for swimming, which may never have been built because even in the middle of summer, the water at Acadia is quite cold.
So, short-span bridge or tunnel through a causeway? There’s no answer, which is a rather un-engineering thing to say.