Say there’s a body of water to be crossed. How do they decide whether to build a bridge or dig a tunnel?
Short answer: They don’t.
That was really useful. Thanks!
What he was trying to say is that engineers are hired guns. They get a project, they make it work, but they don’t decide what the project is. Now, there may very well be engineers on the municipal, state and federal boards that decide to build bridges or tunnels, and they probably consult with professionals before they make their decision, but in the bottom line, it’s elected officials who make the choice.
Why an engineer would recommend to build a tunnel instead of a bridge, or vice versa, THAT I have no idea. It probably has a lot to do with the terrain.
What’s the purpose of the river crossing? Road, rail, both? Other functions (e.g. a bridge can carry a large water main)? Required capacity? Surrounding infrastructure? Geological conditions? Width and depth of the river? Clearances required for shipping lanes? What extremes of weather does the location experience? Any particular environmental or aesthetic concerns?
And those are just a few WAGs…
Depends on which union has the most influence.
I can supply one data point: the Chesapeake Bay Bridge-Tunnel (different from the Chesapeake Bay Bridge) crosses a stretch of water that restricts Norfolk, Virginia’s access to the sea. Norfolk is the United States Navy’s Atlantic Fleet Headquarters, so getting ships through the waterway was a key design requirement. At the time there was no way to make bridges tall enough for aircraft carriers to pass under, given the span of the bay at that point, so instead they sunk a tunnel between some causeways at the deepest part. At high tide, a ballistic missile submarine (our boat with the deepest draft) can sail over the roof of the tunnel with a few feet of clearance.
Just as container ship designers now have to take the width of the Panama Canal into consideration during the design process, any new submarine design would have to take into account the Bridge-Tunnel.
Many cases exist where bridge design was affected in this way - London’s Tower Bridge had to be able to allow large ships to pass.
Another pair of data points: For very short crossings, bridges are far cheaper. It’s much easier to build a 10 metre span, than to dig a 10 metre tunnel, plus dig the approaches on both sides. For very long crossings, unless you can build a viaduct with multiple spans, only a tunnel is feasible. While the Channel Tunnel between England and France was very expensive, a bridge of that length wouldhave been quite impossible.
If the bridge will cross a navigable river, it needs to be a certain height to allow ships to pass underneath. This, in turn, creates a big demand for space on either side of the crossing so that traffic can “ramp up” to the bridge. In New York City, the Hudson River crossings to midtown and downtown are tunnels for this reason. These tunnels (Lincoln and Holland) require much less real estate on either side than if they were bridges.
It’s not a coincidence that the one bridge between New Jersey and Manhattan was built at around 181st street. With Washington Heights on one side and the Pallisades on the other, the George Washington Bridge has a lot of clearance for ships but doesn’t require “ramping up” space on either side.
One word: cost.
The expense of tunnels varies widely as well: a tunnel through solid rock is VERY expensive, and can take years to construct. If you are tunneling though sand and rock, considerably less. In some cases, tunnels can be built in sections, floated to the site, and sunk into trenches in the seabed-that is the cheapest way to build. Years ago, I read about an experimental tunneling method-the tunnel would be MELTED in the solid rock, by electric current flowing from giant electrodes at the tunnel face. i wonder if this was ever tried?
Also the Navy really hates having bridges across the entry to bases on general principle - since an enemy could blow up the bridge and block access to incoming/outgoing ships. When I lived in Groton CT, where there’s a large sub base and the I-95 bridge across the Thames river downstream, there were rumors that (a) a few subs were always stationed outside the mouth of the river so they couldn’t be trapped and (b) the Navy had plans to destroy and remove the bridge themselves in case of war, since they could do it in a controlled fashion and keep access to the base open.
Both of these statements sound like urban legends (or would that be suburban legends?)
The Navy does indeed station a few subs outside the mouth of the river–that is, the ones already at sea! 
Frankly, though, there are no subs moored downstream of the Goldstar Bridge, unless they are being worked on at EB for some reason.
Secondly, I can’t see any benefit to the Navy destroying the bridge themselves. If they were that concerned about subs being trapped, it would be much easier to simply send all of the subs out to sea (that are capable of getting underway, of course). If we were in a hot enough situation that an enemy is likely to attack the bridge, I would be more concerned about the base and the subs themselves being attacked at the pier.
In any event, I think we could remove bridge wreckage in very short order, if it were necessary.
It can work the other way, as well. I have heard that when Huey Long was governor of Louisiana he caused to be built a non-moving highway bridge across the Mississippi River. As a consequence, there is no salt-water traffic on the river above Baton Rouge. Barges and their tugs fit under the bridge just fine, but not the big guys.
Thanks for the responses, gang.
Wikipedia via Answers.com has a nice entry under bridge-tunnel. As others have noted, tunnels are generally more expensive for water crossings, but bridges have problems with clearance and the necessity of ramping cars up to a sufficient height.
Tunnels present ventilation challenges.
The story of NYC’s Holland Tunnel is here. They originally considered a bridge, but chose a tunnel for reasons given oldmanwinter. A tunnel’s superior handling of bad weather was also mentioned.