eli5: Why do some waterways have bridges and others have tunnels?
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Sometimes a bridge is cheaper, and sometimes a tunnel is cheaper. The highway department almost always chooses the cheaper option. There are times where one is impossible, but generally it’s all about cost.
There are quite a lot of factors. Bridges can prevent larger boats from using the waterway. Tunnels reduce the visual and noise impact on people in the area. Tunnels are more vulnerable to flooding but bridges are more vulnerable to high winds and earthquakes. Fires and accidents can be more difficult to deal with in tunnels because it’s harder to keep them ventilated and you can’t use aircraft or boats to approach them.
The cost of building a tunnel depends on the length and diameter, the hardness of the stuff you need to dig through, the risk of hitting pipelines and archaeological sites and so on, and any infrastructure you need for drainage and ventilation. The cost of building a bridge depends on the length and width, what kind of weight it needs to support, what kind of wind and earthquakes it needs to withstand, and what kind of ground is available to build supports into. In both cases there is also the cost of obtaining all of the necessary permissions and approvals.
Generally people will consider a range of possible bridge or tunnel options and weigh up all of these factors.
Some foods are easier to eat with a fork, others with a spoon. It’s the same thing with waterways: sometimes the best solution is a bridge, other times a tunnel. A few examples of bridge vs. tunnel questions are:
– How deep is the water? If it’s shallow, that makes it easier to build a bridge. If it’s deep and narrow, a suspension bridge might work (one tower on each side, hang the bridge in between the two towers). If it’s deep and wide, the best option is usually a tunnel.
– Is the waterway a shipping route? If it is, the bridge needs to be tall enough for them – or you can skip the problem and add a tunnel. A lot of crossings use bridges and tunnels, like the Øresund crossing between Sweden and Denmark and the Chesapeake Bay bridge-tunnel in Virginia.
– Are there high winds and/or icy weather in the area? Some bridges have to partially or completely close when the winds are too strong or the bridge is freezing. In those cases, a tunnel is more reliable.
Tunnels make more sense if there is lots of tall shipping traffic. Cargo ships are VERY tall and if you have to make a very tall bridge or a lift bridge that is constantly opening and closing a tunnel becomes more reasonable.
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