Some context: I row every morning, and due to my rowing club's proximity to a large lake, it is often very windy and rough (which builds character, according to the coaches). I noticed that as we row under bridges, the water seems to "speed up" and the waves grow in height. A similar effect is noted near the breakwaters close to shore, where an area of rough and fast waters seems to be created.

I'm not sure if this is fitting in the Earth Science SE, or if this is too broad or elementary, but I would like to know what causes this!

EDIT: The bridge in question has two rows of three pillars (similar to the below image, except closer together and made of steel) parallel to each other, we pass between the opening.

enter image description here

  • $\begingroup$ I'm in two minds about this question, in some ways it might be better explained in SE Engineering. Something that needs clarification is the type of bridge you mention. A don't see how a bridge without some part of it in the water would affect water height & speed - something like a suspension bride. But bridge with pylons in the water, to support the deck of the bridge I can see how they would interfere with the flow of water under the bridge offering greater resistance to flow, channelizing the flow & thus causing the water the speed up & increase water height. The phenomenon you ... $\endgroup$
    – Fred
    Jul 10 '21 at 4:30
  • $\begingroup$ ... mention might also be due to a co-incidental localized increase in the height of the bed of the river under the bridge causing the thickness of the water to be reduced & thus create higher speed & turbulence, which would result in a higher wave. $\endgroup$
    – Fred
    Jul 10 '21 at 4:32

Water velocity increasing near bridge pylons is literally the Bernoulli principle manifested. Mass balance on either side of a constriction requires that flow speed up within the constriction itself.

Additionally, since nothing confines water at the surface, it is free to rise (and fall as shear stresses dictate), creating local waves...hence fast moving rough water in narrow channels, under bridges, around jetties, and even over sand bars.


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