Bridges and Water Flow
A fast flowing river goes under a bridge which is narrower that the banks either side of the bridge.
Does the level of water under the bridge rise or fall compared to the level prior to going under.
If the river does not have a large amount of turbulence(rapids, whirlpools,
etc.), the water under the bridge is a little lower than the rest of the
river. Because the river has less room under the bridge, the water must
move quicker while in the narrow zone. Fluids have a property discovered by
a gentleman named Bernoulli. When a calmly flowing fluid speeds up, the
internal pressure decreases. The air pushes down on the surface a little
harder than the river pushes up. This makes the surface of the river drop
down a little bit. Another example of this is water flowing calmly from a
faucet or hose(no spray). As the water moves downward, the water speeds up.
This increase of speed causes the stream to become more narrow.
No, the level of water will not rise. However, its speed does.
According to Huygens' Principle, water flowing through an opening, even if
narrower than the original, will pass through without change. the water may
slosh up and down, but the level will not change. What does change is the
The water level behind the constriction (upstream of the bridge) will be
slightly higher than the water level downstream. The water flowing under
the bridge will be moving much faster than the water upstream or
downstream. As a result the same volume of water is flowing at all
positions in the river, upstream, downstream and at the bridge.
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Update: June 2012