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The Sciences

Scour: Why Most Bridges Fail

But Not SimplerBy Kyle HillMay 24, 2013 10:36 PM

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Yesterday the I-5 bridge spanning the Skagit River in Washington had one of its support girders hit by a truck too wide, and the whole scene of contorting metal fell into the water below. Though a number of people and their cars went into the water along with the bridge, thankfully no one was killed. The collapse of the I-5 bridge has sparked a conversation about the failing infrastructure system of the US, but failures due to girders unable to take a truck impact are hardly a common occurrence. In fact, the most dangerous thing to a bridge is that which it typically takes us over.

LocalScour.gif

Water finds a way. It might be a basement, a beach, or a bridge, but water finds a way. Its power comes from a life-giving combination of cohesion, polarity, and fluidity, not to mention weight. A specific arrangement of atoms gives water heft—enough water to fill seven milk jugs weighs more than 62 pounds. Just a foot and a half of it can lift an SUV off the street

. Two hydrogen atoms and one oxygen molecule describe this molecular wave that can toss ships, stop fatal car crashes, and undermine embankments. We brick and mortar ourselves away from nature. The outside world is behind glass, underneath a slab of concrete, running through PVC. Our steel and concrete are suitable enough until nature comes knocking. And when she wants to get in, nothing can stop her. In the attempt to dodge rivers and streams, nature ultimately determines when we can pass. Her toll is simple: if you engineer the bridge correctly, you can pass. If you don’t, this happens: http://www.youtube.com/watch?v=p_uqPR4Ir5o&feature=player_embedded Scour is the largest cause of highway bridge failures in the United States

[PDF]. During years of service, or during a large event like a flood, the river or stream’s molecular soup carries thousands of pounds of sediment and debris with it. This detritus builds up in the channel and at the piers of the bridge, changing the way the water flows around it. This change in flow (seen in the photo above), if unanticipated by the engineers, gives the water a new plan of attack: blast away supporting material. In the case of the video above, the flow of the water is blocked, but like the show, the flow must continue. Churning and swirling water caused by the obstruction removes material from the base of the embankment. The water can’t go over, so it eats a hole through. You can see the exact point (around 25 seconds in) when enough material has been removed for the water to take a different path. The path takes the support of the embankment with it. Without support, the bridge collapses spectacularly. For most bridges, scour is the enemy. Freak accidents like the one in Washington aside, water's consistently erosive force is what our failing infrastructure system has to worry about. Engineer as we might, we are only a few extra molecules away from realizing that far from controlling nature, we merely endure it. --- Image Credit: Local bridge scour

from the USGS

This is an adapted reposting from my Science-Based Life blog

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