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guynextdoor

Floods raise scientific dilemma
Efforts to control rivers can make matters worse elsewhere




Most of us think of floods as acts of nature. Wrong. The biblical story of the Great Flood notwithstanding, after the skies have opened up, scientists say that what humans have done to the environment has a lot to do with whether all that rainwater turns into flood water.

One of the major ways we make flooding worse comes from one of the ways we try to protect ourselves from flooding in the first place. Levees are big earthen embankments built up along river edges, essentially walls that raise the riverbanks higher, so the water has to rise higher before it can spill over. That seems to make sense. But levees don抰 just raise the height of the riverbanks. They also raise the height of the river.

Think of water running down a big wide ditch that抯, say, 50 feet across. Now make that ditch into a narrow little open-topped pipe only 10 feet across. What抯 going to happen when the same amount of water that used to be able to spread out in that nice wide ditch tries to run down that narrower tube? With the walls squeezing it in, the water is going to rise higher as it flows. That抯 what levees do.

Riverbeds also include their floodplains, those wide flat areas next to the riverbanks that handle the extra water when rivers flood out of their central channels. The overall path of a river includes both the main channel and these floodplains. Together, they are like that wide ditch in our example. But levees cut the river off from its floodplains. They force the same amount of water to run down a narrower pipe. The water level rises. That means that when unusually heavy rains come along, the levees that are supposed to protect us have actually already raised water levels closer to flood levels.

Here抯 another problem with levees: Where they抮e high enough to contain the river, great. But when the river finally gets downstream to someplace without a levee, the narrow upstream channel means the river arrives at a much higher level. That means the risk of flooding where no levee has been built is much higher. That抯 what almost happened to St. Louis in the great Mississippi flood in 1993. Many experts say that downtown St. Louis was spared only because levees upstream failed, allowing lots of water to spill out of the river.

It all started in the 1800s
Levee building really got going in America after Congress, back in the mid-1800s, passed the Swamp Act, encouraging the sale of 65 million acres of 搒wamp

Sephiroth

Nature does NOT keep whatever bad things Humans give her.

Humans can pollute the land and water, cut down trees, level hills and such and make it excuses that this is Progress. Nature will keep quiet about it.But when time comes, she will return to Humans what humans have given her.

Good and the bad we do, it comes right back to us. :music: