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Flood Control

 

Flood control has been practiced since ancient times with methods such as reforestation, and the construction of levees, dams, reservoirs and channels diverting floodwater, called floodways.

Levees were used by the ancient Chinese to raise the banks of the Yellow River.  It was hypothesised that because the river was restricted its channel would deepen to contain the flow.  In practice, however, the alluvial soil that used to be deposited over the whole flood plain instead built up on the river bottom, causing the riverbed to rise.  Over time the level of the river rose as much as 21m above the surrounding plain, and in 1887 the flooded river broke through the levees killing over a million people: one of the worst recorded floods ever.  The Middle Ages saw the construction of levees on the Rhine, Danube and Po rivers, but these have since been reinforced with reforestation measures and reservoirs.  Today, the Mississippi river is confined to a narrow channel by levees in order to give it a navigational depth, however the maintaining of this depth is a very expensive process as the channel needs to be cleared frequently.  On a large river, such as the Mississippi, levees are often not sufficient protection against flooding and so other methods such as dams and floodways are also used.

Dams were originally built for water storage, irrigation and power creation.  Recently dams have been built especially for flood control.  Groups of dams built on the headwaters of streams allow for the retention of heavy runoff which can then be released during dry periods.  The dams closest to the origins of the tributaries restrain the floodwaters while the dams further downstream release their reserves and the flood waters are then released into each succeeding dam and finally into the main river.  In the Netherlands a flood control project called the Delta Plan uses a huge storm-surge barrier 9km long that is lowered only when a sea flood is expected.  This same principle has also been applied on a smaller scale across the Thames River.  Floodways are a method of flood control that diverts flood waters so that a controlled area of land is flooded, meaning that others are not.

In many places floods are caused by man's mismanagement of the land through deforestation and intensive cultivation which has led to serious soil erosion and the decreased ability of the soil to retain water.  In these areas flood control is implemented through reforestation and efficient methods of soil management such as contour ploughing and crop rotation.

EXAMPLES

Yellow River:

Much of the problem stems from the high silt content of the river -- in some stretches as much as 60% by weight. Millions of tons of yellow mud choke the channel, causing the river to overflow and change course. In its lower reaches, the river bed has actually become higher than the level of the surrounding countryside. Water is held in by dikes of ever increasing height, some reaching 30 feet and more.

Attempts at controlling the Yellow River were begun as early as the third century B.C. An engineer named Yu came up with the idea of dredging the river to encourage the water to flow in its proper channel. Yu was made Emperor of China for his contribution, but managing the river's silt would continue to be an ongoing challenge.

Over the years, the Chinese have tried to control the Yellow River by building higher levees, digging channels and building dams. Dams have tended to be the most helpful in controlling floods, but the river's thick silt has clogged many of them. Currently, the Chinese are constructing a massive new dam called the Xiaolangdi Multipurpose Dam Project. Boasting 10 intake towers, nine flood and sediment tunnels, six power tunnels and an underground powerhouse, the structure may finally mitigate "China's Sorrow."

Nile : 

This narrow strip along the Nile, together with the delta at the river's northern mouth, is the only farm land Egypt has. Though it totals only three percent of the county's land, it has provided ample food for thousands of years. But recently, a population boom has forced Egyptians to increase their agricultural output.

In 1970, they completed the Aswan High Dam, which stretches across the Nile 600 miles south of Cairo. The dam has effectively stopped the river's annual floods by trapping its waters in a reservoir that is slowly released during the dry season.

Now farmers along the Nile plant crops year round. In fact, the area has become one of the most intensely cultivated pieces of land in the world. Because the Aswan Dam traps 98% of the river's rich sediments and prevents them from flowing downstream, farmers along the Nile must now use large amounts of artificial fertilizer. Another negative side-effect of the dam is that the Nile delta is no longer being built up by the river sediments. As a result, this important agricultural area is now struggling with erosion and dangerously high levels of soil salinity.

Mississippi

After a disastrous flood in 1927, the U.S. Army Corps of Engineers was charged with task of taming the mighty Mississippi. In building the longest system of levees in the world, the Corps successfully minimized flooding and improved the river's navigability. This dramatic achievement spurred millions of Americans to move onto the floodplain, where the soil was fertile and the property was cheap. As a result, much of the river's bordering wetlands have been lost to agriculture and construction.

Today, the Mississippi is outfitted with 29 locks and dams, hundreds of runoff canals, and miles and miles of levees. Most years, the system works remarkably well, but the flood of 1993 washed away the illusion of complete control.  Fully 80% of private earthen levees in the river basin failed. Most federal levees held, saving lives and land -- but sent torrents of water towards less protected field and towns.

 

 


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