Modifications And Channelization Of RivEr Systems To Alleviate Water Shortages

some desert and semiarid regions of the world have undergone rapid population explosions, necessitating the alteration of river courses to bring water to thirsty cities and to provide irrigation to farmlands to feed this growing population. In the American desert southwest, California, and the middle East, riverways have been extensively modified, regulated, and sometimes diverted hundreds of miles from their

5 billion «1987

4 billion ¿1974

5 billion «1987

4 billion ¿1974

2 billion «1925

6,000

G Infobase Publishing

4,000

2,000

Years before present

2 billion «1925

6,000

G Infobase Publishing

4,000

2,000

Years before present

Population curve showing the number of humans on Earth natural course to provide water to places where people prefer to live.

Many examples of the effects of urbanization on flood intensity have been documented from California and the American desert southwest. urban areas like Los Angeles, san Diego, Tucson and Phoenix have documented the speed and severity of floods from similar rainfall amounts along the same drainage basin. These studies have documented that the floodwaters rise much more quickly after urbanization, and they rise up to four times the height of preurbanization, depending on the amount of paving over of the surface. The increased speed at which the floodwaters rise and the increased height to which they rise are directly correlated with the amount of land surface now covered over by roads, houses, and parking lots, blocking infiltration.

In natural systems floods gradually wane after the highest peak passes, and the slow fall of the floodwaters is related to the stream system being recharged by groundwater that seeped into the shallow surface area during the heavy rainfall event. In urbanized areas, however, the floodwaters not only rise quickly but also recede faster than in the natural environment. This is attributed to the lack of ground-water continuing to recharge the stream after the flood peak in urbanized areas.

many other modifications in stream channels have been made in urbanized areas, with limited success in changing nature's course to suit human needs. many stream channels have been straightened. This only causes the water to flow faster and have more erosive power. straightening the stream course also shortens the stream length and thereby steepens the gradient. The stream may respond to this by aggrading and filling the channel with sediment in an attempt to regain the natural gradient.

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