» Erosion

Published: 20-04-2010, 14:42

Erosion

Erosion has made huge changes in the surface of the Earth and is still doing so today. Erosion is the removal of materials from the Earth’s surface by a variety of processes. The material is eventually deposited elsewhere, often far from where it started. Most erosion is caused by the action of wind, water, or ice. Water causes the most erosion. Weathering is the breaking down of rock into smaller particles. Erosion differs from weathering in that erosion involves something moving—wind, rain, or a glacier. During the natural process of erosion, the landscape is changed over thousands or millions of years. Mountains are worn down, valleys are filled in, and rivers change their courses. These changes are gradual, but many of the practices of man speed up the process of erosion and cause serious problems around the world. Construction is one of the biggest culprits. Topsoil, minerals, and nutrients are lost from every construction site. About 80 percent of the erosion in Florida is due to manmade activities. Cutting the forests and plowing the land has also contributed to erosion of the land. Human activities cause topsoil to be lost, along with the minerals and nutrients it contains. This affects agriculture. Erosion causes ugly gullies in the landscape. Materials from water erosion can clog culverts and streams. Recreational areas and residential areas are damaged by erosion. Wildlife may be destroyed and its environment altered to the point that it can no longer support wildlife. Water erosion includes stream erosion, beach erosion, and erosion by flooding. Stream erosion is most common. Streams carry sediment from one place to another. The amount of sediment carried and the amount dropped depend on the speed of the water. Water speed is affected by many factors, including the steepness of the slope and the shape of the channel through which the stream flows. The faster the water moves, the more material it can carry, and the larger particles it can move. When water goes around a bend, sediment is removed from the outer part of the bend and swept downstream. Water flows more slowly on the inside of the curve, so sediment accumulates here as it is dropped by the slowing water. Valleys eroded by streams are V-shaped, as opposed to the U-shaped valleys eroded by glaciers. Heavy rains in spring can increase the velocity of a stream and cause more erosion to take place. Sandy ground erodes more easily than rocky ground. At its most extreme, stream erosion has created wonders like the Grand Canyon. This vast canyon was eroded by the Colorado River, although some scientists now believe other rivers helped cut the canyon. The canyon, which is 227 mi (446 km) long, averages 4,000 ft (1,219 m) deep. Its deepest point is 6,000 ft (1,829 m) and the widest point spans 15 mi (24 km) This erosion took place over a period of millions of years. Beach erosion occurs along ocean shores and can be caused by natural phenomena or by humans. Natural phenomena that can cause beach erosion include currents, storms, earthquakes, winds, waves, and tides.
Erosion

Coastal erosion can cause millions of dollars in property damage, as shown after a coastal storm in Australia.

The gradual movement of the Earth’s tectonic plates can also cause beach erosion. Unlike stream erosion, which takes place over a period of many years, beach erosion can be immediate. This is especially true in the case of a storm. However, even in calm weather, sand may be pulled out into deeper water, causing it to be lost from the beach. Loss of sand is significant, since the beach both protects the land behind it and provides recreation areas and habitat for wildlife. The beach absorbs energy from the sea, and the wider the beach is, the more energy it will absorb before the waves reach landward developments. So when sand is washed away, making the beaches recede, more damage is likely to be done to houses and developments. Global warming also has its effect on beach erosion. It causes the sea to warm up and the polar ice caps to melt. This raises the level of the oceans, causing more erosion. Many houses in Florida have been built too near the beach and erosion has put them at risk. The Cape Hatteras Lighthouse on the Outer Banks of North Carolina is another example of buildings endangered by erosion. When the 4,800-ton (907-kg) lighthouse was built in 1870, it was 1,500 ft (457 m) from the ocean. By 1999, the beach had been eroded to within 150 feet (46 m) of the structure. The Cape Hatteras Lighthouse was moved a quarter of a mile (.4 km) inland from its former site. In Massachusetts, Cape Cod’s oldest lighthouse, the Highland Light, is also threatened by beach erosion. The Atlantic Ocean is 400 ft (122 m) closer than it was when the lighthouse was constructed in 1797. And in just the seven years between 1997 and 2004, 40 ft (12m) of beach disappeared because of harsh winter storms. The lighthouse now stands just 100 feet (30.5) from the water. The Army Corps of Engineers is in the process of moving the Highland Light, a $1.5 million project.

SEA WALLS
Man has tried to curtail beach erosion in several ways. Seawalls have been built off the shore. However, the walls reflect the wave energy back to the sea, and that accelerates erosion. Also the beaches don’t get the sand that is usually eroded from the bluffs along the shore. Groins, which are rock walls built perpendicular to the beach, have also been tried. The sand will gather on the updrift side of the wall. The currents carrying sand are slowed down and slow-moving water can’t carry as much sediment. The result is more sand on the updrift side, but it is removed from the downdrift side. Jim O’Connell, a coastal processes specialist at Woods Hole Oceanographic Institute, points out that the coastline is “all one linked system. If you alter one area, you will be causing an alteration in another.” On some beaches, restorers have tried pumping sand, obtained from deep waters, onto the beaches. It works for a while but is expensive. Robert Dalrymple, a civil engineer at the University of Delaware Sea Grant program calls it the “method of choice these days.”
Erosion

At its most extreme, stream or river erosion has created wonders like the Grand Canyon in the United States.

BREAKWATERS
Building offshore breakwaters is another possible solution. These are long heaps of rocks dumped parallel to the shoe to intercept waves. “Depending on how they are used, they will do fine,” Dalrymple explained, but he also pointed out that visible breakwaters can be an eyesore. He says a submerged breakwater may be the solution, but details of where and how to build them have not yet been worked out. Flooding can also cause erosion, and vice versa. When water overflows the banks of a river or lake, it can erode the land it covers. Flooding is also more likely to occur where land at the edge of a river or lake has eroded away. Spring thunderstorms sometimes cause surface runoff and create large gullies when streams overflow their banks. Some flooding can be prevented. Flood prevention measures include digging drainage ditches, planting ground cover, constructing tile drains, creating and preserving wetlands, and building dams and levees. Related to water erosion is erosion caused by ice. Most erosion by ice occurred during the Ice Ages, when the glaciers moved huge amounts of soil and rock. Hundreds of lakes were created as the glaciers moved slowly across the Northern Hemisphere. Glaciers also built up landforms when they melted, depositing rock, soil, and other materials. Most glacial erosion occurred during the Ice Ages, but glaciers still exist in Antarctica, Greenland, Canada, New Zealand, Chile, and the northwestern United States. Another form of ice erosion occurs each winter when water trickles into cracks in rocks, then freezes. Since water expands when it freezes, the cracks are enlarged and the rocks continue to break down. Water isn’t the only thing that causes erosion. Wind erosion is also a serious problem in many parts of the world, especially in arid or semiarid regions. Agriculture suffers from the effects of wind erosion. During the 1930s, the Great Plains of the United States were subject to disastrous dust storms because of a long dry spell. This period in American history caused great hardship for the people who lived in that area, called the Dust Bowl. Although the Dust Bowl was the worst case of wind erosion, much land in the Great Plains was damaged by wind erosion as recently as 1996. When wind erosion occurs, the lighter, less dense soil near the surface is removed. This is the most fertile part of the soil, so the land is then less productive. Blowing soil can also damage small plants. The dust fills the air, causing poor visibility and pollution. In the United States, wind erosion affects the Great Plains more than any other region, but it is also a serious problem in sandy coastal areas and in cultivated soil everywhere. After the Dust Bowl crisis, experts in prairie agriculture began searching for ways to control and prevent wind erosion. A lot of progress has been made. There are three basic objectives when trying to prevent wind erosion. First, you need to reduce the wind velocity at the surface of the soil. This can be done by planting windbreaks, leaving crop residues in the fields, planting cover crops, and increasing the roughness of the surface. Trapping soil particles is also important. Ridging or roughening the surface of the soil will trap soil particles so they can’t blow away. The third objective is to increase the size of the soil aggregates, which means creating clods on the soil’s surface. Then it will take a stronger wind to move the larger chunks of soil. Maintaining plant cover for the soil helps accomplish these objectives. Bare fields are especially at risk for wind erosion. Farmers often leave stubble standing in the fields over the winter. This helps prevent wind erosion, reduces evaporation, and traps snow, which provides extra moisture for the soil in spring. Erosion isn’t all bad, though. Without it, we wouldn’t have sandy beaches, as the sand is made up of particles eroded from the rocks and soil along the coast. All soil is also made from the weathering of rocks. Erosion by glaciers gave us many of the lakes we enjoy for recreation and fresh water. Glaciers scooped out depressions that filled with water, creating the Great Lakes and many smaller lakes.
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tom boy

25 октября 2011 12:48

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