Soil erosion and soil conservation 英文论文

English For Water ResourcesSoil erosion and soil conservationAbstract: The major movement of soil erosion and soil erosion, soil erosion hazards to humans is the erosion of the first over of the land barren exception, and therefore can not grow crops, farmers bankrupt and had to move to a better place. The main methods to prevent soil erosion is a good method of cultivation. Based on U.S. soil as an example about how to build terraces and other methods.1 Introduction All civil engineers know two main types of soil movement, soil creep or the slow flow of soil, and the faster flows, called slides, slips or avalanches. The faster movement generally moves layers that are deeper than those moved by soil creep.But more important than either of these two types (at least in the quantity of soil removed every year) are soil erosion, the removal of soil by flowing water from ground that is usually dry, and scour the removal of soil or rock from the bed or bank of a river or stream. Forest soil under growing trees generally does not erode because the leaves and dead or broken wood hold up the rain like blotting paper and prevent its attacking the soil. The main action of dead leaves in delaying runoff is that they hold open the drainage channels into the soil. In bare soil that is not protected in this way, the soil pores are quickly blocked during rain by the fine soil grains flowing into them. The rain runs over the surface and not into it, small channels form, to be deepened eventually into the really damaging deep, scoured ditches called gullies.The first disadvantage of soil erosion to mankind is that eroded land is so poor that crops will not grow on it, farmers are ruined, and have to move to better land. This has happened to many thousands of farmers in the United States, which in the last hundred years has probably suffered more form erosion than any other country. The second disadvantage is that soil erosion makes flooding both more severe and more frequent since the water runs off the soil more quickly. In forest areas, 95 percent of the rainfall is absorbed while on bare soil about 95percent may run off. This proportion is called the runoff coefficient. Nearly all civil engineering structures, roofs, roads, and any concrete work have a runoff coefficient of about 95 percent.The damage caused by the soil deposited during flooding in a city street has often been calculated, but that caused by the loss of the same valuable soil from the farmland has not, and to the farmer it is equally serious.With the rapid surface flow caused by the cutting down of the forests, deep channels quickly form in ditches that formerly were small, increasing the speed of erosion in the ditch and forming a gully. The main ways of preventing erosion lie in good farming practice, such as terracing, strip cropping, and ploughing horizontally (contour ploughing ) instead of up or down a slope. We cannot study these in this book, but the civil engineering should be aware of erosion and do what he can to help the farmer. Almost all civil engineering work increases the runoff but civil engineers can try to reduce it by keeping their earth slopes gentle and planted with grass or bushes or trees, particularly in upland areas where slopes are steep and erosion can be most serious. When roads, railways or building foundations break into sloping ground or ground at the foot of a slope, every engineer watches for the danger of earth slips and designs his structures to avoid them but he should also try to conserve soil.The United States soil conservation service has introduced beavers in forest areas to reduce the flow of rivers during heavy rainfall because these animals build dams across the streams, which hold back water at little or no cost to mankind.Soil erosion is the cause of nearly one third of the work of maintaining the main roads in the United States. Similar proportions (or higher) probably apply to roads in all hilly country areas where the rainfall is moderate to high. Particular care is needed in the design and building of culverts because the water can scour both the upstream and the downstream side of the culvert. If the culver slopes steeply downstream, scour can be prevented by building a drop outlet from the culvert (Fig.28.1). This is an outlet with a waterfall into a concrete structure which the falling water cannot scour. An upstream drop inlet is also possible. Tree planting should be so arranged that it does not cause snow to form deep banks.Fig.28.1 culvert under a highway on a steep hillside.The hillside is protected from scour by a drop outlet, a concrete pit with a smooth overflow. It is drained by a small pipe drain if it is necessary to keep down mosquitoes.The toes of stream banks can be cheaply protected from scour by planting trees which like water. These break the force of the water and encourage the deposition of silt. Banks that are too bare to be planted can be protected by laying matting made of brushwood or rushes over them, in which silt can collect and plant growth can start. For the urgent protection of bare banks exposed to strong currents, either large stones or pre-cast concrete slabs or concrete bag-work may have to be placed.Jetty protection of banks is slightly different. Short jetties or piers are built out into the river, upstream of the bank being scoured. The disadvantage of jetties is that even if they do prevent scour on their own bank, they may encourage a corresponding scour on the opposite bank. Jetties may be built from piles driven into the river bed, or by tipping large stones into it, or even with rubbish such as old car bodies.In downstream areas where the river passes through a broad gentle flood plain, civil engineers may be asked to build flood protection works. One such structure is a levee, usually consisting of a long cheap earth bank thrown up as far as conveniently possible from the riverbank. The flood is thus allowed to spread into the land between the river and the levee. The larger the distance between the river and the levee, the greater will be the volume of flood water required to fill it, and the more useful will be the levee. The land thus flooded is usually pasture or other farmland which is not severely damaged by flooding.