A Essay, Research Paper

Rain causes floods, but isn’t always the whole story

While most floods follow heavy rain, or rapid, widespread melting of deep snow, flood forecasters also have to worry about several other factors. Here are the factors that help determine whether a flood is minor or major:

Deep snow cover. Deep snow can melt into a lot of water. Deep snow very rarely causes flooding by itself. Often, heavy rain and rapid warm ups combine with rapidly melting snow to cause major flooding problems. For example, Fargo, S.D. had a 15 inch snow pack at the beginning of March 1994. Fortunately, slowly warming temperatures along with below freezing overnight temperatures through most of March into April reduced flooding from melting snow. Forecasters were originally concerned that the deep snow would pose significant problems during the spring thaw.

Frozen ground. Frozen soil can not absorb as much water as unfrozen soil. Rain or rapid snow melt atop frozen soil can cause flooding that wouldn’t have occurred if the soil were not frozen.

Wet or saturated soil. Saturated soil can’t absorb rain and water from melting snow. The excess water becomes runoff that rapidly flows into rivers and streams. Unsaturated soil acts like a sponge, absorbing some of the water from rain or snow melt. Saturated soil by itself does not cause flooding. Usually, heavy rain or rapid snow melt combined with saturated soil causes the flooding.

?Full reservoirs. Reservoirs are large, mostly man-made basins that hold water for irrigation and drinking. Reservoirs can alleviate river flooding by absorbing and spreading out flood crests flowing down the river. This would reduce the height in which the water rises downstream of the reservoir. If the reservoir is already full, then it can not absorb any water from swollen rivers.

High river and stream levels. Streams or rivers that are already at bankfull can be a precursor to major flooding. Heavy rain or rapid snow melt that flows into an already full river will cause the river to overflow its banks and flood nearby locations. High river levels, such as those in the Ohio Valley in the spring of 1997, make forecasters very nervous anytime a storm threatens to dump heavy rain over the region. A prolonged dry spell, however, can alleviate flooding concerns.

Ice-covered rivers. As rain or melting snow fill river, ice at the surface cracks and breaks up into chunks that float downstream. These chunks of ice can form a dam as they run into barriers, such as bridges, along the rivers. The ice dams cause water to rise rapidly behind them, flooding many upstream locations. If the dam suddenly breaks, water can also flood downstream locations. Large chunks of ice can also damage bridges and other structures. A USA TODAY graphic has more on ice jams.

Widespread, heavy rain. This is perhaps the most important and influential factor of them all. Long periods of heavy rain can cause flooding even if all other factors are unfavorable for flooding. Often, heavy rain is a cause of some of the factors listed above such as wet soils, high stream levels and full reservoirs. The Midwest flooding during the summer of 1993 and the Southeast flooding caused by the remnants of Tropical Storm Alberto in 1994 are a couple of examples of flooding caused by heavy rain.

Whether or not these factors cause flooding often depends on daily weather conditions over the region such as temperature and precipitation. This makes long range flood forecasts very difficult to pin down, which is the main reason why hydrologists often forecast long range flood potential rather than actual long range flood forecasts.

Construction of levees

Levees are natural or artificial embankments along the course of a river. Natural levees are low banks that are produced by the river during floods when the overflowing of the river decreases the speed of the water and permits the deposit of silt. Artificial levees are considerably higher than natural ones and protect the surrounding countryside from floods. On a large river, floods cannot be controlled by levees alone because the waters rise to heights that would overwhelm any embankment. Levees are, however, used to protect portions of the riverbank areas, such as cities and towns, which have a high economic value. The floodwaters are allowed to flow through breaks in the levees over land of low value and are drained off through supplementary channels that are sometimes equipped with secondary levees.

Dams

An effective method of controlling floodwaters is to construct coordinated groups of dams and reservoirs on the headwaters of the streams that lead into the main rivers, so that water can be stored during periods of heavy runoff and released gradually during dry seasons, and thus preventing floods from occurring. During time of high water the dams operate to slow down the flow. The dams closest to the origins of the tributaries restrain the floodwaters while the dams further down slowly release their normal reservoirs and are drained. Then the floodwaters are released to each succeeding dam and are finally emptied into the main river.

Canalization

An other method of protecting the land against floods is building canals, which can drain water away if there is too much water in the river. You can also use these canals for the irrigation of the farmland. Because of that, canalization is a very effective method.

Flood ways

Floods ways are surrounding areas along the river. When the river overflows, the overflowed water will be catched in the floods ways. When the flood is over, the water returns into the river. This is a very old and effective method to prevent floods. The egypts have used it since 750 years B.C.

Reforestation

For vegetable covers, farmer have deforestated the ground for centuries. Which increased in a soil erosion. All the Soil went into the rivers. The rivers got shallow and overflowed the surroundings (river floods). To stop this proces, reforestating was the best solution.

Periodic floods

This type of flood occurs often when many rivers have too much water, forming a area known as the flood plain. These flood plains are usually result form rainings combined with for example melting snow.

Flash floods

Flash floods occur when there is a tremendous rainfall in a small local area. Flash floods occur most in coastal areas, when the wind blowing from the ocean. An example of a flash flood was, when 25 cm of water fell into the Big Thompson River region (US) within four hours.

River floods

When a river has more water than he can hold, he overflows the surrouding areas. This type of flood is called a river flood. The river flood is also called the steam flood. Examples of rivers, which has frequently river floods are the Nile in Egypt and the Mississippi in the US.

Coastal floods

The world’s coastal area are something terrible flooded by the sea, these floods are caused by violent wind, like typhoons, tsunamis or gigantic waves induced by earthquakes beneath the sea level. The coastal flood that was brought about by a tropical cyclone in 1970 killed over a 300.000 people in Bangladesh. Coastal floods have killed milions of people

Seasonal floods

When floods occur in a seasonal pattern it’s called a seasonal flood. In India and China, summer moosons bring heavy rains, causing river overflows. This happens every season.