Comprehension: Floods

     After a great downpour, an otherwise dry land is covered by standing water, which stays for a period of time. This is an example of flood. Flooding is caused by different situations.

     In an area where the soil is sandy, rainwater sinks into the ground readily. This is however not the case if the soil is clayey. This is because clay retains water. As a result, the rainwater does not sink readily. In areas where the soil is clayey, therefore, flooding is possible after a short rainfall.

     In coastal areas like Port Harcourt, Lagos, and other places where the land and sea meet, the level of water in the sea rises and falls. This rise and fall of the sea are referred to as the tide of the sea. Depending on the time of year and the season, tides can be low, high, or very high. Tides are caused by the pull of the Moon and the Sun. Flood caused by seawater is more severe during the rainy season when the water level rises and spreads across the land. There is also an overflow of water from the sea into nearby rivers. Excess water from the rivers also causes flooding of the land nearby.

     Rivers may overflow their banks during the rainy season. If the rain is very heavy, the river may overflow from its banks to neighbouring land, making the land flooded. This occurs more often with large rivers such as River Niger and River Benue.

     There are many ways to prevent flood. Construction of dams along rivers that overflow into neighbouring land can help in preventing flooding. This is because part of the water that is carried by the river is stored in the dam. The speed and the volume of water carried by the river are thereby reduced. This greatly reduces the tendency to flood. This is the case with River Niger in Nigeria, Mississippi and Missouri Rivers in the United States. Flooding can also be prevented through the construction of river or sea embankments. An embankment is an artificial bank usually made with stone to hold back water. Gutters can also be cleared to allow for the free flow of rainwater.

     In urban areas, surface drainage paths should be designed and constructed by competent engineers. The floor of the gutter is made of concrete and designed to have a slope that allows water to flow away easily. The sides are made of concrete and therefore hold back water. The top of the drainage path may be covered with concrete slabs to prevent cars and bicycles running into it or children falling inside it. Another reason is that some people may throw rubbish into the drainage to block it, thereby causing a flood.