Classwork Series and Exercises {Agricultural Science – JSS3}: Flooding

Week 3

Topic: Flooding

Introduction

Flood occurs when a large amount of water covers an area that is usually dry. Flooding may occur after a heavy downpour of rainfall. When a river overflows its banks, flooding may also occur. A flood is an overflow of water that submerges land which is usually dry. The European Union (EU) Floods Directive defines a flood as a covering by water of land not normally covered by water. In the sense of “flowing water”, the word may also be applied to the inflow of the tide.

Flooding may occur as an overflow of water from water bodies, such as a river, lake, or ocean, in which the water overtops or break-slevees, resulting in some of that water escaping its usual boundaries, or it may occur due to an accumulation of rainwater on saturated ground in an areal flood. When floods happen in an area that people live, the water carries along objects like houses, bridges, cars, furniture and even people. It can wipe away farms, trees and many more heavy items.

Drainage Pattern

The drainage pattern in a city or town may be blocked by refuse. When rain falls, water cannot flow freely in a blocked drainage. Hence, the rain water overflows the drain, spreading over adjourning area.

Causes of Flooding

1. Heavy downpour of rainfall
2. Blocked drainage
3. Indiscriminate dumping of refuse into rivers, streams etc.
4. Release of excess water in dams
5. No gutters/drainage system on either side of the roads
   
   Heavy Rains – Each time there are more rains than the drainage system can take, there can be floods. Sometimes, there is heavy rain for a very short period that result in floods. In other times, there may be light rain for many days and weeks and can also result in floods.
   River overflow – Rivers can overflow their banks to cause flooding. This happens when there is more water upstream than usual, and as it flows downstream to the adjacent low-lying areas (also called a floodplain), there is a burst and water gets into the land.
   Strong winds in coastal areas – Sea water can be carried by massive winds and hurricanes onto dry coastal lands and cause flooding. Sometimes this is made worse if the winds carry rains themselves. Sometimes water from the sea resulting from a tsunami can flow inland to cause damage.
   Dam breaking (ruptured dam or levee) (Embankments, known as levees, are built along the side of a river and are used to prevent high water from flooding bordering land) – Dams are man-made blocks mounted to hold water flowing down from a highland. The power in the water is used to turn propellers to generate electricity. Sometimes, too much water held up in the dam can cause it to break and overflow the area. Excess water can also be intentionally released from the dam to prevent it from breaking and that can also cause floods.
   Ice and snow-melts – In many cold regions, heavy snow over the winter usually stays un-melted for sometime. There are also mountains that have ice on top of them. Sometimes the ice suddenly melts when the temperature rises, resulting in massive movement of water into places that are usually dry. This is usually called a snowmelt flood
   

Flooding can be controlled by the following

1. Removing or cleaning materials from the drainage system.
2. Building of high walls to keep rivers from over flowing their banks.
3. Widening of straightening of rivers.
4. Diversion of streams so that they do not overflow into river.
5. Building of dams to hold back water in river.

Effects of Flooding on Community and Farmland

1. The soil becomes waterlogged and often times muddy
2. People cannot travel easily on roads
3. Food crops are damaged
4. Building s may also become submerged in flooded area
5. Where flooding occurs annually, people are moved away from the area.

The following are causes of flooding except (a) tree planting (b) heavy downpour of rainfall (c) blockage drainage (d) Release of excess water in dams  Answer: Tree Planting