15-8 Too Much Water
What Causes Flooding? Rain and People
Heavy rain and rapid snowmelt are the major causes of natural flooding by streams. A flood happens when water in a stream overflows its normal channel and spills into the adjacent area, called a floodplain.
People settle on floodplains because of their many advantages: fertile soil, ample water for irrigation, availability of nearby rivers for transportation and recreation, and flat land suitable for crops, buildings, highways, and railroads.
Case Study: Living on Floodplains in Bangladesh—Danger for the Poor
Bangladesh is one of the world’s most densely populated countries, with 162 million people packed into an area roughly the size of the Maritime provinces. It is also one of the world’s poorest countries. The people of Bangladesh depend on moderate annual flooding during the summer monsoon season to grow rice and help maintain soil fertility in the delta basin. The annual floods deposit eroded Himalayan soil on the country’s crop fields.
The increased runoff of soil, combined with heavier-than-normal monsoon rains, has increased the severity of flooding along Himalayan rivers and downstream in Bangladesh. A disastrous flood in 1998 covered two-thirds of Bangladesh’s land area for 9 months, leveled 2 million homes, drowned at least 2 000 people, and left 30 million people homeless.
How Can We Reduce Flood Risks? Think about Where You Want to Live
Several methods can be used to deal with the risk from flooding:
One is to straighten and deepen streams, a process called channelization. Channelization can reduce upstream flooding, but it removes bank vegetation and increases stream velocity. This increased flow of water can promote upstream bank erosion, increase downstream flooding and sediment deposition, and reduce habitats for aquatic wildlife.
Another approach is to build levees or floodwalls along the sides of streams. Levees contain and accelerate stream flow, but this increases the water’s capacity for doing damage downstream.
Building dams can also reduce the threat of flooding by storing water in a reservoir and releasing it gradually. We can also construct a floodway, like the one in Winnipeg, designed to direct the water around a community with minimal damage.