Natural Hazards: Causes and Effects - Disaster Management Center ...

The decline in agricultural output led to higher levels of unemployment and underemployment.
Masses of destitute rural people migrated to urban areas where job opportunities proved equally
scarce. Added to this was a 50 percent inflation rate fueled by escalating prices of essential
commodities now in short supply. In sum, the flood directly caused deterioration in levels of
output and combined with rising unemployment and inflation to disturb the once moderately
equitable nature of Bangladesh society. 3
Yellow River, China
China’s Yellow River has the dubious distinction of being responsible for more human deaths
than any other individual feature of the world’s surface. The cause for this is the river’s unique
form and configuration. For nearly 4,000 kilometers (2,500 miles) it flows through the mountains
and plateaus of northern China, and on its route through the easily eroded loose soils it picks up
an enormous quantity of silt. The flow of the river may be 40 percent yellow silt (which gives it
its name) when it arrives at Kaifeng. From there it travels another 800 kilometers (500 miles) to
the sea across the great Yellow Plain—essentially a massive alluvial fan, sloping more steeply
than a true delta—which is also 800 kilometers wide and spreads around both sides of the
mountains of Shantung. The river gradient across the plain is far higher than in a normal delta,
but the Hwang Ho, as the Yellow River is known in Chinese, is still unable to carry its sediment
load, and the plain is made of redeposited silt.
From Kaifeng, 15 channels radiate across the plain. Each time the Yellow River tops one of
these, it causes enormous floods before resuming a single channel. The floods have drowned
unbelievable numbers of people on the crowded plain, and the destruction of crops results in
famine and yet more deaths. In only three floods since 1887 the Yellow River has killed over
6,000,000 people.
The levees, which were started over 2,500 years ago, have had to be constantly raised by the
labour-intensive methods for which the Chinese are famous. There is nothing with which to
build them except the silt. The constant raising means that the Yellow River now crosses its
plain about 7.5 meters (25 feet) above the surrounding countryside, between inner and outer
levees that form a belt 19 kilometers (12 miles) wide. The silt is the cause of the problem, for it
is constantly deposited in the river channel. The river rises to yet higher levels and the Chinese
are left with a literally never-ending task of building the levees higher still. Because of this the
Yellow River now has no tributaries for over 640 kilometers (400 miles), and millions of people
live below river level with the constant threat of flooding. There are no hills in the plain, no
escape routes in the event of a flood. And the average area flooded each year is 8,200 square
kilometers (3,000 square miles). Because the plain is below river level, it cannot drain. Regions
stay flooded to the horizon for a year at a time. Once a major levee break lets the river
completely escape, it occupies a braided course perhaps 24 kilometers (15 miles) wide for up to
10 years before it settles itself into a new channel. 4
Geographical Distribution
Flooding is the most universal of natural hazards. It occurs on each continent and is a potential
threat wherever there is rainfall or coastal hazards. With the exception where rainfall is never
more than very light, every watershed is a potential site for flooding. Every coastline that is
vulnerable to tropical cyclones or tsunamis is also at risk to flooding.