Download Chapter C1 Natural Hazards

SLCSS/Form 5/GEO/Chapter C1 / P.1
Chapter C1 Natural Hazards
C1.1
Types of Natural Hazards
Natural
Hazards
Caused by
Natural Processes
Geomorphic
Processes
Eg. earthquake
Biological
Processes
E.g.pests,
diseases
Man Induced
Processes
Meteorological
Processes
E.g. flooding,
drought
C1.2
Effects Brought by Hazards
Losses in Hazards:
1.
2.
3.
Injuries and deaths
Damages on properties
Hindrance of economic development in affected areas
C1.3
Human Responses to Hazards
1.
Human responses before a hazard
prevention of hazards
prediction of hazards
reduction of losses and damages caused by hazards
2.
Human responses after a hazard
emergency relief
restoration
reconstruction
3.
There are great variations in the response to hazards among individuals.
This depends on:
One’s knowledge of the hazard
One’s personal perception towards hazards
One’s level of education
One’s religious belief
Effectiveness of social and political systems
Nature, magnitude, frequency and recency of occurrence of the
hazard
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C1.4
Case Study: Huang He Basin (Problems of Floods and
Droughts)
A. General Cause of Flooding and Drought
Floods and droughts are caused by extreme climatic events resulting form
abnormal precipitation, often of prolonged duration.
Upper course
Middle course
Lower course
Loess Plateau
North China Plain
Huang He Basin
Fig. C1.1 The Huang He Basin
B.
~
Floods in the Huang He Basin
Causes:
1. Great fluctuation in the amount of rainfall
a. In middle Huang He ( i.e. the semi-arid Loess Plateau region), rainfall is
highly irregular with occasional periods of prolonged heavy downpours.
b.
c.
In lower Huang He (i.e. the North China Plain region), heavy summer
rains are often brought by monsoons.
Either of these two causes may lead to a sudden increase in the
discharge of the river
2. Sudden change in gradient
a. The slope gradient of Huang He decreases abruptly as it runs into the
North China Plain from its mountainous middle course.
b. At the point of sudden gradient change, the river’s velocity decreases
SLCSS/Form 5/GEO/Chapter C1 / P.3
sharply, hence the river flow is retarded.
c.
d.
Mass deposition of sediment takes place at this point leading to rapid
silting onto the river bed.
As the river cannot flow smoothly, floods often occur.
3. High silt content
a. Huang He obtains most of its sediment when it flows through the Loess
Plateau.
b. Soil erosion is extremely serious in the Loess Plateau because:
(1)
The plateau is made up of very fine but unconsolidated soil
materials
(2)
c.
d.
The land surface is lack of dense vegetation cover due to the
semi-arid climate
(3)
Poor farming practices
(4)
Widespread deforestation for fuel wood
The large amounts of sediment that enter Huang He cause widespread
deposition on the river bed
Floods then occur readily whenever the discharge is high.
4. Absence of large lakes
a. Along the whole course of Huang He there is no large lake which can
help to:
b.
(1) Regulate the discharge
(2) Retain the large amount of silt
Therefore the lower Huang He which always receives excessive
discharge and sediment is exposed to the threat of floods from time to
time.
5. Silting in flat lower course
a. Rapid deposition takes place on the river bed
b. As the river bed is raised, the capacity of the river reduces resulting in
floods.
~
Effects of Flood
a.
b.
c.
d.
e.
f.
g.
loss of life
loss of properties
damages to crops
local famine resulted from crop failure
spread of diseases
destruction of communication lines
obstruction to navigation as a result of fluctuating water levels
SLCSS/Form 5/GEO/Chapter C1 / P.4
C. Droughts in the Huang He Basin
~
Causes of Drought in the Huang He Basin
Droughts are greatest in the semi-arid areas of the middle Huang He (i.e.
Loess Plateau).
1. Low and variable rainfall
a. the Losses Plateau experiences a typical semi-arid climate
b. The mean annual rainfall is low and highly variable.
2. High rate of evaporation
In summer, the rate of evaporation often exceeds the amount of
precipitation, resulting in water deficits in the soil.
~
1.
2.
3.
4.
5.
Effects of Drought
Shortage of water for cultivation and domestic uses
crop failures
local famine
obstruction of hydro-electric power generation
loss of habitats (e.g. wetlands) for wildlife
6. retardation of economic development
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D. Human responses to Floods and Drought in the Huang He Basin
Beijing
B
C
A
Major dam
Irrigated area
Minor dam
Soil conservation and
afforestated area
Fig C1.2 Water Project in China
Name two important cities A, B and C.
A is Kaifeng.
B is Lanzhou.
C is Xian.
Name three dams X, Y and Z.
X is Longyang Gorge Dam.
Y is Liujia Gorge Dam.
Z is Sanmen Gorge Dam.
*** The Chinese government has launched a series of projects at both
national level and local level.
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National Level:
In the 1950s, a large-scale multi-purpose water scheme, the Huang He
Project, was carried out.
Aims
Projects
1. To control floods in the middle
1. 46 dams are constructed along
and lower courses of Huang He
the river course to regulate
stream flow e.g. the Longyang
Gorge Dam ( 龍 陽 峽 ) , the
Sanmen Gorge Dam (三門峽) and
the Liujia Gorge Dam
2. Flood retention reservoirs are
built behind the dams to retain
flood water.
3. Desilting basins are dredged on
both sides of Huang He to
reduce the amount of sediment
carried downstream.
2. To reduce the effects of drought Networks of canals are constructed
by providing water for irrigation to lead water from the fold
retention
reservoirs
to
the
neighboring farms.
3. To improve rural electrification
Taking full advantage of the huge
dams, hydro-electric power is
generated.
4. To provide cheap water transport 1. Channel flow is regulated and
in lower and middle Huang He
controlled by dams and sluice
gates.
2. The river bed is constantly
dredged to provide a deep
enough waterway.
Local Level:
1. Soil conservation
Implement various local soil conservation projects to reduce soil erosion
and minimize the amount of sediment that may enter Huang He.
a. Afforestation
b. Hillslopes are terraced to reduce water erosion
c. Wind breaks are planted
d. Gully erosion is controlled by filling the gullies with soil
e. Silt collected in the desilting basins is dredged out and used to
replenish the land surface where soil erosions is serious.
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2. In the lower course, many flood-prevention measures are put into
practice:
a. Dykes are built to strengthen the natural levees.
b. River meanders are straightened to quicken the steam flow.
c. The river bed and river mouth are dredged to increase the channel
capacity.
What are the Pros and Cons of building
Huanghe Water Project, Three Gorges
Pros
1. Risk of flood is reduced
2. Problem of drought is lessened
3. Farm production has been increased
through irrigation
4. Farming activities are diversified
5. Using cheap hydro-electric power is
possible
6. Develop tourism industry
Multi-purpose Water Scheme (like
Project in Changjiang)?
Cons
1. Natural landscape has been
greatly altered.
2. Extensive areas of natural
vegetation have been removed
3. Wild animals are forced to
leave their habitats
4. Disturbance brought to
aquatic life
5. Resettlement problem
C1.5 Earthquakes
A. Definition:
Earthquake refers to the sudden trembling of the rocks in the earth crust.
B. Introduction:
1. When an earthquake occurs, seismic waves are transmitted from the
origin onto the surface through the crust.
2. Several features of an earthquake can be identified:
a. Focus
It is the source of an earthquake where seismic waves are
generated.
b. Epicentre
c.
d.
It is the point on the ground surface vertically above the focus.
Body waves
These are seismic waves which travel through the crust.
Surface waves
These are seismic waves moving along the surface of the earth
3. A seismograph is used to record the strength, duration and location of an
earthquake.
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4. Magnitude
a. The magnitude of an earthquake is an indication of the amount of
energy released at the focus.
b. It is commonly measured on the Richter Scale which ranges
earthquakes on a scale from 0 to 9.
5. Intensity (measured by Modified Mercalli Scale)
(source: http://www.weather.gov.hk/gts/equake/mms_e.htm)
a. This is a qualitative measure of earthquake effects based primarily
upon the extent of damages, loss of life and the physical feeling of
people.
b. Earthquake intensity depends on:
(1) the earthquake magnitude
(2) the distance from the epicenter
(3) the local geology
i.e. the weaker the crustal materials, the higher the intensity
6. Frequency
a. More than a million earthquakes shake the earth each year.
b. In general, earthquakes of lower magnitude tend to occur in higher
frequency.
7. Duration
a. Most earthquakes occur in short duration.
b. They usually last for no more than one minute.
C. Major Earthquake Zones
The world’s major earthquakes coincide with plate boundaries / plate
margins.
a. The Circum-Pacific Zone
This zone includes the west coast of the Americas and the island
groups of Asia.
-
-
Colliding plates give rise to young fold mountain belts. Plate
compression and oceanic plate subduction causes friction, both
triggering off earthquakes.
Sliding plates also trigger off earthquakes.
b. The Alpine-Mediterranean-trans-Asiatic Zone
This belt runs through North Africa, Spain , Italy, Greece, Turkey,
Iran , North India and Burma.
Colliding plates give rise to young fold mountain belt and folding
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triggers off earthquakes.
c. Minor earthquakes belts
diverging plates in mid-Atlantic belt
diverging plates beneath East African Rift Valley
Fig C1.3World distribution of earthquake zones
D.
1.
a.
b.
c.
--d.
Causes of Earthquakes
Tectonic movements
Tectonic movements produce most of the world’s major earthquakes.
Therefore plate boundaries are regions of highest frequency of
earthquake occurrence.
Earthquakes of greater magnitude occur
Destructive boundary
Transform fault
Earthquakes of low magnitude occur along constructive boundaries
2.
a.
b.
c.
Man’s Action
The collapse of large, deeply dredged underground mines
Underground nuclear explosion
Careless siting of reservoirs in weak zones
SLCSS/Form 5/GEO/Chapter C1 / P.10
E.
1.
a.
b.
c.
d.
e.
f.
g.
2.
Effects of Earthquakes
The degree of destruction of an earthquake is largely determined by:
magnitude
duration
proximity to epicenter
population density of the affected area
building design and materials
time of the earthquake (people’s activity)
aftershocks if any (points overflow)
Earthquakes cause direct shakings, but they also cause secondary
hazards which may bring even more widespread and catastrophic
damages , for example, landslides, tsunamis and fires
F. Human Responses to Earthquakes
1. Prediction
Earthquakes can be predicted through installing earthquake detecting
systems.
2. Reduction the damages brought about by earthquake
a. To restrict the construction of dams, nuclear power plants
b. To carry out proper land-use planning to minimize development and
settlement in dangerous zones
c.
d.
e.
To improve the structural design of buildings so that they can
withstand earthquake shocks
To provide early warning for evacuation
To educate the public on the proper reactions during and after and
earthquake.