F.7 geography Ch 5 Farming Hazards

It is the interaction of Nature and Man Positive result - considered natural resources, which we either use or conserve. The negative aspects - to make a response in order to minimize risk and danger.

For example, loess is formed by fertile wind- blown deposit which provides good soils for farming in North China. The fiord coast in Norway provides good natural harbours. They encourage the development of fishing industry. The natural environment is unstable. And from the beginning, people have had to learn to deal with the environmental risks and changes produced by the unstable environment.

Natural hazards are events in the physical environment caused by forces of nature which bring harm to man. For example, floods would not be hazards if no human settlements occupied the flood plains. They are merely parts of a regular process of nature.

Below diagram shows the interaction of nature and man. The interaction of nature and man creates both useful resources and hazardous threats for human beings.

(C) FARMING HAZARDS: MEASURING AND ANALYING NATURAL HAZARDS

Geographers use six indicators to measure and analyse the extreme geophysical events that comprise natural hazards. These six indicators are : Geographers use six indicators to measure and analyse the extreme geophysical events that comprise natural hazards. These six indicators are : 1. magnitude ( 強烈程度 ) 2. speed of onset ( 出現時間的速度 ) 3. duration ( 持續期 ) 4. areal extent ( 影響面積 ) 5. areal reliability ( 影響範圍的穩定性 ) 6. frequency ( 頻率 )

(D) FARMING HAZARDS: IMPACT OF NATURAL HAZARDS AND LEVEL OF ECONOMIC DEVELOPMENT We should pay particular attention to the level of economic development in the area concerned, for there is a noticeable worldwide relationship between economic development and the impact of natural hazards. About 95% of disaster related deaths occur among the two thirds of the world's population that occupy developing countries

(E) FARMING HAZARDS - FLOODING

1. WHAT IS FLOODING ? A “ flood ” refers to an abnormally high water level in the river channel, above the critical upper limit of the “ bankful stage ” of the river, such that overbank flow occurs and the adjoining flat ground becomes inundated

Coastal flooding caused by : a. unusual atmospheric conditions, e.g the high seas created by onshore hurricane or tornado winds; or b. earth tremors or volcanic eruptions that set up huge tidal surges.

River flooding, occur anywhere in a river channel, where the flow exceeds bank-full capacity. River flooding, occur anywhere in a river channel, where the flow exceeds bank-full capacity. This can happen when there is very heavy rain or rapidly melting snow, or very rarely, when natural or man-made dams collapse and the impounded water is released. Most rivers are subject to flooding in the lower course.

Distribution: Flooding is especially common on the floodplains of Nile, Huang He (Yellow River), Chang Jiang (Yangtze), Ganges( 恆河 ), Indus, Mekong, Mississippi and Missouri. Low – lying floodplains are attractive to farmers because of the following advantages: Low – lying floodplains are attractive to farmers because of the following advantages:

- They are covered by deposits of alluvium, hence fertile soil, for agriculture. - They are covered by deposits of alluvium, hence fertile soil, for agriculture. - They provide extensive level land for farming. - They provide extensive level land for farming. - The rivers which cross the floodplains provide irrigation water. - The rivers which cross the floodplains provide irrigation water. - And cheap river transport of agriculture products. - And cheap river transport of agriculture products.

Farming and settlements on floodplains, however, can also be subject to great risk, for once in a while the rivers may overflow their banks and cause great damage to crops, livestock, farmhouses and settlement, and even loss of human lives.

FREQUENCY: The study of long periods of records of river stage and past floods enables us to estimate the probability and frequency of floods in a river system. In a floodplain, there is at least once a year unless some flood prevention work has been done, e.g. in the Nile floodplain today. In a floodplain, there is at least once a year unless some flood prevention work has been done, e.g. in the Nile floodplain today. In some floodplains, the annual flood often corresponds with the period of maximum river flow after prolonged monsoon rains or after the melting of snow in the upper courses of the river.

For example, In Huang He and Ganges, flooding may be experienced twice in a year, one in late spring and the other in late summer. This is because these rivers have the upper courses in snow-covered highlands and receive heavy monsoon rains.

Magnitude of flood: It depend on the height of the floodwaters above their mean annual heights. In some area, the magnitude of flood stage may be enlarged by occasional tropical cyclones which bring heavy rainfall in summer months.

2. WHAT IS IT LIKE WHEN FLOODING OCCURS ? Floods are the most common of all natural hazards. It also covers the largest spatial area and causes the greatest loss of life and property compared to other natural hazards. The reasons are obvious. Human beings are attracted to the flood plains because of their natural characteristics - abundant water supply, fertile soil and flat relief. Due to the dense human settlement in the flood plains, the damage potential of floods is great.

Causes of flood: 1. Unreliable, erratic rainfall comes suddenly with high intensity. 2. Melting of snow and glaciers in upper mountain courses results in a sudden increase of meltwater in the rivers. 3. Excessive / large water volume and heavy load or sediment are brought down by rivers and tributaries.

4. Low relief/gentle gradient causes slow flow and deposition of silt / load, thereby rising tiver beds and making channels shallow. 5. There is silting in the river mouth / delta region due to weak tidal currents. 6. Absence of lakes in upper and middle courses to regulate the flow of river. 7. Absence of dams / reservoirs to filter sediments and control river flow. 8. Frequent landslides may dam the river channels and block the movement of water in the rivers. 9. Rapid runoff due to deforestation.

4. DAMAGES OF FLOODING Study the table below showing historic floods around the world.

Effects of flooding on cropping: Massive flooding cause outright and destruction of crops, and excessive prolonged waterlogging. Alluvium though deposited cannot be readily usable. Controlled or small-scale flooding does not bring so much harm and may be beneficial at times – it deposits a lot of alluvium, which can be readily used by crops after the surface water recedes by evaporation and drainage.

5. FLOODING : PEOPLE'S RESPONSES TO FLOODS

Geographers and psychologists studying people's perceptions of natural hazards have gone a long way to help use understand how people will interpret various hazards and respond to public education programmes or hazard warning and preparation schemes.

They have found that people exhibit a range of responses - from accepting the hazard as an Act of God, inevitable and uncontrollable, through trying to predict hazardous events, taking appropriate action to reduce the damage potential, to offsetting the losses through insurance. A range of hazard perceptions and samples of common response in the case of flooding are provided in the following figure.

The hazard research discovered to explain the great variety of perceptions and responses: 1. The past record of hazards, especially magnitude, frequency and tedency; 2. The strength of traditional cultures, especially people's attitudes to the environment; 3. Education standards in the community; 4. Relative levels of wealth and economic development;

5. community awareness and preparations; 5. community awareness and preparations; 6. willingness of local and national government to spend money on long-term hazard prevention and damage reduction schemes compared with short-term emergency relief after a disaster. 6. willingness of local and national government to spend money on long-term hazard prevention and damage reduction schemes compared with short-term emergency relief after a disaster.