Flood Studies in Asia by Remote Sensing
2.0 Floods in Asian Countries
Disasters have global impact. Most of the Asian countries are disaster-prone. The physiography, morphology and other natural conditions have made them vulnerable to disasters and environmental hazards. The major universal disasters include. : floods, droughts, windstorms, earthquakes, volcanic eruptions, environmental degradation etc. These disasters occurs frequently and regularly all over the globe but the Asian countries suffer most because of their less preparedness and less preventive measures available in the countries. Recent floods in the Asian countries have caused havoc in terms of physical damages in property and infrastructural facilities in addition to the loss of human life and life stocks. Satellite Remote Sensing technology have been used for flood monitoring, damage assessment, reconstruction, rehabilitation activities. In the present paper an in-depth study of floods using Remote Sensing and conventional information/data for Bangladesh and China have been made.
2.1 Floods in Bangladesh
Floods frequently and regularly occur in Bangladesh. Through monsoon rains are the major causes of flooding but there are various other factors like rapid runoff, the effect of the confluences of the major rivers, the flat topography of the delta and surges in the Bay of Bengal. Types of flooding in Bangladesh are :
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Flash floods – sharp rice and drop in water levels causing high velocity damaging crops and property.
- Rain floods – high density rainfall over Bangladesh and surrounding area.
- Monsoon floods – over spilling of major rivers – usually rise slowly but extensive damage occurs when the three major rivers rise at a time.
- Storm surge floods – arising from storm surges in the coastal area.
The peculiar geographic location and low topography of Bangladesh are has subjected her flooding from time immemorial. The severe and extensive flooding in the Bangladesh area occurred in the years 1787, 1871, 1885, 1892, 1918, 1922, 1954, 1955, 1963, 1968, 1969, 1971, 1974, 1987 and 1988. Satellite Remote Sensing technology have been used for the study of 1987 and 1988 flooding.
The 1987 flood commenced in the third week of June and lasted until the end of September. The earliest flood hit the northern parts of Bangladesh. The Brahmaputra, the Ganges, and the Meghna reached their respective bankful stages in the last week of July and beginning of August. One of the unique features of the 1987 flood was that the northwestern districts of Bangladesh, which remain normally flood –free, experienced prolonged flooding from the tributaries of the Brahmaputra and the Ganges. In the northwestern region, the Ganges and the Brahmaputra and many of their tributaries exceeded their record water levels, causing extensive over bank inundation (Miah 1988). In the central region, extensive flooding was reported along the lower reaches of the Brahmaputra, Ganges and Meghna rivers. Reports also confirmed deep flooding along the tributaries of the Meghna in the northeastern region. The official estimate of the maximum extent of flooding in 1987 was 57, 300 km2 (40% of Bangladesh)(BWBD 1987). However, the accuracy of this estimate could not be verified through satellite imagery alone since it was based on varied sources of information. More than one-third of Bangladesh population was affected during this flood.
1988 Flood :
The 1988 flood was the most catastrophic flood of this century. The flood commenced in early July, but the record high water levels were achieved relatively late in the summer by rapid synchronization of peak flows in eh Brahmaputra (99,500 m3/sec), and the Meghna (>19,000 m3/sec) within a period of 72 hr between 30 August and 2 September (Matin and Hussain 1988). The combined peak flow of the three rivers (190,800 m3/sec)seceded the 1987 record –
highly flow by a significant margin and, thus, set a new 100 – years record. The flood also set another 100 – year record by inundating an estimated area of 82,000 km2 (57% of Bangladesh), which surpassed the previous records of the extent of 1955 and 1987 flooding 36% and 40%, respectively) by a wide margin. The flood area delineation from an AVHRR image by using visual interpretation and the computed flood areas on different dates varied significantly during the recession period, indicating the dynamic nature of the flood event. These data suggest that the flood started receding slowly through a period of secondary peaks (such as that of 15 September). By 24 September, the extent of flooding was considerably less than of 10 September, although large areas were still under water. More than fifty – percent of Bangladesh population was affected by this flood.
2.2 Floods in China
A legendary flood occurred in China in 2297 BC due to extremely intensive and extended heavy rains. The Huang Ho (Yellow river) and Yangtze rivers overflowed their banks and submerged almost the entire north China. Historical records show that during the period 206 BC to 1960 AD China has experienced more than 1030 major floods. Extensive flooding also occurred in the Yangtze basin in 1870, 1896, 1911, 1931, 1949 and 1954. The catastrophic flooding of 1931 and 1954 was treated as national disasters. The Huang Ho has been called “china’s sorrow” and “The Ungovernable” but now the basin has been turned into “China’s rice granary” and productive watercourse by controlling the flood. The worth-mentioning flooding happened in the Huang Ho valley in the year 1887, 1889, 1921, 1933, 1938, 1949 and 1959.
1991 Flood
Taihu lake area of China (situated between Nanjing and Shanghai) was inundated due to flooding of July 1991. Satellite imagery of March, May, June and July 1991 were used to study the effects of flooding. More than 40 percent of the surrounding lake area was inundated in July 1991 by flooding causing huge loss go agricultural crops and much sufferings of the people in the area.
