Water Hazards, Resources and Management for Disaster Prevention:
A Review of the Asian Conditions
IDNDR 1991-1999

3. SUMMARY OF WATER-RELATED HAZARDS IN THE ESCAP REGION

3.1 General

As has been indicated above, water-related natural disasters in the form of tropical cyclones, floods, landslides and mud flows are periodical occurrences in the majority of the countries of the ESCAP Region. In many places land degradation, the consequence of poor land management, has served to aggravate the seriousness of such disasters.

The available data indicate that whilst not all the ESCAP countries are affected by tropical cyclones, very few of them are free from damaging flood events. These data also indicate that whilst tropical cyclones and associated storm surges are likely to cause the highest numbers of fatalities, floods are the most frequently occurring disaster events and the ones which cause the greatest total amount of damage.

Tsunamis are also the cause of substantial destruction in coastal regions. Elsewhere, landslides and mud flows following very heavy rainfalls may cause considerable damage in both urban and rural communities.

Droughts are a frequently occurring natural disaster in many countries, impacting particularly upon rural communities. Land degradation may exacerbate and prolong the adverse consequences of such events.

Water-related natural disasters are of common occurrence in the Philippines, where they may produce massive devastation. They include cyclones, floods, mass movement and drought. The severity of the impact from such disasters varies from region to region according to geographical location and topographical features. This country lies within the tropical cyclone belt and is affected by cyclones associated with the south-west monsoon during May-September and the north-east monsoon during November-February.

Flooding is the most commonly occurring form of natural disaster and includes both riverine flooding and coastal flooding. Coastal areas are particularly susceptible to flooding from tsunamis, which may be aggravated at high tide periods. Floods often cause tremendous damage to prime agricultural lands and to government infrastructure such as roads, bridges, irrigation dykes and flood-control structures. Landslides are the most commonly-occurring form of mass movement disaster in the Philippines, and may affect pristine, disturbed or developed land areas.

Climatic conditions in Indonesia are dominated by the tropical monsoon which extends from December to May each year. Flooding is the most frequently occurring natural disaster phenomenon, but tidal waves, landslide and droughts may have severe effects on local populations. The occurrence and severity of such events varies widely across the many island regions of which Indonesia is composed.

Generally speaking, the upper watersheds of most large rivers are characterized by very steep slopes which are occasionally subject to very high intensity rainfall. Under such conditions, flash flooding and landslides are common occurrences. On the coastal plains, extensive and protracted flooding occurs from time to time.

On the East Coast of Peninsular Malaysia, and along the coastline of Sabah and Sarawak, flooding is commonly associated with the north-east monsoon during the months of November to February. Intense, localized and short-duration thunderstorms are often the cause of flash flooding on the small but steep watersheds along the West Coast of Peninsular Malaysia.

In urban areas of Malaysia, intensive convective thunderstorms during the monsoon season are often the cause of flash flooding, particularly in Kuala Lumpur. Landslips or mudflows are an occasional consequence of heavy localized rainfall. Generally speaking, however, Malaysia is relatively free from massive flooding caused by severe tropical cyclones.

The Republic of Korea is located in the temperate monsoon region. About two-thirds of the annual rainfall is received during the monsoon period from June to August. During these months, tropical cyclones and intense depressions bring heavy rainfall which often results in major flooding. Prior to the monsoon season, occasional droughts affect the agricultural and industrial sectors and impact upon rural communities. Tsunamis and landslides produce less frequent and less harmful natural disaster events.

Tropical cyclones can occur along the entire Chinese coast and the inland areas adjacent to it. The eastern and southern coastal regions are particularly vulnerable but all inland areas, with the exception of the north-western region, are within the range of cyclone damage. Most of the tropical cyclones affecting China approach from the China Sea. They may cause heavy damage along both large and small rivers, as well as along the coastline. Flooding may also be caused by heavy rainstorms, ice jams or landslides.

Apart from these effects, landslides and mud flows can be problems across substantial areas of China. Many areas are also affected by droughts, often occurring sequentially, which result in severe impacts upon agricultural production and the overall national economy.

In Cambodia, major flooding can be caused by the Mekong River, as a consequence of heavy monsoon rainfalls over its upper catchment. Flash flooding is also common on smaller high-level watersheds across the country. Landslides caused by heavy rainfalls are also a common occurrence on upland watersheds.

Along the coast of Myanmar, widespread damage can result when tropical cyclones coincide with storm surge conditions. Cyclones occur during the months of June to December. Severe tropical storms are also experienced during April, May, October and December.

Flooding during the south-west monsoon may severely affect the lower reaches of the Ayeyawaddy River. Flash flooding is also experienced over upstream tributaries and smaller watersheds as a consequence of heavy rainfall. Damages from landslides, mudflows or droughts are essentially negligible by comparison with the severe damages that results from cyclone and flood events.

India has a long coastline which is exposed to tropical cyclones originating in the Bay of Bengal and the Arabian Sea. These cyclones are usually associated with high winds, torrential rains, flooding and storm surges.

Elsewhere in India, flooding occurs during the monsoon season and is a consequence of heavy rainfalls associated with cyclone events, the monsoons, or intense tropical storms. Flash flooding is a problem on steep watersheds.

Landslides are also a common and frequent form of natural disaster in India, a consequence of heavy rainfalls and land and soil degradation resulting from inappropriate human activities on steep country. The highest incidence of landslide disasters is to be found in the Himalayan Region.

Many parts of India are also subject to severe drought events, a consequence of the erratic occurrence and behaviour of local rainfall conditions. It is estimated that 70 percent of the agrarian districts of India are drought-prone.

Pakistan does not suffer from the damaging effects of tropical cyclones, but is subject to devastating floods originating from monsoon rainfalls and snowmelt. The most widespread flooding occurs on the floodplains of the larger river systems, but upstream flooding resulting from landslides or the blocking of tributaries by glacial dams is also a common problem.

Drought is a common feature of climatic conditions in the arid regions of Southern Pakistan and may have severe adverse effects, not only in economic and social terms but also in its land degradation consequences.

Cyclones, floods and drought are the worst forms of natural disaster to affect Bangladesh, although droughts occur only comparatively rarely. Tropical cyclones originating in the Bay of Bengal are usually associated with heavy rainfalls, strong winds and storm surges. Tidal waves often accompany cyclonic storm events. Tidal wave and storm surge conditions have from time to time been responsible for very heavy loss of life and extensive property damage.

Severe flooding occurs along the main rivers in Bangladesh and smaller tributary streams may also experience serious flooding as a result of intense local rainstorms. Because Bangladesh has only a small area of hilly country, flash flooding or mass movement are not significant problems.

In Vietnam, the northern and central regions are often affected by tropical cyclones during the rainy season, which occurs between July and October. Storm surges may also be a problem along the coastline. Flash flooding occurs on the many small, steep watersheds in the central region, whilst extensive and protracted flooding can be experienced in the Red River Delta region to the north. Although tropical cyclone damage is rare in southern Vietnam, the Mekong Delta region commonly experiences major flooding as a result of heavy rainfall on the upper catchment.

The major forms of water-based natural disaster to affect the Lao People's Democratic Republic are droughts and floods. Whilst tropical cyclones are not a direct threat, they can produce very heavy rainfalls leading to devastating flooding on the many smaller tributaries throughout the country. Flooding along the Mekong River results from heavy monsoon rainfalls during the period of August to September. Droughts may be experienced between May and July, before the arrival of the monsoon season.

In Thailand, major natural disasters are mainly the consequence of flooding caused by heavy rainfalls associated with tropical cyclones. Landslides may also cause severe problems, whilst droughts are a common occurrence in the months preceding the rainy season.

Tropical cyclones and deep tropical depressions may extend across Thailand between May and October. These are associated with very heavy rainfalls which can produce major and protracted flooding along the larger rivers and their flood plains. Destructive flash flooding is also a common occurrence on the smaller watersheds scattered through the country.

The climate of Nepal is largely controlled by the monsoon cycle. The principal flood season coincides with the period of maximum monsoonal rainfall, which usually occurs in August. High altitude watersheds are subject to major flooding from snowmelt. Flash flooding may also occur in the higher watersheds as a result of heavy rainfall. Glacial outburst floods may occur as a result of the sudden release of ponded glacial lakes and landsliding is also a common occurrence in high watersheds.

Sri Lanka may be subject to a variety of natural disaster events, which include tropical cyclones, floods, oughts, landslides and coastal erosion. Heavy rainfalls occur during both the south-west monsoon period, from October to February, and the south-east monsoon period, from May to September. Tropical cyclones occur only rarely, but they can cause severe destruction and heavy loss of life. Floods are a common occurrence and they are often associated with landslips. Flash flooding is experienced on the high watersheds of the central mountain range and its slopes. Drought is also a common occurrence in the northern and eastern districts.

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