Subsidence and Flooding in Bangkok

Bangkok, the capital of Thailand, is situated on flat, low land in the southern part of the Central Plain, one of the main physical units of the country. Through the heart of the city, the Chao Phraya flows from the north and discharges into the Gulf of Thailand, 25 km south of the city centre. The city was founded in 1782, and in its early years numerous klongs (canals) were dug for transportation and defence uses. These canals became corridors of early development, and banks were lined with houses, shop-houses, and temples, etc. With the beauty of its waterway landscape, Bangkok was once dubbed the Venice of the East. Unfortunately, such a resemblance no longer exists as most of the canals have been backfilled to make room for road construction in recent urbanization. The Bangkok metropolis, which at present has a population in excess of 10 million, has expanded rapidly on both banks of the river since 1950. It has encroached into surrounding provinces, covering an area of approximately 60 × 70 km. Owing to its flat topography and close proximity to the sea, flooding threatens the city annually. Modern urbanization has resulted in the drastic destruction or blockage of natural drainage paths, increasing the flood risk to the city. Severe land subsidence from excessive groundwater extraction since the 1960s has intensified the flood risk, as well as creating numerous foundation problems. At present the land surface in some areas is already below mean sea level. The city now has to rely on a flood protection system to prevent inundation. However, its effectiveness is only temporary because land subsidence has not yet ceased. The Central Plain is formed by the Chao Phraya River, the largest in the country. The river basin stretches from the Northern Highland to the Central Plain and covers about one-third of the country (514 000 km2). The Central Plain can be divided into the Upper and Lower Central Plains. The former extends from Tak to Nakhon Sawan Provinces. Four main rivers, namely, the Ping, the Wang, the Yom, and the Nan, which originate in the Northern Highland, traverse the plain and join together at Nakhon Sawan, 240 km north of Bangkok, to form the Chao Phraya River.

Water in Cities

Southeast Asia, with most of its area receiving an annual rainfall of more than 2000 mm, is a region of positive water balance. It is also an area where unfulfilled demand for water is not unknown. Such a contradiction happens at times in its towns and cities. Several Malaysian urban settlements, for example, suffer occasionally from water shortage in a country with an average annual rainfall of about 3000 mm. Kuala Lumpur went through a prolonged period of water shortage in 1998 (Hamirdin 1998) in spite of large allocations made earlier in various five-year plans towards developing water supply infrastructure. Such shortages are common during long dry periods associated with El Niño. Regional water shortages may become more common in future, especially with the rising population and economic expansion. The shortages are the result of an inability to meet the rising demand of water in cities driven by both increasing population and progressive prosperity. Serious shortage occurs in large cities such as Jakarta, Bangkok, and Manila where a significant proportion of their population has no immediate access to municipal potable water. Even where piped connection exists, supplies are not available round the clock and often do not meet the required water quality standards. In many cities the local sources are inadequate and water has to be brought in from rural areas. The demand for water in a city has to be met on both quantitative and qualitative terms. For example, drinking water supplied to households by a municipal administration has to meet a given standard (WHO 1993). Ideally a city should have enough water to drink, to meet industrial demand, and to be able to store an adequate volume under pressure for firefighting and street cleansing. Supplying a city with water requires water sources, a treatment system, a distribution system, and arrangements for treating waste water and its disposal. In this chapter we review the current status of water supply in urban Southeast Asia and the sources that are available, concentrating on the major cities. We indicate the success stories as well as the shortcomings.

Hydrology and Rural Water Supply in Southeast Asia

The East Asian economic turmoil of 1997 and its lingering effects belie the decade of unprecedented economic growth in the Southeast Asian region. This economic boom saw a significant increase in the per capita income of the population of the respective countries and a corresponding rise in the standards of living. The decade also saw increased government spending on infrastructural development of basic amenities, including irrigation extension and rural water supply. The demand for and consumption of water increased significantly in both cities and the rural areas. In contrast to the escalating demand for water by the economies of the Southeast Asian countries, available resources remain limited despite the fact that the region generally receives more rainfall than it loses through evaporation annually. Annual, seasonal, and spatial variations in the rainfall within and between countries on the one hand, and accelerated demands for water from the various sectors of the economy on the other, put a severe strain on the available water resource base. In addition, natural resources in the form of rivers, groundwater storage, and lakes are rapidly diminishing in quality as a result of domestic, agricultural, and industrial waste discharges. In the coastal plains, excessive groundwater abstraction resulting in salt-water intrusion has affected groundwater resources. Inland, and in the watershed areas, rapid and extensive development has been at the expense of forested land, which has given way to new urban centres and residential and industrial complexes, while uncontrolled logging and shifting agriculture have caused the deterioration of the remaining forested ecosystem and natural watersheds. Given these factors, the future water resources scenario of the region seems bleak unless urgent steps are taken to manage seriously the resources in a judicious and sustainable way. Water will certainly feature as an important issue of development in the region in the decades ahead, given that large population concentrations and economic development are located in the lower parts of river basins. This chapter describes the hydrological conditions of the Southeast Asian region and examines the nature and extent of water resources that have been put to use for rural and agricultural development.

Volcanic Hazards in Southeast Asia

Active volcanism in Southeast Asia is associated with marked zones of activity in the Earth’s crust that run through south and east Indonesia and the Philippines. These zones are also characterized by frequent earthquakes and a measurable movement of tectonic plates, often in the order of 5 cm yr−1. The underlying mechanism is that of subduction of oceanic plates below continental plates; the rigidity of the moving plates causes ruptures and shockwise adjustments (earthquakes). The oceanic plate, while being under thrust, sinks down to great depths below the continental plate and in the process loses its rigidity owing to heating and part assimilation into the underlying magma. Earthquakes are caused in the zone where the subducted plate is still rigid. Chapter 1 in this book puts this phenomenon in the regional context. Volcanism in this zone is marked by frequent eruptions, mostly violent and of an explosive nature. It is manifest in distinct belts that comprise all (or nearly all) of the Philippines, and large parts of Indonesia with the exception of, roughly speaking, Kalimantan and Papua. The violence of the eruptions poses threats to human settlements in the surroundings of the volcanoes, to the cultivated lands, and the infrastructure. These threats may occur during and after the actual eruption, and they may indirectly cause other hazards as well. Moreover, volcanoes in apparent dormancy that have not erupted in historical times may still come to life as the interval between eruptions may be very long. In the present chapter these hazards will be discussed. Natural hazards have been defined in four ways, of which the 1982 definition of the United Nations Disaster Relief Co-ordinator (UNDRO) seems appropriate to follow in the context of volcanic hazards (Alexander 1993). UNDRO defines natural hazards as ‘the probability of occurrence within a specific period of time and within a given area of a potentially damaging phenomenon’. A hazard therefore may represent a situation with the possibility of a disaster that may affect the population and the environment which are in some degree of vulnerability.

Accelerated Erosion and Sedimentation in Southeast Asia

Periodic attempts to plot global distribution of erosion and sedimentation usually attribute most of Southeast Asia with a very high sediment yield (Milliman and Meade 1983). The erosion rates and sediment yield figures are especially high for maritime Southeast Asia. Milliman and Syvitski (1992), for example, listed 3000 t km−2 yr−1 for the archipelagos and peninsulas of Southeast Asia. They provided a number of natural explanations for the high erosion rate: location near active plate margins, pyroclastic eruptions, steep slopes, and mass movements. This is also a region with considerable annual rainfall, a very substantial percentage of which tends to be concentrated in a few months and falls with high intensity. Part of Southeast Asia (the Philippines, Viet Nam, Timor) is visited by tropical cyclones with heavy, intense rainfall and possible associated wind damage to existing vegetation. The fans at the foot of slopes, the large volume of sediment stored in the channel and floodplain of the rivers, and the size of deltas all indicate a high rate of erosion and episodic sediment transfer. This episodic erosion and sediment transfer used to be controlled for most of the region by the thick cover of vegetation that once masked the slopes. When vegetation is removed soil and regolith de-structured, and natural slopes altered, the erosion rates and sediment yield reach high figures. Parts of Southeast Asia display striking anthropogenic alteration of the landscape, although the resulting accelerated erosion may be only temporary, operating on a scale of several years. Over time the affected zones shift, and slugs of sediment continue to arrive in a river but from different parts of its drainage basin. The combination of anthropogenic alteration and fragile landforms may give rise to very high local yields. Sediment yields of more than 15 000 t km−2 yr−1 have been estimated from such areas (Ruslan and Menam, cited in Lal 1987). This is undoubtedly towards the upper extreme, but current destruction of the vegetation cover due to deforestation, expansion of agriculture, mining, urbanization, and implementation of large-scale resettlement schemes has increased the sediment yield from < 102 to > 103 t km−2 yr−1.

Vegetation

Southeast Asia is not a natural biogeographical unit: it extends well north out of the tropics in Myanmar, while the eastern boundary bisects the island of New Guinea. It is also divided in two by one of the sharpest zoogeographical boundaries in the world, Wallace’s line (Figure 7.1; Whitmore 1987). There is, however, one important unifying feature that distinguishes it from most other regions of the tropics: Southeast Asia is a region of forest climates. Only on the highest mountains in Papua and northern Myanmar is the climate too cold for forest and, with the possible exception of some small rain-shadow areas, it is nowhere too dry. Elsewhere the only permanent non-forest vegetation in the region before the human impacts of the last few millennia was on coastal cliffs and beaches, seasonally flooded river plains, active volcanoes, and perhaps some small inland areas on soils too poor to support forest. Today, however, as a result of human impacts, forest occupies less than half of the region, with various anthropogenic vegetation types occupying the rest. The recognition of Southeast Asia, as defined here, as a separate political and geographic entity is very recent, so it is not surprising that there has been no previous account of the vegetation of the whole region. Van Steenis (1957) gave a general account of the vegetation of Indonesia, while Whitmore (1984) concentrated on the tropical evergreen forests of the region, with only a brief description of the vegetation of drier climates. Champion (1936) described the principal forest types of Myanmar, while Vidal (1997) covered the vegetation of Thailand, Cambodia, and Lao PDR. Numerous other publications describe smaller areas or specific vegetation types. To a first approximation, the potential natural vegetation of the region (Plate 1) up to about 20°N is controlled by two main environmental gradients: a horizontal gradient of water availability and a vertical, altitudinal gradient. Water availability is determined largely by the amount and distribution of rainfall, with the length of the dry season the most important factor, although the water storage capacity of the soil becomes increasingly significant at the drier end of the gradient.

The Geological Framework

This chapter outlines the principal geological features of the region, extending from Myanmar and Taiwan in the north, southwards to include all the ASEAN countries, and extending as far as northern Australia. The present-day lithospheric plates and plate margins are described, and the Cenozoic evolution of the region discussed. Within a general framework of convergent plate tectonics, Southeast Asia is also characterized by important extensional tectonics, resulting in the world’s greatest concentration of deep-water marginal basins and Cenozoic sedimentary basins, which have become the focus of the petroleum industry. The pre-Cenozoic geology is too complex for an adequate analysis in this chapter and the reader is referred to Hutchison (1989) for further details. A chronological account summarizing the major geological changes in Southeast Asia is given in Figure 1.2. The main geographical features of the region were established in the Triassic, when the large lithospheric plate of Sinoburmalaya (also known as Sibumasu), which had earlier rifted from the Australian part of Gondwanaland, and collided with and became sutured onto South China and Indochina, together named Cathaysia. The result was a great mountain-building event known as the Indosinian orogeny. Major granites were emplaced during this orogeny, with which the tin and tungsten mineral deposits were genetically related. The orogeny resulted in general uplift and the formation of major new landmasses, which have predominantly persisted as the present-day regional physical geography of Southeast Asia. The Indo-Australian Plate is converging at an average rate of 70 mm a−1 in a 003° direction, pushed from the active South Indian Ocean spreading axis. For the most part it is composed of the Indian Ocean, formed of oceanic sea-floor basalt overlain by deep water. It forms a convergent plate margin with the continental Eurasian Plate, beneath which it subducts at the Sunda or Java Trench. The Eurasian continental plate protrudes as a peninsular extension (Sundaland) southwards as far as Singapore, continuing beneath the shallow Straits of Malacca and the Sunda Shelf as the island of Sumatra and the northwestern part of Borneo.

The Climate of Southeast Asia

Southeast Asia lies between the continental influence of the rest of Asia to the north and the more oceanic influence of the Indian and Pacific Oceans to the south and the east respectively. While its overall net energy balance is very much determined by its latitudinal position, which is approximately between 20°N and 10°S, the locational factors referred to above largely give the regional climate its distinctive character. Within the broad latitudinal extent defined above, the Southeast Asian region has often been conveniently separated into two sub-areas: continental and insular Southeast Asia. In some ways these sub-regions represent a valid delineation into the more seasonal climatic region influenced by the monsoon system of winds and the uniformly humid equatorial climate. The former comprises Myanmar, Thailand, Lao PDR, Cambodia, and Viet Nam, while the latter includes Malaysia, Singapore, Indonesia, and the Philippines. The continental Southeast Asia experiences greater seasonality, more extremes in both temperature and rainfall, and more pronounced dry spells; whereas the insular parts, termed the ‘maritime continent’ (Ramage 1968), with a much greater expanse of sea than land (the sea area of Indonesia, for example, is four times its land area), have more equable climate. The northern and southern continental interactions in winter and summer and the differential heating due to the asymmetric character of the two sub-regions give rise to the monsoon development (Hastenrath 1991), which, to a large extent, influences the rainfall characteristics of the region as a whole. In a sense, more than temperature variations, this monsoonal influence gives the Southeast Asian climate its distinctive character. Figure 5.2 shows the two monsoon wind systems that affect Southeast Asia. In addition to these annual reversals of the monsoon winds, the seasonal migration of the Intertropical Convergence Zone (ITCZ)—closest to the Equator during the northern hemispheric winter and farthest north during summer—is a significant factor in influencing the monthly weather regime of the region. Being a belt of low-pressure trough coinciding with the band of highest surface temperature, the ITCZ attracts the moist easterlies from both hemispheres towards its trough resulting in uplift of air, intense convection, and precipitation. This whole process provides a mechanism for the transfer of latent heat from the low to the higher latitudes (Houze et al. 1981; Hastenrath 1991).

The Urban Environment in Southeast Asia

As elsewhere, the major cities of Southeast Asia suffer from traffic congestion, air pollution, water supply shortages, garbage disposal inefficiencies, and sewage treatment inadequacies (Barrow 1981). Such problems are not confined to the capital cities and other centres of over a million population. They are prevalent, and often worse, in hundreds of smaller towns of a thousand to a million inhabitants. Most such urban centres have a large proportion of poor, ill-housed people who have difficulty in doing anything to improve their environment. At the same time, the bigger cities will also have some select, well-managed, often walled and gated, suburbs where the quality of housing and water and sanitation services is excellent. However, all social groups may be vulnerable to the air pollution and disease risks associated with a generally poor urban environment. Floods, landslides, and subsidence also do not distinguish the wealth or social status of their victims. These multiple, overlapping urban environmental problems are a response to a complex set of causes or drivers. The character of cities and towns in tropical Southeast Asia is driven in part by the types of human activity within and around them and in part by the environment in which they are situated. The hot and often humid climate has increasingly led to changes in house design from buildings with verandas and arcades designed to be cooled by natural air flows, to more boxlike structures dependent on air conditioning. The exhausts from the air conditioners inevitably add heat to the outside air, warming the immediate urban environment, often making the narrow streets of many cities hotter and more uncomfortable than they otherwise would be. The design and character of buildings are governed by environmental, aesthetic, functional, and cost considerations. In part building styles reflect the type of shelter needed and in part they make statements about their owners and the activities which go on inside them. It is the same with the settlement as a whole. A town or village has features that help it to cope with the natural environment around it, especially heavy rains and strong winds.

Hazards and Risks at Gunung Merapi, Central Java: A Case Study

Of the 1.1 million people living on the flanks of the active Merapi volcano in Java (average population density: 1140 inhabitants per km2), 440 000 live in relatively high-risk areas prone to pyroclastic flows, surges, and lahars. The sixty-one reported eruptions since the mid-1500s killed about 7000 people. For the last two centuries the activity of Merapi has alternated regularly between long periods of lava dome extrusion and brief explosive episodes with dome collapse pyroclastic flows at eight- to fifteen-year intervals. Violent explosive episodes on an average recurrence of twenty-six to fifty-four years have generated pyroclastic flows, surges, tephra falls, and subsequent lahars. The current hazard zone map of Merapi (Pardyanto et al. 1978) portrays three areas, termed the forbidden zone, first danger zone, and second danger zone, based on progressively declining hazard intensity. Revision of the hazard map has been carried out because it lacked the details necessary to outline hazard zones with accuracy (in particular the valleys likely to be swept by lahars), and excluded some areas likely to be devastated by pyroclastic density currents, such as the 22 November 1994 surge. In addition, risk maps were developed in order to incorporate social, technical, and economic elements of vulnerability (Lavigne 1998, 2000) in the decision-making progress. Eruptive hazard assessment at Merapi is based on reconstructed eruptive history, based on eruptive behaviour and scenarios combined with existing models and preliminary numerical modelling (Thouret et al. 2000). The reconstructed past eruptive activity and related damage define the extent and frequency of pyroclastic flows, the most hazardous phenomenon (Camus et al. 2000; Newhall et al. 2000). Pyroclastic flows travelled as far as 9–15 km from the source, pyroclastic surges swept the flanks as far as 9–20 km away from the vent, thick tephra fall buried temples in the vicinity of Yogyakarta 25 km to the south, and subsequent lahars spilled down radial valleys as far as 30 km to the west and south. At least one large edifice collapse has occurred in the past 7000 years (Camus et al. 2000; Newhall et al. 2000).