|Environmental Changes on the Coasts of Indonesia (UNU, 1980, 53 pages)|
The changes that we have been able to identify on the coasts of Indonesia include short-term changes related to such events as earthquakes, landslides, volcanic eruptions, tsunami, and major river flooding; medium-term changes such as the gradual or intermittent advance or retreat of shorelines over periods of several decades, notably on deltaic sectors; and long-term changes such as land uplift or subsidence, or sea-level rise and fall, which proceed at rates of a few millimetres to a few centimetres per century. Short-term changes can be seen as various kinds of natural hazard, which people living on or near the coast should be aware of in terms of planning and development, and capable either of avoiding, or diminishing by appropriate action. Medium-term changes, which can greatly modify the nature and extent of coastal resources over a period of a few years, must also be acknowledged by planners, and allowed for, or countered by practical schemes, in the course of development in coastal regions. Long-term changes, over periods of a few centuries, are of scientific and cultural interest, but are not directly relevant to the pressing problems of survival, nutrition, and welfare of peoples inhabiting the Indonesian coastal zone. In this account, our emphasis is on short- and medium-term changes: their immediate and ensuing effects; and the actual and potential modes of human response to them.
Earthquakes in the coastal zone are apt to be even more damaging then earthquakes inland, especially in low-lying areas where the disturbance may result in flooding by the sea, or overwash from rivers and estuaries, or the catastrophic release of water from broken dams upstream. On deltas and coastal plains the prospects of evacuation to higher ground are very limited. In addition, earthquakes trigger landslides and mudflows on steep coastal slopes and these may devastate or sweep away village settlements on or near the shore: again, the prospects of escape are limited, except for those who can get away on boats.
Earth tremors occur very widely through Indonesia and many parts of the coastline have been affected by them, directly or indirectly. We have made reference to earthquake damage in recent years on the coasts of Sumbawa and on Yapen and Biak, north of Irian Jaya.
Volcanic eruptions are more localized in Indonesia, but, nevertheless, several sectors of coastline have been blanketed by ash or invaded by lava in recent years and the subsequent movement of materials of volcanic origin down rivers has frequently been accompanied by flooding and the inundation of farmland and settlements by water from which sediments are deposited. On the other hand, volcanic activity has served to maintain the fluvial sand critical to maintaining a prograding shoreline. The central south coast plain of Java, for example, would probably be subject to shore erosion were it not for the maintenance of its beach fringe by fluvial sand supplies regenerated by periodic eruptions of the Merap; volcano north of Yog Yakarta.
Earthquakes and volcanic eruptions are also responsible for the generation of tsunami, when sea level is raised briefly by a series of large waves transmitted from the centre of disturbance through adjacent sea areas to reach sectors of coastline. The dimensions of these waves are determined partly by the severity of the generating disturbance, partly by the depth and configuration of the sea areas through which they are transmitted, and partly by the aspect and transverse profile of the coastline upon which they are received. Thus the Krakatau explosion in 1883 immediately generated waves up to 30 metres high on the surrounding shores of Sunda Strait but, as these spread out into the adjacent seas, they were modified by encounters with reefs and shoals and by refraction around headlands and across shallow, nearshore areas. Diffraction into the Java Sea reduced them to about a metre in Jakarta Bay and along the deltaic shorelines to the east. Given a knowledge of the depth and configuration of sea areas around a centre of disturbance, it is possible to draw wave refraction diagrams that will predict the height of a wave-generated by a coastal or submarine earthquake or violent eruption- as a proportion of the wave height initially generated and to estimate how long after a disturbance tsunami will reach various parts of the Indonesian coastline.
The effects of a tsunami are obviously related to the configuration of the coast. Waves break heavily over reefs and rocky-shore platforms, and against cliffs and steep coasts; they overwash beaches, beach ridges, and low dunes, scouring away sand and inundating low areas to the rear; they race up tidal inlets and river estuaries and, if they overtop natural or artificial levees, there is extensive salt water flooding of adjacent low-lying terrain. There is, of course, extensive damage to coastal settlements; boats are ripped from their moorings, overturned, and sunk; bridges are swept away and canal sluices disrupted; fishponds are washed over, rice and other crops ruined by sea flooding, trees uprooted, buildings and other structures destroyed. The damage may be less serious than that associated with Australian cyclones or China Sea typhoons, but the sudden rise of sea level, coupled with onshore wave movements, can be extremely destructive in low-lying coastal terrain. Moreover, it is very difficult to avoid these effects. It is necessary to inhabit and utilize these highly productive lowlying coastal areas, and it is fortunate that, although earthquakes and volcanic eruptions are frequent in Indonesia, major tsunami effects are relatively rare and localized. When they do occur, the changes in coastal landforms and associated landscape and land-use features that result are likely to be enduring.
River flooding occurs frequently in the wet season in deltaic areas and the lower reaches of valleys and is sufficiently regular for adjustments of land use, settlement, and routeways to be made in relation to the patterns and depths of inundation, for levees to be built to confine floodwaters, and for drainage canals to be cut to disperse them. Nevertheless, exceptional floods occur from time to time, and these adjustments may then prove inadequate. It is probable that the incidence and extent of flooding have increased following deforestation and the introduction of grazing and cultivation to sloping areas within river catchments, which has led to accelerated runoff and increased discharge as well as to soil erosion and augmented sediment yields. This has been offset to some extent by dam construction, which can reduce flooding as well as sediment yield downstream. Attempts to reduce the effects of flooding and to diminish soil erosion also include slope terracing, gully damming, and revegetation and afforestation in headwater regions. A more comprehensive programme of soil and water conservation would certainly reduce the flood hazard in lower reaches of valleys and in deltaic regions.
One cause of exceptional flooding, not strictly a natural hazard, is the deluge that follows the collapse or destruction of dams impounding reservoirs upstream. Such catastrophes may result from earthquakes or may be due to failure of an engineering structure. The rush of water downriver causes erosion and devastation along the valley and sudden flooding in deltaic areas. This is what happened in southern Java in 1966 after the collapse of the Sempor Dam, north-west of Kebumen, sent vast quantities of water downstream past Gombong to the coasts near Karangtawang.
However caused, river floods result in deposition of sediments in the inundated land areas and in the sea areas off river mouths. Subsequent reworking of this sediment in shallow coastal waters leads to some of it being added to beaches, the finer silt and clay settling on tidal mudflats and in mangroves on sheltered sectors: destructive effects are thus followed by constructive changes.
The loss of coastal terrain as the result of erosion is of particular importance when the land margin is densely populated and intensively utilized. Fishing villages are often located on or immediately behind beaches in Indonesia and, where erosion is in progress, these are undermined. Alternatively, they may be forced to retreat when the sandy shoreline is driven back by washover processes. Larger towns are usually sited farther inland, so that problems of maintaining them on sandy shorelines are less common than in countries where seaside towns have developed behind recreational beaches, but beach erosion has become a problem with the growth of tourism in Bali;, where some of the coastal hotels are already reporting losses of their beach areas.
Some beaches are backed by sandy beach terrain on which dry-land farming has developed and, where erosion becomes prevalent, this useful and productive land may be gradually pared away. More common, especially in Java, is the situation where shoreline recession is undermining and breaching the walls that impound fishponds, opening them up to marine incursion and rendering them useless for productive management. Brackish-water fishponds are usually excavated in the mangrove fringe on low-lying coasts, especially on delta margins. On some prograding sectors of deltas (e.g., on the Cimanak) fishpond construction has begun on accreting mudflats even before mangroves have become established. Inevitably, when erosion begins, as the result of diversion or decay of a river outlet or a diminution of sediment yield from a river mouth, brackish-water fishponds that were constructed so close to the sea are soon dissected and cut away. Erosion of fishponds occurred near the mouth of the Cidurian River after it was diverted by canal cutting in 1927, and similar erosion is in progress at Sungaibuntu on the receding sector of the Citarum delta (Plate 2), and at Kedungsemat, near Jepara. On some sectors, erosion started after the clearance of a mangrove fringe exposed the fishpond banks to wave attack.
There would certainly be less damage if the land margin were left in a natural condition, with the mangrove fringe intact and beach woodland uncleared, but in Java ( where pressure on land resources is so intense) this is unrealistic. On the other hand, anti-erosion works such as concrete or boulder walls or artificially nourished beaches would be expensive to construct and maintain along the receding shores of deltas (where the problem may be compounded by gradual subsidence) and less expensive wooden structures are rarely adequate to the task. Where a protective mangrove fringe is present it should be conserved, if necessary with plantings of seedlings to fill any gaps that may start to develop. It can be argued that the utilization of newly prograded areas offsets the losses where erosion is in progress and that the net gain of land on the deltas of northern Java means that, taken overall, the problem of losses through coastal erosion is not serious. But problems of land tenure are complex in a situation of gains in one area and losses in another, and there are difficulties in achieving social equity by way of compensation and equivalent land allocation to those who directly sustain losses due to coastal erosion without unnecessarily disadvantaging those in a position (socioeconomic as well as locational) to benefit from the acquisition and development of new land produced by delta shore progradation.
A partial solution could be reached by more careful management of distributary discharge of water and sediment in deltaic regions in order to maximize the possible supply of material to developed shore sectors that would otherwise begin to erode, at the expense of some diminution in the rate and extent of land gains on prograding sectors. This would involve the cutting of new distributary canals to "feed" sectors actually or potentially eroding and the diversion of some of the water and sediment yield at present going into distributaries that lead to prograding sectors. The necessary pattern could only be designed after careful geomorphological analysis, including budgeting and flowpath charting of discharge and sediment yield, and it would need to be modified in response to natural or Man-induced changes in the river system.
In view of the obvious difficulties caused by coastal erosion, it may seem surprising that deposition can also lead to problems in the Indonesian coastal environment. Certainly, an abundance of deposition is an asset when it adds to the limited land area of a densely populated country, especially if the newly formed land can soon be in productive use. On the other hand, sedimentation can shallow inlets, estuaries, and areas off river mouths, impeding navigation and requiring the use of dredging to maintain navigable approaches to harbours. This is a problem in the vicinity of the major port of Surabaya. Dredging of channels is a recurrent expense, and difficulties also arise over where to dispose dredged material. In some cases it can be used for land reclamation, but if it is dumped offshore there is a risk that it will be washed back into navigation channels, or into other sectors where it is disadvantageous: in Jakarta Bay there is a risk that muddy material dumped offshore by dredging will start to accumulate on the sandy beaches of Jakarta Utara, reducing their recreational value.
The shallowing of river mouths by siltation is a major problem in Indonesia, especially in eastern Sumatra and northern Java. It impedes navigation, diminishes the local dry-season water supply, and increases the extent of subsequent river flooding in adjacent low-lying areas. As we have seen in the Cisadane, Citarum, and Cimanuk deltas, this siltation can lead to the initiation of new distributaries, either by channel bifurcation at the river mouth or by lateral overspilling of floodwaters some distance upstream.
As deltaic areas are prograded by deposition the transverse gradients of their surface diminishes, so that drainage becomes more difficult, longer channels being needed to conduct water off the land. The risk of damage by floodwaters also increases as these very flat depositional areas are developed. As the shoreline progrades, coastal structures such as brackish-water fishponds may become difficult to manage because of increasing remoteness from sea water. As new fishponds are constructed to seaward, the inner ones may be abandoned, or converted into rice-fields irrigated by fresh water instead of sea water. We understand that such conversions have taken place on the Solo delta, in the course of progradation, but have been unable to find detailed records. The necessity of such land-use changes and associated structural conversions requires efforts and expenses that are directly due to coastal deposition.
Sedimentation in coastal waters may impede the growth of corals, and reduce the productivity of fisheries by making the water turbid and by blanketing seagrass areas, thus diminishing breeding and feeding environments. This is the case whether the sedimentation is natural or whether it is the outcome of man-made changes such as induced soil erosion, the diversion of river mouths, or the dumping of dredged materials.
Reference has also been made to the problems arising from the attachment of the island of Dua to the mainland by progradation of the C;ujung delta, the island being a wildIife reserve that now no longer has the natural protection of relative inaccessibility by land.
The problems arising from coastal deposition may be less severe than those due to coastal erosion but they can be important locally. Management of sediment yield to the coast, mentioned previously, could reduce the problems arising from deposition as well as from erosion.
Mining and Quarrying
Removal of sand and gravel from beaches or nearshore sea-floor areas for use in constructional works has led to problems in many parts of the world, such depletion being followed by erosion of the beach and its hinterland. This material resource can only be used on sectors that are receiving natural replenishment of sand and gravel; in other words, where a renewable resource is being harvested. Elsewhere, the removal of such material is likely to initiate disequilibrium and produce a deficit in the shore-nearshore zone, where water deepens and larger wave action moves in to erode the shoreline. An example of this has occurred on the coast at Cilincing and Marunda, east of Jakarta where, as has been noted, erosion accelerated after sand had been removed from the beach.
Quarrying of coral limestone, especially from nearshore reefs, has also led to disequilibrium and shoreline erosion, the most notable example being on the east coast of Bali, where reef-quarrying has been followed by rapid erosion. Extensive mining of coral limestone has also taken place on reefs in the Thousand Islands, north-west of Jakarta Bay, at Balikpapan in Kalimantan, and at Sengkidu on Bali. Coral blasting by dynamite has been widely used in Indonesia as a means of obtaining fish. Such operations are disadvantageous in that they generate turbidity and fine-sediment blanketing in adjacent waters, which can damage the growth of corals and marine vegetation, and impoverish the local fishery. About one-third of the coral area around Pombo Island (a proposed nature reserve) has been destroyed in this way, and damage is already extensive in many other areas (Kvalvagnaes and Halim 1979).
Although dunes are of limited extent on humid tropical coasts, they are present locally around Indonesia, chiefly behind the beaches of sand of volcanic origin exposed to strong waves and relatively strong wind action on the south coast of Java and the south-west coast of Sumatra. In most of these areas they retain a stabilizing cover of scrub or woodland but on the central south coast of Java, west from Parangtritis, this is either sparse or absent as a result of grazing by sheep and goats and the harvesting of firewood (Verstappen 1957). In consequence, the dunes are mobile, and in places have spilled inland across farmed terrain, including rice-fields (Plates 7 and 8). Several small farming settlements in this region, recorded on early Dutch maps, have disappeared, probably as the result of sand drifting.
Stabilization of these dunes could probably be achieved by planting grasses and shrubs, providing that all grazing animals were removed and the collection of firewood prohibited. However, the coast sands are rich in heavy minerals, notably titaniferous magnetite (Subandro and Djumhari 1972, Suyitno 1978), and farther west, towards Cilacap, they are being extracted by open-cast mining to produce rutile and iImenite. The exploitation of these mineral resources to the rear of the beach could profitably precede any stabilization by planting and it may be better to allow mining to proceed before the drifting sand, which at present carries very little vegetation, is stabilized within a newly developed and restored coastal landscape.
The mangrove resources of Indonesia are very substantial, mangroves occupying more than 3.6 million hectares of coastal swampland, 2.9 million of which are in Irian Jaya. There is also considerable diversity, with up to 40 species present, although many are rare and localized. Rhizophora, Avicennia, and Sonneratia species are widespread, forming extensive coastal forests in some sectors, especially in northeastern Sumatra, Kalimantan, and the northern and western shores of Irian Jaya.
Natural changes on mangrove coasts include both seaward advance, with pioneer species spreading forward on accreting mudflats, and recession as the result of nearshore and shoreline erosion. Where an advance is taking place, successional changes are usually in progress in the forest to the rear, with displacement by other swamp vegetation (e.g., nipa palms) at the landward margin as accretion raises the substrate above high-tide level. The mangrove community thus migrates seawards on prograding coastlines.
Losses by erosion may be balanced by gains elsewhere, especially on deltaic sectors where there have been complex patterns of progradation and recession over recent decades, but it is likely that the mangrove resources have been much diminished, especially in Java, as the result of land reclamation projects and the construction of brackish-water fishponds. In the absence of Man's interference, mangroves would be very extensive on the deltaic coasts of northern Java, whereas they are, in fact, confined to a narrow and intermittent fringe. In addition to the reduction of the mangrove area it is likely that many species, which existed only in the larger mangrove forest complexes, are now very rare, if not extinct.
Mangrove resources are of value for several purposes. They provide a source of timber, including firewood, for use by local people; they have been exploited for charcoal, especially in Sumatra and Kalimantan, and are being harvested extensively for woodchip production; they are a productive habitat for fish, crustaceans, and waterfowl that can be cropped as a food supply; and they contribute to the stabilization of coastal terrain by the sheltering effects of their canopy and the binding effects of their roots, promoting accretion and contributing organic matter to terrain that would otherwise remain mobile under the action of waves and currents.
With intensive pressure on land and water resources it is inevitable that parts of this mangrove area have been removed to make way for farming and the construction of brackish-water fishponds. Under the First Five-Year Development Plan (1969 to 1974) for Indonesia almost 200,000 hectares of mangrove were reclaimed for agriculture and aquaculture and a much larger area has been reclaimed under the Second Five-Year Development Pian (1974 to 1979). As we have noted, it is advisable to leave (or to plant) a mangrove fringe on the seaward side of these constructions to reduce the risk of erosion by wave action. Some attempts have been made to plant Rhizophora in and around fishponds to provide shelter, shade, and a nutrient supply, especially from leaf litter. Elsewhere, mangroves persist in areas where fish are reared, but this form of multiple use depends on realization that thinning and replanting are necessary to maintain a mangrove cover that is expected to yield timber and firewoods as well as fish.
Some mangrove areas have been placed under forestry management which tends to favour selected species; others have been declared reserves in the hope of maintaining species diversity as well as samples of natural mangrove habitat and associated ecosystems. It is difficult to judge, in the present state of knowledge of Indonesian mangroves, whether these reserves are adequate and whether particular forms of management are necessary, for example, to maintain a sufficient stock of the rarer species.
Finally, it is necessary to refer to projects where coastal land has been reclaimed by enclosing and filling the intertidal zone, and parts of the nearshore zone. The largest such project in Indonesia is at Jakarta Ancol; a new land area 1 kilometre wide has been thus reclaimed on the southern shores of Jakarta Bay. Such projects obviously modify the regimes of waves and tides in nearshore waters and there can be problems in maintaining the seaward margin, subject to relatively strong wave action at high tide. On the Jakarta coast it has been necessary to build sea walls and groynes (Plate 13) and it would be useful to develop and maintain a protective artificial beach which would be of recreational value in this area. Elsewhere, if the reclaimed land is of industrial or agricultural value, rather than part of a recreational complex as at Jakarta Ancol, a more useful approach might be to establish a protective mangrove fringe seaward of the reclaimed zone.