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close this bookEnvironmental Changes on the Coasts of Indonesia (UNU, 1980, 53 pages)
View the documentPreface
View the document1. The Indonesian coastal environment
View the document2. The changing coastlines of Indonesia
View the document3. Problems in the changing coastal environment
View the document4. Some tasks for future research
View the documentReferences
View the documentOther UNU publications


This report on environmental changes on the coasts of Indonesia has been prepared to provide a background for further study and research. The need for such a review became apparent when graduate training courses in coastal resources management were initiated in Indonesia by the United Nations University, in association with the Indonesian Institute of Science (LIPI), in 1979. Although there is a substantial scientific literature on the coasts of Indonesia, especially from Java and Sumatra, with major contributions by such workers as H. T. Verstappen, J. H. F. Umbgrove, and H. D. Tjia, it exists in widely scattered papers and monographs, many of which are not readily accessible to graduate students and research workers on Indonesia. Moreover, material relevant to the question of changes that have occurred in recent centuries around the coasts of Indonesia, and especially to changes now in progress, occurs only patchily within this scientific literature, much of which is concerned with longer-term geological and geomorphological information.

It therefore seemed appropriate for us to review the available information on coastal changes in Indonesia and provide a readily accessible summary which can be used by graduate students and research workers on the next phase of study of coastal dynamics. On the one hand, there is a need for much more basic information from the less well known sectors of Indonesian coastline, especially in Kalimantan, Sulawesi, Nusatenggara, and Irian Jaya. On the other hand, there is a need for updating and intensification of research already carried out in Java and Sumatra. We hope that, by providing this report, we can stimulate this further research.

Another aim is to review the features of the changing coastlines of Indonesia for wider recognition. In 1972, the International Geographical Union (IOU) set up a Working Group to compile world-wide data on rates and patterns of coastal change, and in 1976, when a preliminary report on this project was issued, it became clear that Indonesia was one of the regions from which much more information was required. We have, therefore, made comparative studies of past and present maps, charts, and air photographs (including satellite photographs], in order to identify sectors where changes have taken place, and to supplement and update information previously published. With the support of the National Oceanological Institute (LON) in Jakarta, and the United Nations University through its Natural Resources Programme- we have been able to visit sectors of the Indonesian coastline to conduct field studies. It will be obvious that our work is of a preliminary nature, and that there is a need for many research workers to study sectors of the long and intricate coastlines of the Indonesian archipelago that have so far received little if any attention.

The report consists of four chapters, the first of which deals with the environmental factors and processes that have influenced coastal evolution in Indonesia. With this as background, the second chapter summarizes information available on Indonesian coastal features, and the patterns and rates of documented shoreline changes. The third chapter examines some of the problems that have arisen as the result of these changes, and the fourth outlines the research tasks that should now be undertaken, including more detailed monitoring of active changes, the assessment of Man's impacts on coastal systems, and the need to identify sites of special scientific interest that should be conserved and managed as coastal reserves for educational and research purposes.

We are grateful to the sponsoring organizations, the United Nations University and the National Oceanological Institute of Indonesia, for enabling us to carry out this work, and especially Dr. Aprilani Soegiarto, Director of the National Oceanological Institute, for his support and encouragement of our investigations. We also thank Mr. Karel Giebel and the staff of the map room of the Koninklijk Instituut voor de Tropen in Amsterdam for assistance with historical maps of Indonesia, Dr. Verstappen of the Institute for Aerial Survey and Earth Sciences in Enschede for geomorphological data, and Dr. Tjia of Universiti Kegangsaan, Malaysia, for comments on sea-level changes and permission to reproduce Figure 3. Mr. Ken Boston of Melbourne State College and Mrs. Michele Barson of the University of Melbourne also contributed valuable information obtained during visits to the Netherlands. We acknowledge the assistance of Mr. Robert Bartlett and Mr. Ken Pohlner, Department of Geography, University of Melbourne, in the preparation of diagrams and photographs, and the cartographers of Lembaga Oseanologi Nasional for their help in drafting the sequential maps.

1 March 1980

E. C. F. Bird
O. S. R. Ongkosongo

1. The Indonesian coastal environment

Indonesia consists of about 13,700 islands, with an intricate coastline whose length has been estimated as just over 60,000 kilometres by Soegiarto (1976). The islands show considerable diversity of coastal features, related partly to contrasts in the geology and geomorphology of the hinterland and the bordering sea floors, and partly to variations in adjacent marine environments. In terms of global tectonics the Indonesian archipelago occupies the collision zone between the Indo-Australian, Pacific, and Eurasian plates. It is a region of continuing instability, marked by frequent earthquakes and volcanic eruptions. Its mountain ranges are areas of Cainozoic uplift, augmented by large volcanic constructions, and its bordering seas are underlain by unstable shelf areas, especially towards the Java Trench, the subduction zone that lies to the south of the Indonesian island-arc.

Indonesian coastlines show the effects of past and present tectonic instability, volcanic eruptions, and changes of sea level. There have been upward and downward movements of the land, often accompanied by tilting or faulting; outpourings of volcanic lava and ash have influenced coastal features both directly and indirectly; and vertical movements of sea level have resulted in complicated sequences of emergence and submergence of island coastlines. Many characteristics of Indonesian coastal landforms are related to their development under tropical -especially humid tropical-conditions, and it will be useful first to consider these features briefly.


Although much of Indonesia lies within the humid tropical zone, some parts have sub-humid and even semi-arid climates. Climatic characteristics are determined largely by the position of the Intertropical Convergence zone (ITC), a zone of unstable air and heavy rainfall which migrates north and south over Indonesia, crossing the equator in May and November each year, and reaching latitudes of about 15 south in January. Indonesian climatic stations generally show a more pronounced wet season when the Intertropical Zone of Convergence is to the south, when westerly winds prevail; the dry season occurring after it migrates away to the north, and winds move around to the south-east. Generally the southern part of the archipelago has a smaller mean annual rainfall than the rest of the country, partly because of a reduction in the water content of westerly winds as air masses move to the east, and partly because of the influence of drier air brought in from the Australian region by winds from the southeast during the dry season. In additon, afternoon showers caused by local intensive heating are common, and may occur even in the dry season. The pattern of rainfall is influenced by the orographic factor, notably where moist air is forced upwards as it moves eastwards across mountain ranges, particularly in Sumatra, Java, and Irian Jaya. The wettest coastal areas are thus found to the west of the ranges, and the relatively dry areas in the "rain shadows" to the east (Sukanto 1969).

Winds are generally light to moderate, the most vigorous being the south-easterlies in the dry season. The cyclones of northern Australia and the typhoons of the South China Sea do not reach Indonesia, although waves generated by these disturbances are occasionally transmitted into Indonesian coastal waters.

In terms of Koppen's classification, most Indonesian coastal regions are in Category A, with mean temperatures in the coolest month of at least 18°C., but a few sectors have a sufficiently long and dry winter season to be placed in the semi-arid category BS. Truly humid tropical coasts (Af), with a mean rainfall of at least 60 millimetres in the driest month, are extensive around Sumatra and Kalimantan, in southern Java, much of Sulawesi, and the islands to the east. They give place to monsoonal lam) climates, with a short dry season compensated by a large annual rainfall, along the north coast of Java (Jakarta, Semarang, Bangkalan) and in several minor sectors around Sulawesi, including Ujung Pandang; and to somewhat drier savanna (Aw) climates in the rain shadow areas of northeastern Java (Surabaya, Pasuran) and the islands to the east, and around Timor (Dill, Kupang), which is much influenced by dry air masses arriving from Australia. Sectors dry enough to warrant semiarid (BS) classification are limited, but occur on the north coasts of Lombok and Sumba. Much more information from coastal stations is necessary before climatic sectors around the Indonesian archipelago can be delimited accurately.

The interior uplands record substantially higher rainfall than most coastal regions, so that river systems carry a very large runoff from the high hinterlands.

General Geomorphology

Landscapes in humid tropical environments are subject to the intense chemical and associated biological weathering of rock formations that proceeds under perennially warm and wet conditions. This has led to the formation of deep mantles of decomposed rock material, mainly silt and clay, and in places these are up to 30 metres thick. Away from coastal cliff exposures, natural rock outcrops are rare: they are found locally on resistant sandstones, some limestones, and recently formed lava flows.

The natural vegetation cover is tropical rain forest, with a dense canopy and a thick organic litter that protects the ground from the direct erosive effects of heavy rainfall. A subsurface network of roots also binds and stabilizes the upper part of the weathered mantle. This luxuriant vegetation tends to hold the weathered mantle in place, but on steep slopes the rapid runoff that occurs during heavy rain may wash away surface material even where the vegetation is dense, and landslides and mudflows frequently scar the forested hillsides.

Fluvial Sediments

Runoff is thus typically laden with fine-grained sediment, silt and clay produced by weathering, but in steep areas the streams incise their valleys and derive sand, or even gravel, from the less-weathered underlying rock formations. Coarser sediment is also derived from the lava and ash produced by volcanic eruptions. On steep volcanic slopes lahars are formed, when torrential rainfall saturates and mobilizes masses of pyroclastic debris, which flow down into the valleys. Streams also derive sand and gravel when previously constructed volcanic structures are dissected by runoff.

The combination of steep elevated hinterlands of deeply weathered rock, recurrently active volcanoes, and frequent heavy rainfall produces large river systems that carry substantial quantities of sediment down to the coast. Deposition of this material has built extensive deltas and broad coastal plains, especially in Java, Sumatra, Kalimantan, and Irian Jaya. The lithology of outcrops within each catchment determines the nature of the weathered mantles and strongly influences the composition of sediment loads carried downstream by the rivers. As Meijerink (19771 has shown, the sediment volumes per square kilometre per year from catchments dominated by sedimentary formations are much greater than those from volcanic catchments (Table 11. Where sandy material is carried down to the coast it is reworked by waves and deposited as beach formations along shorelines adjacent to river mouths: the most extensive of these are on the south coast of Java. Silts and clays are incorporated in tidal mudflats and coastal swamps, and deposited in lowlyingareas on and around river deltas.

TABLE 1 Influence of Lithology on Sediment Yields, as Observed for a One-Year Period (based on Meijerink 1977)

Catchment Drainage area km² Sediment yield ton/km² /year Source
Ciliwung 130 250 - 375 Rutten
Rambut 4.5 532 van Dijk
Banyuputih 225 750 - 1,000 Rutten
Brantas 10,000 875 - 1,500 Rutten
Mainly volcanic      
Citarum 73,000 800 - 1,200 Modified after Soemarwoto
Cimanuk 3,000 1,000 - 2,000 Rutten
Mixed vole-Sedimentary  
Tandjum 210 750 - 1,000 Rutten
Cilamaya 225 2,500 - 3,500 Rutten
Lusi 860 2,500 - 3,500 Rutten
Serayu 700 3,500 - 4,500 Rutten
Cilutung   7,500 Nedeco
Jragung 101 4,000 - 6,250 Rutten
Cacaban 7.9 6,600 Rutten
Pengaron 41 9,250 - 12,500 van Dijk

Runoff has undoubtedly been accelerated and sediment yield increased in many parts of Indonesia as the result of the modification or removal of the natural vegetation cover (Table 21. Many formerly forested areas now carry more open plantations, or have been cleared for cultivation or grazing. Soil erosion has become a widespread phenomenon, and the material lost from deforested terrain augments the loads carried downstream in the river systems. Moreover, accelerated runoff increases the frequency and extent of river flooding in the valleys and out over the coastal plains and deltas. These effects are most evident in densely populated and intensively utilized regions, particularly in Java, but a similar sequence of events can be demonstrated in other parts of Indonesia.

TABLE 2 Runoff from Small Plots under Different Vegetation on Java (based on Meijerink 19771

Vegetation Station Elevation
a s l
Mean annual Rainfall Runoff in % of rainfall during observation period
Land Use min max Average
Bare soil 4 stations, W Java       25 - 55  
  Janlappa 100 3,000     32
  Monggot 150 2,200     42
Dry cultivation Janlappa 100 3,000 13 19.5 16.2
  Ciwidej 1,750 3,200 4.7 17.3 10.6
  Klakah 100 3,200 - - 11.9
  Ngadisari 2,000 1,500 10.1 10.3 10.2
  Cobarrondo 1,500 1,800 - - 2.0
Young forest Ciparaj - - 2.1 20.6 9.5
plantation Monggot 150 2,200 9.0 10.7 9.5
< 21/2 years Monggot 150 2,200 2.8 7.5 2.8
Grass, mainly Monggot 150 2,200 2.5 8.9 5.4
alang-alang Klakah 200 3,200 -   5.0
(Imperata) Janlappa 100 3,000 0.35 7.6 3.9
  Cobarrondo 1,500 1,800 0.2 3.0 1.6
  Ciwidej 1,750 3,200 0.1 1.7 0.5
  Arcamanik 1,300 2,500 0.28 0.28 0.28
Bomboo grass 4 stations, - - - 10 - 20 -
  W Java     - 5 - 10 -
Jungle Janlappa 100 3,000 1.0 4.3 2.6
  Klakah 200 3,200 - - 2.1
  Ciwidej 1,750 3,200 0.35 2.7 1.5
Teak Monggot 150 2,200 - 8.0 -
Mahogany Monggot 150 2,200 2.4 4.6 3.6
Thinned Pinus Arcamanik 1,300 2,500 3.2 3.2 3.2
Rain forest Ciparaj 1,000 4,000 3.5 12.3 6.2
Rain forest Bogor - - - - 2.4
Rain forest Arcamanik 1,300 2,500 0.55 - 2.4
Rain forest Ciwidej 1,750 3,200 0.4 2.4 1.5
          4.5 1.3

Although much of the sediment supplied to the coast under humid tropical conditions has been delivered by rivers, material has also been derived from the erosion of cliffs along the shore, and from the sea floor. Sediment has been carried onshore from shallow coastal waters to many parts of the world's coastline, and it is likely that the deltas and coastal plains around Indonesia incorporate in their stratigraphy marine sediments that originated in this way. For example, marine clays are known to underlie parts of the extensive swampy lowlands of south-eastern Sumatra.


Beaches of sand and gravel are extensive around the coasts of Indonesia, especially near the mouths of rivers delivering this kind of material, adjacent to cliffs of sandstone or conglomerate, and along shorelines to the rear of fringing coral reefs. Beach sediments of volcanic origin are typically black or grey; those of coralline origin white or yellow. Quartzose sands are of very localized occurrence in relation to quartz-arenite outcrops along the coast and within hinterland river catchments.

Sandy beaches are typically backed by swash ridges, and multiple beach ridges occur on sectors that have intermittently prograded. Coastal dunes are poorly developed in the humid tropics generally, and in Indonesia they occur only on a few sectors, notably in southern Java, where the fluvially nourished beaches near Yogyakarta are backed by dune topography, and locally in southwestern Sumatra. Beach ridges and dunes carry a woodland formation dominated by Casuarina, Pandanus, Calophyllum, Inophyllum, and Barringtonia species, usually with planted or self-seeded coconut palms. Many beaches show evidence of erosion with backshore cliffing and undercutting of vegetated terrain but this is not the case where there is a continuing supply of sandy, or gravelly, sediment to maintain or prograde the shoreline.


Shorelines of depositional coasts are typically sandy or swampy, and in the humid tropics swampy sectors are usually occupied by mangroves, which colonize the upper part of the inter-tidal zone. Once established, mangroves can protect the coast from wave scour and may promote the accretion of sediment to build up new depositional terrain to high-tide level. On accreting shores, the mangroves spread forwards, and as deposition attains high-tide level nipa palms, or rain forest, or freshwater swamp vegetation, move in from the rear. The constructive and protective value of mangroves is often demonstrated where they have died back, or been cut down, exposing the substrate which is then rapidly eroded by wave scour.

Steep and Cliffed Coasts

Cliffed coasts are relatively rare in the humid tropics, partly because of the great extent of deltas and coastal plains formed by deposition in front of hilly, or mountainous, terrain; partly because of the protection of fringing and barrier reefs of coral on many sectors; and partly because of the general absence of strong wind and wave action in these environments. Where high country extends to the coast it terminates in steep coastal slopes with features similar to those of valley-sides inland, usually with a dense vegetation cover extending down almost to high-tide level. The slope base has often been undercut by wave action, which has removed weathered material to expose rocky outcrops or boulder accumulations on the shore. Because of this basal undercutting, landslides occur frequently on steep coastal slopes.

Steep coasts of this kind are extensive in Indonesia, especially around Sulawesi and the islands to the east. On the other hand, cliffs have developed along sectors of the southern coasts of Sumatra, Java, and the islands east to Sumba, which are exposed to the relatively strong wave action generated across the Indonesian Ocean.* They are best developed on sandstones, limestones, and outcrops of volcanic rock. While most cliffs are formed by wave attack, some are due to catastrophic changes such as earthquakes and volcanic eruptions. The explosive eruption of Krakatau in Sunda Strait in 1883 left high cliffs cut in volcanic materials on the residual islands.

Rocky shore outcrops are subject to intense physical, chemical, and biological weathering. Limestone outcrops become intricately pitted and honeycombed (Plate 4), and similar disintegration can be seen on sandstones and volcanic rocks exposed to the action of surf and spray, salt corrosion, solution by rain water and percolating ground water, and the effects of wind scour. Such features are well known on humid tropical coasts, but little attention has been given to them in Indonesia.

Coral Reefs

Reefs built up by coral and associated organisms occur extensively in Indonesian waters, especially in the Flores and Banda Seas (Darwin 1842, Davis 1928, Molengraaf 1929, Kuenen 1933, Umbgrove 1947). Coral growth requires clear warm water, with temperatures that do not fall below 18 C., and salinity within the range 27 to 38 parts per thousand. Such conditions are widely satisfied in the seas around Indonesia, the chief exceptions being off the mouths of rivers, where salinity is diluted and the sea made turbid by the discharge of suspended sediment loads. Thus coral reefs are sparse in the shallow, often muddy seas south and west of Kalimantan, and in the sediment-laden waters off the deltaic coastline of northern Java. Nevertheless, there are scattered coral reefs in Jakarta Bay, in the clearer water seaward of the muddy areas off river mouths.

Other factors inhibiting reef growth include active vulcanicity, where coastal waters are frequently invaded by lava and ash, as around Manada-tua, the active volcano off northern Sulawesi, and the deposition of large quantities of sediment off coasts where the slopes of recent or active volcanoes are undergoing mass movements downslope or rapid dissection by streams. Examples are Ternate, Tidore, and Makian in the Moluccas, Gunong Ija, south of Flores, Sangeang, north-east of Sumbawa, and Ruang, north of Sulawesi. In each case the reefless island shores are bordered by beaches of black volcanic sand and gravel, or cliffs and rocky shores of lava, with only scattered coral growth in the nearshore zone.

Tectonic instability has also interfered with reef develop" meet, either by maintaining an unstable substrate, or by raising coral growths and reef formations out of the sea as emerged features (Verstappen 1960). Subsidence, leading to submergence of reefs, may stimulate upward growth, as in the classical sequence whereby a fringing reef developed along an earlier coastline becomes an outlying barrier reef off a submerged coast, and where reefs fringing high islands become encircling reefs enclosing a lagoon with a partly submerged central island (an "almost-atoll") and eventually, with further subsidence, atolls surrounding a lagoon on the site of a completely submerged island. Davis (1928) and Molengraaf (1929) quoted examples of each of these features from the Indonesian region. Gunong Api is an island in the Banda Sea with an encircling fringing reef; Goram, to the north-east, has a ring of reefs around a lagoon with a central island; and there are numerous atolls built up to present sea level from subsided foundations in deep water south of Sulawesi: Kalukalukuang, Postillion, Sabalana, Sapuka, Paternoster, and Zandbuis are good examples. Major barrier reefs include the Great Sunda Reef, which rises from submerged shelf margins south-east of Kalimantan, the reef east of Sulawesi, and the similar reefs off the south-west coast of Sumatra, which curve out towards the islands off Batu and Banjak. Submerged reefs have also been charted in sea areas west of Irian Jaya.

Emerged reef features are widespread in the Indonesian region. They are found in northern Sumatra, along the south coast of Java, and especially around Sulawesi and along the islands east of Bali, notably Sumbawa and Timor. They are common in eastern Indonesia, particularly in the Banda Sea, where there are uplifted atolls such as Manowolko and Matabello. Kafiau, off Irian Jaya, is an uplifted almost-atoll, with rimming hills of reef limestone around a hollow that encircles an interior upland, while Salajar, south of Sulawesi, is the emerged half of a tilted atoll, with gentle slopes to the west and a steep descent to the east, where a volcanic foundation is exposed. Muna Island off Sulawesi, Pomana Island off Nusatenggara, and Satengar to the north of Sumbawa are reef patches that have been raised out of the sea by earth movements to form islands. There are also many islands with bordering stairways of uplifted reef terraces, Timor being the largest and most elevated.

In addition to uplifted reefs, coralline islands include low sand cays deposited on offshore reefs and more complex depositional islands, with coarser remnants of coral shingle on the side exposed to stronger wave action, sand cays to leeward, and intervening shallow lagoons, in which mangroves may grow. It is possible that the coralline sediment which forms these low islands is the outcome of erosion of coral reefs that had been built up during phases of slightly higher sea level, and were exposed to wave attack when the sea subsequently fell. Material is also broken from the submerged slopes of coral reefs, especially during tsunami, or periods of strong wave action, when the delicate structures of staghorn coral are dislodged and thrown up onto the reef as cylindrical fragments of coral shingle. Another source of coral shingle is the debris generated when a reef is quarried for limestone. Larger coral boulders known as "negro-heads" littered the reef-fringed shores of Sunda Strait after the tsunami that resulted from the Krakatau explosion in 1883; one of them had a volume of 300 cubic metres. The tsunami from the Paloweh eruption in 1928, drove similar large coral boulders up onto the nearby shores of Flores.

Algal structures (mainly of Lithothamnium) locally surmount coral reefs and are better developed on the southern coasts of Indonesia, which are subject to surf generated by ocean swell and south-easterly wave action. Algae thrive in wellaerated surf, and tend to build rims awash at high tide and exposed at low tide along the outer edges of fringing reefs in these relatively high-energy environments. By contrast, fringing reefs bordering inner sea areas in Indonesia generally lack algal rims and are built out at slightly lower levels.

Processes in Coastal Waters

The shaping of these various coastal features has been much influenced by the wave regime in Indonesian coastal waters. A strong swell transmitted from the Southern Ocean moves in from the south-west to the south coasts of Sumatra and Java. Farther east it becomes attenuated and weaker, having crossed the broad north-west shelf of Australia, but it can still be detected on Timor. A Pacific swell moves southwards through the Philippine Sea to reach the north coast of Irian Jaya, weakening as it diffuses through the Moluccas. Otherwise, wave action in Indonesian coastal waters is determined by local winds. Between April and November south-easterly winds are dominant over sea areas south of Indonesia and waves from this direction are important along the south-facing coasts of Java, Bali, Lombok, and Sumbawa, and on the south coast of Timor. At this season, winds over the Java Sea are easterly to northeasterly, and there are lighter breezes from various directions in the equatorial zone to the north. In the wet season the winds over Indonesian waters are gentler and more variable but typically westerly (Table 3).

TABLE 3 Prevailing Wind Directions at Selected Coastal Stations in Indonesia

STATION Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Tabing W W SW SW W W W W W W W W
Tandjung Pinang N N NE NE SW S S S S S N N
Pangkalpinang N N N NE S S S S S E E N
Bengkulu NE NE NE S NE NE NE S S S S S
Pusakenegera NW W W S S SE SE SE SE SE SE SE
Semarang - Maritim W W W NW E N E SE E NW NW NW
Kalianget W W W E E E E SE E E SE N
Surabaya Perak W W W E E E E E E E E N
Supiado-Pontianak W N W W E S S S S E W W
Banjarmasin W W W W E E S S S E W W
Balikpapan N N N N N NW S S S S S N
Denpasar W W W E E E SE SE SE SE SE W
Hasanudin E E NW NW E E S NW NW NW SE E
P.G. Bone Arasoe N W N N S SE SE SE SE SE SE SE
Palu N N N N N N N N N N N N
Rembiga W W W E E SE SE SE SE SE W W
Ambon , N N N N S SE SE SE SE SE S S
Kaimana N N N NW S S S S S S S NW

With the exception of the southern shores of Sumatra, Java, and the Lesser Sunda islands, which receive a southwesterly ocean swell and relatively strong southeasterly wave action in the winter, the coasts of Indonesia are exposed only to low-wave energy. As has been noted, the tropical cyclones which produce short-term high-energy conditions on the coasts of northern Australia, and around the South China Sea, do not reach Indonesia, although waves generated by these disturbances are transmitted into Indonesian coastal waters south of Java and the Lesser Sunda islands, including Timor, and into the sea between western Kalimantan and the islands south-east from Singapore.

Tidal movements in Indonesian waters result from impulses arriving from the Pacific and Indian Oceans. One wave moves into the Straits of Malacca from the north-west, augmenting its range and generating strong currents as the configuration narrows. Another arrives from the South China Sea, diverging through the narrow waters west of Kalimantan, and producing an interacting system with the Malaccan tides south of Singapore. Tides from the Pacific Ocean advance through the South Philippine Sea to the north coast of Irian Jaya, and penetrate the straits around the Moluccas, while tides from the Indian Ocean move through the waters south of Java to the Timor and Arafura seas, augmenting in the coastal waters south of Irian Jaya. Within the Java and Banda seas there are minor but complex tides, the patterns being related to deep basins within the intricate configuration of the eastern Indonesian archipelago.

Spring tide ranges (Fig. 1 ) are a metre or less on the south west coast of Sumatra, but they increase to more than 3 metres in the narrows of the Straits of Macassar. On the south coast of Java they are 1 to 1.5 metres, but less than 1 metre on the north coast, except in the Straits of Madura where Surabaya has 1.7 metres. They are up to 1 metre on the south-west coast of Kalimantan, and somewhat larger (up to 2.8 metres) on the east coast. Sulawesi has small tide ranges, exceeding 1 metre only on the north coast and in Teluk Bone, the southern gulf. Tide ranges of about 1 metre are typical in eastern Indonesia, but they exceed 2 metres on the south coasts of Sumba, Flores, and Timor as the result of augmentation through the Timor Sea, where the Australian coast to the south records tide ranges that locally exceed 10 metres. North-east of the Arafura Sea tide ranges of more than 5 metres occur in estuarine inlets along the southern coast of Irian Jaya, where tidal bores are generated, moving upstream as steep waves as the tide rises.

Tidal oscillations are also complicated by wind action. Northeast winds over the China Sea build up the water level south of Singapore by as much as 0 5 metres between January and March, while south-east winds raise winter sea levels a similar amount along the southern coasts between Timor and Java.

In addition to these regular tidal and seasonal alternations there are irregular surges generated by earthquakes and volcanic eruptions in the Indonesian region. These tsunami are occasionally devastating, causing shoreline erosion, displacing material from coral reefs, overwashing beaches, and flattening mangrove fringes. The most severe tsunami so far recorded was that generated by the Krakatau explosion in 1883, when waves reached up to 30 metres on the adjacent shores of Sunda Strait, washing away the lighthouse on Java Head. Lesser surges were then experienced all round the coasts of Indonesia. In 1979, massive landslides occurred on the coasts of Lomblen and Nusatenggara as the result of a tsunami.

FIG.1 Mean spring tide ranges in the Indonesian region, based on data from British Admiralty Tide Tables volumes 2 and 3

Changing Levels of Land and Sea

So far we have dealt mainly with processes effective with the sea at its present level, but a study of the coastal features of Indonesia soon encounters evidence that the sea has stood at different levels in relation to the land at various times in the past. On some sectors there are emerged coral reefs and terrace features that formed when the sea stood at a relatively higher level; others show the effects of submergence, in the form of drowned mouths or embayments over drowned lowlands. Some of the changes are due to eustatic movements of the sea surface, resulting from world-wide changes in ocean volume; others to uplift or depression of the land margin as the result of earth movements, which have been particularly marked in the Indonesian region (Katili and Tjia 1969, Tjia 1968, Umbgrove 1949, Verstappen 1974, Zen and Sudarmo 1977). Until the sequence of eustatic movements has been established, it will not be possible to elucidate the extent to which these various vertical changes are due to land movements independent of sea-level oscillations. Meanwhile it should be noted that terraces along river valleys or on coastal sectors are not necessarily due to land uplift; they may be due, at least in part, to eustatic lowering of sea level.

World-wide sea-level oscillations occurred during Quaternary times, the sea falling to relatively low levels during the Glacial phases of the Pleistocene and rising to relatively high levels during the Interglacial phases. The fact that the sea floor of the Sunda Shelf region was exposed to sub-aerial denudation during low-sea-level phases, its topography retaining valley patterns shaped by river action, and that its submergence resulted from the world-wide sea-level rise that accompanied late Pleistocene deglaciation was first perceived by Molengraaff and Weber (1920). East of Sumatra the islands of Bangka and Billiton show Quaternary terraces 50,25, and 6 to 8 metres above present sea level (Tjia et al. 1977), which probably correspond with the higher sea levels attained during the Pleistocene Interglacial phases.

Twenty thousand years ago, during the Last Interglacial, world sea level was at least 100 and perhaps as much as 140 metres lower than it is now. At that stage Java, Sumatra, and Kalimantan were areas of higher country rising from the broad plains of the emerged sea floor on an enlarged Malaysian Peninsula: according to Verstappen (1975b) its land area increased by about 3 million square kilometres. To the south-east, an enlarged Australia was linked with New Guinea by way of the emerged Torres Strait land bridge. In between, Sulawesi and the eastern Indonesian archipelago persisted as slightly larger islands amid deeper seas and straits (Fig. 2). A deep channel extended down through the Straits of Macassar between Kalimantan and Sulawesi, and on through the gap between Bali and the island of Lombok. This became known as "Waliace's Line" after Alfred Russel Wallace reported in 1856 that it formed a biogeographical boundary between Asian and Australasian faunas which could not migrate across this late Pleistocene strait. Later research has shown that the division is, in fact, a broader zone, the sea area between the enlarged Malaysian and Australian land masses, the biota of the eastern Indonesian archipelago being transitional in character between that of Malaysia and Australia (Hooijer 1975).

According to Verstappen (1975b) the climate of the Indonesian region was cooler and drier (with longer dry seasons) and also windier during the late Pleistocene low sealevel phase, but the uplands remained humid tropical, and carried rain forest. Currents in the Indonesian seas were modified and disrupted, and although the temperature of the sea fell 4 to 5 C., it remained warm enough for coral growth, the waters being less turbid because of reduced rainfall and runoff. Verstappen questioned the correlation of river valley incision with low sea-level phases of the Pleistocene, on the grounds that Interglacial climate and vegetation were also conducive to stream entrenchment, especially in the hinterland, where the snow line was lowered several hundred metres and vegetation zones became vertically compressed, protective rain forest giving place to less dense vegetation on many slopes. However, it should be remembered that relatively high sea levels in Interglacial phases set downward limits to valley incision, and that in coastal regions there has been aggradation of valleys previously cut down to lower, glacial phase, sea levels, as well as submergence and infilling of their former extensions across the neighbouring sea floor.

There is morphological evidence for terraces of Quaternary age both above and below present sea level, probably marking stillstands in sea level relative to the land. Tjia (1975) listed several former coastline levels in Malaysia and parts of Indonesia, but the determination and dating of such features is still largely speculative, especially in the tectonically unstable Indonesian region. In terms of environmental changes, interest centres on the Holocene sequence of land- and sea-level changes, during and since the world-wide marine transgression that began about 18,000 years ago, accompanying climatic amelioration and the partial melting of polar and upland ice sheets.

FIG. 2 Land and water areas in the Indonesian region during the Late Pleistocene (Last Glacial) low sea-level phase. Malaysia(M), Sumatra (S), Java(J), and Kalimantan (K) are on an enlarged south-east Asian peninsula, separated by islands in deeper water from a continental area bearing Australia(A) and Irian Jaya (l) (based on Verstappen 1975b)

This Holocene marine transgression brought the sea up to approximately its present level about 6,000 years ago, since when there have been only minor oscillations. The evidence for Holocene changes of land and sea level in Indonesia and Malaysia has been examined in a number of papers by Tjia (e.g., Tjia 1970a, 1975; Tjia et al. 1972, 1975,1977), who has put forward an oscillating Holocene sea-level curve, based largely on Malaysian evidence, with several peaks of sea level up to 3.5 metres above the present, diminishing in amplitude through to the modern level (Tjia et al. 1977). There is certainly good evidence of a higher Holocene sea level along the east coast of peninsular Malaysia, where oyster beds are found up to 3.5 metres above present sea level, but on Indonesian coasts the evidence is more variable, indicating the complicating effects of upward and downward movements of the land. Thus, on the west coast of Sulawesi at Pangkajene, molluscs between 4,000 and 5,000 years old have been found in position of growth attached to Eocene limestone cliffs 5.75 to 6.50 metres above present sea level: even if these grew during an episode of higher Holocene sea level, they must have been uplifted to attain their present height.

Emerged shoreline features of Holocene age have been widely observed on Indonesian coasts by Tjia (Fig. 3) and have been deduced in recent work in the Straits of Malacca (Geyh et al. 1979) and at points along the north coast of Java, but it is not clear which of them formed in relation to a higher sea-level stand on stable coast sectors and which owe their present elevation, at least partly, to tectonic uplift. West of Kalimantan there are coastal terraces up to 2 metres above present sea level on the islands of Tambelan and Bunguran. On Tambelan dead corals attached to rock outcrops just above sea level and Tridacna specimens found above their normal growth level have been dated within a range of 5,200 to 5,500 years BP (Haile 1970).

Evidence of rapid Holocene uplift at rates of up to 10 millimetres/year has been deduced from radiocarbon dating of coral reef terraces in Sulawesi, on Timor and Sumbawa, and bordering the islands of Tomea, south-east of Sulawesi, and Selu, off Yamdena. Saubi Island, east of Madura, and Satengar Island, a coral cay north of Sumbawa, also show Holocene uplift, while the tilted reef terraces on Biak Island north of Irian Jaya indicate transverse displacement in the course of uplift that has continued into Holocene times (Tjia et al. 1974). Active subsidence is less easily demonstrated, but it is very likely that isostatic subsidence is in progress in the areas occupied by large deltas, where the massive sediment loads depress the underlying crust. Sectors of deltaic shoreline not maintained or advanced by river-mouth deposition and longshore accretion are likely to develop inlets and embayments as submergence accompanies subsidence.

Much more detailed research is necessary to establish the location and extent of the various raised shorelines in Indonesia, and to separate the effects of sea-level oscillations from those of tectonic uplift or depression. Attempts to establish average rates of uplift or subsidence in Indonesia, or over regions within Indonesia (Tjia 1970b, Tjia et al. 1974) are of little value because the regional pattern is so complex and variable, but measurements of rates of vertical movement at specific sites (cf. Chappell and Veeh 1978) are of much interest as a basis for deciphering patterns of differential uplift and subsidence.

FIG. 3 The extent of emerged Holocene shoreline features in Indonesia, according to Tjia (1975): sectors showing such features are marked in black

FIG. 4 Diagram analysing advancing and receding coasts in terms of emergence, deposition, submergence, and erosion (based on Valentin 1952)

Changes of sea level result in horizontal movements of the shoreline, which advances as emergence takes place and retreats during submergence. These effects can be offset by erosion or deposition: a rapidly eroding shoreline may continue to recede during a phase of emergence, and a rapidly accreting shoreline could continue to advance even when submergence is taking place. These possibilities are summarized diagrammatically in Fig. 4. In order to assess the nature and extent of coastal changes in Indonesia in Holocene times it would be useful to determine the alignment of the shoreline 6,000 years ago, when the marine transgression established approximately present sea level. However, this is difficult, partly because much of the evidence has been eroded or concealed by deposition, and partly because there have been minor oscillations of sea level during the past 6,000 years, accompanied by tectonic displacements of some Indonesian coastal areas within this period.

Impact of Man

The attempt to assess the extent to which environmental changes are due, directly or indirectly, to Man's activities in Indonesia is complicated by the fact that the islands have long been populated, and that it is difficult to establish what conditions would have been obtained naturally. Indeed, since the hominid Pitbecanthropus modjokertensis was living in Java at least 1.9 +0.4 million years ago (Jacob and Curtis 1971), and there is evidence of his occupation of existing coastal regions about 1.2 million years ago, Man has evidently been present during the oscillations of land and sea level discussed previously. When sea level fell during the Last Glacial phase of the Pleistocene, Man presumably occupied emerged lands seaward of the present coastline. As the sea rose, he must have retreated to the upland areas that then persisted as islands, and during the past 6,000 years he has adjusted his occupance to advances and recessions of the shoreline.

Nevertheless, the population of Indonesia was relatively small, and apparently stable, when Dutch explorers and colonists arrived in the sixteenth century. Its rapid increase during and since this phase of European influence has resulted in the locally extreme pressures of population on resources now seen, especially in northern Java. Although there was port construction in the Jakarta area at least by the fourth century AD, and coastal settlements depending on fishing and trade have certainly existed for many centuries, the impact of dense populations on the coast has been comparatively recent. The cutting of canals; the embanking, damming, and diversion of rivers; the large. scale replacement of hinterland forests by grazed and cultivated land, with consequent increases of runoff and soil erosion; the destruction of vegetation that formerly stabilized coastal dunes; and the modification of mangrove areas for agriculture and arguaculture, are all major impacts that have developed mainly within the past century. There has also been mining of tin, bauxite, nickel, and iron ores in coastal regions, and within the last decade there has been rapid expansion of oil production from wells mainly in the coastal and nearshore zones. Reference will be made subsequently to the effects of beach mining and the quarrying of coral reefs. Industrialization has proceeded in the Jakarta and Surabaya regions, and near Cilacap, alongside an estuarine embayment on the south coast of Java. In consequence, parts of the Indonesian coastline show evidence of changes on a scale much greater than those that took place in preceding centuries; indeed, throughout the period that the sea has stood at approximately its present level. And some sectors that have so far been little modified, particularly in eastern Sumatra, are beginning to receive the impacts of people arriving from densely populated areas under the Indonesian government transmigration schemes.


The most insidious form of Man's impact on the Indonesian coastal environment is pollution (Soegiarto 1975, 1976). In a broad sense this includes additional sedimentation due to soil erosion, and to mining and dredging activities. More specifically, chemicals derived from the fertilizers, pesticides, and herbicides that have been used increasingly in recent years to improve agricultural productivity, especially in rice fields, have seeped or flowed into rivers, and thence to estuaries and coastal waters, including brackish-water fishponds. The fertilizers can lead to excessive nutrient concentration, resulting in algal blooms that impoverish or destroy the habitats of fish and crustaceans; the toxic chemicals intended to kill weeds and pests can also destroy organisms that live in coastal waters. As 98 per cent of marine fish production in Indonesia is derived from traditional artisanal fisheries centred mainly in coastal and estuarine waters, this is a serious problem. It has been compounded recently by the discharge of toxic chemicals, including such heavy metals as cadmium and mercury as dissolved salts, into waters draining from industrial areas, particularly in the Jakarta Bay region. Petrochemical wastes and oil spills have also had additional adverse effects on marine ecosystems, fouling brackish-water fishponds and tainting fish caught in estuaries or nearshore waters subject to this pollution. These various forms of pollution constitute an unwelcome environmental change in the coastal environments especially near the more densely populated regions of Indonesia.

The foregoing account of factors and processes influencing the evolution of Indonesian coastal features, their present dynamics, and the impact of Man's activities sets the scene for a systematic review of our knowledge of the changing Indonesian coastline.

2. The changing coastlines of Indonesia

Although there has been geomorphological research on several parts of the Indonesian coastline, the coastal features of Indonesia have not Yet been well documented. The following account-based on studies of maps and charts, air photographs (including satellite photographs), reviews of the published literature, and our own traverses during recent years-is a necessary basis for dealing with environmental changes on the coasts of Indonesia. Coastal features will be described in a counter-clockwise sequence around Sumatra, Java, Kalimantan, Sulawesi, Bali and the eastern islands, and Irian Jaya. Inevitably, the account is more detailed for the coasts of Java and Sumatra, which are better mapped and have been more thoroughly documented than other parts of Indonesia. In the course of description, reference is made to evidence of changes that have taken place, or are still in progress.

Measurements of shoreline advance or retreat have been recorded by various authors, summarized and tabulated by Tjia et al. (1968). Particular attention has been given to changes on deltaic coasts, especially in northern Java (e.g, Hollerwoger 1964), but there is very little information on rates of recession of cliffed coasts. Measurements are generally reported in terms of linear advance or retreat at selected localities, either over stated periods of time or as annual averages, but these can be misleading because of lateral variations along the coast and because of fluctuations in the extent of change from year to year.

Our preference is for areal measurements of land gained or lost or, better still, sequential maps showing the patterns of coastal change over specified periods. We have collected and collated sequential maps of selected sites and brought them up-to-date where possible.

Coastal changes can be measured with reference to the alignments of earlier shoreline features, such as beach ridges or old cliff lines stranded inland behind coastal plains. In Sumatra, beach ridges are found up to 150 kilometres inland. The longest time scale of practical value is the past 6,000 years, the period since the Holocene marine transgression brought the sea up to its present level. Radiocarbon dating can establish the age of shoreline features that developed within this period, and changes during the past few centuries can be traced from historical evidence on maps and nautical charts of various dates.

These have become increasingly reliable over the past century, and can be supplemented by outlines shown on air photographs taken at various times since 1940. Some sectors have shown a consistent advance, and others a consistent retreat; some have alternated. A shoreline sector should only be termed "advancing" if there is evidence of continuing gains by deposition and/or emergence, and "retreating" if erosion and/or submergence are still demonstrably in progress (Fig. 4).

Coastal changes may be natural, or they may be due, at least in part, to the direct or indirect effects of Man's activities in the coastal zone and in the hinterland. Direct effects include the building of sea walls, groynes, and breakwaters, the advancement of the shoreline artificially by land reclamation, and the removal of beach material or coral from the coastline. Indirect effects include changes in water and sediment yield from river systems following the clearance of vegetation or a modification of land use within the catchments, or the construction of dams to impound reservoirs that intercept some of the sediment flow. There are many examples of such man-induced changes on the coasts of Indonesia.

Reference will also be made to ecological changes that accompany gains or losses of coastal terrain, and to some associated features that result from man's responses to changes in the coastal environment.


Incidental references to some of the coastal features of Sumatra were included in Verstappen's (1973) geomorphological reconnaissance, but there has been no systematic study of this coastline. Verstappen's geomorphological map (1:2,500,000) gives only a generalized portrayal of coastal features: it does not distinguish cliffed and steep coasts, the extent of modern beaches, fringing reefs, or mangrove areas, but it does indicate several sectors where Holocene beach ridge plains occur.

Sumatra is 1,650 kilometres long and up to 350 kilometres wide, with an anticlinal mountain chain and associated volcanoes bordered to the east by a broad depositional lowland with extensive swamp areas along the Straits of Malacca. Off the west coast the Mentawai Islands constitute a "non-volcanic arc," consisting of uplifted and tilted Tertiary formations, their outer shores being generally cliffed -facing the predominant south-westerly swell transmitted across the Indonesian Ocean-while the inner shores are typically lower and more indented, with embayments fringed by mangroves. There are emerged coral reefs and beach ridges, especially on the outer shores, and the possibility of continued tilting is supported by the disappearance of islets off the coast of Simalur even within the present century (according to Craandijk 1908: quoted by Verstappen 1973). There are, however, contrasts between the islands, the relatively high island of Nias (summit 886 metres) being encircled by emerged reef terraces suggestive of uplift with an absence of tilting, while Enggano is tabular, steep-sided, and reef-fringed. Much more detailed work is needed to establish the evolution of these island coasts, and the effects of recurrent earthquakes and tsunami. At this stage, no information is available on rates and patterns of shoreline changes taking place here.

The south-west coast of mainland Sumatra is partly steep along the fringes of mountainous spurs, and partly low-lying, consisting of depositional coastal plains. Swell from the Indonesian Ocean is interrupted by the Mentawai Islands and arrives on the mainland coast in attenuated form. It is stronger to the north of Calang, where there are surf beaches bordering the blunted delta of the Tuenom River, and south-east of Seblat, where there are steep promontories between gently curving sandy shorelines backed by beach ridges and low dunes, interrupted by such blunted deltas as the Mana, the Seblat, and the Ketuan.

Coral reefs are rare along the central part of the south-west coast of Sumatra because of the large sediment yield from rivers draining the high hinterland, but to the south there are reef-fringed rocky promontories. Pleistocene and Holocene raised beaches and emerged coral reefs are also extensive, especially on headlands near Krui and Bengkulu, where reefs raised 30 metres above the present sea level have been truncated by the recession of steep cliffs. Farther south the coast shows the effects of vulcanicity on the slopes of Rajabasa. The Krakatau explosion of 1883 generated a tsunami that swept large coral boulders onshore and produced a fallout of volcanic ash that blanketed coastal features and augmented shore deposits. Near Cape Cina the steep coasts of Semangka Bay and Tabuan Island are related to en echelon fault scarps that run north-west to south-east, and the termination of the coastal plain near Bengkulu may also result from tectonic displacement transverse to this coastline. Farther north, the Indrapura River turns parallel to the coast to follow a swale behind beach ridges before finding an eventual outlet to the sea with the Batang River.

Padang is built on beach ridges at the southern end of a coastal plain that stretches to beyond Pariaman. The extensive shoreline progradation that occurred here in the past has evidently come to an end, for there are sectors of rapid shoreline erosion in Padang Bay, where groynes and sea walls have been built in an attempt to conserve the dwindling beach. North of Pariaman the cliffed coast intersects the tuffs deposited from the Manindjau volcano, and farther north there is another broad swampy coastal plain, with associated beach ridges built by wave action reworking fluvially supplied sediment derived from the andesite cones, Ophir and Malintang, in the hinterland. Towards Sirbangis this plain is interrupted by reef-fringed headlands of andesite on the margins of a dissected Pleistocene volcano. Beach erosion has become prevalent in the intervening embayments between here and Natal, and Verstappen (1973) suggested that the swampy nature of the coastal plain here could be due to recent subsidence, which might also explain the present recession of the coast. Broader beach ridge plains occur farther north, interrupted by Tapanuli Bay, which runs back to the steep hinterland at Sibolga. Musala Island, offshore, is another dissected volcano. Next comes the broad lowland on either side of the swampy delta of the Simpan Kanang, in the lee of Banyak Island, and beyond this the coast is dominated by sandy surf beaches, backed in some sectors by dune topography, especially in the long, low sector that extends past Meulaboh.

At the northern end of Sumatra the mountain ranges break up into steep islands with narrow straits scoured by strong tidal currents. Weh Island is of old volcanic rocks, terraced and tilted, with emerged coral reefs up to 100 metres above sea level. Uplifted reefs are also seen on some of the promontories of the northern Sumatran mainland. At Kutaraja the Aceh River has filled an intermontane trough, but the deltaic shoreline has been smoothed by waves from the north-west, coming down the Bengalem Passage between Weh and Peunasu islands, so that the mouths of distributary channels have been deflected behind sand spits and small barrier islands. Beach ridges built of fluvially supplied sediment form intersecting sequences near Cape Intem, where successive depositional plains have been built and then truncated, and there is an eastward drift of beach material along the coast towards Lhokseumawe.

Within this sector Verstappen (1964a) examined the coastal plain near the mouth of the Peusangan River. He concluded that a delta had been built out north of Bireuen, only to be eroded after the Peusangan was diverted by river capture 8 kilometres to the south (Fig. 5). Following this capture, the enlarged river has built a new delta to the east. Patterns of truncated beach ridges on the coastal plain commemorate the shorelines of the earlier delta, which also retains traces of abandoned distributary channels and levees on either side of a residual creek, the Djuli. At the point of capture the Peusangan valley has since been incised about 20 metres, but the old delta was clearly built with the sea at its present level, and so piracy must have taken place within the past 6,000 years, after the Holocene marine transgression had brought the sea up to this level. The new delta has developed in two stages (A, B in Fig. 5), the first indicated by converging beach ridges on either side of an abandoned river channel, the second farther east, around the present mouth. Dating of these beach ridges could establish rates of coastal advance and retreat in this area, and show when the river piracy took place.

South-east from Cape Diamant the low-lying swampy shores of the Straits of Malacca have sectors of narrow sandy beach interspersed with mudflats backed by mangroves, which also fringe the tidal creek systems to the rear. As the Straits narrow the tide ranges increase, and river mouths become larger, funnel-shaped estuaries bordered by extensive swamps instead of true deltas. The widest estuary is that of the Kampar River, where the tide range is sufficient to generate tidal bores that move rapidly upstream. The river channels are fringed by natural levees, and patterns of abandoned levees may be traced throughout the swamps. Locally there has been tectonic subsidence- marked by the formation of lakes amid the swamps-as on either side of the Siak Kecil River and south of the meandering Rokan estuary where lakes which formed along an abandoned river channel as it was enlarged by subsidence are now shrinking as the result of swamp encroachment.

FIG. 5 Changes near the mouth of the Peusangan River, northern Sumatra, following its diversion by capture. Beach-ridge patterns indicate the trend of an old delta, now eroded, north of Bireuen and two stages in development of a new delta to the east: at A a lobe that has been truncated by erosion, and at B a developing modern delta (based on Verstappen 1973)

In the narrower part of the Straits of Malacca there are elongated shoal and channel systems, and some of the shoals have developed into swampy islands, as on either side of the broad estuary of the Mampar. Verstappen (1973) suggested that the Bagansiapiapi Peninsula and the islands of Rupat, Bengkalis, and Tebingtinggi may be due to recent tectonic uplift, and the Rupat and Pajang Straits to alignments of corridor subsidence. The islands have extensive swamps, but their northern and western coasts are fringed by beach ridges possibly derived from sandy material on the sea floor during the shallowing that accompanied emergence. Farther south the Indragiri and Batanghari estuaries traverse broad swamp lands, in which they have deposited large quantities of sediment derived from the erosion of tuffs from volcanoes in their headwater regions. These very broad swamp areas have developed in Holocene times with the sea at, or close to, its present level. The rapidity of their progradation may be related to several factors: an abundance of fluvial sediment yield derived from the high hinterland by runoff under perenially warm and wet conditons; the luxuriance of swamp vegetation which has spread rapidly forward to stabilize accreting sediment, and has also generated the extensive associated peat deposits; and the presence of a broad, shallow, shelf sea, on which progradation may have been aided by tectonic uplift.

In eastern Sumatra, progradation appears to have been very rapid within historical times, but there is not yet sufficient information to permit detailed reconstruction and dating of the shoreline sequences. Studies of early maps, the accuracy of which is uncertain, and interpretations of descriptions by Chinese, Arab, and European travellers led Obdeijn (1941) to suggest that there had been progradation of up to 125 kilometres on the Kuantan delta since about 1600 AD. In further papers, Obdeijn (1942a, 1942b, 1943, 1944) found supporting evidence for extensive shoreline progradation along the Straits of Malacca and in southern Sumatra. In the fifteenth century Palembang, Djambi, and Indragiri were ports close to the open sea or a short distance up estuarine inlets (Van Bemmelen 1949). More recently, the shoreline of the Djambi delta prograded up to 7.5 kilometres between 1821 and 1922, while on the east coast the fishing harbour of Bagansiapiapi has silted up, and the old Sri Vijayan ports are now stranded well inland (Verstappen 1960, 1964b).

Witkamp (1920) described hillocks up to 4 metres high occupied by kitchen middens containing marine shell debris and now located over 10 kilometres inland near Serdang, but these have not been dated. Tjia et al. (1968) quoted various reports of beach ridges up to 150 kilometres at Air Melik and Indragiri, which were former shorelines, but such features are sparser on these swampy lowlands than on the deltaic plains of northern Java. Commenting on the rarity of beach ridges, Verstappen (1973) suggested that the sandy loads of the rivers are largely deposited upstream, so that only finer sediment reaches the coast to be deposited in the advancing swamp lands. Some beach ridges were derived from sediment eroded from the margins of drier "red soil"-ta/ang-particularly around former islands now encircled by swamps, as in the Mesuji district. If progradation has been aided by emergence one would expect beach ridges to be preserved as surface features, for where progradation has been accompanied by subsidence (as on most large deltas) the older beach ridges are found buried as sand lenses within the inner delta stratigraphy. The Holocene evolution of the lowlands of eastern Sumatra still requires more detailed investigation, using stratigraphic as well as geomorphological evidence.

Patterns of active erosion and deposition alongside the estuaries north of Palembang have been mapped by Chambers and Sobur (1975). The changes are due partly to estuarine meandering, with undercutting of the outer banks on meander curves as the inner banks are built up. Towards the sea there has been swamp encroachment, for example along the Musi-Banjuasin estuary, which is bordered by low natural levees breached by orthogonal tributary creeks. The shoreline on the peninsula north of Sungsang is advancing seawards, and there is active progradation along much of the southern coast of Bangka Strait.

Bangka Island rises to a steep-sided plateau with a granite interior: like the Riau and Lingga islands to the north it is geologically a part of the Malaysian Peninsula. Pleistocene terraces occur up to 30 metres above present sea level on Bangka, and its northern and eastern shores have coralfringed promontories and bays backed by sandy beach ridges, but the southern shores, bordering Bangka Strait, are low and swampy, with mangrove-fringed channels opening on to shoaly seas. Belitung is morphologically similar, but has more exposed coasts, with sandy beach-ridge plains extensive south of Manggar on the east coast, facing the south-easterly waves from the Java Sea. Both islands have tin-bearing alluvial deposits in river valleys and out beneath the sea floor, where such valleys extended and incised during glacial low sea-level phases and were submerged and infliled as the sea subsequently rose.

South of Bangka the east-facing coast of Sumatra consists of beach ridges backed by swamps and traversed by estuaries. Lobate salients such as Cape Menjangan and Cape Serdang are beach-fringed swamps rather than deltas, but beach ridges curve inland behind swamps on either side of the Tulangbawang River, where progradation has filled an estuarine gulf. At Telukbetung the lowlands come to an end as mountain ranges intersect the coast in steep promontories bordering Sunda Strait.

Sunda Strait

In 1883 the explosion of Krakatau, an island volcano in Sunda Strait (Fig. 6), led to the ejection of about 18 cubic kilometres of pumice and ash, leaving behind a collapsed caldera of irregular outline, up to more than 300 metres deep and about 7 kilometres in diameter (Fig. 7).

FIG. 6 Krakatau, an island volcano in Sunda Strait which exploded in 1883, leaving three residual islands around a deeper submerged crater, within which a new volcano, Anak Krakatau, has formed

FIG. 7 Krakatau and adjacent areas before and immediately after the explosive eruption in August 1883

The collapse caused a tsunami up to 30 metres high on the shores of Sunda Strait and surges of lesser amplitude around much of Java and Sumatra (Verbeek 1886). Marine erosion has cut back the cliffs produced by the explosive eruption: at Black Point on Pulau Krakatau-Ketjil, cliffs cut in pumice deposited during the 1883 eruption had receded up to 1.5 kilometres by 1928 (Umbgrove 1947). Since 1927 a new volcanic island, Anak Krakatau, has been growing in the centre of the caldera, with phases of rapid enlargement and outward progradation in the 1940s and early 1960s (Zen 1969).

Sunda Strait is bordered by volcanoes, the coast consisting of high volcanic slopes, with sectors of coral reef, some of which have developed rapidly in the century since the Krakatau explosion destroyed or displaced their predecessors. Panaitan Island consists of strongly folded Tertiary sediments, with associated volcanic rocks, and has a sandy depositional fringe around much of its shoreline. Similar rocks form the higher western part (Mount Payung) of the peninsula of Ujong Kulon, the rest consisting of a plateau of Mio-Pliocene sedimentary rocks. This peninsula is a former island, attached to the mainland of Java by a depositional isthmus (Verstappen 1956). It is cliffed on its south-western shores, but the southern coast has beaches backed by parallel dune ridges up to 10 metres high, covered by dense Pandanus scrub, the beach curving out to attach a coral island at Tereleng as a tombolo. The northwest coast has cliffs up to 20 metres high, passing into bluffs behind a coral reef that lines the shore past Cape Alang and into Welkomst Bay. Volcanic ash and negro heads on and behind this reef date from the Krakatau explosion, when a tsunami washed over this coast. Verstappen (1956) found that notches up to 35 centimetres deep had been excavated by solution processes and surf swash on the coral boulders thrown up onto this shore in 1883. This is rapid compared with solution notching measured by Hodgkin (1970) at about 1 millimetre/year on tropical limestone coasts. Within Welkomst Bay there are mangrove sectors, prograding rapidly on the coast in the lee of the Handeuleum reef islands. The geomorphological features of Sunda Strait deserve closer investigation, with particular reference to forms that were initiated by catastrophic events almost a century ago (cf. Symons 1888).


An island about 1,000 kilometres long and up to 250 kilometres wide, Java is threaded by a mountain range which includes several active volcanoes. To the north are broad deltaic plains on the shores of the Java Sea; to the south steeper coasts, interrupted by sectors of depositional lowland, face ocean waters.

FIG 8. The coastal outline of noth-western Java as shown on 1883 - 1885 topographic maps (above) and on 1976.

The west coast of Java is generally steep, except for the Bay of Pulau Liwungan, where the Ciliman River enters by way of a beach-ridge plain. Near Merak the coast is dominated by the steep slopes of the Karang volcano which descend to beachfringed shores. Panjang and Tunda islands, offshore, are of Miocene limestone, but the shores of Banten Bay are lowlying and swampy, with some beach ridges, widening to a deltaic plain of the Ciujung River. This marks the beginning of the extensive delta coastline built by the silt-laden rivers of northern Java. There are protruding lobes of deposition around river mouths and intervening sectors of erosion, especially where a natural or artificial diversion of the river has abandoned earlier deltaic lobes, or sediment yield has been reduced by dam construction. A patchy mangrove fringe persists although there has been widespread removal of mangroves, in the course of constructing tambak (brackishwater fishponds), and in places these are being eroded. Some sectors are beach-fringed and the prevalence of northeasterly wave action generates a westward drifting of shore sediment. Fig.8 shows the pattern of change on the north coast of West Java detected from comparisons of maps, drawn between 1883 and 1885, and 1976 Landsat imagery: there has been seaward growth of land in the vicinity of river mouths, and smoothing and recession of the shoreline in intervening sectors.

There was rapid progradation of the Ciujung delta after the diversion of its lower course for irrigation and flood-control purposes. Growth of the new delta led to the joining of Dua, a former island, to the Javanese mainland, and this has raised problems of wildlife management, for the island had been declared a bird sanctuary in 1973, before it became so readily accessible from the land. Immediately to the west there have been similar changes on the Cidurian delta since 1927, when an irrigation canal was cut, and a new outlet established 4.5 kilometres west of the old natural river mouth. Comparison of outlines on air photographs showed that over an 18-year period the new delta built up to 2.5 kilometres seawards at the mouth of the artificial outlet, while the old delta lobe to the east was cut back by wave action which removed the mangrove fringe and eroded fishponds to the rear (Verstappen 1953a).

Changes have also taken place on the large and complex delta built by the Cisadane River. Natural breaching of levees by floodwaters led to the development of a new outlet channel, and when delta growth began at the new outlet the delta preciously built around the old river mouth began to erode, the irregular deltaic shoreline being smoothed as it was cut back (Verstappen 1953a).

Numerous coral reefs and coralline islands (the Thousand Islands) lie off Jakarta Bay, and many of these have shown changes in configuration during the past century. As a sequel to the studies by Umbgrove (1928,1929a,1929b). Zaneveld and Verstappen (1952) traced changes with reference to maps made in 1975,1927, and 1950.

Haarlem have grown larger as the result of accretion on sand cays and shingle ramparts, but there are also sectors where there has been erosion or lateral displacement of such features on island shorelines. In general the shingle ramparts have developed around the northern and eastern margins, exposed to relatively strong wave action, while the sand cays lie to the south-west, in more sheltered positions. Verstappen (1954) found changes in the position of shingle ramparts before and after 1926, on these islands, which he related to climatic variations. In the years 1917-1926 easterly winds predominated, with the ITC in a relatively northerly position because the Asian anticyclone was weak, and wave action built ramparts on the northern and eastern shores; after 1926 westerly winds became dominant, with the ITC farther south because of stronger Asian anticylonicity, and waves built new ramparts of shingle on the western shores (Verstappen 1968).

There is evidence of subsidence on some of the coral islands, such as Pulan Pugak, where nineteenth-century bench-marks have now sunk beneath the sea, while others have emerged: Alkmaar Island, for example, has a reef above sea level undergoing dissection. Some of the islands have been modified by the quarrying of coral limestone for use in road-making and buildings in Jakarta. This quarrying augmented the supply of gravel to shingle ramparts, but several islands that were quarried have subsequently been reduced by erosion: Umbgrove (1947) quoted the example of Schiedam, a large low-wooded island on a 1753 chart, reduced to a small sand cay by the 1930s.

The features of Jakarta Bay were described in a detailed study by Verstappen (1953a). The shores are low-lying, consisting of deltaic plains with a mangrove fringe inter rupted by river mouths and some sectors of sandy beach. Between 1869 and 1874 and 1936 and 1940 as much as 26 square kilometres of land was added to the bay shores by deltaic progradation, mainly on the eastern shores (Fig. 9). Detailed comparisons of maps made between 1625 and 1977 show the pattern of advance at Sunda Kelapa, Jakarta (Fig. 10). Inland, patterns of beach ridges mark earlier alignments of the coast during its irregular progradation, the variability of which has been related to fluctuations in the position of river mouths delivering sediment (Fig. 11). The beach ridges diverge from an old cuspate foreland at Tanjung Priok, across the deltaic plains of the Bekasi-Cikarang and Citarum rivers to the east.

FIG. 9 The extent of accretion and abrasion on the shores of Jakarta Bay between the periods 1869-1874 and 1936-1940 (based on Verstappen 1953a)

FIG. 10 The pattern of coastal advance at Sunda Kelapa, Jakarta, between 1625 and 1977

Pardjaman (1977) published a map based on a comparison of nautical charts made in 1951 and 1976 which showed substantial accretion along the eastern shores of the bay, especially alongside the mouths of the Bekasi and Citarum rivers. This was accompanied by shallowing off the mouths of these rivers. Along the southern shores at Jakarta a fringe of new land a kilometre wide has been created artificially for recreational use by reclaiming the mangrove zone and adjacent mudflats. On the other hand, removal of lorry-loads of sand from Cilincing Beach resulted in accelerated shoreline erosion. In the 65 years between 1873 and 1938 the shoreline retreated about 50 metres but in the 24 Years between 1951 and 1975, with sand extraction active, it went back a further 600 metres (Pardjaman 1977).

East of Jakarta the Citarum River drains an area of about 5,700 square kilometres, including mountainous uplands, plateau country, foothills, and a wide coastal plain, with beach ridges up to 12 kilometres inland. It has built a large delta (Fig. 12), which in recent decades has grown northwestwards at Tanjung Karawang, with subsidiary growth northwards and southwards at the mouths of the Bungin and Blacan distributaries. At present the river heads north from Karawang, and swings to the north-west at Pingsambo, but at an earlier stage it maintained a northward course to build a delta in the Sedari sector. This has since been cut back, leaving only a rounded salient, along the shores of which erosion is continuing (Plate 1). The shores are partly beach-fringed, the beaches showing the effects of westward longshore drifting, which builds spits that deflect creek mouths in that direction. Eroding patches of mangrove persist locally and, north of Sungaibuntu, there is erosion and dissection of fishponds (PIate 21.

FIG 11 The beach-ridge pattern in the hinterland to the south of Jakarta Bay. Data from Verstappen 1953a and the Geological Survey of Indonesia 1970

FIG. 12 The Citarum delta, showing former courses of the river and the pattern of beach ridges indicative of earlier shorelines (based on Verstappen 1953a)

According to Verstappen (1953a) the Citarum delta prograded by up to 3 kilometres between 1873 and 1938, although sectors of the eastern shore of Jakarta Bay retreated by up to 145 metres. After the completion of the Jatiluhur Dam upstream in 1970 a marked slackening of the rate of progradation of the deltaic shoreline was noted at the mouth of the Citarum. BY contrast, growth on the neighbouring Bekasi delta accelerated after 1970. It was decided that dam construction had diminished the rate of sediment flow down the Citarum River because of interception of silt in the impounded reservoir whereas the sediment yield from the undammed Bekasi River had increased. Such reduction of the rate of progradation has been widely recognized on many deltaic shorelines, following dam construction within their catchments, and the onset of delta shoreline erosion is a phenomenon that has also been documented widely around the world's coastlines (Bird 1976). There is little doubt that the rate and extent of delta shoreline progradation will diminish and that shoreline erosion will accelerate and become more extensive as further dams are built in the catchments of the rivers of northern Java. This will be accompanied by increasing penetration of brackish water into the river distributaries and the gradual spread of soil salinization into deltaic lands.

East of the Citarum delta are the extensive depositional plains built up by the Cipunegara River (Fig. 13). The Cipunegara has a catchment of about 1,450 square kilometres, with mountainous headwater regions, carrying relics of a natural deciduous rain forest and extensive tea plantations; a hilly central catchment with teak forest, rubber plantations, and cultivated land; and a broad coastal plain bearing irrigated ricefields. The river meanders across this plain, branching near Tegallurung, where the main stream runs northwards and a major distributary, the Pancer, flows to the north-east. An 1865 map shows the Cipunegara opening through a large lobate delta, the Pancer having a smaller delta to the east, but when topographical maps were made in 1939 the Pancer had developed two large delta lobes extending 3 to 4 kilometres out into the Java Sea while the Cipunegara delta had been truncated, with shoreline recession of up to 1.5 kilometres. Aerial photographs taken in 1946 showed further advance on the Pancer delta, and continued smoothing of the former delta lobe to the west (Hollerwoger 1964). Tjia et al. (1968) confirmed this sequence with reference to the pattern of beach ridges truncated on the eastern shores of Ciasem Bay and the 1976 Landsat pictures show that a new delta has been built out to the north-east (Fig. 14). Along the coast the mangrove fringe (mainly Rhizophora) has persisted on advancing sectors but elsewhere has been eroded or displaced by the construction of fishponds.

FIG. 13 Stages in the evolution of the Cipunegara delta since 1865 (including data from Hollerwoger 1964)

FIG. 14 The deltaic coastline east and west of the Cipunegara showing the pattern of beach ridges indicative of stages in shoreline evolution (based on Tjia et al. 1968)

Third in the sequence of major deltas east of Jakarta is that built by the Cimanuk River (Fig. 15). The Cimanuk and its tributaries drain a catchment of about 3,650 square kilometres, the headstreams rising on the slopes of the Priangan mountain and the Careme volcano, which carry rain forest and plantations. There has been extensive soil erosion in hilly areas of the central catchment following clearance of the forest and the introduction of grazing and cultivation, particularly in the area drained by the Cilutung tributary (Van Dijk and Vogelzang 1948). The Cimanuk thus carries massive loads of silty sediment down to the coast: of the order of 5 million tonnes a Year (Tjia et al. 1968). The broad coastal plain bears extensive rice-fields, with fishponds and some residual mangrove fringes along the shoreline to the north. The river meanders across this plain, the distributary Rambatan diverging north-westwards near Plumbon.

Hollerwoger (1964) traced changes on the delta shoreline with references to maps made in 1857, 1917, and 1935, and air photographs taken in 1946. Examination of beach-ridge patterns, marking successive shorelines, shows that before 1857 the Cimanuk took a more northerly course and built a delta lobe (Fig 16). BY 1857 this was in course of truncation, and the Cimanuk mouth had migrated westwards to initiate a new deltaic protrusion. Between 1857 and 1917 large delta lobes were built by the Cimanok and the Rambatan but an irrigation channel, the Anyar Canal, had been cut from Losarang to the coast, diminishing the flow to the Rambatan, and a new delta began to grow at the canal mouth, out into the embayment between the Cimanuk and Rambatan deltas. BY 1935 this embayment had been filled, the shoreline having advanced about 6 kilometres in 17 years, while erosion had cut back the adjacent Rambatan delta. Continued growth occurred at the mouth of the Anyar Canal and the Cimanuk between 1935 and 1946, by which time the Rambatan delta shoreline had retreated up to 300 metres.

During a major flood in 1947 the Cimanuk established a new course north-east of Indramayu, and a complex modern delta has since grown here (Plate 3). Stages in the evolution of this modern delta are shown in Fig. 17. At first there was only a single channel, but three main distributaries-the Pancer Balok, Pancer Payang, and Pancer Song-have developed as the result of levee crevassing, and each of these shows further bifurcations resulting from channel-mouth shoal formation, as well as the cutting of artificial lateral outlet channels (Tjia 1965; Hehanussa et al. 1975; Hehanussa and Hehuwat 1979). Since 1974 the Pancer Balok has replaced the Pancer Payang as the main outlet. Erosion has continued on the northern lobe where the present coastline shows an enlargement of tidal creeks, probably the result of compactionsubsidence.

On the east coast, south of Pancer Song, there has been erosion in recent decades. Sand drifting northwards has been intercepted by the oil terminal jetty at Balongan, and the shoreline north of the jetty is retreating rapidly. According to Purbohadiwidjojo (1964) Cape Ujung, to the south, was an ancient delta lobe, but there is no evidence that any channel led this way. Tjia (1965) suggested that it might be related to a buried reef structure, but there is no evidence of this either. In fact, the cuspate promontory is situated where one of the earlier beach ridges has been truncated by the present shoreline. Patterns on the 1976 Landsat picture suggest that the cape is at the point of convergence of two current systems in the adjacent sea area, but it is not clear whether the pattern is a cause or a consequence of present coastal configuration.

FIG. 15 Evolution of the Cimanuk delta between 1957 and 1974. The modern delta (Fig. 17) is northeast of Indramayu (based on Hehanussa et al. 1975).

FIG. 16 Earlier shorelines of the Cimanuk delta as indicated by beach-ridge alignments

FIG. 17 Stages in the growth of the modern Cimanuk delta between 1947 and 1976 (based on Hehanussa and Hehuwat 1979)

FIG.18 Growth of the Bangkaderes delta since 1853 (including data from Hollerwoger 1964)

FIG. 19 Growth of the Sanggarung and Bosok deltas since 1857 (including data from Hollerwoger 1964)

FIG. 20 Growth of the Pemali delta since 1865 (including data from Hollerwoger 1964)

Although it has only a relatively small catchment (250 square kilometres) the Bangkaderes has built a substantial delta (Fig. 18) on the coast south-east of Cirebon. This is because of its large annual sediment load, derived from a hilly catchment where severe soil erosion has followed forest clearance and the introduction of farming. An 1853 map showed a small lobate delta but by 1922 two distributary lobes had been built, advancing the shoreline by up to 2.7 kilometres. Air photographs taken in 1946 show continued enlargement of the eastern branch, extended by up to 1.8 kilometres seawards, and erosion of the western branch, which no longer carried outflow (Hollerwöger 1964).

A few kilometres to the east are the Sanggarung and Bosok deltas (Fig. 19). The Sanggarung has a catchment of 940 square kilometres, and rises on the slopes of the volcanic Mt. Careme. The headwater regions are steep and forested and partly farmed land, and the coastal plain consists largely of rice-fields, with fishponds to seaward and some mangrove fringes. An 1857 survey showed a delta built out northeastwards along the Bosok distributary, and between 1857 and 1946 deposition filled in the embayment to the east, on either side of the Sebrongan estuary, and there was minor growth on the Bosok delta; to the north west the Sanggarung built out a major deltaic feature, with several distributaries leading to cuspate outlets. The coastal lowland here has thus shown continuing progradation of a confluent delta plain without the alternations that occur as the result of natural or artificial diversion of river mouths (Hollerwoger 1964).

The Pemali delta (Fig. 20) also showed consistent growth between an 1865 survey, 1920 mapping, and 1946 air photography (Hollerwöger 1964). The river drains a catchment of about 1,200 square kilometres, with forested mountainous headwater regions and extensive hilly country behind the swampy coastal plain. The delta grew more rapidly between 1920 and 1946 than it had over the 56 years preceding the 1920 survey, possibly because of accelerated soil erosion in hilly country as the result of more intensive farming.

The growth of the Comal delta to the east has shown fluctuations (Fig. 21).When it was mapped in 1870 the Comal (catchment area of about 710 square kilometres) was building a lobate delta to the northwest but by 1920 growth along a more northerly distributary had taken place. The river then developed an outlet towards the north-east, leading to the growth of a new delta in this direction by the time air photographs were taken in 1946. The earlier lobes to the west had by then been truncated. In this, as in the other north Java deltas, growth accelerated after 1920, probably as a result of increasing soil erosion due to intensification of farming within the hilly hinterland (Hollerwöger 1964).

The Bodri delta (Fig. 22) is the next in sequence. The Bodri River rises on the slope of the Prahu volcano, and drains a catchment of 640 square kilometres. Again the mountainous headwater region backs a hilly area, with a depositional coastal plain, mainly under rice cultivation. An 1864 survey shows the Bodri opening to the sea through a broad lobate delta which had grown northwards to Tanjung Korowelang at the mouths of two distributaries when it was remapped in 1910. Thereafter a new course developed, probably as the result of canal-cutting to the north east, and by 1946, when air photographs were taken, a major new delta had formed here, prograding the shoreline by up to 4.2 kilometres. Meanwhile, the earlier delta at Tanjung Korowelang had been truncated and the shoreline smoothed by erosion (Hollerwoger 1964).

East of Semarang large-scale progradation is thought to have taken place in recent centuries. Demak, a sixteenth-century coastal port, is now about 12.5 kilometres inland behind a prograded deltaic shoreline. Continuing progradation is indicated by the small delta growing at the mouth of a canal cut from the River Anyar to the sea, but otherwise the coastline north to Jepara is almost straight at the fringe of a broad depositional plain. According to Niermeyer (1913: quoted by Van Bemmelen 1949) the Muria volcano north-east of Demak was still an island in the eighteenth century, when seagoing vessels sailed through the strait that separated it from the Remang Hills, a strait now occupied by marshy alluvium. This inference, however, needs to be checked by geomorphological and stratigraphical investigations.

FIG. 21 Growth of the Comal delta since 1870 (including data from Hollerwoger 1964)

FIG. 22 Growth of the Bodri delta since 1864 (including data from Hollerwöger 1964)

FIG. 23 Shoreline changes south of Jepara between 1911 and 1972, showing the evolution of the delta at the mouth of Wulan canal

The shoreline of the Serang delta, south of Jepara, changed after the construction of the Wulan Canal in 1892, which diverted the sediment yield from the Kedung River to a new outlet, around which a substantial new delta has been formed. In 1911 this was of cuspate form, but by 1944 it was elongated, and by 1972 it had extended in a curved outline northwards, branching into three distributaries (Fig. 23). Between 1911 and 1944 the new delta gained 297 hectares, and from 1944 to 1972 a further 385 hectares, including beach-ridge systems and a seaward margin adapted for brackish-water fishponds.

Beyond Jepara the coast steepens on the flanks of Muria, but the shores are beach-fringed rather than cliffed. To the east the Juwana River opens on to the widening deltaic plain behind Rembang Bay, but at Awarawar the coast consists of bluffs cut in Pliocene limestone. Tuban has beaches and low dunes of quartzose sand, supplied by rivers draining sandstones in the hinterland, but otherwise the beaches on northern Java are mainly of sediments derived from volcanic or marine sources. Hilly country continues eastwards until the protrusion of the Solo River delta.

The modern Solo delta (Fig. 24) has been built out rapidly from the coast at Pangkah since a new artificial outlet from this river was cut at the beginning of the present century (Verstappen 1977). Comparisons of the outlines of the Solo delta shown on 1 :50,000 topographical maps made in 1915 and 1936 and on air photographs taken in 1943 and 1970 indicated #award growth of 3,600 metres in 1915 to 1936, a further 800 metres between 1936 and 1943, and 3,100 metres between 1943 and 1970; in real terms the delta increased by 8 square kilometres in the first period, 1 square kilometre in the second, and a further 4 square kilometres in the third (Verstappen 1964a, 1977). The rate of progradation of such a delta depends partly on the configuration of the sea floor, for as the water deepens offshore a greater volume of sediment is required to produce the same increase in surface area. It also depends on the rate of fluvial sediment yield, which has here increased following deforestation and intensified land use within the catchment, so that larger quantities of silt and clay have been derived from the intensely weathered volcanic and marry outcrops in the hinterland: the average suspended sediment load is 2.75 kilograms per cubic metre. Much of the silt has been deposited to form levees, while the finer sediment accumulates in bordering swamps.

The features of this delta include a relatively smooth eastern shoreline backed by parallel beach ridges and fronted by sand bars, the outlines determined by northeasterly wave action during the winter months. As this is also the dry season, there has been a tendency for distributaries and creeks formed on the eastern side of the Solo to be blocked off by wave deposition and silted up, the outcome being that the channels opening north-westwards have persisted to carry the bulk of the discharge and sediment yield from the Solo in the wet season, so that the delta has grown more rapidly in this direction. Mangroves (mainly Rhizophora spp.) are patchy and eroded on the eastern shore, but broad and spreading seawards between the distributary mouths on the more sheltered western shore. The tide range is small (less than 1 metre), but at low tide the mudflats exposed on the western shores are up to 200 metres wide. The rapid growth of such a long, narrow delta, protruding more than 20 kilometres seawards, is related partly to the shallowness of the adjacent sea and the consequent low-wave energy conditions and partly to the predominance of clay in the deltaic sediment, which is sufficiently cohesive to form persistent natural levees projecting out into the Java Sea.

FIG. 24 The evolution of the Solo delta between 1915 and 1970 (from Verstappen 1977)

Between 1915 and 1936 there was some lateral migration of the Solo River, marked by undercutting of banks on the outer curves of meanders, and a new outlet channel (3 in Fig. 24) was initiated, probably as the result of flood overflow and levee crevassing on the meander curve. A small delta formed here, but by 1970 it had been largely eroded leaving only a minor protuberance on an otherwise smoothly prograded eastern coast. The effects of canal construction are well illustrated where a channel, cut between 1936 and 1943 from a distributary (2 in Fig. 24) to irrigate rice-fields, increased drainage into an adjacent creek (5 in Fig 24) which then developed levees that grew out seawards. However, by 1970 this, too, had been cut back. A similar development farther south (4 in Fig. 24) converted a creek into a minor distributary of the Solo, with its own sub-delta lobe by 1943, but progradation of mangrove swamps (largely replaced by fishponds) has proceeded rapidly on this part of the western coastline, and by 1970 the distributary, although lengthened, protruded only slightly seawards. In the course of its growth, the Solo delta has incorporated the former island of Mangari, which consists of Pliocene limestone (Verstappen 1977).

East of the broad funnel-shaped entrance to Surabaya Strait the Bangkalan coast of north-west Madura Island shows several small mangrove-fringed deltas on a muddy shoreline. The north coast of the island of Madura is remarkably straight, with terraces that show intermittent emergence as the result of tectonic uplift. The hinterland is steep, with areas of Pliocene limestone, but the shore is generally beachfringed, with some minor dunes to the east. The southern coast of the island is depositional, with beaches of grey volcanic sand that culminate in a recurved spit at Padelegan. Coastal waters are muddy, but outlying islands, such as Kambing, have fringing reefs and derived beaches of pale coralline sand. Tide range increases westwards, and the Baliga River enters the sea by way of a broad, mangrovefringed tidal estuary, bordered by swampy terrain, with a narrow beach to the west.

Surabaya Strait shows tidal mudflats, scoured channels, and estuarine inlets indicative of relatively strong current action, and there has been extensive reclamation for fishponds along the mangrove-fringed coast to the south. In the fourteenth century ships could reach Mojokerto, now 50 kilometres inland on the Brantas delta, which continues to prograde around its distributary mouths. The southern shores of Madura Strait are beach-fringed, the hinterland rising steeply to the volcanoes of Bromo and Argapura. Beach sediments are grey near the mouth of rivers draining the volcanic hinterland, pale or cream near fringing coral reefs, and white in the Jangkar sector, where quartzose sands are found.

The east coast of Java is steep, with streams radiating from the Ijen volcano, but to the south a coastal plain develops and broadens. This consists of low beach ridges built mainly of volcanic materials derived from the Ringgit upland. The Sampean delta is fan-shaped, accreting on its western shores as erosion cuts back the eastern margin. The Blambangan Peninsula is of Miocene limestone, and has extensive fringing reefs backed by coralline beaches, with evidence of longshore drifting on the northern side, into the Straits of Bali.

The south coast of Java is dominated by wave action from the Indonesia Ocean, and receives a relatively gentle southwesterly swell of distant origin and stronger locally generated south-easterly waves that move shore sediments and deflect river outlets westwards, especially in the dry winter season. It is quite different from the north coast of Java, being dominated by steep and cliffed sectors and long, sandy beaches rather than protruding deltas. There is very little information on the extent of shoreline changes in historical times, and we cannot accept the statement of Tjia et al. (1968, p. 26) that abrasion rates along the south coast must have been much higher than those on the deltaic northern shoreline because of the more powerful wave action from the Indonesian Ocean: changes on this rocky and sandy coast will have been relatively slow.

The Bay of Grajagan is backed by a sandy barrier enclosing a river-fed estuarine lagoon system with an outlet to the sea at the western end, alongside the old volcanic promontory of Capil. Farther west the coast becomes indented, with cliffed headlands of Miocene sedimentary rock and irregular embayments, some with beaches and beach ridges around river mouths. Nusa barung is a large island of Miocene limestone with a karstic topography and a cliffed and isletted southern coast; its outlines are related to joint patterns and southward-tilting (Tjia 1962). It modifies oceanic wave patterns on the sandy shores of the broad embayment to the north in such a way as to generate longshore drifting from west to east so that the Bondoyudo River has been deflected several kilometres eastwards to an outlet behind a barrier spit leading to a cuspate foreland with multiple beach ridges (Fig. 25).

FIG. 25 Longshore drifting and the evolution of a cuspate foreland in the lee of Nusa barung, a limestone island off the south coast of Java

The coastal plain then narrows westwards and gives place to a steep indented coast on Miocene sedimentary formations, including the limestones of Kendeng, with bolder promontories of andesite near Tasikmadu. At Puger and Meleman there are beach-ridge systems surmounted by dunes up to 15 metres high, with a thick vegetation cover, in sequence parallel to the shoreline. These interrupt the predominantly karstic limestone coast (Plates 4, 5, and 6), with cliffed sectors and some fringing reefs, that continues westwards to Parangtritis. Near Baron the limestone cliffs are fronted by shore platforms exposed at low tide and flat-floored notches, cut in the base of cliffs and stacks, testify to the importance of solution processes in the shaping of these features (Plate 4). Locally, beaches of calcareous sand and gravel occupy coves, and where these occur an abrasion ramp may be seen at the rear of the shore platform. At Baron a river issues from the base of a cliff and meanders across a beach of black sand that has evidently been washed into the valley-mouth inlet by ocean waves (Plate 5), the sand having come from sea-floor deposits supplied by other rivers draining the volcanic hinterland.

At Parangtritis the cliffs end, and the broad depositional plain of central south Java begins. The Opak and Progo rivers, draining the southern slopes of the Merapi volcano, are heavily laden with grey sands and gravel derived from pyroclastic materials. During floods these are carried into the sea to be reworked by wave action and built into beaches with a westward drift (Plates 7 and 8). The coastal plain has prograded, with the formation of several beach ridges separated by swampy swales. No measurements of historical changes are available, but our reconnaissance in November 1979 found evidence of sequences of localized progradation at the river mouths followed by westward distribution of part of the prograded material. It appears that the alignment of the shore is being maintained, or even advanced seawards, as the result of successive increments of fluvial sand supply. Finer sediment, silt and clay, is deposited in bordering marshes and swales, or carried into the sea and dispersed by strong wave action.

On some sectors, especially near Parangtritis, the beach is backed by dune topography, typically in the form of ridges parallel to the shoreline and bearing a sparse scrub cover (Plate 9). At Parangtritis there are mobile dunes up to 30 metres high, driven inland by the south-easterly winds (Plate 10). The presence of mobile dunes, unusual in this humid tropical environment, may be due to a reduction of their former vegetation cover by sheep and goat grazing, and by the harvesting of firewood (Verstappen 1957).

Whereas the present beach and dune systems consist of incoherent grey sand, readily mobilized by wind action in unvegetated areas, the older beach-ridge systems farther inland are of more coherent silty sand which can be used for dry-land cultivation. The silt fraction may be derived from airborne (e.g., volcanic dust) or flood-borne accessions of fine sediment, or it may be the outcome of in situ weathering of some of the minerals in the originally incoherent sand deposits.

At Karangtawang the depositional lowland is interrupted by a high rocky promontory of andesite and limestone, the Karangboto Peninsula. There are extensive sand shoals off the estuary of the Centang River, which washes the margins of the rocky upland, and there appears to have been rapid progradation of the beach to the east-and also in the bay to the west-where sand has built up in front of a former sea cave which used to be accessible only by means of ropes and ladders when men descended the cliff to collect birds' nests. Rapid accretion may have been stimulated here by the catastrophic discharge of water and sediment that followed the collapse of the Sempor Dam in the hinterland in 1966.

The sandy and swampy coastal plain resumes to the west of the Karangboto Peninsula, and extends past the mouth of the Serayu River. In this sector it has been disturbed by the extraction of magnetite and titanium oxide sands; in places, the beach ridges have been changed into irregular drifting dunes, while dredged areas persist as shallow lagoons.

On either side of the mouth of the Serayu River the coastal plain has prograded by the addition of successive sandy beach ridges separated by marshy swales. The sediments are of fluvial origin, reworked and emplaced by wave action, and progradation has enclosed a former island as a sandstone hill among the beach ridges. According to Zuidam et al. (1977) the coastal plain shows a landward slope at a number of places where the streamlets flow land-wards instead of seawards, and this is presumed to be due to very recent differential tectonic movements.

The geomorphological contrast between the irregular deltaic coast of northern Java and the smooth outlines of depositional sectors on the south Java coast is largely due to contrasts in wave-energy regimes and sea-floor topography. The sediment loads of rivers flowing northwards and southwards from the mountainous watershed are similar, but the finer silt and clay, deposited to form deltas in the low-energy environments of the north coast, are dispersed by high-wave energy on the south coast. The coarser sand fraction seen in beach ridges associated with the north coast deltas is thus concentrated in more substantial beach and dune formations on the south coast. The contrast is emphasized by the shallowness of coastal waters off the north coast, which reduces wave energy, as opposed to the more steeply shelving sea floor off the south coast, which allows larger waves to move into the shoreline. Nevertheless, silt and clay carried in floodwaters settles in the swales between successively built beach-ridge systems along the southern coast, and in such embayments as Segara Anakan, and, as we have noted, it may also have been added to the sandy deposits of older beach ridges inland.

The nature and rate of sediment yield from rivers draining to the south coast vary with the size and steepness of the catchment, with geological features such as catchment Ethology, and with vegetation cover. In the Serayu River basin, deforestation has accelerated sediment yield and increased the incidence of flooding in recent years. Meijerink (1977) found that the annual sediment yield from catchments dominated by sedimentary rocks was ten times that of catchments with similar vegetation and land use on volcanic formations, the contrast being reflected in the nature and scale of depositional features developed at the river mouth.

West of the Serayu River the sandy shoreline, backed by beach ridges, curves southwards to Cilacap, in the lee of Nusakambangan, a high ridge of limestone and conglomerate, with precipitous cliffs along its southern coastline. Extensive mangrove swamps threaded by channels and tidal creeks border the shallow estuarine embayment of Segara Anakan (Fig. 26), which receives large quantities of silty sediment from the Citanduy River. At the eastern end, strong tidal currents maintain a navigable inlet for the port of Cilacap, which stands on the sandy barrier behind a shoaly bay. A meandering channel persists westwards, leading through the mangroves to Segara Anakan, which has a larger outlet through a steep-sided strait to Penandjung Bay. Changes in the configuration of Segara Anakan between 1900 and 1964 were traced by Hadisumarno (1964), who found evidence for rapid advance of mangroves into the accreting intertidal zone. He reported surveys made in 1924, when the average depth (ignoring deeper tidal channels) was 0.5 to 0.6 metres, and 1961, when it had shallowed to 0.1 to 0.2 metres, the tidal channels having deepened. Mangrove advance is exceptionally rapid here, and much of the shallow lagoon is expected to disappear as mangroves encroach further in the next two decades. The features and dynamics of Segara Ankan are being studied in Phase II of the UN University LIPI Indonesian coastal resources management project in 1980-81.

FIG. 26 The rapidly silting estuarine embayment of Segara Anakan, shrinking in area as a result of mangrove encroachment, still has a tidal channel, Kali Kembangkuning, linking it to an eastern outlet at Cilacap

West of Segara Anakan the beach ridge plain curves out to the tombolo of Pangandaran, where deposition has tied an island of Miocene limestone (Panenjoan) to the Java mainland (Fig. 27), and continues on to Cijulang, where the hinterland again becomes hilly. Beaches line the shore, and many of the rivers have deflected and sand-barred mouths. At Genteng a beach-ridge plain develops, curving out to a tombolo that attaches a former coralline island, and beach ridges also thread the depositional lowlands around the mouths of the Ciletuh and Cimandiri rivers flowing into Pelabuhanratu Bay. The beach ridges indicate past progradation, but no information is available on historical trends of shoreline change in this region. West of Pelabuhanratu the coast steepens, but is still fringed by surf beaches, some sectors widening into depositional coastal plains with beach and dune ridges and swampy swales, including the isthmus which ties Ujong Kulon as a peninsula culminating in Java Head


The Indonesian coasts of Kalimantan have received very little attention from geomorphologists, and there is no information on rates of shoreline change in historical times. The western and southern coasts are extensively swampy, with mangroves along the fringes of estuaries, inlets and sheltered embayments. The hilly hinterland approaches the west coast north of Pontianak, where there are broad tidal inlets, and to the south depositional progradation has attached a number of former volcanic islands as headlands. The Pawan and the Kapuas rivers have both brought down sufficient sediment to build substantial deltas (Tjia 1963) but in general the shoreline consists of narrow intermittent sandy beaches backed by swamps, with cuspate salients in the lee of islands such as Gelam, or reefs as at Tanjung Sambar. South of Kendawagan a ridge of Triassic rocks runs out to form the steep-sided Betujurung promontory and the hills on Bawat and Gelam islands.

The south coast is similar, with a number of cuspate and lobate salients, most of which are swampy protrusions rather than deltas. Sand of fluvial origin has drifted along the shoreline east and west from the mouth of the Siamok, to form the straight spit of Tanjung Bandaran to the east, partly enclosing mangrove-fringed Sampit Bay, and the recurved spit of Tanjung Puting to the west. Near Banjarmarsin, ridges of Cretaceous and Mio-Pliocene rock run through to form the promontory of Selatan where the swampy shores give place to the more hilly coastal country of eastern Kalimantan.

The east coast has many inlets and swamp-fringed embayments, the chief contrast being the large Mahakam delta, formed downstream from Samarinda (Fig. 28). Coarse sandy sediment derived mainly from ridges and valleys in the Samarinda area is prominent in the delta, which has numerous distributaries branching among the swampy islands (Magnier et al. 1975, Allen et al. 1976). Other rivers draining to the east coast open into funnel-shaped tidal estuaries, as at Balikpapan and Sangkulirang, and Berau, Kajau, and Sesayap in the north-east (Tjia 1963); as has been noted, tide ranges are higher on the east coast of Kalimantan than on the south and west coasts. At Balikpapan, shoreline erosion has resulted from the quarrying of a fringing coral reef, but the rate and extent of this erosion have not been documented.

FIG. 27 The tombolo at Pangandaran, southern Java, a depositional isthmus attaching Panenjoan, formerly an island, to the mainland

FIG. 28 The large Mahakam delta, built by fluvial and marine deposition on the east coast of Kalimantan


The coasts of Sulawesi have also received little attention from geomorphologists but it is known that this island has been tectonically active. In contrast with the low-lying swampy shores of Kalimantan there are long sectors of steep coast, often with terraced features indicating tectonic uplift or tilting, especially where coral reefs have been raised to various levels up to 600 metres above present sea level, some of them transversely warped and faulted. Rivers are short and steep, with many waterfalls and incised gorges, and there are minor depositional plains around the river mouths. Fringing and nearshore coral reefs are extensive, and along the shore there are sectors of beach sand, with spits and cuspate forelands, especially in the lee of offshote islands, as at Bentenan. It is likely that progradation is taking place where rivers drain into the heads of inlets and embayments, especially on the east coast, where mangrove fringes are extensive, but no details are available. Volcanic activity has modified coastal features locally, for example on Menado-tua -the active volcano off Menado in the far north of the island- and erosion has been reported at Bahu, but again there are no detailed studies. South and south-east of Sulawesi there are many uplifted reef patches and atolls, as well as islands fringed by raised reef terraces Binongko, for example, has a stairway of 14 reef terraces, the highest 200 metres above sea level (Kuenen 1933), and Muna is a westward-tilted island with reef terraces up to 445 metres above sea level (Verstappen 19601.

Bali and Nusatenggara

The northwestern coast of Bali consists of Pliocene limestone terrain, the shores having yellow beach sands and some fringing reefs. A lowland behind Gilimanuk becomes a narrowing coastal plain along the northern shore, giving place to a steeper coast on volcanic rocks near Singaraja. Out to the north, the Kangean islands include uplifted reefs and emerged atolls.

In the eastern part of Bali the coast is influenced by the active volcanoes, specially Agung, which generate lava and ash deposits that move downslope and provide a source of sediment that is washed into the sea by rivers, particularly during the wet season (December to April). These sediments are then distributed by wave action to be incorporated in grey beaches. Sanur beach is a mixture of fluvially supplied grey volcanic sand and coralline sand derived from the fringing reef (Tsuchiya 1975,1978). At Sengkidu the destruction of a fringing reef by collecting and quarrying of coral has led to erosion of the beach to the rear, so that ruins of a temple now stand about 100 metres offshore, indicating that there has been shoreline erosion of at least this amount in the past few decades following the loss of the protective reef (Praseno and Sukarno 1977). Similar erosion is in progress on Kuta and Sanur beaches.

South of Sanur, in the lee of the broad sandy isthmus that links mainland Bali to the Bukit Peninsula (of Miocene limestone) to the south, spits partly enclose a broad tidal embayment with patches of mangrove on extensive mudflats. This peninsula has a cliffed coast with caves and notches, stacks rising from basal rock ledges and extensive fringing reefs; beaches occupy coves and south of Benoa beach deposition has resulted in the attachment of a small island to the coast as a tombolo (Plate 11).

West of the isthmus, ocean waves determine the curvature of beach outlines, and there has been erosion in recent decades on either side of the protruding airport runway at Denpasar. The beach here, in the lee of a fringing coral reef, is of pale coralline sand, backed by low dunes. It gives way northwards to grey sand of volcanic origin, with beaches interrupted by low rocky promontories and shore benches. Longshore drifting to the north-west is indicated by spits that deflect stream mouths in that direction (Plate 12), and as wave energy decreases, in the lee of the Semenanjung promontory of south-eastern Java, the beaches become narrower and gentler in transverse gradient.

At the north-western end of Bali the Gilimanuk spit shows several stages of growth from the coast at Cejik, to the south, interspersed with episodes of truncation. Verstappen (1975b) suggested that growth occurred during phases of dominance of westerly wave action and truncation when south-easterly waves were prevalent, the variation being due to wind regimes associated with long-term migrations of the ITC, but stages in the evolution of this spit have not Yet been dated.

Many of the features found on Bali are also found on the similar Lesser Sunda islands to the east, but few details are available. Cliffs of limestone and volcanic rock extend along the southern coasts of Lombok, Sumbawa, and Sumba but elsewhere the coasts are typically steep rather than cliffed and often have fringing coral reefs. There are many volcanoes, some of them active: Inerie and Iya in southern Flores and Lewotori to the east have all erupted in recent times and deposited lava and ash on the coast, as has Gamkonora on Halmahera. Rivers have only small catchments, and depositional lowlands are confined to sheltered embayments, mainly on the northern shores. Terraces and emerged reefs indicative of uplift and tilting are frequently encountered on these eastern islands (Davis 1928). On Sumbawa uplifted coral reefs are up to 700 metres above sea level, attached to the dissected slopes of old volcanoes and, on Timor, reef terraces-much dissected by stream incision-attain 1,200 metres above sea level, the higher ones encircling mountain peaks that were once islands with fringing reefs or almost-atolls with reefs enclosing lagoons that had a central island.

Chappell and Veeh (1978) have examined raised coral terraces on the north coast of Timor and on the south coast of the adjacent volcanic island of Atauro, where they extend more than 600 metres above sea level. Dating by Th230-U234 established a sequence of shoreline features and fringing reefs developed during Quaternary oscillations of sea level on steadily rising land margins. On Atauro, where the stratigraphy is very well displayed in gorge sections cut through the terrace stairway, the shoreline of 120,000 Years BP is 63 metres above present sea level. Correlation with other such studies, notably in Barbados, New Guinea, and Hawaii, suggests that the world ocean level was then only 5 to 8 metres above the present, which indicates a mean uplift rate of about 0.5 metres per 1,000 years in Atauro. At Manatuto, Baucau, and Lautem in north-east Timor, dating of similar terraces indicates a similar uplift rate but at Hau, just east of D;l;, the shoreline of 120,000 years BP is only 10 metres above sea level indicating a much slower rate of land uplift, only 2 to 4 centimetres per 1,000 years.

Another emerged almost-atoll is seen on Rot;, southwest of Timor, where the enclosing reefs have been raised up to 200 metres, the highest encircling interior hills of strongly folded sedimentary rocks. Kissar, north-east of Timor, has a stairway of five reef terraces, the highest at 150 metres above sea level. Leti, to the east of Timor, has been uplifted in two stages to form reef terraces 10 metres and 130 metres above sea level, and similar features are seen at 10 to 20 metres and 200 to 240 metres on the nearby island of Moa. Yamdena is an island bordered by high cliffs of coral limestone cut into the outer margins of a reef that has been raised 30 metres out of the sea.

North of the Banda Sea, Seram has a coral reef 100 metres above sea level, and Ambon, which consists of two islands linked by a sandy isthmus, has reefs at heights of up to 530 metres. Gorong, south-east of Seram, is an atoll uplifted in several stages to 300 metres, and now encircled by a lagoon and a modern atoll reef Obi and Halmahera also have upraised reef terraces up to 300 metres above sea level. In the Aru Islands, Verstappen 11960) described cliffs fronted by shore platforms that had been submerged as the result of tectonic subsidence, but uplifted atolls also occur in this region.

A great deal of research is required to establish the nature of coastal features in eastern Indonesia. Some of the reconnaissance accounts are misleading: cliffs have been taken as evidence of recent uplift, and mangrove-fringed embayments as indications of recent subsidence; and it is possible that too much emphasis has been given to catastrophic events, such as earthquakes, volcanic eruptions, and tsunami, in the interpretation of coastal features.

Irian Jaya

Tectonic movements have undoubtedly influenced coastal changes in parts of Irian Jaya, both of steep sectors, mainly in the north, and in the extensive swampy lowlands to the south. Verstappen (1964a) compared a 1903 map of Frederik Hendrik island (Yos Sudarso), near the mouth of the Digul River on the south coast, with maps based on air photographs taken in 1945, and found evidence of substantial progradation, which he attributed to recent uplift in a zone passing through Cape Valsch (Fig. 29). Frederik Hendrik Island is mainly low-lying, with extensive reed-swamps, and its bordering channels are scoured by strong tidal currents but the Digul River opens into a broad estuary, and under present conditions it relinquishes most of its sediment load upstream as it traverses extensive swamps and recently subsided areas between Gantentiri and Yondom. In consequence it is not now building a delta into the Arafura Sea.

On the north coast of Irian Jaya the Mamberamo has built a substantial delta, but in recent decades this has shown little growth; indeed, the western shores show creek enlargement and landward migration of mangroves, while the eastern flank is fringed by partly submerged beach ridges with dead trees, all indicative of subsidence (probably due to compaction) and diminished sediment yield from the river. Verstappen (1964a) related this diminished yield to an intercepting zone of tectonic subsidence that runs across the southern part of the delta, marked by a chain of lakes and swamps, including an anomalous mangrove area (Fig. 30). The largest of the lakes, Rombabai Lake, is adjacent to the levees of the Mamberamo, and at one point the subsided levee has been breached during floods and a small marginal delta has grown out into the lake.

FIG. 29 Historical progradation on the island of Yos Sudarso, south-west coast of Irian Jaya. Based on Verstappen (1964a)

FIG. 30 The Mamberamo delta on the north coast of Irian Jaya, showing the transverse zone of subsidence containing Rombobai Lake and freshwater swamps invaded by mangroves from the west (based on Verstappen 1964a)

The islands west of Irian Jaya show evidence of tectonic movements, Waigeo being bordered by notched cliffs of recently uplifted reef limestone, while Kafiau is essentially an upraised almost-atoll with hills of coral limestone ringing an interior upland.

In September 1979 a major earthquake (force 8 on the Richter scale) disturbed the islands of Yapen and Biak, north of Irian Jaya, initiating massive landslips on steep coastal slopes, especially near Ansus on the south coast of Yapen. According to the United States Geological Survey it was the strongest earthquake in Indonesia since the August 1977 tremor on Sumbawa which had similar effects. Tsunami generated by these and other earthquakes were transmitted through eastern Indonesia but there have been no detailed studies of their geomorphological and ecological consequences.


This review of Indonesian coastal features has indicated the variety of forms that exist within this archipelago, the bestdocumented sectors being the north-eastern coast of Sumatra and the north coast of Java, both of which show evidence of substantial changes within historical times. It is hoped that geomorphological studies will soon provide much more information on the other sectors, which at this stage are poorly documented and little understood.

3. Problems in the changing coastal environment

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.

Natural Hazards

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.

Coastal Erosion

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.

Coastal Deposition

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).

Dune Areas

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.

Land Reclamation

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.

4. Some tasks for future research

As the foregoing account has shown, there are major gaps in our basic knowledge of the Indonesian coastal environment. Although highly generalized geological and topographical maps of the Indonesian archipelago are available at scales of 1 :2,000,000 there is only limited coverage at larger scales outside Java. Geological maps covering Java are available on three sheets at the 1: 5,000,000 scale and parts of the north and south coast are mapped on the 1: 100,000 scale. There are also geological maps of Bali, Sumbawa, and Sumba at 1 :250,000, but apart from the Padang area there are no largescale geological maps of the Sumatran coast. Kalimantan is partially covered by 1 :500,000 geological mapping; Sulawes; only on the 1 :1,000,000 scale apart from four sectors at 1 :250,000, while the eastern islands and Irian Jaya are not yet covered by recent maps from the Geological Survey of Indonesia. As geological mapping proceeds it may be possible to include more detailed coverage of coastal regions, showing the extent and nature of beaches, beach ridges, and dunes; of cliffed coasts; and of coral reefs at, above, and below present sea level.

It would be useful to have the equivalents of Verstappen's 1 :2,500,000 geomorphological map of Sumatra for the rest of Indonesia, but for coastal work it is necessary to include details of cliffed and steep coasts, beaches and related features, reefs, and mangrove swamps. Such information should preferably be published on a much larger scale, at least 1 :50,000 (the scale of the engineering geology maps of the Jakarta area), which could then be assembled in the form of an atlas of strip maps, each showing coastal sectors 30 kilometres long and up to 10 kilometres wide. There is little doubt that many important features of the Indonesian coastline have escaped reconnaissance attention and would come to light in the course of assembling such a survey.

Once the nature and pattern of Indonesian coastal land-forms has been fully documented it may be possible to arrive at a more accurate picture of the extent of changes that have occurred in recent decades. Our investigations of this topic have been impeded by the limited availability of detailed surveys and aerial photographs that can be used to establish past configurations for comparison with present outlines, in order to establish linear and areal measurements of the extent of change over specified periods. Evidence has been presented of the extent of such changes in deltaic regions, mainly in Java, but there are many sectors of the Indonesian coastline where the changes that have occurred cannot be gauged from cartographic, photographic, and documentary evidence available at present. In the course of archival searches we found many interesting historical maps and charts, especially those compiled by Dutch surveyors in the nineteenth and early twentieth centuries, but were disappointed to find only a few sectors with coverage sufficiently reliable to establish the extent of subsequent coastal changes. It is possible that additional historical data bearing upon this question will be found as the result of further inquiries.

More satisfactory information on trends of coastal change can be obtained by monitoring shoreline positions in relation to fixed survey locations. Studies of this kind have been initiated by Hehuwat and Hehanussa on the Cimanuk delta and in a number of other locations by the Directorate of Hydraulic Engineering (Bandung). These result in measurements of shoreline changes that are much more accurate than those obtainable from air photographs or cartographical comparisons without such ground control. In Java especially, a comprehensive programme of delta shoreline monitoring-using fixed survey location that can be identified on aerial photographs and used as a basis for field measurements to shoreline positions-would be of particular value in determining rates of change and particularly in providing an early warning of sectors that, having previously prograded, show signs of the onset of active erosion. Given such warning, it should be possible to plan modifications of land use in sectors under threat of erosion or to devise some kinds of anti-erosion procedures, before actual damage is done to brackish-water fishponds, farmland, routeways, or built-up areas. The possibility of managing water and sediment flow to delta shorelines, put forward in the previous chapter, would require detailed survey data of this kind as a basis for designing canals and determining optimal shoreline locations for their outlets.

As has been indicated, Indonesian coastlines are subject to various natural hazards, including the direct and indirect effects of earthquakes and volcanic eruptions, tsunami, and river flooding. Assessments of the risk require detailed mapping of centres of disturbance, such as the location and alignments of fault zones along which earthquake epicentres have been located, and the distribution of active and temporarily quiescent volcanoes. With reference to these it is possible to derive maps showing hazard levels around the more populous parts of the Indonesian coastal zone, for example the heights likely to be attained by tsunami generated from a coastal or submarine disturbance. Such information could be of particular value in planning settlement sites and routeways in coastal regions and devising security procedures such as warning systems and evacuation drills for coastal sectors where the risk is high, for example, on the shores of Sunda Strait where a repetition of the 1883 tsunami (attaining 30 metres above sea level) must be counted a possibility. The extent of river flooding in coastal regions can be established in relation to gauging station levels, and maps could be prepared of the depth of flooding to be expected during predictable floods of specific recurrence interval.

An array of ecological changes is known to accompany natural and artificial modifications of the coastal margin: progradation often leads to the seaward advance of mangroves and successional migration of mangrove communities, whereas erosion leads not only to loss of vegetation but also increased landward penetration of marine influences, especially salt water, leading to the replacement of freshwater and vegetation by halophytic communities and to the die-back of trees and shrubs that cannot tolerate high levels of groundwater salinity or occasional inundation by floodwaters that have become brackish. Few maps have been made of coastal vegetation and, although mangrove species have been listed at a number of sites, information on their distribution through Indonesia remains scanty and quite inadequate for assessing the extent to which they occur and can be conserved within coastal reserves declared for this purpose. Systematic listing and mapping of mangrove species, and of other coastal vegetation such as salt marsh and beach/dune woodland, is necessary as a basis for botanical conservation on the coasts of Indonesia.

There are similar limitations in available data on Indonesian coastal zoology. While some information is available on species of economic value, especially fish and crustaceans, in the coastal waters of Java and Sumatra and to a lesser extent elsewhere in Indonesia, there is only scattered and fragmentary data on the rest of the fauna, even the bird life having received little attention. There is considerable scope for surveys of animal ecology in the coastal zone, especially on the smaller islands, and in the swampy environments of Irian Jaya.

The distribution of coral reefs in the Indonesian region has been indicated on hydrographic charts and we have reviewed some of their features, as described and discussed by Davis (1928), Molengraaf (1929), Kuenen (1933), Umbgrove (1947), and others. While much detailed observation has been carried out, and the biota of Indonesian coral reefs widely investigated, the richness, variety, and profusion of coralline features in this region are so great that an immense amount of research is still necessary on the evolution, ecology, and dynamics of these reef structures. One field of study concerns the initiation of reefs and their rates of growth, a topic that may repay investigation on the volcanic shores of Sunda Strait, where reefs that began to form after the Krakatau explosion show nearly a century of growth. Another is the response of reef corals to the effects of sedimentation, especially the accumulation of silt and clay discharged into the sea by rivers, and of pollution. These topics can conveniently be studied on the reefs in Jakarta Bay.

In Chapter 2 reference was made to the numerous raised reefs, found at a variety of levels, especially in eastern Indonesia. Many of these have been dissected by rivers, and on such islands as Atauro, off Timor, it is possible to study sections showing the internal structure of fringing reefs attached to volcanic slopes, now uplifted out of the sea. Chappell and Veeh (1978), who have illustrated these sections, have also applied uranium-thorium dating to establish the ages of reefs. Particular attention is given to reefs about 120,000 years old, which are thought to have formed when the sea stood between 5 and 8 metres above its present level. The altitude of such reefs above or below the 5- to 8-metre level is thus an indication of the extent of subsequent uplift or depression of the land margin. Mapping of the present level of the 120,000-year-old reef formation will generate a picture of the late Quaternary tectonic history of the Indonesian region and help to resolve some geophysical questions concerning the movements of bordering plates.

In the course of such research, some coastal sites will prove to be of particular scientific importance, either because they show clearly features and relationships that are typical of a wider area or because of their uniqueness. This applies not only to geological and geomorphological features but also to sites that show characteristic or rare botanical, zoological, and archaeological features. The location, delimitation, documentation, and management of such sites is essential for their maintenance for further scientific research and for educational purposes. Inventories of such sites have been compiled in a number of countries in Europe and the Americas, as well as in parts of Australia and New Zealand, and it is necessary now to document them in Indonesia. It is obvious that such sites as Krakatau in Sunda Strait, the dunes of Parangtritis, and the uplifted coral reef terrace stairway on Atauro Island are of international significance, for they are already established in scientific literature, but many other coastal sites are of scientific importance and their mapping and documentation should be carried out systematically, preferably within a framework of data collection organized by LIPI.

Returning from the consideration of such broad scientific problems to more specific and applicable research in the coastal zone, we regard the practical problems of brackishwater fishpond construction and management as being of paramount importance in Indonesia. The origin of fishpond cultures of this kind is uncertain: there have certainly been brackish-water fishponds on parts of the north coast of Java for several centuries. In the past few decades these have spread along much of the deltaic shoreline on this island and to a few scattered sites elsewhere in Indonesia. Since they produce a valuable source of animal protein, chiefly in the form of milkfish and shrimps, their design and management are of great practical importance. Some aspects of landwater interactions relevant to the management of brackishwater fishponds are being investigated by the United Nations University's Natural Resources Programme, in association with LIPI and the Bogor Agricultural University (IPB), but there is really a need for a comprehensive research programme involving coastal geomorphology, engineering, hydrology, ecology, and climatology, as well as the socioeconomic aspects of tambak cuIture in Indonesia.

Existing brackish-water fishponds become hypersaline, or even dry out in the dry season and are subject to inflow of hinterland water that is commonly eutrophic, polluted, or bearing toxic chemicals, notably pesticides, used on the ricefields. These effects seriously limit the productivity of the fishpond ecosystem. Provision of a sea-water supply is largely based on gravitational inflow by means of channels that are apt to become silted or sealed off by wave-deposited sand at their mouths during the dry season, when northeasterly wave action is relatively strong. The design of the fishponds and their sea-water supply could be rationalized in relation to inflow sources that can be maintained, particularly if simple pumping devices can be utilized.

Reference has also been made to problems of stability of areas converted to fishponds where the mangrove fringes have been cleared away and wave erosion facilitated. In addition to the necessity of conserving the mangrove vegetation, it may be possible to use mangroves in and around the fishponds to improve shelter, shade, and nutrient supply, benefiting the fishery at the same time as maintaining mangroves for timber and firewood production.

Our hope is that existing studies can be amplified and extended to the point where practical contributions can be made to the design and management of brackish-water fishponds. In socioeconomic terms we see this as the most important single problem in the changing environment of the Indonesian coastal zone.


The impetus for this study derived initially from the work of the International Geographical Union's Working Group on the Dynamics of Shoreline Erosion. Between 1972 and 1976 this Working Group sought world-wide data on the extent of advance or recession of shorelines over the past century. Published information on the world scale was inevitably patchy, some sectors having been repeatedly mapped, air photographed, and studied in great detail, while others were known only at a reconnaissance level, with limited data. Apart from the work of Verstappen, Hollerwoger, and Tjia, mainly in Java and eastern Sumatra, Indonesia fell into the second category, and the Working Group had to be content with a relatively brief statement (Bird 1976a)..

In 1976 the Working Group was expanded into a broader Commission on the Coastal Environment and charged with the task of world-wide reporting on five topics: changes in progress on mangrove and salt marsh coasts, the effects of artificial structures on shorelines, the dynamics of coastal dune systems, and the requirements for delimiting coastal sites of scientific interest, in addition to further work on sandy shoreline changes. In practice, the Commission has been led to consider a wide range of problems of coastal management arising from the environmental changes that it has been documenting.

The present study has sought to document and review existing knowledge of environmental changes in the Indonesian coastal zone, and to examine some of the management problems and research needs that arise from awareness of these changes. Reference has also been made to such features as changing mangrove systems, the effects of land reclamation, the problems of the mobile dunes in south-central Java, and the need to locate and document coastal sites of scientific interest, especially in the less-known eastern Indonesian region. The study has thus developed an Indonesian contribution to the world-wide projects of the IGU Commission on the Coastal Environment.

It is acknowledged that there are many gaps in our information on the coastal features of Indonesia and the environmental changes that are, or have recently been, taking place. This reflects the great length and complexity of the archipelago coastline; the fact that access to many sectors remains difficult; and the relatively early stage in development of Indonesian scientific resources in this field. The United Nations University, through its coastal resources management project, is endeavouring to stimulate further surveys and research work at the same time as training and giving experience to young Indonesian scientists embarking on various kinds of coastal research. The initial project (1979 to 1980) was based in Jakarta with emphasis on the deltaic coast eastward to Cirebon and field studies on the Cimanuk delta. In succeeding years, similar projects are to be conducted elsewhere in Indonesia. In each case coastal data are assembled for an introductory programmatic workshop (Bird and Soegiarto 1980) and refined and supplemented in the course of the year's work. One difficulty has been the lack of background material for use by Indonesian scientists. Our hope is that the present study will provide a framework for this research and for further coastal-data collection during the next few Years.


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__________,1960. "On the geomorphology of raised coral reefs and its tectonic significance," Zeitsch. Geomorph. 4.

__________,1964a. "Geomorphology in Delta studies," ITC. Delft Publ. B. 24.

__________,1964b. "Geomorphology of Sumatra," Journ. Trop. Geogr. 18: 184191.

__________,1968. "Coral reefs: wind and current growth control," in Encyclopaedia of Geomorphology, R.W. Fairbridge (ed.): 1 97202.

__________,1973. A Geomorphological Reconnaissance of Sumatra and Adjacent Islands (Indonesia), Groningen.

__________,1974. The Effect of Quaternary Tectonics and Climatas on Erosion and Sedimentation on Sumatra, ITC, Enschede, The Netherlands.

__________,1975a. "Landforms and inundations of the lowlands of South-Central Java," ITC Journ. 75/4: 611-520.

__________,1975b. "On palaeoclimates and landform development in Malaysia," in Modern Quatornary Research in Southeast Asia, G.J. Bartstra et al. (eds.): 3-35.

__________,1977, Remota Sensing in Geomorphology, Elsevier, Amsterdam, Witkamp, H., 1920. "Kjokkenmoddinger ter Oostkust van Sumatra." Tijdschr. Kon. Ned. Aardrijisk. Genoot. 37: 572-574.

Zaneveld, J.S., and Verstappen, H., 1952. "A recent investigation on the geomorphology and the flora of some coral islands in the bay of Djakarta," Journ. Sci. Research 3: 58-68.

Zen, M.T., 1969. "The state of Anak Krakatau in September 1968," Bull. Not. Inst. Geol. Min. Bandung, 2(1): 15-24.

__________, and Sudarmo D. R.P., 1977. "Seismicity and tectonic pattern of the Indonesian region," Assoc. Indon. Geol Ann. Conv., Bandung.

Zuidam, R.A. van, Meijerink, A.M.J., and Verstappen, H., 1977. "Geomorphology of the Serayu River Basin, Central Java." in Serayu Valley Project, Final Report 2: 3-24, ITC, Enschede, The Netherlands.

Other UNU publications

Development Projects in the Sudan: An Analysis of Their Reports with Implications for Research and Training in Arid Land Management

by HeinzUlrich Thimm

An analysis of the successes and failures of eight development projects. In describing the conflicts between public and private interests, nomads and sedentary farmers, animals and crops, it illustrates the difficulties in planning and executing arid land development schemes. It also examines the practical implications of the study and suggests topics for further research.

ISBN 91-808-0042-6
50 pages, 21.4 x 28 cm, paper-bound. US$3.00

Proceedings of the Khartoum Workshop on Arid Lands Management

Edited by J.A. Mabbutt

A compilation of papers focused particularly on understanding the causes and effects of management strategies to control and, if possible, reverse desertification. For countries with substantial dryland areas the assessment of existing knowledge and the training of personnel in its application are prerequisites of development.

ISBN 92-808-0044-2
97 pages, 21.4 x 28 cm, paper-bound. US$5.00

Conservation and Development in Northern Thailand: Proceedings of a Programmatic Workshop on Agro-Forestry and HighlandLowland Interactive Systems

Edited by Jack D. Ives, Sanga Sabhasri, and Pisit Voraurai

Includes eight papers on the physical, social, and economic relationships between highland and lowland and on the problems posed by the change from shifting cultivation to a more settled type of agriculture. Additional reports analyse research and training needs.

ISBN 92-808-0077-9
114 pages, 214 x 28 cm, paper-bound US$10.00

Proceedings of the Jakarta Workshop on Coastal Resources Management

Edited by C.F. Bird and A. Soegiarto

The collected papers that make up these proceedings were presented at the September 1979 workshop which inaugurated the UNU training course on coastal resource management. Relating primarily to the north coast of Java, they provide a multi-faceted and up-to-date discussion of various coastal resources such as mangroves and marine fisheries, as well as more general papers on model development, humid tropical deltas, water-quality assessments, and socio-economic studies. In most cases the authors analyse the implications of their work for coastal resources management.

ISBN 92-808-0130-3
106 pages, 21.4 x 28 cm, paper-bound. US$9.00

Rural Energy Systems in the Humid Tropics: Proceedings of the First Workshop of the United Nations University Rural Energy Systems Project

Edited by W.B. Morgan and R.P. Moss

The papers presented are concerned with energy supply and demand, especially for fuelwood in rural areas. Concentrating primarily on southwestern Nigeria, they point out the urgent need for an understanding of the social and ecological implications of rising energy prices and fuelwood shortages even in the more humid, supposedly wood-rich zones of developing countries.

ISBN 92-808-0093-0
56 pages, 21.4 x 28 cm, paper-bound US$5.00

Social and Environmental Aspects of Desertification

Edited by J.A. Mabbutt and A.W. Wilson

Contains a summary of the papers presented at a meeting of the International Geographical Union's Working Group on Desertification in and around Arid Lands. First results from the UN University's research projects and a summary of work accomplished under Unesco's auspices are included. The tremendous variation in causes and effects of desertification is clearly brought out, as is the value of adopting an interdisciplinary approach.

ISBN 92-808-0127-9
40 pages, 21.4 x 28 cm, paper-bound US$4.00

Spatial Analysis for Regional Development: A Case Study in the Bicol River Basin of the Philippines

by Dennis A. Rondinelli

This case study in the Philippines explains how a variety of scientific techniques were selected and used to analyse the spatial linkages within the Bicol River Basin, and to describe the inequalities in services, income, and opportunities within the region. Valuable insights are then provided as to how development projects, by incorporating such information, could be designed to discourage the massive urban migration and increasing inequality that consistently accompany development efforts.

ISBN 92-808-0166-x
45 pages, 21.4 x 28 cm, paper-bound US$4.00

Bedouins, Wealth, and Change: A Study of Rural Development in the United Arab Emirates and the Sultanate of Oman

by R. Cordes and F. Scholz

The sudden and unprecedented flow of wealth to the oilproducing countries in the Middle East has triggered an extremely rapid process of modernization which has had varying impact in the rural areas. By analysing in two contrasting case studies the extent to which these changes can take place without disrupting traditional systems of values, land-use, and social organization, the authors draw conclusions that have a much wider applicability.

ISBN 92-808-0143-0
65 pages, 21.4 x 28 cm, paper-bound. US$9.00

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Agro-Forestry Systems in Latin America: Proceedings of a Workshop Held in Turrialba. Costa Rica March 1979

As a form of land-use agro-forestry has tremendous possibilities that have yet to be studied or explored in depth. This publication, the proceedings of a workshop jointly sponsored by the United Nations University and the Centro Agronomico Tropical de Investigacion V Enseñanza, Costa Rica, presents over thirty scientific papers on the integration of trees with field crops and/or animals, primarily in Latin

America: both small-scale traditional systems and large industrial estates. Topics covered include a classification of agro-forestry systems, experimental design, economics, land rehabilitation, erosion, and the dissemination of techniques. Available from the publisher, CAT/E (Centro Agronomico Tropical de Investigacion V Enseñanza), Turrialba, Costa Rica.