THE INDONESIAN COASTAL ENVIRONMENT
Eric C. F. Bird and Otto S. R. Ongkosongo
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.
Climate
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. (full text)
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.
Climate
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. (full text)
