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Geology

Otto S. R. Ongkosongo

The northern part of West Java is formed by an alluvial lowland, which is usually called the coastal plain of Jakarta. The plain covers almost the whole coastal zone of West Java. On the most western tip it is almost absent, limited by the Gede (595 m) mountain, but to the east it spreads extensively north of Karawang, to about 43 km wide (Geological Survey of Indonesia 1977). It consists largely of alluvial river deposits and lahars from the volcanoes in the hinterland. From east to west the length of the alluvial lowland plain is about 290 km.

The north coast of West Java contrasts with the south coast from the points of view of elevation, morphology, coastal development, Ethology, and wave characteristics. The north is low and relatively flat, with some bays and capes; the coast has a curved outline with rapid coastal accretion, consisting largely of alluvial river deposits, and with relatively weak waves. The south coast is mostly hilly and rocky, with sea cliff and notched coast; the sea bottom shelves steeply, the coastline is relatively straight, and the waves are strong.

The northern coastline of West Java forms the southwest shore of the Java Sea, with general depths nowhere exceeding 60 m. The 5 m isobath lies generally about 2 km offshore, but in some places, for example off the cape of Ujung, it is more than 10 km. East of Tanjung Prick, Pasir Putih, Eretan Wetan, and Cirebon it lies at about 0.75 km, 2.7 km, 1 km, and 1.5 km offshore, respectively. The beach slopes of these four beaches are generally very gradual (less than 1 per cent) to low water line, while at high water line they are less gradual (1 - 3 per cent). The beach widths at low water are, respectively, 10 - 70 m, 50 m, more than 100 m, and possibly 40 m, in these areas.

The currents in the open waters of the Java Sea and along the coast of West Java are mostly monsoon drift. In addition there is a slight current setting generally toward southsouthwest. The monsoon current sets westward from May to the end of October and eastward in January and February. In November a monsoon current of 2 knots in strength is sometimes encountered. There is little or no current during the transition periods (locally known as kentering) between the two monsoon seasons described above.

In Jakarta, from July to September, the mean wind directions are as follows: at 0900 hours, 147 true; at 1400 hours, 33°; and at 1800 hours, 84 . In January and February, the mean directions at the same hours are 256, 331°, and 308°. In the vicinity of Cirebon, during the east monsoon, a dry wind from the south to southwest usually sets in between 1900 and 2100 hours, and continues strongly until sunrise; then it decreases in force and dies away by 0900 or 1000 hours. Two hours later a northeast to northerly wind starts, usually weak, but occasionally increasing to a stiff breeze in the afternoon. In the case of the latter, it usually shifts to the east. The south and southwest winds are strong and locally called kumbang. During the west monsoon the wind blows from west to west-northwest.

Based on Koppen's classification, Schmidt and Ferguson (1951) mapped the climatic condition of West Java. The average rainfall over the Java Sea has been reported by Wyrtki (1956), and is presented in Table 3.

FIG.1. Comparison of the north coast of West Java between 1883 - 85 and 1976. The map shows the shallow bottom and silted area distribution, and the trend of probable coastline changes in the future.

FIG.1. Comparison of the north coast of West Java between 1883 - 85 and 1976. The map shows the shallow bottom and silted area distribution, and the trend of probable coastline changes in the future. - continue

The Growth of the Coastal Plain

The evolution of the north coast of West Java has been studied by several authors such as t'Hoen (1929), Verstappen (1953a), Hollerwoger (1964), Tjia (1964,1965), Tjia et al. (1968), Hehanussa et al. (1975), and Pardjaman (1977). If we make a comparison between the 1883 - 85 maps (Stemfoort and Ten Siethoff 1883 - 85) and the 1976 LANDSAT photo, we may draw the area with rapid accretion during the intervening period (Fig. 1). Rapid changes occur along the big river mouths, especially the Ciujung, Cidurian, Cisadane, Ciasem, Cipunegara, Rambatan, and Cimanok deltas. The formation of bays on the north coast is due to the big river mouths forming capes, leaving the bays with relatively small rivers far behind (Umbgrove 1929).

Verstappen (1953a, 1954a) mentioned that the whole alluvial plain has been formed in the last 5,000 years and is therefore of Holocene age. Although this figure was primarily based on his extrapolation from data on coastline progression, this is probably the oldest datum in the geomorphic history of the West Java coastal plain. Marks (1956) reported the analysis of sediments from a bore hole in Kebayoran Baru, Jakarta. The sediments from up to 253 m depth chronologically comprise four marine zones indicative of the glacial period. The marine sediments are rich in fossils of inter-tidal to shallow-water environments, but the only age indication is a jaw fragment of Sus brachygrathus, a mammal, from a depth of 51 - 52 m, which possibly indicates a Middle Pleistocene age.

The former coastline is indicated by lines of beach ridges. Verstappen (1953a), the Geological Survey of Indonesia (1975), and Sandy (1976) have described ancient beach ridges around Jakarta Bay, some of which lie more than 10 km inland, for example at Pulogadung. Furthermore, Ongkosongo (1979) has studied the strandline advance in the Jakarta area from 1625 to 1978. Verstappen reported changes between 1873 and 1938, and Pardjaman (1977), the advance from 1951 to 1975. Verstappen drew an abrasion and accretion map of the coast, including the coastal advance from the cape of Kait in the west as far as Ciasem Bay in the east.

To the east, coastal development around Ciasem Bay has been investigated by Tjia et al. (1968), who recognized the development of the coastline from 1725 to 1946 by assuming an annual rate of accretion of 50 m. Here the beach ridges can be recognized up to 9 km inland. Hollerwöger (1964) discussed the evolution of the Cipunegara Delta from the 1865 and 1934 topographic maps and the 1946 aerial photographs.

Further to the east, the evolution of the Cimanuk Delta complex has been investigated by t'Hoen (1929), including changes from the bay north of Kandanghaur to north of Indramayu, from 1877 to 1914 - 15. Hollerwöger (1964) discussed the outlines of the Cimanuk Delta in 1857, 1917, 1935, and 1946, and Tjia (1965) examined delta development up to 1946. Tjia et al. (1968) described the Cimanuk Delta in 1857,1917,1935,1940, and 1946, and finally Hehanussa et al. (1975) extended the works of Tjia (1964,1965), and emphasized the new birth of the Comanuk Delta in 1947 and discussed its evolution until 1975.

Purbo-Hadiwidjojo (1964) suggested that the cape of Ujung, north of Cirebon, was a former river mouth or delta of the ancient Cimanuk, which was a tributary of the large Pleistocene drowned river system in the Java Sea. This river system was mentioned by several authors, for example, by Tuyn (1932) and Bemmelen (1949). This suggestion should be indicated by the existence of an ancient river course off Ujung Cape. Relatively older soils are found on Ujung Cape in comparison with the Indramayu area, as shown by the 1961 soil map compiled by the Soil Research Institute in Bogor. Tjia (1965) made field observations as well as an aerialphotograph study, and came to the conclusion that Ujung Cape was not once part of a delta but was related to the presence of some resistant substratum, probably a buried patch of coral reef. The old map of West Java (Fig. 1) shows that from 1883 to 1885 the cape was as protuberant as the present time, which corresponds with Tjia's extrapolation.

The rate of accretion of the north coastal plain of West Java, from the Ciujung River in the west to Ciwaringin, north of Cirebon, has been reported by Tjia et. al. (1968) and extended by Ongkosongo (1979a). The coastal changes are of the order of +416.6 m/year to-333.3 m/year. Figure 1 shows the trend of coastline changes in the north coast of West Java from the period 1883 - 85 to 1976. It also presents the 1976 siltation distribution from some important rivers that debouch into the southwestern coastal waters of the Java Sea.

In the offshore areas of Northwest Java, coral reefs occur, especially to northwest of Jakarta. Many of these form coral the islands and are located near the mainland of Java. Due to the rapid of the north coast, some of the islands or reefs advance were buried by sediments. A buried coral reef was found in Harbour (Verstappen 1953a, 1977b), and a Tanjung Priok fossil tombolo formed by corals in Kramatpanjang, Jakarta, serial photographs, was checked by field clearly shown on investigation (Verstappen 1954, 1977a). Some smaller coral Sedari Cape and Ciasem Bay are known as islands between Sedari and Sedulang reefs.

North-northeast of Jakarta Bay are the Thousand Islands, a group of 108 coral reefs. The reefs lie on a west-northwest to south-southeast trending ridge about 30 m below sea level. Deep east-to-west gullies cut across this ridge, and one of them, near Payung Island, reaches a depth of 93 m. Umbgrove (1947) suggested that the ridge might represent an anticline in the basement strata,and that the gullies are probably the result of the strong sea currents between the Java Sea and the Strait of Sunda. The seismic section across the ridge shows that it is due to the high basement beneath the Seribu Islands, commonly called the Seribu Platform. The existence of the ridge is supported by the north-south block fault system west and east of the platform, and a gentle anticline in the strata above the basement (Koesoemadinata and Pulunggono 1975).

Factors Governing the Coastline Changes

Ongkosongo (1979a, b) has mentioned the natural and human factors which influence the Indonesian coastline directly or indirectly. Some of the factors, probably ail of them, exist in this region. However, investigations should be carried out to obtain accurate figures. Some of the existing and important factors influencing the coasts are discussed below.

1. Sea waves and currents

River mouths have a tendency to form capes jutting far out into the sea. Obviously this cannot continue for any length of time since the surf will attack these projecting parts of the coasts from three sides, one from offshore and two alongshore. Only in relatively calm water can this development freely continue, forming a bird's-foot delta, for example the new Cimanuk Delta.

Normally, however, there is a certain balance between silt supply on the one hand and surf action on the other, with the bird's-foot delta and the arcuate as extreme formations (Verstappen 1953a). The lack of delta formation on the south coast of Java is because of the heavy breakers of the Indonesian ocean, where the currents remove all the mud, sand, and other sediments brought to the sea by the rivers. On the north coast, the surf is weaker,the tides and currents are relatively small, the sea bottom is shallow, and so deltas may be formed. The tides here are generally less than 1 m (Janhidros 1978). Delta formation is a land augmentation, but many deltas are being reduced by the action of the sea (Hollerwöger 1964), for example the Cidurian and Rambatan deltas.

With a powerful surf, the coastline will develop a regular course. In Jakarta Bay, Verstappen (1953a) concluded that the regularly curving coastline exists in the western half of the bay (which is in contrast with the coastline of the eastern half, with its river deltas projecting far into the sea) because waves and currents produced by north, northeast, and easterly winds are generally stronger than those generated by westerly winds. The difference between the tapering form of Bekasi Delta and the broad Angke Delta can only have been caused by the difference in the force of the surf.

2. Sedimentation

In most cases sedimentation promotes land advance because it can exceed coastal subsidence. Rapid sedimentation usually occurs at the mouths of rivers. When the conditions of waves and currents and the sea floor are not suitable for delta formation, the deposition is usually distributed along the coast, sometimes in one direction, generally with the prevailing wind. According to van Galen (1947) the tendency of big rivers on the north coast of Java, e.g., the Citarum, Cimanuk, and Cipunegara, to curve west is due to the prevailing east monsoon. The sedimentation, however, mostly occurs during the western monsoon because the river floods then bring 80 - 90 per cent of the annual sediment yield to the sea. The formation of beach ridges therefore occurs mostly east of the river mouth (Verstappen 1953a).

Umbgrove (1929a, b) proposed that the lack of coral islands in the eastern part of Jakarta Bay was because of siltation by the Citarum River in the west monsoon. Verstappen (1953a) proved however that siltation was not the main cause. Water transparencies showed that silt content was limited mostly to the nearshore areas, and some of the coral islands in the western part of Jakarta Bay are located in front of river mouths. Measurements of water transparencies by the National Institute of Oceanology (LON) in 1975 - 79 in Jakarta Bay also gave the same trend, although wider in distribution than the measurements made by Verstappen 24 years before. Sediment grain size measurements offshore in Jakarta Bay by LON show that most of the bottom sediments are mud and sand. The latter also occur on and near the coast, for instance on the beaches of Muara Karang, Ancol, Tanjung Priok, and Marunda. Off the Cimanuk Delta, Hehanussa etal. (1975) found that clay exists only 1.5 - 3 km from the river mouth.

Interpretation of the LANDSAT 1976 picture also proved that turbid or very shallow water is limited to within 4 km, and generally less than 2km, from the coastline. However, the picture was taken in the eastern monsoon in June, when there is more probability that the turbid water area will extend offshore from river mouths in the rainy western monsoon season.

The photos also show the pattern of sedimentation and the probable direction of evolution of the coast. Tables 4 and 5 present data from some selected rivers debouching from the north coast of West Java (Census and Statistical Office 1978).

3. Wind

Umbgrove (1928, 1929a, b) and later Verstappen (1953a, b,1954b, 1968) and Verstappen and Zaneveld (1952) demonstrated the effects of wind on the geomorphology of the Seribu Islands. Cays consisting of fine coral sand occur on the lee side and shingle ramparts built of coarse material are found on the windward side of the reefs. The orientation of the sand cays on the southwest side of the reefs in the period from 1917 - 26 to 1925 - 44 demonstrates that the northeast wind has the strongest influence on these islands.

The effects of the monsoon on the changes along the north coast of West Java mentioned by van Galen (1947), but Verstappen (1953a, 1954b, 1968) investigated in more detail the processes at work on the coastline, and concluded that winds produce the currents that cause sedimentation on the beach.

4. Human impact

Ongkosongo (1979a, b) mentioned the impacts of man on the coast of Indonesia, some of them on the north coast of West Java. Change in a river mouth due to the making of new irrigation canals usually results in abrasion around the old river mouth and accretion around the new river mouth. Some examples are the Ciujung River, Cidurian River (Verstappen 1953a), and Cimanuk River (Hollwerwöger 1964). Due to the cutting of an irrigation canal in 1927, in a period of 18 years the new Cidurian Delta, 4.5 km west of the old mouth, grew 2.5 km, while the shoreline 2 km east of the former mouth was abraded.

In the period from 1873 to 1938, the Citarum River in its Tanjung Gembong estuary accreted about 45 m/year and the Pecah, 46 m/year (Verstappen 1953). After the Jatiluhur Dam was constructed upstream, the accretion from 1950 - 75 was decreased to 40 m/year and 44 m/year, respectively (Pardjaman 1977).

The silting of the dammed water in Jatiluhur Reservoir . is clearly seen on the LANDSAT photograph of 1976. In contrast, in the neighbouring Bekasi and Cikarang rivers, catchments with no dam constructions upstream, the annual accretion rate increased from 15 to 50 m during the same period (Pardjaman 1977).

During the period 1873 - 1938, Cilincing Beach in Jakarta retreated about 50 m, or at a rate of only 0.76 m/year. From 1951 to 1975, the retreat of the strandline increased to 600 m, or an average of 24 m/year. The changes were especially obvious in the period 1972 - 75, with a retreat of about 260 m or an annual recession of about 87 m. This was because of sand mining by the local population, which amounted to about 200 to 300 trucks of sand per day. This is the main cause of shoreline recession, because sedimentation from the rivers was not sufficient to balance this loss. The rapid retreat of the coast was also due to some extent to mangrove deforestation (Pardjaman 1977).

In contrast to the effect of beach mining, land reclamation will prograde the land seaward. In Jakarta, land has been reclaimed in the Pluit and Muara Karang area for housing, recreation, and the power station project. Sea walls and other types of beach protections, such as jetties, groins, and breakwaters, will stabilize the shoreline, which in Jakarta, for example, extends from Muara Angke in the west to around Kalibaru in the east, about 20 km (Ongkosongo 1979c). The higher rate of sedimentation on the coast west of the Sunda Kelapa Canal ("Oude Haven Kannal") than to the east was largely due to the construction of two breakwaters, as shown in the coastline evolution map made by Ongkosongo (1979c). Hehanussa (1975) showed that construction of breakwaters for an oil terminal at Balongan (northwest of Cirebon) led to the abrasion of the adjacent coast.

Table 3. Amount of Rainfall in mm over the Java Sea Averaged from 7 Coastal and Island Stations (including Labuhan and Edam Islands) according to Wyrtki in 1956 (Average Number of Observations in Year: 33)

Table 4. Lengths and Catchment Areas of the Rivers Flowing to the North Coast of West Java (Census and Statistical Office 1978)

Table 5. Data from Selected Rivers Debouching to the North Coast of West Java (Census and Statistical Office 1978)

References

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