|Proceedings of the Jakarta Workshop on Coastal Resources Management (UNU, 1980, 106 pages)|
|3. Main papers and discussions|
P. E. Hehanussa and F. Hehuwat
Geomorphologically, the province of West Java can be divided into three zones: the northern coastal plain, the volcanic backbone, and the southern coastal area. There is not much coastal plain on the south coast, the land-sea boundary usually being represented by cliffs or hilly to mountainous country rising directly from the sea. The area is sparsely populated. The volcanic backbone, forming the mountainous part of the island, represents the results of volcanic activity throughout the Quaternary, which was characterized by continuous shifting of the eruption centres. Consequently, a number of volcanic belts can be distinguished, active at different stages of
Quaternary history. Interspaced with the volcanoes, a number of plains of different origin can be distinguished, the most conspicuous being those which represent former intramontane lakes (the plains of Bandung and Cianjur). Population density varies a great deal in the volcanic province, ranging from very dense in the intra-montane plains to sparse in the rugged mountainous parts. At least half of West Java's population occupies the northern coastal plain, which represents slightly less than one-third of the total land area of the province.
In the discourse to follow, an analysis of the morphogenesis of the coastal plain is attempted and its implications for the development efforts of the area are discussed.
The Northern West Java Coastal Plain
Physiographically, the coastal plain can be divided into:
The landward limit of salt water intrusion can be used as a division between the two units (Fig. 1), although they grade into each other. As the tidal range along the northern coast of Java is generally less than 1.0 m, the configuration of the coastal plain has been, for the larger part, determined by the main fluviatile systems in the area. Superimposed upon these alluvial processes were the effects of eustatic movements and the regional geological structure, which, for the most part, reflects the underlying Plio-Pleistocene structural grain, of which some elements continued to be active in Recent times (Hehuwai 1972). The main rivers debouching into the Java Sea in the area between Cirebon and Jakarta are from east to west: the Cimanok, Cipunegara, Citarum, Ciliwung, and Cisadane.
The result of the interaction between the riverine and the marine systems has resulted in a coast which is generally prograding at rates varying from a mere 3.6 m per year for the Angke distributary of the Ciliwung, to an astonishing 204.0 m per year for the Cimanuk Delta (Fig. 2). The coastal types indude the deltaic areas which can be divided into distributary lobes or fans and inter-distributary bays, and the inter-deltaic areas, in which we can distinguish between straight barrierbeach stretches and the embayments.
Stratigraphy of the Northern West Java Coastal Plain
Detailed investigation of the Quaternary stratigraphy of the coastal plain is still lacking, but a preliminary stratigraphic subdivision based on water-well logs has been established for the Jakarta artesian basin (Soekardi and Koesmono 1973), in which nine lithologic units have been distinguished (Fig. 3). The division was made using key horizons (clay layers, quartzsand beds, etc.), marine fossil fragments, and groundwater characteristics as the distinguishing criteria. The thickness of the Quaternary in the Jakarta artesian basin generally exceeds 250 m and locally it exceeds 300 m. Attempts have been made to correlate the stratigraphic units with the eustatic rise and fall of the sea level during the Pleistocene, but in the absence of radiometric dates, these attempts are not too reliable. Three main sediment assemblages make up the Quaternary section of the coastal plain, namely:
To get a better understanding of the mechanisms and processes involved in the formation of the coastal plain, studies in recent sedimentation have been carried out in the Cimanuk Delta area (Hehanussa etal. 1976) and in Jakarta Bay (Siregar and Hadiwisastra 1977).
Based on the Ethology and faunal content (mainly foraminifera and ostracods) a number of units can be distinguished, namely (Fig. 4):
The faunal assemblage does not reflect only the Ethology of the sub-stratum; it is also dependent on such factors as salinity, water depth, I;ght penetration, and temperature. The palaeontological and lithological data obtained from this study have been used to assign a facies interpretation to the sedimentary sequences encountered in water-well logs of the region,
The study of Jakarta Bay proceeded along the same lines as the Cimanuk Delta study, but it was confined to the offshore area. This study also had an additional objective, which was to examine changes in the nearshore environment as a result of different climatic conditions (rainfall and wind direction). Two main biotopes have been distinguished in Jakarta Bay, the "open shelf" and the "embayment" biotopes (Fig. 5), and during the rainy season it is possible to distinguish between an "inner" and an "outer" embayment biotope (Hehuwat 1977).
From the foregoing discussions it can be seen that the coastal plain of northern West Java was formed by prograding sediments of deltaic and inter-deltaic facies. The spatial arrangement of the various facies is the resutt of a preexisting geologic structural grain, upon which there has been a delicate interplay between the river and the sea systems. Any man-made alteration such as the destruction of the mangrove forest, the dredging of sand from the river channel or from the beach, or the building of structures will disrupt the existing balance. This can result in a coastal plain in which different lithologies ( with different physico-chemical characteristics) are in juxtaposition in both the horizontal as well as the vertical sense. Development of such an area should take into account all these factors.
We will here focus on two aspects, groundwater and soil/ sub soil characteristics.
The Jakarta artesian basin produces groundwater from aquifers at different depths. According to their depth of occurrence we can distinguish the following categories of aquifers (Soekardi and Purbo-Hadiwidjojo 1979): A-up to 60 m, B-between 60 - 150 m, C-between 150 - 225 m, D-exceeding 225 m.
Groundwater is only produced from the aquifers A, B, and C, as the water of category D is usually mineralized. Each category furthermore has its own groundwater characteristics: Category A: salty north of a line that runs from Tanah Abang to Klender; south of this line the content of Fe and Mn is rather high, usually exceeding 0.3mg/l. Specific capacity varies between 50 - 100 I/minim drawdown. Category B: better quality groundwater, but with a high organic content, in excess of 10 mg/l. Specific capacity is small, and varies between 30 - 70 I/min/m drawdown. Category C: the most important water producer; quality in general is good and the specific capacity is between 50 - 100 I/minim drawdown.
It has been estimated that the groundwater potential of the Jakarta artesian basin amounts to 39 million m³ per year, but there are indications that the actual production exceeds the sustainable yield.
It has been estimated that the groundwater potential of the Jakarta artesian basin amounts to 39 million m³ per year, but there are indications that the actual production exceeds the sustainable yield. Excessive utilization of this resource results in salt-water encroachment and the lowering of the piezometric surface (Hehanussa 1979). Before the Second World War, salt water was only encountered in wells in the coastal zone at depths of less than 60 m, while today, aquifers of the B category are known to produce salt water (Fig. 6). A notable lowering of the piezometric surfaces can be observed from aquifers in the B and C categories (Fig. 7). In the foreseeable future this deterioration is bound to continue, because of the continuing decline of forest cover in the hinterland, the extension of the wet-rice-field acreage, and the spread of urbanization of Jakarta southward, all of which will result in diminished recharge of the aquifer system.
The different mechanisms of sedimentation which created the coastal plain of northern West Java gave rise to different subsoil characteristics, as is shown on the engineering geological map of the Bogor-Jakarta area (Fig. 8). These varied characteristics, as well as the availability of building material, should be taken into consideration in the planning of the infrastructure development of the area.
The coastal plain of northern West Java was formed through the progradation of sediments carried by the rivers debouching into the Java Sea. The resulting patterns of sediment distribution are attributed to the interplay between the riverine and the marine systems. The whole process took place blanketing a Plio-Pleistocene unconformity surface, of which some of the structural elements continued to be active in Recent times. To understand the spatial relationships of the sediment assemblages in both horizontal and vertical senses, studies in Recent sedimentation have been carried out in the areas of the Cimanuk Delta and Jakarta Bay. The genetic history of the coastal plain has profound implications for the development of the area and the ,management of its resources.
The repercussions on groundwater resources and civil engineering aspects have been discussed to serve as examples.
Hehanussa, P. E.,1979. Penyusupan air laut ke dalam cekungan artois Jakarta (Saltwater encroachment in the Jakarta artesian basin). Paper presented at 5th Ann. Conv. Indon. Geol. Assoc.
________; S. Hadiwisastra; and St. Djoehanah 1976. Sedimentasi delta baru Cimanuk (Sedimentation in the new Cimanuk Delta). Geol. Indon. 311):21 - 35.
Hehuwat, F., 1972. The significance of zircon and rutile distribution pattern on the Sunda shelf. UN ESCAP Rept. 9th Session CCOP, pp.164 - 1 71.
________1977 Penyebaran biotop foraminifera Teluk Jakarta berdasarkan analisa "cluster" (The distribution of foraminiferal biotopes in the Bay of Jakarta based on cluster-analysis). Hiset Geol. Pertamb. 1 (1):1 - 3.
Sireger, M. S., and S. Hadiwisastra 1977. Penyelidikan sedimen Teluk Jakarta (Investigations on the sediments of the Bay of Jakarta). in Teluk Jakarta: sumber daya, sifat-sifat oseanologi serta permasalahannya. Lembaga Oseanologi National-LIPI Jakarta, pp.107 - 137.
Soekardi and M. Koesmono 1973. Pengamatan neotektonik den morfogenesa di daerah daratan Jakarta (Neotectonic and morphogenetic observations in the Jakarta area). Unp. Rept., Geol. Survey of Indonesie, no.1799.
Soekardi,R., and M. M. Purbo-Hadiwidjojo 1979. Cekungan artois Jakarta (The Jakarta artesian basin). Geol. Indon. 2 (1): 25-28.
Muluk: Are you suggesting that the plan for Cengkareng Airport is wrong?
Hehuwat: No, but they have to consider the deeper geological structure of this area, in relation to other possible uses of the site.
Koesoebiono: What factors increase the salinity of tambaks?
Hehanussa: The problem of increased salinity in the tambaks may partly be due to the encroachment of saline water from below.