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3. Main papers and discussions

Model development for integrated utilization of land-water interactive resource systems in the coastal part of the citarum watershed
Environmental problems related to the coastal dynamics of humid tropical deltas
Morphogenesis of the northern coastal plain of west Java between Cirebon and Jakarta: Its implications for coastal zone management
The oceanographic features of the coastal region between Jakarta and Cirebon
Socio-economic studies in java in the context of a coastal resources evaluation
The mangrove ecosystem of the northern coast of west Java
The marine fishery resources of the north coast of west Java
The interpretability of landsat colour composite images for a geographical study of the northern coastal zone of west Java
Water-quality assessement of the cimanuk watershed

 

Model development for integrated utilization of land-water interactive resource systems in the coastal part of the citarum watershed

Chairul Muluk, Ngadiono, Koesoebiono, and Soeratno Partoatmodjo

Introduction

A watershed is a natural ecosystem which is separated topographically from adjacent watersheds. As a system the watershed may be divided into three sub-systems, namely the upland, lowland, and coastal sub-systems which have the following major functions:

  1. the upland sub-system as the main water catchment and flow regulator,
  2. the lowland sub-system as the main water distributor and water consumer, and
  3. the coastal sub-system as a water-based resource system.

Being connected, the condition of the resources in the coastal sub-system is affected by the resource utilization decisions made in the two upper sub-systems of the watershed Furthermore, resource utilization in the coastal sub-system is affected not only by the influx of freshwater channelled through both natural and man-made channel systems into the area, but also by the marine system.

The boundary between the upland and the lowland subsystems is the "reservoir belt," which is a line connecting the lowest location above sea-level potential for reservoir constructions. The boundary between the lower and the coastal sub-systems is difficult to identify and is assumed to be the line connecting the locations potentially affected by salt intrusion.

Based on the land-use in the coastal part of the Citarum watershed, four agricultural production systems, namely food and cash crop agriculture, aquaculture, animal husbandry, and forestry, can be identified. These production systems can be termed the agro-ecosystem in which land-water interaction plays an important role. Response of the agro-ecosystem is affected by the state of interaction among these four components. The output of the agro-ecosystem in the coastal zone is a result of the interaction of the agro-ecosystem, human resources (social, cultural conditions", and the technological and economic conditions prevailing in the area.

Therefore, determining strategies to optimize resource use aimed at providing the majority of the people with basic necessities beyond their minimal sustenance level involves manipulating the agro-ecosystem, human resources, and the technological and capital inputs.

Theoretical Framework

In developing the coastal part of the Citarum watershed it is imperative that integrated development of three components, i.e., the agro ecosystem, human resources, and capital and technological aspects, should be achieved.

Agro-ecosystem

The agro-ecosystem is the agricultural production system and comprises four components: food and cash agriculture, aquaculture, animal husbandry, and forestry. Among the components, the interaction through land is a matter of competition for space, while within each component the landwater interaction may affect, adversely or Positively, its production process. However, among the components water acts as an integrator due to its ability to flow from one to the other. Besides these interactions, the possibility exists for inter-component man induced interactions (Fig. 2). Figure 1 shows the theoretically derived interactions among components.

Human resources

Human resources are the part of a society which has the potential or actual capability to manage available natural resources within the system to provide sufficient products and conditions to sustain or improve the livelihood of that society. The efficiency or productivity of the human resources in achieving these objectives is determined by three major characteristics, namely, the quality and quantity of the human resources, and the social institutions. Improvements in quality may be achieved through training and education, nutrition, etc. Creation of better employment opportunities and wages will increase the productivity of available human resources, while the social institutions can promote the capability to innovate and to participate in supporting development programmes (Fig. 3). Ultimately, manipulation of these activities creates better chances of achieving improvement in the quality of life and the distribution of income per capita.

FIG.1. Theoretical land - water interactions in the agro - ecosystem ( - = transferred through water; * = man - induced)

FIG. 2. The agro - ecosystem

FIG. 3. Development strategies in human resources (Hidayat 1979, modified)

FIG. 4. Theoretical interactions between the agro-ecosystem, human resources, and technology and capital in the coastal part of the Citanum watershed

Technology and capital

Technology applied to and capital needed in resource utilization should be relevant to conditions of the existing human and natural resources, should increase the efficiency of the activities to obtain beneficial outputs, and should minimize the harmful or non-utilizable outputs of each process. Both technology and capital may already exist as a part of the human resources or may be introduced into the system.

Interaction of the major components

Interactions of the three major components related to integrated development of resource utilization in the coastal part of the Citarum watershed are shown in Figure 4. This figure describes the activities or decisions in resource (agroecosystem) utilization or exploitation in obtaining products and making them available directly or indirectly to the society. It also describes the decisions or activities needed to improve the productivity of the human resource and the capability to sustain or improve the resource base, which will provide not only the society's needs but also improved environmental conditions.

These decisions or activities require a certain level of input of available or introduced technology and capital. The result of interactions occurring in the system forms the output of the systems and this will affect other systems

Management strategies

Based on the foregoing discussions it can be stated that development strategies are aimed at improving conditions of the coastal part of the Citarum watershed. Indicators of improved conditions of the coastal part of the Citarum watershed are among others increased production and minimized waste of the agro-ecosystem, and improved social and economic levels of the society.

Therefore, the development strategies are decisions or activities which will simultaneously increase production and improve the social and ecological conditions. These decisions are among others exploitation, handling, processing and recycling, distribution, regeneration, rehabilitation, conservation, education, technology and capital, protection, and aesthetics (see Fig. 4). These expressions may be formulated as follows:

maximize:

St = St-1 + ft (St-1, dt)

subject to

dt = decisions or activities,

(social development),

(ecological development),

where

St = production in period t
ft = function in period t
dt = (d1, d2,...,dn) = vector of decision variables

Description of the Coastal Part of the Citarum Watershed

Introduction

The coastal part of the Citarum watershed covers an area around 351 km˛. It forms a relatively flat area, with slopes of less than 5 per cent, the highest elevation being 2 m above sea level. Ninety-five per cent of the area has poor drainage and low water-infiltration capability. The land capability for crop production is identified as good, and the prevailing soil type (96 per cent) is alluvial.

The distribution for agriculture-related land-use amounts to:

agriculture: 17.5 %
forest: 46.6 %
aquaculture: 15.5 %
swamps: 17.6 %

The remaining ±3 per cent is the area used for human settlements.

Based on Oldeman's classification the agroclimatic type prevailing in the area is type E, with one to two wet months (rainfall greater than 200 mm) (Fig. 5). The average annual rainfall is 1,300 mm.

The water available in the area is used not only for agricultural purposes (in the broadest sense of the word) but also for domestic purposes. Aside from rainfall, the water supply is provided through channel systems from the Citarum River (Fig. 6). Specifically in the case of coastal aquaculture, sea water is provided through natural and manmade channel systems using tides as a driving factor (Fig. 7).

The population density in the area is 360 persons/km˛, and the population (1975) is 126,404, out of which 61 per cent forms the potential labour force, composed of 44 per cent males and 56 per cent females. The average annual population increase rate, estimated from 1971 to 1975, is 2.6 per cent.

Formal education is offered through general and religious primary schools. The latter are involved not only with religious and general education but also with vocational education relevant to the conditions in the area. Children can enter school from age six, but only 15 per cent of those eligible receive primary education. Rural social institutions such as village cooperative units are involved in the capital raising and technology dissemination necessary for resource exploitation, processing, and distribution activities, and also for sustaining and improving the resource base and the environmental conditions.

In general it can be stated that only in the field of agriculture, through government programmes, is the application of improved technology (e.g., the use of pesticides, fertilizers, and high-yielding and pest-resistant varieties) relatively intensive. This is also true in the case of making available the capital needed to obtain the technology. However, traditional technology and capital formation institutions are already rooted in the society of the area and should be considered in development.

The agro-ecosystems

1. Agriculture

Rice fields, of which 68 per cent are irrigated, constitute 80 per cent of the agricultural land of the area. The land capability of the area is identified to be good for the development of cash and food-crop agriculture. However, its topography, soil conditions, and elevation cause problems in its development. Sedimentation of drainage canals causes floods during the rainy season, and makes this area unsuitable for agriculture in the following season. The area is also potentially exposed to sodium hazards. However, improvement of channels, both irrigation and drainage channels, will increase the ricefield area. In addition, the use of fertilizers and pesticides increases the annual rice production per ha from 3.3 to 4.0 T. However, the poor infrastructure in the area limits the distribution of pesticides and fertilizers. Only 24 per cent of the farmers are exposed to these inputs.

Co-operative units and other institutions involved in processing and distribution of products are also involved in providing capital, pesticides, and fertilizers to the farmers.

FIG. 5. Climatic types in the downstream part of the Citarum watershed

FIG. 6. Kabupaten (County) Karawang irrigation network (1977)

FIG. 7. Types of potential aquatic resource utilization in Kabupaten (County) Karawang (1977)

2. Aquaculture

The condition of the area available for aquaculture only permits the development of brackish-water aquaculture, which is also termed tambak culture. The greater part (60 per cent) of the tambak culture is executed with the tumpangsari method in the mangrove forest.

Areas exposed to floods which become unsuitable for agriculture and recently accreted areas are potential areas for tambak culture. However, where improvements of irrigation and drainage channels are made, increasing the rice-field area and also areas exposed to marine erosion, the area of the tambak tends to decrease. The average annual production of the tambak ranges from 130 kg/ha to 200 kg/ha. This low productivity reflects the simple, traditional technology used and the insufficiency of both capital for proper care and seeds for the production process.

3 Animal husbandry

The animal husbandry in the area involves the raising of cattle, sheep, goats, pigs, and poultry (chickens and ducks). Large animals with a density of 0.3 head/ha are mainly used as draught animals in soil preparation. Piggeries are restricted to farmers of Chinese origin, and wastes from this activity are used in dry-land food-crop cultivation. Waste and by-products from the agriculture components form the principal feed for the animals raised.

Water supplies available in the area may be unsuitable for this production process, especially during dry spells. Vaccination and improvement of breed are conducted to increase the productivity of poultry and cattle.

4. Forestry

Mangrove forest forms the principal forest of the area, and it provides energy in the form of firewood for the inhabitants in the surrounding area. In mangrove areas land accretion is promoted and becomes an area of conflict of interest. The local inhabitants tend to transform this area into tambak, while the Forestry Service and the local government tend to preserve the area as a forest. However, there are concessions made to the local inhabitants to practice aquaculture using the tumpangsari method.

Programmes to rehabilitate and create a green belt along the shore have been established. Difficulties in implementing these programmes are caused by conflict of interest and ecological conditions inherent in certain localities (e.g., marine erosion caused by strong sea currents and wave action).

Alternative Decisions in Coastal Area Development

At the present stage of the study, the data base constructed from primary and secondary sources is not large enough for the important decisions on integrated utilization of the area's resources that are aimed at increasing production and improving the social and ecological conditions of the area. Therefore, the existing conditions as stated in the foregoing section and the reasoning behind the qualitative models expressed in Figures 1, 2, 3, and 4 were taken as the basis for the formulation of hypothetical decisions leading to the stated objectives.

These decisions are categorized as follows:

1. Decisions within the agro-ecosystem to increase production This category includes those decisions that are based on the interactions as shown in Figure 2. Thus, examples of decisions are

2. Decisions related to exploitation e.g.

3. Decisions related to handling e.g.

4. Decisions related to processing e.g.

5. Decisions related to distribution e.g.

6. Decisions within the society

Within this category are decisions aimed at improvement of ideas, activity, and productivity of human resources through

7. Decisions related to regeneration, rehabilitation, and conservation of the agro-ecosystem e.g.

8. Decisions related to recycling of supplementary outputs e.g.

9. Decisions related to protection and aesthetics e.g.

10. Decisions for the mobilization of technology, capital, and human resources from outside the coastal sub-system e.g.

Additional field work and data collection are necessary to

  1. verify the present models as presented in Figures 1, 2, 3, and 4;
  2. verify the above-mentioned decision categories;
  3. verify the type and degree of interactions existing in the area, and existing constraints.

Based on this information a set of decision alternatives can be formulated as a basis for developing a model for integrated resource utilization in the area. This model should become the guideline in resource utilization which minimizes wastes and maximizes production for each time span decided, and within existing constraints.

References

Adiramta, E. R.;Sunarto;Said Rusli; and E, Kusumah 1971 - 1972. Penggunaan teknologi baru oleh petani padi di Kabupaten Karawang. Laporan penelitian kerjasama Badan Pengendali Binas Nasional den Institut Pertanian Bogor.

Adiramta, E. R.; A. Gafur; Sujadi; and Hatomi 1971-1972. Menuju kearah diversifikasi usaha dalam Usahatani, suatu pendekatan dalam model pembangunan pertanian Kabupaten Karawang. Kerjasama Direktorat Perentjanaan den Pengembangan Direktorat Djendral Pertanian den Institut Pertanian Bogor.

Anwar, A., 1969. Wilayah potensi pertanian den sumber-sumber lain Daerah Kabupaten Karawang. Laporan Survey sebagai Landasan Penyusunan Pola Dasar Pembangunan Daerah Kabupaten Karawang. Kerjasama Karawang - Institut Pertanian Bogor.

__________1971. Karawang potensi pertanian dalam pembangunan regional. Proyek Kerjasama Pemerintah Daerah Kabupaten Karawang- IPB.

Azzaino, Z., and Soeharnis 1969. Wilayah potensi pertanian den sumber-sumber lain Daerah Kabupaten Karawang. Laporan Survey sebagai Landasan Penyusunan Pola Dasar Pembangunan Daerah Kabupaten Karawang. Kerjasama Karawang-Institut Pertanian Bogor.

__________1970. Faktor-faktor strategic jang harus diperhatikan dalam mentjiptakan pembangunan pertanian jang progresif di Lokalita Wadas dalam hubungan dengan perentjanean pembangunan Kabupaten Karawang. Suatu pendekatan decision making process dalam Lokalita. Proyek kerjasama Pemerintah Daerah Kabapaten Karawang-IPB.

Birowo, A.T., and A. Gafur 1973. Peranan pertanian dalam pembangunan ekonomi daerah di Kabapaten Karawang. Laporan penelitian kerjasama Direktorat Jendral Pertanian dengan Institut Pertanian Bogor.

Muluk, C.; S. T. H. Wardayo; Koesoebiono; E. Manan; D. R. O. Monintja; M. l. Effendie; and S. Sosromarsono 1976. Studi Penentuan Kriteria kualitas lingkungan perairan den biotik. Panitia Perumus den Rencana Kerja Bagi Pemerintah di Bidang Pengembangan Lingkungan Hidup. Proyek Pengelolaan Sumber- Sumber Alam den Lingkungan Hidup,

Ruddle,K., and T. B. Grandstaff 1978. The international potential of traditional resource systems in marginal areas. Technological Forecasting and Social Change 11: 119 - 131. Elsevier, New York.

Soewardi, B.; S. Ngadiono; Partoatmodjo; and M. Soekandar 1978. Studi pembinaan model pengelolaan wilayah Daerah Aliran Sungai. Buku I den Buku II. Panitia Perumus den Rencana Kerja Bagi Pemerintah di Bidang Pengembangan Lingkungan Hidup. Proyek Pengelolaan Sumber-Sumber Alam den Lingkungan Hidup.

Soewardi,B., and Ngadiono 1979. Watershed management: an analysis through modelling. Paper presented at the Programmatic

Workshop on Agro-Ecosystems in the Framework of Watershed Management, 18 - 20 June 1979, Bogor, Indonesia. Center for Natural Resource Management and Environmental Studies Bogor Agricultural University.

Sumawidjaja, K.; C. Muluk; T. H. Supomo; Wardoyo; Koesoebiono; Daniel R. O. Monintja; and G. W. Atmadja 1977. Survai ekologi perikanan Daerah Aliran Sungai: aspek-aspek penyelamatan perikanan di perairan umum. Bagian Il: Daerah Aliran Sungai Citarum. Proyek Penyelamatan Perairan Umum Direktorat Jendral Perikanan. Departemen Pertanian-Institut Pertanian Bogor.

Discussion

Burgers: Please explain the use and ownership of these resources.

Muluk: The use of water for aquaculture purposes is regulated by water resources regulations issued since the Dutch colonial period. Although disposal of waste into the water system is prohibited, we actually have no law yet. Piggeries cannot be located near the water system area due to religious reasons. On the ownership system we have land reform, which stated that one person cannot own more than 2 ha of land, but the water is public property. As you know, the accretion rate of land is very fast, particularly in the north coast of Java. The question is who owns that new land, the government or the people?

Hehuwat: Accretion is rapid, and no cadastral map exists, but strictly speaking, the new land up to a certain distance inland from the coastline is owned by the government. If we apply strictly the government law on the new area, then there will be a conflict with the local people.

Bird: Please explain the tumpangsari method.

Muluk: It is a system where the mangrove forest and aquaculture can be in the same area, so that two crops are cultivated from the same land; a kind of multiple cropping.

 

Environmental problems related to the coastal dynamics of humid tropical deltas

Eric C. F. Bird

Deltas are numerous and extensive on coasts with humid tropical (Koppen Am) climates, partly because the high runoff resulting from heavy rainfall supplies rivers with large quantities of sediment derived from the outcrops of deeply weathered rock formations that have developed under hot, wet conditions in the hinterlands, and partly because the prevalence of low to moderate wind energy, due to relatively weak wind action over coastal waters, has permitted the growth and persistence of these protruding depositional landforms. Studies have been made of the geomorphological, hydrological, and ecological features of a number of humid tropical-zone deltas (e.g., Unesco 1966), with Particular attention to the changes, both natural and man-induced, that take place on and around them (Verstappen 1964). The present paper reviews the coastal dynamics of humid tropical deltas in terms of environmental problems that have arisen in the course of man's development and utilization of these areas. It provides a basis for investigating these problems on the extensive deltaic coast east of Jakarta, where deposition from a number of rivers, including the Citarum, the Cipunegara, and the Cimanuk, with headwaters in the uplifted steep and high ranges to the south, has built up a broad deltaic lowland, with a seaward margin consisting mainly of swampy terrain fringed by narrow sandy beaches (Bird and Ongkosongo 1980).

Such deltas attained their present form during and since the Holocene marine transgression, which began about 20,000 years ago, when the sea was at least 100 m below its present level, and came to an end about 6,000 years ago with the attainment of the present stillstand. On humid tropical coasts, rapid and abundant fluvial deposition has generally offset the effects of submergence which elsewhere persist in the form of drowned valley mouths and coastal embayments. Drilling has shown that deltas are deep wedges of sediment, largely of fluvial origin, but with intercalations of marine sediment, mainly fluvial deposits reworked by waves and currents in the nearshore zone. The stratigraphy of a delta usually indicates a history of gradual or intermittent subsidence, evidently a localized isostatic response of the earth's crust to the accumulation of a large sedimentary load. Continuing subsidence, perhaps augmented by a slight rise of the world sea level, explains why sectors of deltas that are not still receiving sediment, either of fluvial origin or after marine reworking, commonly show active shoreline erosion. The internal structure and stratigraphy of a delta is of much scientific interest, and of economic importance in terms of the disposition of water-bearing, oil-bearing, or mineral-bearing formations; but in terms of environmental problems an understanding of the processes that are changing the delta surface and its seaward margins is of more practical value.

Delta Dynamics

Deltas are low-lying terrain, with gentle transverse gradients (a few centimetres per kilometre). River channels often divide into distributaries as they approach the sea, and distributary channels are apt to be variable in form and dimensions, waxing and waning through time, and subject to diversion and closure, especially at their mouths. The rivers carry water and sediment to the deltaic shore and out into the adjacent sea, the flow being related partly to fluvial discharge and partly to the effects of tidal action entering the river mouth and of waves and currents in the nearshore zone. In addition, the delta shoreline may be interrupted by tidal creeks, which are often relics of earlier distributary mouths cut off by deposition upstream. These inlets are subject to the regular ebb and flow of tidal sea water, but receive fluvial runoff and sediment occasionally during episodes when the delta is inundated by major river flooding.

Sediment delivered by rivers is usually a mixture of sand, silt, and clay; it is carried to the river mouths, especially during floods, and deposited in the form of channel shoals and offshore bars. The sand fraction is sorted out by wave action and distributed along the shore as beaches and spits by waves and associated currents. This longshore drifting is responsible for the deflection, and sometimes the closure, of tidal creeks along the delta margin. Nearshore waters are frequently discoloured by the discharged fluvial load of suspended silt and clay, which gradually settles in calm water environments offshore, or in inlets and embayments along the coast, where it builds up tidal mudflats colonized by mangroves.

The pattern of sedimentation is influenced by the positions of river mouths. Distributaries carrying a substantial sediment load develop lobes at their mouths Changes in the position of river mouths occur both naturally, as the result of deflection along the shore or diversion upstream, and as the result of engineering works. New deltaic lobes are initiated at the diverted or deflected outlet, and earlier lobes may then start to erode away. The deltaic shoreline is thus dynamic, prograding on sectors that are directly or indirectly supplied with sediment, and being cut back on sectors where the sediment supply has diminished. Studies of historical maps and charts, and successions of air photographs, in comparison with existing outlines, show the patterns of gain and loss along deltaic shorelines in the past, and can be used as a means of predicting where future changes are likely to occur.

The factors that influence delta dynamics may be summarized as follows:

  1. Water discharge, related to the incidence and pattern of rainfall in the river catchment. During episodes of flooding, large quantities of sediment move downstream and the salinity of water at the mouths of rivers and in the adjacent sea is diminished, while in relatively dry periods the lower reaches of the rivers may become brackish. Finer sediment, especially clay, remains in suspension in fresh water, but is flocculated and precipitated as the water becomes brackish Reduction of fluvial discharge, due to dam construction or diversion of rivers upstream, results in a diminished incidence of flooding, a reduced sediment yield, and increased salinity at the mouths of rivers.
  2. Fluvial currents, generated by river discharge, scour channels in estuaries and produce patterns of shoal deposition splaying outward through the nearshore zone. An outflowing current can act as a "breakwater," interrupting the longshore drifting of sediment by waves and currents, and resulting in accretion on the updrift side of a river mouth.
  3. Fluvial sediment yield, the nature and abundance of which is determined by the surficial geology of the catchment region. In the humid tropics, rock formations generally show deep weathering, due mainly to intense chemical decomposition under the prevailing warm and wet conditions. Granitic rocks and sandstone outcrops yield predominantly sandy sediment, whereas slates, shales, and volcanic rocks weather to yield silts and clays. The rate of sediment yield is a function of runoff, slope gradients, and the density of vegetation cover within the catchment. It can be increased by tectonic uplift or volcanic activity in the hinterland, or by reduction of the vegetation cover, either as the result of natural changes (landslides, bushfires, desiccation), or as the outcome of man's activities, especially deforestation and the introduction of grazing or cultivation. On the other hand, sediment yield can be reduced by the construction of weirs or dams that impound river water and trap sediment, or by the diversion of rivers into canal systems which disperse the sediment load. An increase in sediment yield from rivers is followed by coastal accretion, shallowing the nearshore zone and prograding the deltaic shoreline, while a decrease results in nearshore deepening and shoreline erosion. It should be noted that the sediment accumulating on a deltaic shoreline may include material carried in by wave action from the sea floor (the bulk of which is reworked fluvial sediment derived from preceding episodes of floodwater discharge) and material brought along the coast by longshore drifting from adjacent sectors (such as cliffed headlands or eroding coastal plains).
  4. Nearshore processes include the rise and fall of tides and currents associated with these movements, wave and current action generated by winds over coastal waters, and swell waves of distant origin which may reach the deltaic coast. Where the tide range is large on deltaic shores, broad inter tidal flats are exposed at low tide, and the associated strong tidal currents shape a complex shoal and-channel topography in estuaries and across the nearshore zone; wave effects are diminished, and shorelines develop an intricate, highly indented configuration, as on the shores of the Irrawaddy Delta, where the spring tide range attains 6 m. On most humid tropical deltas the tidal range is much smaller, and these features are less well developed. Wave action is important, first in sorting the sediment by dispersing silt and clay and concentrating the sand fraction in the nearshore zone, and then in carrying it shorewards and distributing it alongshore as beaches and spits. The outcome is a smoothing of the outline of the delta shore. Associated current action transports the finer sediment, silt and clay, until it reaches calm water, where it is deposited on shoals or in sheltered inlets and embayments.
  5. Shore vegetation, especially mangroves, which colonize the upper inter tidal zone on sectors of shoreline that are sheltered from strong wave or current scour, promote sedimentation and the accumulation of organic materials to stabilize the backshore in the form of a depositional terrace. Mangroves also colonize intertidal shoals in estuaries, building them up as depositional islands that divide the channel into distributaries. They readily colonize muddy substrates, and can also grow on stable sandy terrain within the upper inter-tidal zone; they are adapted to tidal conditions, each species showing variations in tolerance of depth and duration of submergence, substrate mobility, and salinity. Where a sediment supply is sustained, mangrove encroachment advances the shoreline and reduces inlets and embayments until the residual tidal creeks become well defined, and often deeper. Such encroachment is usually marked by a successional zonation of mangrove species, followed by a fresh-water swamp forest to the rear as sedimentation builds up the substrate to the limits of tidal submergence. Where the sediment supply is reduced, mangroves cease to spread, and may die back or be eroded away as nearshore waters deepen and the shoreline begins to retreat. If the mangrove fringe is cleared away by man, either to obtain timber and associated products or to establish access for boat landings, erosion of the previously deposited sediment ensues.
  6. Changes in kind or sea level may result from subsidence of the delta region due to isostasy or the compaction of sediments (especially peats) within the delta, to tectonic movements such as warping or tilting of the delta region, or to eustatic rise or fall of the world's ocean surface. Submergence impedes the growth of a delta, reducing the extent of depositional gains and initiating or accelerating shoreline erosion. Most deltas are subsiding, but if emergence occurred, as the result of localized tectonic uplift for example, shoreline progradation would be accelerated, and channels within the delta would become incised.

Environmental Problems

These geomorphological, hydrological, and ecological processes give rise to a number of problems for the people who develop and utilize land and water resources on deltaic coasts. In the humid tropics, most deltas have been intensively modified to sustain large human populations, and the problems of natural or man-induced coastal change are often severe. Some key problems are listed:

  1. Coastal erosion results in the loss of important resources such as the mangrove fringe, used as a source of timber and fuel and as a habitat from which fish, birds, and crustaceans may be obtained. As erosion proceeds, productive systems such as brackish fish ponds and rice fields, developed where former swampy terrain has been reclaimed, are invaded by the sea and rendered useless. Villages built immediately behind the beach are damaged, and must be relocated as the coastal terrain is cut back. Eventually, continuing erosion intersects and destroys older beach ridges and cheniers on which settlements and areas of dry-land crop cultivation have developed. As has been noted, such erosion is usually either the outcome of a change in the position of a channel mouth, or a reduction of fluvial sediment yield following construction of dams and weirs, or canals upstream. Attempts to halt shoreline erosion by building sea walls along the shore, or by putting in groynes in the hope of retaining a protective beach, are of little value. Suitable stone or concrete materials are rarely available, and wooden structures are soon washed away. As shoreline recession is the outcome of a progressive deepening of nearshore waters, and consequently an increase in the size of breaking waves, only very massive and expensive structures could maintain such a shoreline once this erosion has started. It is important to avoid actions likely to initiate erosion, such as clearance of the seaward fringe of mangroves or the dredging of nearshore areas, and to be aware that such upstream activities as weir and dam construction, the dredging or diversion of river channels, the building of artificial river levees to restrict flooding, or the excavation of canals for transport, drainage, or irrigation purposes, may initiate or accelerate shoreline erosion and produce environmental problems on the delta coast. Some of these activities may lead to progradation elsewhere, forming new coastal land to offset the losses by erosion, but this is not always the case, and where it is, the transference of settlements and agricultural/ aquacultural systems from an eroding to an accreting area raises many practical as well as social problems.
  2. Coastal deposition is a much less serious problem but it can have adverse effects for fishing communities based on the shoreline, and may impede navigation. It may be difficult to maintain sea-water inflow to brackish fish ponds where deposition shallows or seals the mouths of river channels and tidal inlets, and behind prograding sectors the fish ponds may have to be abandoned or converted to other uses as they become more remote from the sea water supply.
  3. Channel changes on estuaries and tidal inlets include lateral migration, which poses problems for riverside communities whose villages and farmland are undermined, and shallowing by sedimentation, which may lead to bank erosion to maintain the cross-sectional area necessary to conduct downstream flow. Shallowing also impedes navigation, and may diminish fishery resources.
  4. Salinity regimes are determined by the interaction of freshwater runoff and sea incursion, and have effects on sedimentation and the ecology of shore and nearshore organisms. Changes in the salinity regime occur when fluvial discharge is modified, or when the pattern of river mouths and tidal creeks alters. Such changes are followed by ecological responses in coastal vegetation and animal communities, including fisheries. An increase in salinity in the lower reaches of rivers can be damaging to rice fields, pastureland, and fresh-water vegetation; it can spoil the water supply available for domestic and irrigation purposes; and it may lead to the development of an excessive salt content in brackish fish ponds. A reduction in salinity is less harmful, since it results in a seaward migration of ecological zones, but it might raise problems in maintaining a sea-water supply to brackish fish ponds.
  5. Coastal hazards are particularly severe on deltas, because it is difficult on low-lying terrain to escape the effects of storm surges, tsunamis, and river flooding, which frequently kill many people and animals and damage or destroy buildings and other structures, farmlands, and fish ponds. In addition, control of pest and disease organisms is often difficult on deltaic coasts, particularly where they occupy habitats (e.g., mangrove swamps) that should be conserved for shoreline protection or as a nursery for fish and crustaceans. Man-made hazards include water pollution, especially in the vicinity of ports and industrial areas, and the effects of toxic chemicals, which are introduced to farmed areas to control pests and diseases in crops and which become adverse if they pass into estuarine and nearshore fisheries and brackish fish ponds. The possibility of a sea-level rise due to the warming of the world's climate (e.g., by the melting of arctic ice as a consequence of large-scale river diversions in Siberia) would have drastic effects on all deltaic coasts. In northern Java a sea-level rise of 1 or 2 m would permanently submerge the brackish-water fish ponds and lead to salinization of ricefield areas to landward. The loss of habitable land and agricultural productivity would be severe in terms of living standards in this region.

Conclusion

There is thus a wide range of environmental problems related to the coastal dynamics of humid tropical deltas dynamics which, in turn, are the outcome of an interacting system of geomorphological, hydrological, and ecological processes. This system has been greatly modified by man's activities in these densely populated and intensively utilized deltaic regions. An understanding of the dynamics of humid tropical deltas is necessary as a basis for coastal management and land use strategies designed to maintain the productivity of these areas, and to provide the information needed to solve the environmental problems that have arisen. This Programmatic Workshop and Training Course aims to promote this understanding, and to initiate research on the dynamics of the coastal fringe of a Javanese delta.

References

Bird, E. C. P., and O. S. R. Ongkosongo Environmental changes on the coasts of Indonesia. UN University. (In preparation.)

Unesco 1966. Scientific problems of humid tropical zone deltas and their implications. Proceedings of the Dacca Symposium, p. 422.

Verstappen, H, T. 1964. Geomorphology in delta studies. 1. T. C. Publications, B 24, Delft, p. 24.

Discussion

Collier: On several ports of the Java deltas, tambaks are being constructed on the mudflats as soon as they accumulate, before they are colonized by vegetation. What effects could this have on delta dynamics?

Bird: Unvegetated delta shores are less stable than those with a mangrove fringe. I would expect these tambaks to suffer storm damage, and to be readily eroded away if there is a change in river mouth positions.

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