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.