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Động lực bồi tụ, xói lở bờ và sự thay đổi hình dạng đảo san hô Trường Sa - Deposition and erosion dynamics and shape change of the Spratly coral island
The risks of coastal erosion and sedimentation are deeply anxious for many local authorities and communities. Despite a lot of limitations, the archived research results have established a scientific basement to orient the further investigation, and some initial resolutions of controlling these risks have been proposed.
Increasing intensity and suddenness, the coastal erosion has expanded on the whole country, and in all coastal geosystems such as open coasts, deltas, estuaries, lagoons and islands. The risks of unusual sedimentation such as siltation of shipping channels, filling river mouths and lagoon inlets, and moving sand dunes have been also increased remarkably. The deep causes of sedimentation and erosion risks are related to coastal evolution, meteo-hydrology factors and human impact in the both coastal zone and catchment. The endogenous dynamics play the basement role, and are deep cause, generally. The exogenous dynamics are often principal and direct cause relative to the litho dynamic processes, which make concentration or dispersion of the sediments. In the many situations, the sedimentation risks are accompanied and caused by sedimentary supplied from neighboring. The tendency of coastal erosion and sedimentation will be very complicate in the future.
These risks have made the very heavy consequences, included damages of human beings, properties and land; degradation of coastal environment and ecosystems; unsustainable development with the investment of low effectiveness and small scale; emigration; and unstable thought in life and production of communities. From long time, the control of them has been interested. However, the implemented resolutions have been passive, responded to concrete situations and lacked definite scientific bases. The existing researches have exposed a lot of limitations, especially applied availability. The control of these risks is a very complicated task, and investment for research needs to be strengthened for responding to the requirements in practice.
The control of coastal sedimentation and erosion need to be implemented by the synchronous and comprehensive resolutions, from macroscopic to concrete range, direct and indirect, construction and non-construction, and soft and hard constructions, corresponding to each coastal site. A comprehensive resolution must be in the framework of integrated coastal management, combining with catchment management, and to have both immediate responses and long-term strategy. The priority is given to the construction resolutions combined with other benefits and a special importance is paid to the combination of controlling coastal erosion and sedimentation. It needs to be applied modern resolutions, and building the resolutions suitable to Vietnam situation. It is also necessary to strengthen legislative basement for coastal protection; to build data base; and to establish a network of observing and monitoring coastal erosion and sedimentation regularly for the purpose of detecting and warning risks, and having the timely decisions.
During last decades, coastal morphology of Vietnam have been changed deeply by the deposition and erosion processes. It can be distinguished 8 types of coastal morphological changes (CMC). They have been created by the both natural evolution and human activities direct and indirect. Recently, the unusual changes in meteorology, hydrology and sea level rise have taken an important role for CMC. Apart from the increase of the land resource in the deltas, most of CMC has made the negative impact on coastal inhabitants, economy and environment. Linked with CMC there have existed the serious coastal risks such as floods and inundations, erosion and port siltation. CMC is very important for the social - economic development of a developing country as Vietnam.
Truong Sa Islands consists of the complex systems of coral reefs developed on the positive landforms on seabed, with most of their bases located at 1500-2500m depth where the earth crust of style continent is about 24-26km thick. In geneal, the coral reef system in Truong Sa Islands is developed in multiform and rich in style, including atolls, coral banks, coral shoals, semi-submerged coral reefs and coral iIslands/cays.
In terms of morphology, a emerged-island includes island part elevated above sea level , island coast, reef platform, underwater cliff and slope, and seabed. At the submerged coral reefs, there exist the lagoons often seen at the middle of the reef, reef platform, underwater cliff and slope, and seabed. Every kind of landform is featured by its own morphological characteristic.
The surface of the emerged island is covered by coral limestone, coral clastic sedimentary rocks and unsolided coral sediments. Especially, at some islands, Guano with high P205 has been found. In Truong Sa Island, in the lime composition of coral bone, the mineral Aragonite has been replaced by Calcite, suggesting that there is a strong environmental condition change in this area, as consequence leading to re-crystallise of the mineral. The sediment in coral reefs include bounder, large peblles- bounder, medium pebbles, small pebbles, large sand, medium sand, and small sand.
Underground fresh water exists at some islands (Islands of Truong Sa Nam Yet etc.), and can be used for living and domestic activities. The underground water table has seasonal variability, and daily fluxtuation with tidal phase.
The area and shape of island surface varies with the meteo-hydrological conditions of areas, increase in summer and decrease in winter. The change of the island’s profile and morphlogy occurs in 4 trends: tidal hour, the tidal cyle, the monsoon and long-term change.
The Islands are living habitats for some typical marine ecosystems in tropical sea such as corals (367 species, belonging to 84 genus, 26 families), sea-grasses (5 species), and other marine fishery resources (524 fish, 111 seaweed, 265 species of invertebral animal, and 5 species of sea turtles).
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