Tsunamis Hurricanes And Neotectonics As Driving Mechanisms In Coastal Evolution

Author : Silva, E. I. L.
Publisher : IWMI
ISBN 13 : 9290907789
Total Pages : 126 pages
Book Rating : 4.2/5 (99 download)

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Download or read book Lagoons of Sri Lanka written by Silva, E. I. L. and published by IWMI. This book was released on 2013-03-01 with total page 126 pages. Available in PDF, EPUB and Kindle. Book excerpt: Sri Lanka, an island in the Indian Ocean, has lagoons along 1,338 km of its coastline. They experience low-energy oceanic waves and semidiurnal microtidal currents. The Sri Lankan coastal lagoons are not numerous but they are diverse in size, shape, configuration, ecohydrology, and ecosystem values and services. The heterogeneous nature, in general, and specific complexities, to a certain extent, exhibited by coastal lagoons in Sri Lanka are fundamentally determined by coastal and adjoining hinterland geomorphology, tidal fluxes and fluvial inputs, monsoonal-driven climate and weather, morphoedaphic attributes, and cohesive interactions with human interventions.Most coastal lagoons in Sri Lanka are an outcome of mid-Holocene marine transgression and subsequent barrier formation and spit development enclosing the water body between the land and the sea. This process has varied from one coastal stretch to another due to wave-derived littoral drift, sediment transport by tidal fluxes, fluvial inputs and wave action or, in other words, sea-level history, shore-face dynamics and tidal range as the three major factors that control the origin and maintenance of the sandy barrier, the most important features for the formation and evolution of coastal lagoons with their landward water mass. In certain stretches of Sri Lanka’s coastline, formation of the barrier spit was very active due to shore-face dynamics that resulted in chains of shore parallel, elongated lagoons. They are among the most productive in terms of ecosystem yield and show some similarities to large tropical lagoons with respect to sea entrance, zonation, biodiversity and ecosystem services. However, some of them become seasonally hypersaline due to lack of freshwater input and high evaporation. Functions and processes of some of these water bodies are fairly known. There are a fair number of small back-barrier lagoons of different shapes and sizes whose origin goes back to sea-level history. They are located on low-energy coasts with prominent beach ridges and restricted hinterland geomorphology. Mixing processes of these landward indentations are hindered by elevated sand dunes, and their salinity increases due to poor freshwater input and high evaporation leading to seasonally hypersaline conditions. These sedimented lagoons, primarily confined to the southeastern coast of the island, are biologically the least productive, with limited ecosystem values and services. Another group of moderately elongated semicircular, slightly large lagoons in the same coast, formed exclusively by submergence due to mid-Holocene sea-level rises, do not receive sufficient freshwater input leading to seasonally hypersaline conditions. They are also biologically unproductive but some are ecologically important since they provide habitats conducive to migratory birds. In contrast, some lagoons on the southern coast receive sufficient freshwater via streams draining the wet zone, maintain more estuarine salinities, exhibit rich biodiversity and serve as functional resource units. Lagoons formed by mid-Holocene submergence and recession of water level with simultaneous chain barrier formation on the high energy southwest coast, which includes cliffs, small bays and headlands, show peculiar configurations and link channel characteristics. Some of these irregular water bodies have clusters of small isles and luxuriant mangrove swamps with high biodiversity but not very rich in catadromous finfish and shellfish species due to the restricted nature of the entrance channel and nondistinct salinity gradients. The barrier-built, seasonally hypersaline lagoon complex in the Jaffna Peninsula, the largest lagoon system in the country with multiple perennial entrances show extremely narrow salinity ranges towards the upper limit of salinity. The main lagoon is elongated and the shore parallel to eastward and southward extensions is connected by narrow channels. The other lagoon in the Jaffna Peninsula is elongated, shore parallel and ribbon-shaped and receives tidal water throughout the year but freshwater is received only from precipitation and surface runoff. Even though the lagoons in the peninsula are extremely rich in ecosystem heterogeneity their hydrology and hydrodynamics have been severely disturbed by infrastructural development for transportation and by attempts to create a freshwater river for Jaffna. There are a few virgin lagoons of moderate size also on the northern coast, south of the Jaffna Peninsula on both the east and west sides. They look very typical tropical lagoons rich in biodiversity and biological production but their structure, functions and values are virtually unknown in scientific or socioeconomic terms. The lagoons located on the east coast are not numerous but relatively large in extent. They are also an outcome not only of mid-Holocene sea-level rises but of submerged multi-delta valleys or abandoned paleo estuaries. When inundated, the multi-delta valley configuration became elongated and is shore parallel with a smooth seaward shoreline; both shorelines become irregular when coastal waves are weak, and internal waves are created by the action of local winds. Configuration of a lagoon formed by inundation of an abandoned river valley is irregular with a long entrance channel extended landward. These lagoons are highly productive with a variety of associated ecosystems, large open water areas and wide perennial sea entrances. When the lagoon is too much elongated, zonation is prominent due to fewer entrance effects. Lagoons form a particular type of natural capital which generates use values (fish, shrimp, fuelwood, salt, fodder, ecotourism, anchorage, recreation, etc.) and nonuse values (habitat preservation, biodiversity, ecosystem linkages, etc.) contributing positively towards improving the human well-being. Of many values of lagoons in Sri Lanka, only the extractive values are generally utilized at present, by way of fish and shrimp catches, salt production and use of mangrove for various purposes. Besides, coastal lagoons generate a range of nonextractive use values and nonuse values, which could add towards the total economic value. Misuse has taken place at several instances when “use” adversely affects the status of the resources or the health of the ecosystem due to vulnerability and poverty, population pressure, urbanization, development activities and multi-stakeholder issues. The status of lagoon resources shows that the resources in the majority of Sri Lankan lagoons still remain satisfactory, somewhat good or very good. Nevertheless, concerns for management of lagoons in Sri Lanka exist only where “use values” (extractive values, such as fish and shrimp) exist. There is no evidence of resources management in lagoons for inspirational, scholarly values or tacit knowledge of the same. Management for use values exhibits several stages from zero management to comanagement via community management and state intervention. Most of Sri Lanka’s lagoons have the potential for generating high extractive and nonextractive use values which could improve the human well-being, while maintaining resources sustainability. Unfortunately, these potentials have not been understood or “seen” yet by the relevant authorities, although a few instances of exploring this potential were noticed.