Browsing by Author "Gunaratne, GL"

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item: Article-Abstract
Effect of inlet morphometry changes on natural sensitivity and flushing time of the Koggala lagoon, Sri Lanka
Gunaratne, GL; Tanaka, N; Amarasekara, P; Priyadarshana, T; Manatunge, JMA
The changes of natural sensitivity in terms of lagoon morphometry, particularly the inlet morphometry and flushing properties of the Koggala lagoon, Sri Lanka, are presented in this paper. The morphometric assessment is based on analysis and consideration of morphometric characteristics for three scenarios of the lagoon, namely: (I) before the construction of the old groyne system or time period before 1996 (Koggala Scenario I-KS I); (2) existing situation or time period after 2007 (Koggala Scenario 2-KS2); and (3) future scenario with respect to proposed groyne interventions for the mouth width of 20 m, as proposed by Gunaratne et al. (J Environ Sci 22(6):813-819, 2010) (Koggala Scenario 3-KS3). The morphometric assessment provides a simple means of defining natural sensitivity or vulnerability of individual scenarios of Koggala lagoon to external loads and anthropogenic activities. We found that KS I and KS3 scenarios are relatively more sensitive than KS2, whereas KSI has the highest natural sensitivity, and KS2 has the lowest natural sensitivity or the most robust conditions. In quantifying the tidal flushing of Koggala lagoon, the concept ofThe changes of natural sensitivity in terms of lagoon morphometry, particularly the inlet morphometry and flushing properties of the Koggala lagoon, Sri Lanka, are presented in this paper. The morphometric assessment is based on analysis and consideration of morphometric characteristics for three scenarios of the lagoon, namely: (I) before the construction of the old groyne system or time period before 1996 (Koggala Scenario I-KS I); (2) existing situation or time period after 2007 (Koggala Scenario 2-KS2); and (3) future scenario with respect to proposed groyne interventions for the mouth width of 20 m, as proposed by Gunaratne et al. (J Environ Sci 22(6):813-819, 2010) (Koggala Scenario 3-KS3). The morphometric assessment provides a simple means of defining natural sensitivity or vulnerability of individual scenarios of Koggala lagoon to external loads and anthropogenic activities. We found that KS I and KS3 scenarios are relatively more sensitive than KS2, whereas KSI has the highest natural sensitivity, and KS2 has the lowest natural sensitivity or the most robust conditions. In quantifying the tidal flushing of Koggala lagoon, the concept of The changes of natural sensitivity in terms of lagoon morphometry, particularly the inlet morphometry and flushing properties of the Koggala lagoon, Sri Lanka, are presented in this paper. The morphometric assessment is based on analysis and consideration of morphometric characteristics for three scenarios of the lagoon, namely: (I) before the construction of the old groyne system or time period before 1996 (Koggala Scenario I-KS I); (2) existing situation or time period after 2007 (Koggala Scenario 2-KS2); and (3) future scenario with respect to proposed groyne interventions for the mouth width of 20 m, as proposed by Gunaratne et al. (J Environ Sci 22(6):813-819, 2010) (Koggala Scenario 3-KS3). The morphometric assessment provides a simple means of defining natural sensitivity or vulnerability of individual scenarios of Koggala lagoon to external loads and anthropogenic activities. We found that KS I and KS3 scenarios are relatively more sensitive than KS2, whereas KSI has the highest natural sensitivity, and KS2 has the lowest natural sensitivity or the most robust conditions. In quantifying the tidal flushing of Koggala lagoon, the concept of flushing half-life (Tso% h) was adapted as the optimum measure of flushing time. Flushing half-life was calculated for KS2 and KS3 scenarios. The flushing half-life ranges from 9 to 37 h (1.5 days) for KS2 and from 12 to 72 h (3 days) for KS3. Flushing half-life suggests that the exchange rate between the lagoon and the ocean is controlled by the balance between fresh water inflow and the seawater inflow. Applications of the morphometric classification and flushing half-life presented in this article can be used to help guide management and policy-making decisions for the coastal environment of Koggala lagoon and other coastal water bodies of Sri Lanka.
item: Article-Full-text
Restoration of Koggala lagoon: Modelling approach in evaluating lagoon water budget and flow characteristics
(Elsevier and Science Press., 2010) Gunaratne, GL; Tanaka, N; Amarasekara, P; Priyadarshana, T; Manatunge, J
Groyne system modification is described related to restoration efforts to Koggala lagoon, Sri Lanka. The large-scale unplanned sand removal at the lagoon mouth shifted the formation of sand bar towards the lagoon and made adverse effects on its ecosystem. After the removal of the natural sand bar, groyne system was constructed to avoid sand deposition in the lagoon and to protect the highway bridge (across the lagoon outlet channel) from the wave attack. The existing groyne system resulted the lagoon mouth being permanently open to sea which in turn led to many environmental problems. Groyne system modification is proposed in this study to reduce the sea water intrusion. Water budget and two-dimensional depth averaged hydrodynamic model were developed for understanding the hydrologic and flow characteristics of the lagoon. Numerical experiments was performed at lagoon mouth area for two cases: (1) existing condition and (2) proposed rubble mound groyne system condition. Comparison of results was obtained for both cases to describe flow pattern at lagoon mouth. Results further showed, the width should be reduced to a maximum of 40 m. Proposed mouth width (40 m) pushed the salting factor towards 0.5 from 0.68. Salting factor reduction with the groyne modification may result a predominant influence of fresh water which may in turn lead lagoon to a fresh water ecosystem.
item: Conference-Full-text
Water balance and renewal time of rekawa lagoon, Sri Lanka; a restorative approach
(2013-11-19) Gunaratne, GL; Priyadarshana, T; Manatunge, JMA; Tanaka, N; Yasuda, S
Rekawa Lagoon is a chocked and shallow coastal water body located in the southern coast of Sri Lanka. It is relatively unusual in that the major freshwater input, Kirama-oya river connects through the constricted channel much closer to the inlet at seaward end. A causeway was constructed, around 700 m from the lagoon inlet to the inland, across the constricted channel with an effort to link a secluded Kapuhenwala village with the rest of the area which in turn greatly reduced the volume and speed of water entering and leaving the lagoon system. Construction of the causeway led to many environmental problems with poor flushing efficiency and hampering to and fro movement of the prawns in the lagoon. The aim of this study was to evaluate the present situation and propose alternative management scenarios for improvement of water flow and lagoon ecosystem. The implications of different development stages of the causeway were discussed in terms of field measurements supported by modeling to describe the water balance and the water renewal time. The alternative of modifying the existing causeway was proposed to increase the free water flow at the inlet that favors recruitment of juvenile shrimp species.