Effect of inlet morphometry changes on natural sensitivity and flushing time of the Koggala lagoon, Sri Lanka

Abstract

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.