Browsing by Author "Amarasekara, GP"

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item: Conference-Full-text
The Current status of density stratification of Koggala lagoon
(2013-11-12) Furusato, E; Amarasekara, GP; Priyadarshana, T; Tanaka, N
A field observation was conducted on 22nd November 2011 to estimate the present state of density stratification of Koggala lagoon. Vertical variation of salinity, water temperature and dissolved oxygen were measured with neap and ebb. The same measurements were taken from inflow streams. The observations indicate that main water body of the lagoon is salinity stratified despite the strong or partial mixing at the mouth. Particularly, strong saline stratification is prominent in the deep central part of the lagoon. Firstly, the current hydraulic state and then bulk parameter of Koggala lagoon is analysed. The relationship between the mixing state and the values of parameters of Koggala lagoon agree with Fisher’s plot. The effects of modification of lagoon mouth are discussed based on these parameters. Temporal and spatial complexities of lagoon mouth affect transport and mixing of saline water and intrusion into the lagoon. In future, a modified bulk parameter will be needed for better understanding of stratification behaviour of the Koggala lagoon.
item: Article-Abstract
Impact of rubble mound groyne structural interventions in restoration of Koggala lagoon, Sri Lanka; numerical modelling approach
Gunaratne, LG; Tanaka, N; Amarasekara, GP; Priyadarshana, T; Manatunge, JMA
Physical processes of the lagoon are influenced by structural interventions. Understanding the complex reality of physical processes sometimes difficult with field observations thus a model provides a simplified abstract view. Two dimensional hydrodynamic model is used to describe, restoration efforts to Koggala lagoon, a combined freshwater and estuarine complex of rich ecosystem on the southern coast of Sri Lanka. The lagoon mouth was naturally closed by a sand bar which controlled the seawater intrusion. Due to large-scale sand removal at lagoon mouth, formation of the sandbar shifted towards the lagoon. After the removal of natural sand barrier, rubble mound groyne structures were built to avoid sand deposition in the lagoon and to protect the highway bridge from the sea wave attack. Construction of the groyne resulted in the lagoon mouth being permanently open which in turn led to many environmental problems with saline intrusion. The aim of this study is to evaluate the current situation of the lagoon and propose alternative structural interventions for minimization of seawater intrusion and subsequently improve lagoon ecosystem. Hydrological parameters were investigated and mathematical models for hydrodynamic behavior of the lagoon were applied in order to describe the lagoon physical processes and flow characteristics. Existing rubble mound structures were redesigned in order to minimize the seawater intrusion. Numerical simulations were carried out for two different mouth widths (40 m and 20 m) with appropriate structural interventions. Existing salting factor for the lagoon is 0.68 and numerical simulation results showed salting factor for 40 m and 20 m openings are 0.61 and 0.54 respectively. This shows the mouth width can be reduced up to 20 m in order to obtain a slating factor close to 0.5, which indicates the predominant influence of fresh water which in turn leads lagoon to a fresh water ecosystem.