Impact of rubble mound groyne structural interventions in restoration of Koggala lagoon,Sri Lanka; numerical modelling approach |
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Authors: | Gayan Lakendra Gunaratne Norio Tanaka G P Amarasekara Tilak Priyadarshana Jagath Manatunge |
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Institution: | (1) Institute for Environmental Science and Technology, Graduate School of Science and Engineering, Saitama University, 255 Shimo-okubo, Sakura-ku Saitama, 338-8570, Japan;(2) Faculty of Fisheries and Marine Sciences & Technology, University of Ruhuna, Matara, Sri Lanka;(3) Department of Civil Engineering, Faculty of Engineering, University of Moratuwa, Moratuwa, Sri Lanka; |
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Abstract: | 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. |
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