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.
