Tilak Priyadarshana

Restoration of Koggala lagoon: modelling approach in evaluating lagoon water budget and flow characteristics.

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.

Mud Crab (Scylla serrata) population changes in Koggala Lagoon, Sri Lanka since construction of the groyne system

This study was conducted to determine the present status of the Mud Crab (Scylla serrata Forskal) population in Koggala Lagoon, including details on size at maturity, sex ratio and carapace width - body weight relationship. This Crab is the most common edible Crab species found in the lagoon, comprising 93.10% of total catch from crab pots and 89.86% of total catch from crab nets. Sex ratios (male:female) were 1:0.86 and 1:0.82 in crab pots and nets, respectively. Female crabs of various sizes were randomly sampled to determine size at maturity. The results indicate that size at maturity of females was 12.17 cm carapace width. Size at maturity of males could not be determined using external characteristics. Population parameters were estimated from length-frequency data collected in 2009 and 2011. Asymptotic carapace length (L∞) and growth constant (K) were estimated to be 20.12 cm, 0.91 yr−1 in 2009 and 19.27 cm, 3.2 yr−1 in 2011, respectively. Exploitation rates were 0.32 in 2009 and 0.44 in 2011. There was a significant difference in Crab size between harvests by the crab nets versus traditional crab pots. Results indicate that S. serrata population is declining due to over-fishing as of increased fishing gear efficiency from traditional crab pots to modern crab nets. Commercial catches of crab net and crab pots comprised 85.7% and 72.0% of immature females, respectively, based on the size at first maturity revealed by this study. In order to maintain natural health of the Mud Crab population, immediate management action is needed to regulate fishing in Koggala Lagoon.

The relationship between significant wave height and Indian Ocean Dipole in the equatorial North Indian Ocean

Based on reanalysis data, we find that the Indian Ocean Dipole (IOD) plays an important role in the variability of wave climate in the equatorial Northern Indian Ocean (NIO). Significant wave height (SWH) in the equatorial NIO, especially over the waters southeast to Sri Lanka, exhibits strong interannual variations. SWH anomalies in the waters southeast to Sri Lanka correlate well with dipole mode index (DMI) during both summer and autumn. Negative SWH anomalies occur over the oceanic area southeast to Sri Lanka during positive IOD events and vary with different types of IOD. During positive prolonged (unseasonable) IOD, the SWH anomalies are the strongest in autumn (summer); while during positive normal IOD, the SWH anomalies are weak in both summer and autumn. Strong easterly wind anomalies over the southeast oceanic area of Sri Lanka during positive IOD events weaken the original equatorial westerly wind stress, which leads to the decrease in wind-sea waves. The longer wave period during positive IOD events further confirms less wind-sea waves. The SWH anomaly pattern during negative IOD events is nearly opposite to that during positive IOD events.