Seasonal Closure of Chilaw Inlet in Sri Lanka -Physical Processes and Mathematical Modelling-

Abstract: Chilaw inlet, with the affiliated lagoon, is located on the west coast of Sri Lanka about 120 km north of the capital Colombo. Each year during the summer monsoon, which lasts from May to September, the inlet closes due to a sand spit forming across the inlet. The closure results in impeded access to the sea for the fishermen, who use the lagoon as anchorage for their boats, and in deteriorated water quality in the lagoon. The theory behind inlet closure was studied in the present investigation in order to understand the governing physical mechanisms related to closure at Chilaw inlet. A measurement campaign, lasting from May to July (2006), was also performed resulting in topographic maps describing the morphological changes at the inlet during the summer monsoon. Increased longshore sediment transport during the summer monsoon 2006 and cross-shore sediment transport from an ebb shoal, located seaward of the inlet, initiated spit formation, which by early July led to an inlet closure. By the end of July a breaching of the spit occurred due to a combination of decreased longshore sediment transport and large amounts of wave energy working on the spit. Based on wave data and topographic maps two mathematical models were developed, one basic and one more advanced. The sediment pathways at the inlet were identified and quantified, and through a reservoir-type model the spit growth was simulated at the inlet. Both models described the observed spit growth fairly well but, since the advanced model involves more assumptions and coefficients than the basic model, the latter on is judged to be the most reliable and suitable with regard to the data available. In order to obtain a better result with the advanced model more physical processes have to be measured and quantified.