Influence of manmade effects on geomorphology, bathymetry and coastal dynamics in a monsoon-affected river outlet in Southwest coast of Sri Lanka

Abstract

A complex coastal environment with a river outlet, a sand spit barrier and an estuary of such coastal features is controlled by wave-induced hydrodynamics and sediment dynamics. The coastal dynamics patterns can be changed by natural or anthropogenic activities. This study is mainly focused on understanding the significant changes of geomorphology, bathymetry, and the coastal dynamics originated due to human influences in a monsoon-affected Kalu Ganga (River) outlet in Sri Lanka. In this research project, bathymetric variations before and after the collapse of sand spit barrier were analysed to understand the extent of the erosional and depositional effects. High-resolution satellite images in Google Earth Pro were also used to qualitatively analyse beach boundary changes before the collapse of the sand spit barrier. Temporal and spatial changes of beach boundary positions after collapse of the sand spit barrier were measured using high precision GPS surveying in river outlet including the evolving sand spit barrier. Nested wave model (Delft3D modelling suite) was applied to understand the wave climate changes. The data obtained from all methods were analysed to understand the geomorphological, bathymetric and coastal dynamic changes of the study area. The results show significant and widespread deepening of bathymetry up to 1–2 m extending as far as 2 km offshore from the river outlet initiated after the collapse of the sand spit barrier. Further, the study shows separation from the mainland and buckling of the sand barrier was initiated by anthropogenic activity coupled with the release of riverine floodwater and strong waves during southwest monsoon season. The weakened and buckled sand spit barrier was migrated and welded to the mainland during calm weather of northeast monsoon. This has resulted complete change of coastal dynamics in the Kalu Ganga (River) outlet area. Since the sediment dynamics and hydrodynamics completely changed, even after 2 years, the sand spit barrier across the river outlet has not been recreated naturally. Therefore, important structures in Kalutara coastal area were threatened due to severe erosion. This study shows understanding such coastal morpho-dynamic and hydrodynamic changes are vital to implement proper coastal prevention management strategies.