Monsoon Influence on Longshore Sediment Transport and Budget Imbalance along the Western Coast of Sri Lanka: A Source-to-Sink Analysis

Abstract

Coastal environments are dynamic systems shaped by the interplay of marine processes, terrestrial inputs, and human activities. The present study investigates the spatial and temporal variability of monsoon-driven sediment dynamics along the Western Coastal Region of Sri Lanka, with particular attention to longshore sediment transport, riverine sediment supply, coastal geomorphology, and sediment budget conditions. An integrated methodological framework was employed, including thermoluminescence analysis of feldspar grains to trace sediment transport pathways, estimation of sediment discharge from Western Coastal rivers, assessment of coastline characteristics using indentedness indices, and sediment budget analysis at both meso-cell and regional scales. Seasonal variations resulting from monsoonal wave climate and river flow were also incorporated. The results demonstrate a dominant northward longshore sediment transport regime, as indicated by decreasing thermoluminescence intensities with increasing distance from sediment sources. Riverine inputs, especially from the Kalu River, constitute the primary source of coastal sediment supply. Nevertheless, excessive river sand mining has substantially reduced sediment availability, with extraction rates exceeding natural replenishment rates. Geomorphological analysis identifies a predominantly straight coastline, which promotes efficient sediment redistribution under swell-wave conditions. Sediment budget analysis reveals a shift from historical equilibrium to current deficits across all coastal cells, attributed to reduced fluvial inputs, offshore losses linked to sea-level rise, and disruptions from coastal infrastructure. Seasonal analysis further indicates that peak sediment discharge during monsoonal periods coincides with high-energy wave conditions, thereby limiting effective beach accretion. These findings indicate that the Western Coastal Region sediment system has lost its natural equilibrium and is experiencing ongoing degradation. Current mitigation measures, such as beach nourishment, offer only temporary relief and do not address the fundamental sediment imbalances. The study highlights the need for integrated, source-to-sink coastal management strategies, including regulating sand mining, restoring sediment pathways, and incorporating climate change considerations into coastal planning. This research advances the understanding of sustainable sediment management in monsoon-dominated coastal environments.