Mechanisms and seasonal variability of sediment transport in a small tropical mountainous estuary-coastal system: Insights from the Selangor River, Malaysia

Abstract

Small- and medium-sized mountainous rivers (SMRs) are critical pathways for delivering terrestrial sediment to coastal systems, particularly in the tropical regions where their dynamics are highly sensitive to anthropogenic influences. This study investigates the sediment transport processes and ultimate deposition patterns in the Selangor River estuary, Malaysia - a representative tropical macrotidal estuary characterized by complex nearshore topography and weak Coriolis effects. By combining field measurements (2023–2024) with sediment analysis, this research reveals three key findings. First, approximately 73.5 % of the annual sediment discharge (19.75 × 10⁴ t) accumulates in estuarine tidal flats, with the northern tidal flats receiving twice the deposition of the southern tidal flats. Second, distinct seasonal transport regimes were evident: (i) during the dry season wave-driven sediment resuspension and tidal forces result in northwestward-southeastward transport, (ii) during the wet season stratified flows lead to significant sediment-freshwater decoupling, where river plumes disperse offshore while terrestrial sediments remain trapped near the estuary. Third, the unique sediment trapping mechanism, different from both large river systems and wave-dominated tropical SMRs, resulting from the interaction of strong tidal currents, seasonal river discharge, and offshore water intrusion form the Strait of Malacca. These findings provide a new conceptual framework for understanding sediment retention in tropical macrotidal estuaries, with implications for coastal management under climate change and intensifying anthropogenic pressures.