Sediment Transport Processes in the Kelani River Basin, Sri Lanka: Formation Process of Bed Material Size Distribution

This study investigates sediment transport processes in the Kelani River Basin, Sri Lanka, focusing on the formation of bed material sediment size distributions. Sediment transport processes during the flood events in 2016 and 2018 are evaluated using two approaches: assuming an equilibrium condition (Case 0) and evaluating basin-scale sediment transport using a distributed Rainfall–Sediment–Runoff (RSR) model based on the unit channel (Cases 1 and 2). Case 1 considers sediment transport only inside river channels, while Case 2 considers sediment supply at upstream end unit channels. The results indicate significant sediment deposition in the downstream reach, particularly downstream of Location 7. In Case 1, the sediment size distribution downstream rapidly coarsens, while in Case 2, considering sediment supply, the bed sediment size distribution downstream is close to the observed one, regardless of flood magnitude. This suggests that with sufficient sediment supply, the bed sediment size distribution forms based on channel conditions such as width and slope. Case 0 shows a similar trend in sediment transport rate to Case 2, demonstrating the applicability of this simple approach. In conclusion, this study has revealed the formation process of the present sediment bed conditions, which provides insight into effective and sustainable river management, including sand mining activities.