The present study describes a river channel management method for restoring riverine environments degraded by sand mining in rivers. Specifically, three conditions that must be met for a restored river channel in the lower reach of the Kelani River in Sri Lanka were proposed: (1) flood discharge capacity of the channel for a given flood, (2) prevention of saltwater intrusion, and (3) creation of a diverse physical environment. The allowable mining volume satisfies the three conditions, while continuing to mine sand was discussed based on the sediment budget calculations in the target river reach. In this case, the amount of sediment stored in the target reach and its variation are determined by the amount of sediment supplied to the target reach, the amount of sediment discharged from the target reach to the sea, and the amount of sediment excavated. This means that the dynamic equilibrium channel of the target reach is determined by the amount of sediment supplied and the amount of sediment excavated. The amount of sand mined when the dynamic equilibrium channel meets the three conditions of the restored channel is a candidate for the allowable amount of sand mined. One of these, the most desirable one, is set as the allowable mining volume. As described above, we proposed a method to develop a restoring reach taking the sediment budget and associated hydraulic and hydro morphological conditions in the target reach into consideration.

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.

The present study discusses sediment transport processes in the drainage basin of the Kelani River using the RSR model which is able to evaluate bedload and suspended load and their sediment size distribution at any cross-sections of stream channels. This model was applied to the 2016 and 2018 flood events to investigate temporal and spatial characteristics of sediment transportation. Simulated results show that the accumulated bedload volumes are comparable to the accumulated suspended load in these flood events. Besides, they show that the material of the suspended and bedload tend to mix in the peak flood stage, and the field data on the upstream and downstream sediment size distribution are similar to those of bedload and suspended sediment, respectively.