Characteristics of Bed Profiles Due to Sediment Transport in a Debris River

Sediment transport in debris rivers is a unique phenomenon of sediment transport in general. This is related to the complexity of the characteristics of the material transported and the nature of the flow that transports sediment. This work aims to predict potential bed profile changes in one of the debris rivers in Indonesia: the Sombe-Lewara River in Central Sulawesi Province. The control of debris flow in this river has been carried out simultaneously by the relevant government agencies by building sediment control structures along the river section. The study was carried out by hydraulic simulation at the intersection of the Sombe River and Lewara River using the HEC-RAS Model with two input data: average discharge and bed load gradations at both upstream boundaries. The average discharge was obtained by rainfall-runoff transformation using the HEC-HMS Model due to the limited discharge data in the study area, while the sediment gradation data was obtained from laboratory test results based on bed load sample data. Due to the limitations of the observation data, the parameter optimization is only applied to the HEC-RAS model, especially the Manning roughness coefficient by comparing the observed water level elevation with the simulated water level elevation. The results of the study indicate that the bed profile elevation in the upstream confluence of the river tends to increase and the bed profile downstream of the confluence tends to decrease. This is closely related to the transport intensity and bed slope upstream and downstream of the river confluence. The results of this study can be used as a reference for handling sedimentation in debris rivers.