Flow and Suspended Sediment Division at Two Highly Asymmetric Bifurcations in a River Delta: Implications for Channel Stability

Summary

The division of sediment at river bifurcations results from the complex interaction between three-dimensional flow, planform, and channel bed morphology, as well as the heterogeneity of the bed material. Sediment division processes cannot be incorporated in their full complexity in scale experiments and are difficult to reproduce with numerical models. Field measurements are thus necessary to advance our understanding of those processes in river deltas. However, such measurements are rare. We present measurements of the flow and sediment division at two tidally influenced bifurcations of the Kapuas River, a large sand-bedded suspended load-dominated river in West Kalimantan, Indonesia. At both bifurcations, a smaller channel branches off from the side of the main river, which makes the planform strongly asymmetric. The planform of both bifurcations has been stable at least since the end of the nineteenth century when the region was mapped for the first time. Based on our measurements, we explore possible factors that stabilize the bifurcations. We measure the flow velocities with a boat-mounted acoustic velocity profiler and determine the sediment concentration from acoustic backscatter, calibrated against water samples. The side branch of the first bifurcation receives a proportionally lower fraction of sediment than water. In contrast, the side branch at the second bifurcation receives a proportionally higher fraction of sediment than water. A comparison of flow velocity and suspended sand concentration indicates that the bed material sorting strongly influences the division of sediment, in particular at one of the bifurcations, which is situated in a meander bend.