Dynamics of the Karnali River Bifurcation in Nepal

The Karnali River in Nepal bifurcates into two major branches (i.e., the eastern Geruwa branch and the western Kauriala branch), as it flows out from the Himalayan foothills onto a low relief area in southern Nepal, where it has created an alluvial fan of order 1000 km². Its dynamics are governed by the natural geomorphological processes of an alluvial fan. The eastern Geruwa branch, which until 2009 used to be the dominant branch regarding its share of the upstream water discharge, now receives a minor share of the water discharge. The reducing discharge in the Geruwa branch has decreased heterogeneity and suitability of wildlife habitat in its floodplains, which constitutes a significant area of Bardiya National Park. The dynamic river branches exhibit a high level of braiding, switching of the dominant channel, and an uneven discharge partitioning between the bifurcates. Our objective is to provide insight on how the system is affected by and will respond to anthropogenic interventions, especially the discharge distribution between the Geruwa and Kauriala branches. The switch in the flow partitioning since 2009 seems to be associated with an intense monsoon season. Besides this, embankments along the Kauriala branch, discharge intakes for irrigation, and unmanaged sediment mining may have affected the partitioning of flow and sediment flux over the Karnali River bifurcates. Furthermore, plans to develop multiple hydropower projects upstream will likely affect the system in the future. We study the impact of these factors on the discharge partitioning between the Geruwa and Kauriala branches, and in particular the flow rate in the eastern Geruwa branch, as the latter is the lifeline for wildlife in the Bardiya National Park, using field surveying/monitoring and numerical models. For this purpose, we have performed an intensive field campaign for data collection and have set up numerical models of various levels of complexity. We have measured cross-sectional profiles and spatial variation of the bed surface grain size distribution. Our observations reveal that bed level in the upstream Geruwa branch is higher than that of the upstream Kauriala branch. Furthermore, we observe river bend sorting in the bifurcation region, which results in a larger bed surface grain size in the upstream Geruwa branch. We have set up a one-dimensional hydrodynamic model to simulate the effects of interventions on the flow partitioning at the Karnali River bifurcation, as well as a two-dimensional hydro-morphodynamic model to study the impact of bend sorting and other two-dimensional aspects on the flow partitioning, as well as sediment deposition in and possible closure of the Geruwa branch.