The research objective of WP4 is threefold. First, to understand the hydro-morphodynamics of the Karnali River system within its alluvial fan, and second to assess the reasons behind the dominant channel switching from the Geruwa to the Kauriala branch of the Karnali River system. Furthermore, the quantification of the hydrodynamics of the Karnali River system in its alluvial fan under the influence of anthropogenic effects and climate change is of importance.

Figure 1: The background image represents the bifurcation region during a low-flow period. The photographs of the bed surface sediment were taken during the fieldwork of 2023. The bed surface in the inner bend is finer  than the outer bend. The outer bend, which is also the entrance to the Geruwa shows significant deposition of coarse sediment consisting of boulders of about 50 cm diameter.

To get grip on the hydrodynamics we analysed the archival discharge time series at Chisapani from 1962 to 2019, sourced from the Department of Hydrology and Meteorology (DHM), Nepal. The discharge of the Karnali River at Chisapani varies significantly, ranging from 173 m³/s during the dry season to 17,900 m³/s at the peak of the monsoon. Furthermore, by using satellite imagery, we estimated the water distribution between the Kauriala and Geruwa branches of the Karnali system from 1972 to 2021. The results show a significant reduction in water surface area within the Geruwa branch since 2009. The braiding index (a metric used to characterized multi-threaded river systems, calculated by dividing the sum of lengths of all active channels, including secondary channels, by the length of the main channel in the river reach) of both branches decreased between 2001 and 2015 compared to the 1990-2000 period. Furthermore,  the Geruwa branch shows more braiding than the Kauriala, which means Geruwa is less stable.

In 2022 and 2023, we conducted two successful field campaigns. There we collected river cross-section data, sediment samples, and photographs from various locations along both branches. We also analysed the sediment-size distribution in both branches. Additionally, real-time water level sensors were installed in the Kauriala and Geruwa branches to monitor the water discharge distribution between them. Based on these field observations, we found that large sediment particles deposit right downstream of the Karnali bifurcation at the entrance of the Geruwa branch, obstructing flow into the Geruwa branch. It seems to be reasonable that bend sorting effects and the double peak flow event in 2009 have triggered the deposition of coarse sediment at this location. Subsequent monsoon floods seem to have reinforced this effect, leading to the gradual closure of the Geruwa branch.

Through numerical flow and sediment routing model the effects of bend sorting and monsoon peak flows on sediment transport and deposition
at the bifurcation were investigated. Initial results support our reasoning of sediment deposition at the entrance of the Geruwa. Further development
of the model is ongoing.

Figure 2: Conceptualization of the bend sorting effect which led to the deposition of coarse sediment at the upstream end of the Geruwa branch and hence gradual closure of the Geruwa branch.

Outreach: Connections have been established with local authorities in the project area, facilitating the sharing of the data and outcomes. A seminar on the relevance of river morphology to wildlife habitats has been organised at Mid-Western University, Nepal. We utilized laboratory facilities and provided training for staff and students from the College of Engineering and Management, Nepalgunj, Nepal. Collaboration with the Karnali River Management Project to share human resources and data has been beneficial during the fieldwork.

Ongoing work: Currently, we are working on a numerical model to understand the sediment deposition processes and influence of the peak flows. Additionally, further development of the model to study influence of climate and anthropogenic changes is also planned.

Researchers involved: Kshitiz Gautam (PhD Candidate, TU Delft), Mathieu Roebroeck (MSc. TU Delft), Marijn Wolf (MSc., TU Delft), Mo De Jong (MSc., TU Delft), Rahil Ahmad (MSc., University of Stuttgart), Marijn ten Zweege (BSc., TU Delft)

Figure 3: Image taken during low flow conditions in November 2023 showing part of the  bifurcation area with the main channels of the Geruwa and Kauriala branches.