Sediment Transport and Flood Risk: Impact of River Confinement with Embankments on River Morphology and Flood Dynamics in Sediment-rich Himalayan Rivers

Human settlements have historically concentrated near rivers due to their transportation benefits and fertile lands, despite the inherent flood risks. To mitigate these flood risks, societies have implemented various interventions, including flood control structures such as embankments. Although these structures can reduce the frequency of small or moderate floodplain inundation, which can support economic growth, they also create a false sense of security, leading to increased settlement in floodplains. This, in turn, can exacerbate the impact of high flood events that exceed the design capacity of flood protection structures. The increasing frequency of flood hazards, driven by changing climatic conditions and changes in land use, raises critical questions about whether these flood control structures alone can serve as long-term sustainable solutions for flood mitigation. In this context, this research investigates how flood control embankments and sediment transport affect river morphology, channel capacity, and flood inundation by simulating various extreme flood scenarios in Himalayan river reaches in Nepal. The results show that river embankments can reduce the extent of floods for low-flow or high-frequency floods, up to the designed discharge. However, in rivers with moderate to high sediment transport rates, the construction of embankments and channel confinement can significantly alter sediment mobility, potentially increasing downstream flood risks and compromising embankment stability during extreme events. This research highlights the importance of evaluating multiple aspects of river embankments, particularly their impact on river morphology, sediment mobility, and flood risk management in sediment-rich rivers undergoing rapid urbanisation and climate change. In these contexts, sediment transport effects should be considered in embankment design and floodplain planning.

Keywords: River embankments, Sediment transport, Flooding, River morphology, Himalayan Rivers