Sediment transport modelling using MIKE 11: A case study of koshi river

Abstract

This study was initiated to better understand the sediment transport capacity of a reach on Koshi River at downstream of Koshi Barrage in a length of 21 km approximately. Koshi River, a vital watercourse in the Himalayan region, plays a pivotal role in the lives of millions of people residing in its basin. The primary objective of this research is to gain a deep understanding of the hydrodynamic and sediment transport patterns within the Koshi River, particularly focusing on seasonal variations and sedimentation rates. These insights are critical for informed decision-making and the sustainable management of this vital natural resource. One of the standout findings from this study is the discernible monthly variation in water discharge observed within the river. The discharge data, acquired through hydrodynamic simulations, exhibits a characteristic sinusoidal pattern, akin to wave-like phenomena seen in the natural world. Within this pattern, the peak discharge occurs during the monsoon season, coinciding with heavy rainfall and an abundant inflow of water. Conversely, the pre-monsoon and post-monsoon periods witness significantly lower discharge levels, attributed to reduced rainfall and diminished water inputs. This seasonal variation in discharge is crucial for understanding the river's response to climatic changes and its impact on the surrounding ecosystem. Furthermore, an analysis spanning from 2003 to 2012 reveals a consistent decline in water discharge within the Koshi Basin. This temporal trend holds significant implications for local ecosystems, water resources, and communities dependent on the river, necessitating adaptive measures to address the consequences of this decline. The sediment transport component of this study unveils the intriguing sedimentation patterns within the river. It is evident that the Koshi River model domain has experienced substantial sedimentation over time. This accumulation of sediment, including sand, silt, and other particles, can significantly alter the riverbed's morphology, with far-reaching implications. The research identifies the upper reach of the Koshi River as a significant contributor to the river's sediment load. This region, characterized by extensive sediment erosion, underscores the importance of understanding sediment dynamics in both upstream and downstream sections. The analysis also reveals that the rate of sedimentation varies significantly along the river's length. While the upstream portion exhibits lower sedimentation rates, the middle and lower reaches experience higher rates of sediment deposition. This non-uniform distribution highlights the need for tailored management strategies along different river segments. Notably, the simulation indicates a gradual decrease in the rate of sedimentation over time, a finding with potential implications for river health and long-term riverbed projections.