Study of morphological changes and socio-economic impact assessment: a case study of Koshi River

The Koshi River is a dynamic river; due to its frequent course change, it presents a challenge in terms of long and recurring flood hazard. The braiding pattern can be easily visualized in the Koshi River, and interlacing of channels is spread over a width of about 5 to 6 km. Koshi is the 3rd largest Himalayan River, originates from the central Himalayas in Nepal, and flows through the State of Bihar in India. Koshi River is also known as the Sorrow of Bihar due to its flooding behavior causing huge loss of life and property every year. About 21,000 km2 of agricultural land has been affected by the annual flood caused by Koshi River. Average discharge of River Koshi is approximately 2166 m3/s. A stretch of approximately 90 km of Koshi River near Koshi barrage at Birpur has been considered in this study. The objective of this study is to investigate the braiding pattern of the river and assess the spatial and temporal variation of braiding intensities. Plan Form Index (PFI) has been used as the braiding indicator. For calculating PFI, satellite imageries over a period of 47 years, from 1972 to 2019, were used in the study. The satellite imageries were processed to assess the channel morphological changes. Image rectification, filtering, noise reduction, and image classification were carried out using image processing software. Koshi River was digitized and river watershed map, stream network map, and elevation map were prepared in geographic information system environment. The results indicate that braiding has decreased with time as shown by the PFI values. Braiding intensities have increased in majority cross-section points at both upstream and downstream of Koshi Barrage. It may be concluded that heavy braiding of Koshi River is the one of the main causes of severe yearly land loss. This study can be integrated in the future for better sustainable solution of the river problems.