Kosi River - Hydrologic analysis

Abstract: The Kosi River, called “the Sorrow of Bihar”, lies in Nepal and northern India, where it has caused devastating floods throughout the years. The river is known for its dynamic flows and high sediment load, leading to one of the largest alluvial fans in the world as well as avulsive shifting. The monsoonal dynamics in the region causes high fluctuation in the river discharge, with almost annual floodings, often with destructive consequences. In 1963-1964 the avulsive behavior of the river was prevented by the construction of a barrage and two embankments along both sides of the river course in Nepal. In 2008 there was a breach in one of the embankments and the consecutive flood led to one of the worst disasters in India, effecting over 3 million people and ruining huge areas of land from future agriculture. The management of this flooding has been highly criticized in literature, and the river management and flood mitigations of Kosi River are frequently discussed. Many projects in India and worldwide are addressing the issues with Kosi River, and this report is born out of a cooperation between Lund University and the National Institute of Technology in Durgapur, India, aiming to develop a decision support system for the Kosi River. The objective of the report is to make a background study to the problems related to the Kosi River, as well as a hydrological analysis of parts of its catchment. This is done through analysing data as well as studying relevant literature. The results from this report will hopefully be found useful as background material for further and more in-depth studies on the Kosi river. Data purchased from the Department of Hydrology and Meteorology, Nepal, was studied through: a double-mass analysis of precipitation data; a flood frequency analysis of the river and some of its tributaries; trend analyses on discharge, precipitation and temperature; as well as an analysis of river routing and rainfall-runoff relations for a sub-basin of the catchment. Trends in precipitation indicated that the number of no rainfall days was decreasing, but that the annual total rainfall trend was either decreasing or increasing depending on the location in the catchment. The temperature did not indicate on any specific trend. The discharge data mainly showed on decreasing trends, with the exception of one station. The study of the sub-basin showed that precipitation and evapotranspiration within the sub-basin had little effect on the discharge leaving the sub-basin, as these parameters were in of a magnitude 10 times less than the discharge. From the river routing it could be concluded that there was no time delay within the sub-basin, at least not any delay larger than a day. As data was temporally and spatially limited the analysis had to be rough and generalizing and all trends must be treated carefully. The double-mass analysis did however indicate that the precipitation data was of decent quality. To improve the accuracy of the studies in the Kosi River catchment, more data is needed, as well as an improved cooperation between different agencies and universities that are working with problems concerning the river.