ID MATHEMATICAL MODEL STUDIES FOR PREDICTION OF LONG TERM BED LEVEL CHANGES IN KOSI RIVER REACH FROM BARRAGE TO 47 Km. DOWNSTREAM

Abstract

River Kosi is the third largest Himalayan river, rises in Tibet and after traversing a distance of 720 km. in Nepal and India, joins river Ganga at Kursela. Three main tributaries in the Himalaya viz. Sun Kosi, Arun Kosi and Tamur join together at Tribeni in Nepal and river below Tribeni is Known as Kosi. River Kosi below Tribeni flows through a deep gorge over a length of about 10 km. until it debouches into the plain at Chatra. River below Chatra built up its plain and flows through several channels spread over a width varying from 6km. to 16 km. Kosi river near Bhimnagar, which is 42 km. below Chatra, enter North Bihar in India. Total distance from Bhimnagar to Kursela where river Kosi falls into river Ganga is about 260 km. The important tributaries those join Kosi river in this reach are Trijuga, Balan, Kamala and Bagmati. The history of the river reveals that, between years 1731 and 1954, the river shifted from east to west for a distance of 112 km. After shifting, it leaves destruction and devastation in its wake, ruining towns and villages, covering agricultural land with sand, turning wide depressions into marshy land ultimately making countryside un-inhabitable and unhygienic. In order to provide relief to the areas affected by Kosi river in North Bihar and Nepal, Kosi project was undertaken in the year 1955. the project comprised:- 1) Construction of flood embankment on both the bank confine flood spread. 2) Construction of barrage near Bhimnagar in Nepal to cater for an annual irrigation of 1.05 m.ha. and power generation of 20,000 kw. Construction of flood embankments was completed in the year 1959 and barrage was commissioned in the year 1963. Because of construction of embankment, the river flows between it and the sediment/silt start depositing, due to that there is savior attack on embankment. To avoid attack batteries of spur was constructed along the embankments. In this thesis rate of aggradation/degradation was carried out. The rate of aggradation in Kosi river within the flood embankments was worked out using HEC-RAS-4 version. 10 daily average inflows was established on the basis of daily discharge data available for the year 1948 to 1966 and recent data from 2001 to 2003. Using sediment concentration and discharge data water and sediment flow relationship was established. This relationship was used for getting the sediment inflow at the upstream boundary. For downstream boundary condition, gauge discharge data available was used. Using these .boundary conditions the bed levels were predicted for successive years.