FLOOD ESTIMATION FOR BAGAMATI RIVER AT PANDHERA DHOBAN, NEPAL

Abstract

Nepal is a landlocked country with a total area of 147,181 km2. situated between India and China. It is located in the middle belt of the Himalayas. The hills and mountains occupy nearly 113200 km2 (77%) of the total area. Monsoon contributes 75 percent of the total rainfall The floods of unprecedented magnitude alongwith big landslides and debris flows were experienced in central and eastern Nepal due to incessant rainfall for three days i.e. from 19 to 21" July 1993. The severe floods in Bagamati river caused heavy damages to barrage and to the infrastructure of Bagamati Irrigation .Project. Different agencies estimated the peak flood of July 1993 and submitted their findings to His Majesty's Government of Nepal. These agencies recommended reassessment of the design flood for the rehabilitation of barrage. In this study, an attempt has been .made to estimate the design flood of Bagamati river at Pandhera Dhoban site using deterministic and statistical approaches. Linear Perturbation Model (LPM) developed by Nash and Barsi (1983) has been applied to this basin and is used for extension of the runoff record. The rainfall and runoff data for the period 1972 to 1992 have been used in :. calibration and validation mode. The performance of Linear Perturbation model for this basin is satisfactory as efficiency in calibration and validation mode are 62.87% and 67.35%. In the deterministic approach the unit hydrograph theory is used to estimate the design flood. The design flood computed by this approach is 10853 cumecs. In the statistical approach; flood frequency analysis has, been applied to annual flood series for the period 1965 to 1992. Floods of different return periods have been estimated. In both graphical and analytical approach, the Gumbel (EV1) distribution is found to be the best fit distribution. The 100 years return period flood works out to be 11203 cumecs. The design flood of 11203 cumecs is thus. recommended for future extension of rehabilitation work of barrage as well as for flood protection work.