Rainfall Runoff Modelling and Inundation Analysis of Bagmati River at Terai Region of Nepal

Climatic variability, unplanned land use pattern and encroachment into the flood plain are affecting the hydrology of Bagmati river basin of Nepal. In this study, methodology to improve present flood management system with non-structural countermeasures has been elaborated. Study of hydrological condition, basin scale rainfall runoff modeling and inundation analysis is helpful to prepare community level flood risk management techniques. In order to develop the relationship between rainfall and runoff and hence to develop flood forecasting model, simple statistical tools for hydrological analysis and least square methods of best fit technique is applied within the available historical rainfall and stage data (1980-2004). Flood frequency analysis is performed and floods of different return periods are identified. To minimize the risk from flood, different level inundation of the predicted flood should be known at community level. Hence inundation analysis is performed based on available digital data, satellite imageries, and GIS based numerical models (Arc GIS, its extension HEC-GeoRAS and HEC-RAS software) together with some field observation data. Real time flood forecasting model and inundation depth identifying technique for lower Bagmati watershed are the main outcomes of this study. Results of this analysis are understandable even to the community level people and such approach can be applied to other river basins as well for non structural countermeasure of flood disaster mitigation.