Integration of Remote Sensing and GIS with Flood Simulation Model for Flood Hazard Mapping in the Bagmati River, Nepal

Nepal, occupying the central part of the Hindu-Kush Himalayan belt covers an area of 147,181 square kilometers between India and China.About 6000 rivers and rivulets drain Nepal. Rugged topography, young geology and monsoon climate, all combine to produce high rate of runoff,erosion and sedimentation rendering the country highly vulnerable to water induced disasters and flood is one of the main recurring natural hazards. The Terai (plain area in the southern part) amounting to only 17% of the total area of the country is regarded as the granary of Nepal and the problems of flooding in this region are of utmost concern.The objective of this study is to integrate flood simulation model and remote sensing (RS) data with the available topographic and socioeconomic data in a geographic information system (GIS) environment for flood hazard mapping of Bagmati River in Terai region. Hydrologic model in combination with digital elevation model (DEM) is useful in delineating the inundation area extent and estimating the flood depths in areas where images capturing the peak flood events are not available. Flood hazard maps were prepared for a 50-year flood considering inundation area, flood depth, land use type and population affected. An areaof 360 km2 of the 87 Village Development Committees (VDC) was estimated to be affected by flood with 183 km2 having a flood depth of greater than1.0 m. The total area of 87 VDCs is 808 km2 and nearly about 45% of the total area was inundated by the flood. Satellite images were used to update the existing land use map in the flood affected areas. A flood hazard ranking system was designed by combining flood extend, flood depth, land use types and population using the matrix multiplication method. Ranking was done from 1 to 18, designating hazard ranks from 1-6 as low, 7-12 as medium and 13-18 as high. It was found 17 VDCs under high hazard category, 19VDCs under medium hazard category and 51 VDCs under low hazard category respectively. The right bank of the river between 10 km and 30 km was identified as the main spilling reach for structural countermeasures.