FLOOD INUNDATION MAPPING OF BABAI RIVER BASIN USING HEC-RAS AND ARC-GIS, NEPAL

Abstract

Floods are the natural calamity that arises from the excessive accumulation of water within an area caused by heavy rainfall , snow melts, river overflow and other factors. Floods are the most notorious natural disaster that every nation is facing threat each year causing huge impact to life and properties . Nepal’s lying in central Himalayan region which is one of the most vulnerable regions to natural disaster ranks in 12th position in terms of various natural calamities and 30th position in terms of vulnerability to flood risk. Due to its rugged topography, weak geological formations relatively high slope, unplanned land use, excessive increase of population and haphazard development activities causes the outburst of glacial flood, concentrated monsoon rains, flash floods and sheet flooding in the plain areas. The Babai River basin lies at the mid-western region of Nepal at Lumbini province experiences the medium to large scale flooding frequently. The river originates at Dharapani of Dang District at in between the foot hill of Mahabharat range and Shivalik range. The river flows from East to west up to the Gothi region of Bardiya and traverse toward South through the fertile plain of Bardiya and enters to India from Bhada (outlet Point) and finally it converges to Ghaghara in India which ultimately joins river Ganga. The area of Basin up to outlet point at Bhada is 3430.71 Km2.The average annual rainfall over the basin is about 1500 mm and out of this 80 percent occurs within four months of monsoon season(June to September). The river traverse through the narrow gorge between the forest area and carries huge amount of sediments within them deposit in the southern plain which rises the bed level of the channel thus over topping the banks even at the small flood thus damaging the agricultural land and settlements near at the flood plains. The basin mainly experienced huge damages by flooding during 15th August 2014 and 13th August 2017. A detailed study is needed to mitigate the risk of flooding and further mitigation planning for the concerned authorities.