FLOODPLAIN MODELING AND MAPPING USING GIS & HEC-RAS ON DOWNSTREAM REACH OF DIJO RIVER, ALAGE, ETHIOPIA

Abstract

Flood is the most dominant common, continuously frequent and destructive natural disaster in the world. It causes loss of life and property, displacement of people, breaking of socio-economic activities and loss of fertile agricultural lands. Dijo River flood affects Alage ATVET Collage during rain seasons. The main objective of this research is to map the flood inundation area on downstream reach (Alage ATVET collage) of Dijo River. GIS 10.3, HEC-Geo RAS, HEC- RAS 5.0.6, excel 2016 and easy fit 5.5 software’s are used for this research. primary data’s (total station surveying data, field observation) and secondary data (Dijo and furfuro stream flow data and Manning’s n value) are used to conduct the research. Flood frequency analysis, pre-processing, processing/model execution and post-processing methods are used and got 101.22m3 /s, 164.6 m3 /s, 175.8 m 3 /s, 207.58 m3 /s and 227.83m 3 /s peak discharge for 2, 10, 25, 50 and 100- years return period respectively. 27 x-section cut lines are drawn on 4km river reach geometry, among these 55.6 % of the x-sections are severely flood venerable x- sections from all return period peak flood discharges and to both side of the river reach. The flood inundation area of 2, 10, 25, 50 and 100 years return periods were 5702, 8154.71, 8499.96, 9271.9 and 9851.84m2 respectively. This area includes both Alage ATVET collage and neighboring kebeles of the collage. Therefore, the responsible body makes remedial measures to control the flood like repair and increase the height of the exiting earthen dykes at upstream of alage Bera Bridge, construction of new dykes and levees at the chefe agricultural area, grassland area and desilting siltation-filled x-section, afforestation from upper watershed of the river