ESTIMATION OF FLOOD MAGNITUDE AND INUNDATION MAPING OF DAYMA’AD RIVER

Abstract

Flooding is defined as a natural occurrence caused by an unusually high level of flow over land or along a coastline, resulting in significant damage. A flood happens when water covers a large area of land, usually low-lying. When a river breaches its banks, the worst floods occur. Flood is one of Ethiopia's biggest natural hazards, wreaking havoc on human lives, property, and cattle in many sections of the country. It is common to many parts of Ethiopia every year causing a lot of losses to human lives as well as damage to property. In frequently number of years, in frequently number of years. Dayma’ad River’s flood has been caused losses of human lives and property in Jigjiga city. It has taken one event caused by dayma’ad stream On May 30 2008 in Jigjiga town at least 25 people were died after heavy rains that cause flash flood 45 people were hospitalized and the flash floods have swept away several houses and damaged huge properties, The aim of this study is to estimate the amount of peak flood and delineate flood inundation map for the study areas that can be affected by extraordinary floods The data required for this study were obtained from the Ministry of Water, Irrigation & Electricity National Meteorological Agency, Jigjiga City Administration, field survey and Ethiopian Road Authority. The collected data were hydrological and soil data, land use land cover data, Topographic Map, DEM and ERA Manual. The software like Arc View GIS, HEC-HMS, HEC-RAS and HEC-GeoRAS were used for the analysis. The peak discharge were calculated by using Arc GIS, HEC-GeoRAS, HEC-HMS SCS-CN. The basin data is pre-processed by Arc-GIS 10. Arc- hydro and HEC-GeoHMS and exported to HEC HMS 4.5 that was used for generation of rainfall-runoff model The flood Inundation map shows the area extent to be delineated as buffer zone with using GIS, HEC-GeoRAS and HEC-RAS, The performance of the model was calculated using Nash-Sutcliffe model efficiency coefficient and coefficient determination. The validation result of the model shows that the model is valid for simulation of the rain fall runoff transformation. The result of the calibration of monthly flow showed that there was good agreement between the measured and simulated average monthly low with Nash-Sutcliffe efficiency and coefficient of determination value of 0.684 and 0.697 respectively.. The result model validation coefficient was 0.665 for Nash-Sutcliffe model efficiency coefficient and 0.682 for coefficient determination. Based on the analysis of the result the peak discharge at the outlet Dayma’ad river was found to be 294.7m3 /s in25 return period. The stream networks are the area that the community living with fear due to an expected flood hazard from the upstream