Flood Risk Mapping And Assessment In Middle Awash Sub-Basin, Ethiopia

Abstract

In Ethiopia, River Flood Is A Major Hazard Causing Significant Damage To Lives And Properties. Middle Awash Sub-Basin (MASB) Has Experienced Severe Flooding By The Overflowing Of Awash River. The Objective Of This Study Is To Generate Flood Hazard, Vulnerability And Risk Maps And DoTheir Assessment On MASB With The Application Of Hec-Ras, Arcgis And Hec-Georas. River Flow Data Of Seven Gauge Stations Were Analyzed For Estimating Flood Quantiles On Gauged And Ungauged Sites By Applying Regional Flood Frequency Analysis (RFFA) Based On L-Moment-Based Index Flood. From Five Candidate Distributions, Pearson Type Iii (Pe3) Was Identified As The Best Fit Regional Distribution For Masb And The Estimated Regional Flood Quantiles Are 0.956, 1.32, 1.53, 1.73, 1.96, 2.12, 2.28, 2.48 For 2-, 5-, 10-, 20-, 50-, 100-, 200-, And 500-Year Floods Respectively. Based On The Regional Quantiles And The Index Flood, At-Site Quantiles Are Estimated And Inputted To Hec-Ras As Profiles For The Generation Of Inundation Extent, Depth And Flow Velocity. Hec Ras Was Calibrated Using Flow And Water Level Data Of Awash 7 Kilo Station And It Performed Very Good (Nse = 0.70), Good (Pbias = -13.80) And Very Good (R2 = 0.99) With Adjusted N Value Of 0.042 For The Main River Channel. Comparing Historical Flood Extent From Satellite Image And Hec Ras Simulation Is Used In Model Validation And Resulted Overlapping Percentage And The F Statistic Values Of 94.74 And 86.67 Respectively. Flood Risk For Each Land Unit Is Computed As A Product Of Hazard And Vulnerability. A Considerable Increase In Inundated Area Is Found With An Increase In Return Periods. The Majority Floodplain Area Of Masb Is Under Medium And High Hazard Class.Out Of The 80 Land Units Considered, 25 Land Units Scored A Vulnerability Factor Of Less Than 1 (48.98%) And The Remaining 26 And 29 Land Units Scored 1 To 3 (27.77%) And Above 3 (23.26%) Respectively. High And Sever Risks Are Observed On Small Areas Of The Basin Covering A Total Percentage Of 10.48 To 12.41% For 2- To 500-Year Flood. Towns Such As Haro Adi, Melka Worer, Bole, Doni, Awash Melkasa, And Metehara Have Experienced High And Sever Flood Risk For The Different Return Periods. The Study Addressed That Flood Risk Of The Land Units Can Be Varied Even After There Is No Change In Flood Hazard Intensity. The Study Can Provide Supportive Information In Identifying Priority Areas In The Flood Risk Zones Of Masb Which Is Particularly Useful When There Is A Budget And Time Constraint. It Can Be Also A Source Of Baseline Information For Future Studies