Flood Mapping and Mitigation Measures for Upper Awash River

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Addis Ababa University


This study focuses mainly on the flood mapping and proposes mitigation measures by using the application of arc-Gis based hydrologic model (HEC-HMS) for estimation of peak discharge and hydraulic modeling (HEC-RAS) for estimation of water surface profile for different return periods. The DEM (30m*30m) of the study area was used to extract the physical characteristic of a watershed using Geo-Spatial Hydrologic modeling extension HEC-GeoHMS. The satellite data analysis was used in the calibration and validation of the Awash Belo flooded area. The six indices were selected in water area extraction from Landsat-7 Geotiff like NDWI, AWEI, NDVI, NDMI, WRI, and NDMI. Among them, NDWI was the best fit with simulated flow and used for both calibration and validation for events of 9/3/2001 and 8/25/2001 respectively. Finally, different mitigation alternatives were proposed to take corrective measures for Awash Belo flood-damaged area. The appropriate mitigation measure for the Belo flood plain was selected based on three main criteria after their design like flood risk, Environmental impact, and balance of the two criteria. Even if, it was difficult to decide the efficient alternative because some alternative contradicts each other, the multi-criteria decision analyses were used. The two-stage channel modification was best in the riskoriented decision but least in environmentally oriented decisions. The one-stage channel modification was good in environmental impact-oriented whereas least preferred in riskoriented. The levee in combination with channel modification was good in environmental oriented but, less in risk-oriented. The balance of all environmental and flood risk gives higher priority for levee construction and less priority for channel modification. The peak discharge developed at upstream flood plain was 82.8 and 100 m /s for 50 and 100 years respectively. The area inundated by this peak was 500.6 ha and 505 ha for 50 and 100 years return period respectively without any protection measure. The area falls to 52.27 and 57.28 hectares when the levee construction mitigation measure was applied for 50 and 100 years return period. Scenario analysis confirms the superiority of levee construction to other proposed alternatives with the highest rated value of 0.733 as well as strong weakness of two-stage channel modification with the least preferred rated value of 0. Although increasing the size of the channel or decreasing its roughness can lead to a reduction in flood level because of additional channel capacity, channel modification could also have a negative effect such as increasing inflow velocity and stream power. Therefore, from the different alternatives, the construction of a levee has been proposed as the best alternative remedial measure to protect the flood from the damaged area.