Flood Risk Mapping (Case Study of Ketar Watershed Ziway-Dugda Woreda, Ethiopia)
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Date
2020-12
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Addis Ababa University
Abstract
Flood is probably the most devastating, widespread, and frequent natural hazard of the
world that producing many socioeconomic and environmental consequences within the
affected floodplains. In Ethiopia, there is a lot of areas under flood problem and Ketar
river floodplain located at the downstream of Abura gauging stations is among the most
frequently flood-affected area for which flood risk mapping is so important. The
objectives of this study were to model rainfall-runoff using HEC-HMS for Ketar
watershed and flood risk mapping by using HEC-RAS and HEC-GeoRAS for Ketar river
floodplain, located downstream of Abura gauging station in Ziway-dugda woreda,
Ethiopia. The HEC-HMS software requires daily hydrological data of 16 years Collected
from a National Meteorological Agency of Ethiopia for rainfall-runoff modeling. A
normal-ratio method was used for filling missing values of precipitation data and data
consistency was checked bya double mass curve. The initial and constant loss for
precipitation loss model, Clark unit hydrograph for excess precipitation transformation
to direct run-off, monthly constant for base flow modeling, and Muskingum for flood
routing modeling were chosen.Gage weight meteorological method was selected to
assign the weighted precipitation to each sub-basins using the Thiessen polygon method.
Among the collected 16 years of hydrological data for rainfall-runoff modeling, 11 years
(1988-1998) were used in model-calibration and 5 years (1999-2003) were used in
model validation. The model-performances were evaluated using performance
measuring techniques including Nash Sutcliff Efficiency (NSE) and Coefficient of
Determination (R2). NSE during calibration and validation was 0.72 and 0.67
respectively whereas R2 during these two processes was 0.87 and 0.81 respectively
Flood frequency analysis was conducted using HEC-HMS’ frequency storm method for
2, 5, 10, 25, 50,and 100 year return periods. The peak flood for each respective return
periods were (198.5, 224.1, 242.9, 251.8, 267.5, and 283.5 m3/s). Flood risk mapping
was modeled for a peak flood of each return period using HEC-RAS, HEC-GeoRAS,and
Arc GIS software; and resulted in the inundated areas of 2.86, 2.91, 2.94, 2.95, 2.98, and
3.01 km2 respectively.
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Keywords
Flood risk mapping, HEC-HMS, HEC-RAS, Arc GIS, Rainfall-Runoff Modeling