Rainfall-Runoff Modeling using HEC-HMS for Flood Inundation Mapping: The Case of Borkena Watershed, Upper Awash Basin, Ethiopia

Abstract

Flooding is a recurring challenge in the Borkena watershed, causing significant damage to lives, livelihoods, and infrastructure. Accurate rainfall-runoff modeling is crucial for understanding hydrological processes, predicting floods, and implementing effective water resource management strategies. The objectives of the study were to simulate rainfall-runoff processes using HEC-HMS, perform flood frequency analysis using historical hydrological data, and develop a one-dimensional steady-state hydraulic model using HEC-RAS to generate flood inundation maps for selected flood scenarios in the Borkena watershed. The HEC-HMS requires daily weather and hydrological data for rainfall-runoff modeling, and 30 years of meteorological data (1994-2023) and 16 years of hydrological data (1995-2010) were collected from the Ethiopian Meteorology Institute and the Ministry of Water and Energy, respectively. After filling in the missing values, data consistency and homogeneity were checked using the double mass curve and XLSTAT software. Other parameters, such as curve number and basin lag time, were generated using HEC-Geo HMS, SCS-CN, SCSUH, Recession, and Muskingum-Cunge methods for precipitation loss modeling, precipitation transformation to direct runoff, baseflow modeling, and flood routing, respectively. The hydrological model was calibrated from 1995 to 2004 and validated from 2005 to 2010. The model performance was evaluated using performance measurement techniques, including Nash Sutcliffe Efficiency (NSE= 0.765&0.704), Coefficient of Determination (R2 = 0.804&0.786), and Percent of Bias (PBIAS = -5.67% & 14.46) during the calibration and validation periods, respectively, indicating a very good performance for rainfall-runoff simulation in the Borkena Watershed. Flood frequency analysis was conducted for return periods of 2, 5, 10, 25, 50, and 100 years, resulting in corresponding peak discharges of 98.3, 144.3, 176.7,219.1, 251.4, and 284.2m??/s. These flood peaks were then used in HEC-RAS to model flood inundation, yielding inundated areas of 8.8, 10.1, 11.9, 13.6, 16.4, and 18.6Km??, respectively. The overall result of this study successfully demonstrates a robust methodology for simulating rainfall?��?runoff and flood inundation in the Borkena watershed using an integrated HEC-HMS and HEC-RAS model. The strong model performance and inundation maps provide essential tools for flood risk management and hydrologic planning in the watershed.