Flood Risk and Flow Variability Assessment at the Railway Drainage Structures: A Case of the Ethio – Djibouti Railway Line, Ethiopia

Abstract

Awash River Subbasin is one of the major river flood-prone areas in Ethiopia where the most significant part of the railway section is constructed. There is tendency of this railway line being affected by flooding. The study performed the flood risk analysis and flow variability at some selected drainage structures within Sebeta and Adama Section of this line. The climate and observed flow data were sourced from the National Meteorological Agency of Ethiopia and the Ministry of Water, Irrigation and Energy of Ethiopia respectively, and used for hydrological model parametrization, calibration, and validation. DMC, Linear Regression, and Thiessen Polygon were used for climate data consistency check, filling of missing data, and estimation of areal data respectively. The rainfall-runoff was simulated with an HEC-HMS hydrological model using a 20years (2000 – 2019) rainfall data. The model calibration and validation results using NSE, PBIAS, and R², were found within the statistically acceptable range for the surface runoff simulation. This study developed Rainfall Intensity – Duration Frequency Curve for the selected catchment as an input for the flood frequency analysis in the model. The coefficient of variance was estimated to select the flood discharge structures with the flow variabilities. The flood risk was assessed by comparing the modeled and the as built designed Q₁₀₀ (m³/s) extracted from the design documents. About 46% of the drainage structures are found prone to flooding at a T_100years. This study recommends the use of the methodological procedure adopted in this study for other section of this line by other researchers.