Effects of Flow Variability on Scouring of Bridges and Culverts between Sebeta and Adama Section of Ethio - Djibouti Railway Line

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Addis Ababa University


This case study area is found within Uplands, Upper Valley, and Western Highlands of Awash river subbasins which is one of the major river flood-prone areas in part of Oromia region where the most significant part of the railway section is found. This study simulated the rainfall-runoff of the hydrological processes in the selected catchment using HEC-HMS hydrological model. The simulation was achieved using the ground-based rainfall datasets for the meteorological gauge stations within the catchment (Uplands, Upper valley, and Western Highlands of Awash river subbasins) for a period of twenty years (2000 – 2019); sourced from the National Meteorological Agency of Ethiopia. The model was calibrated with the observed time-series datasets between 2005 and 2014 at Awash Melka Kuntire, Awash @Hombole, and Mojo @MojoVI hydrological gauging Stations. These stations were selected not only because they are directly linked with the selected railway section but also the availability of full records. These datasets were sourced from the Ministry of Water, Irrigation, and Energy of Ethiopia. The model performance shows that NSE, PBIAS, and R2 were respectively 0.932, 14.94%, and 0.9994 at Awash @Melka Kuntire; 0.923, 16.25%, and 0.9994 at Awash @Hombole; 0.859, 12.42%, and 0.992 at Mojo @MojoVI. It has also been established that there is flow variability in all the selected drainage structures with the coefficient of variance ranged from 0.645 to 1.034. The study has also developed Rainfall (mm) Duration Frequency and Rainfall Intensity (mm/hr) – Duration Frequency Curves for the selected catchment which was used as design rainfall as input for flood frequency modeling in HEC-HMS. The comparison results between the estimated peak flows and the designed peak flow extracted from the design documents, Q100 (m3/s), showed that about 40% of the drainage structures are likely to experience flooding once every 100 years. However, this could only be true if and only if the probability distribution used for peak flow estimation is the same as that used for the design of peak flows in the document. The results of scour depth comparison for all the structure types indicated that the flow variability (in peak flows) has an effect on scouring depths and it varies at different return periods. Also, this study has estimated the median stone size for riprap materials and found it to be about 1.58 m as a mitigating measure to ameliorate the high risk of scouring (about 60%) estimated around a pier at bridge location K97+817.22 (Mojo-Adama No.3 Bridge). This study thereafter generated some relationship equations for the slab culverts and the NSE and R2 were found to be (0.77, 0.90) and (0.82, 0.83) for scenario 1 (Estimated Q100) and scenario 2 (Design Q100) respectively. The study, thereafter, recommended the use of the generated scour depth equations for slab culverts in the other section of this railway line.



Flood risk, scour depths, riprap size, flow variability, hydrological model, slab culvert, railway, Awash river sub-basins