Hydrologic and hydraulic assessment of scour problems at bridge sites in Tigray region, northern Ethiopia

This study was conducted to assess scour problems at three selected bridge sites in the Tigray region, northern Ethiopia, namely the Ilala, upper Geba, and Tankwa bridges. Smart-GIS, ArcGIS, Hydrologic Engineering Centre Hydrologic Modeling System (HEC-HMS), and Hydrologic Engineering Centre River Analysis System (HEC-RAS) were used for the river morphology, hydrologic, and hydraulic analysis. The riverbanks were subjected to higher degradation and instability problems. The estimated 100-year flows were 272.5, 940.4, and 249.9 m3/s for Ilala, upper Geba, and Tankwa bridges, respectively. Flow depth calibration were resulted in model performance with Nash Sutcliffe Efficiencies of 0.90, 0.88, and 0.87 for Ilala, upper Geba, and Tankwa bridge sites, respectively. Scour depths were 1.3, 1.6, and 0.8 m at contracted sections; 3.2, 3.0, and 2.2 m at the piers; 2.6, 3.6, and 2.2 m at the left abutment; and 1.6, 3.0, and 2.1 m at the right abutment of Ilala, upper Geba, and Tankwa bridges, respectively. The use of riprap at the bridges resulted in the reduction of scour depths by 60–100%. Overall, the hydrologic and hydraulic models performed well.