Bridge Scour Assessment on Nile Tributaries in South Sudan During Extreme Wet Season Events

Bridge scour remains one of the leading causes of bridge failure globally, yet it is critically under-studied in sub-Saharan river systems, particularly in the complex hydrological environment of South Sudan's Nile tributary network. This study presents a comprehensive assessment of local and contraction scour phenomena at five representative bridge sites located on primary Nile tributaries — the Sobat, Bahr el Ghazal, Pibor, Kangen, and Akobo rivers — during the extreme wet season events of 2019–2023. Hydrological data, field measurements, and numerical modelling were integrated to estimate maximum scour depths using the HEC-18 framework, modified Richardson-Davis equations, and a newly proposed South Sudan Scour Index (SSSI). The results indicate that scour depths during peak flood events ranged from 1.8 m to 6.3 m across bridge types, with masonry arch and Bailey bridges exhibiting the highest vulnerability. Statistical correlation analysis revealed that peak discharge velocity (r = 0.89, p < 0.01) and sediment median grain size (D50) are the dominant predictive variables. A regional scour prediction model calibrated to Nile tributary hydraulics is proposed, offering a practical design tool for bridge engineers operating in tropical African river environments. Recommendations for scour countermeasures, real-time monitoring, and adaptive bridge design standards under extreme climate scenarios are provided.