Morphodynamic alterations around the bridge opening and the process of scouring under the influence of hydraulic flow parameters on selected bridges in the south Ethiopia regional state, Sodo to Konso highway

Abstract

Numerous bridges in southern Ethiopia are greatly affected by Morphodynamic changes in the alluvial channel caused by hydraulic flow parameters. These bridges are vital for enabling traffic flows between communities in the region. In this study, Morphodynamic alterations around bridge openings were conducted using the hydraulic flow parameters based on the field and laboratory investigations were conducted in selected bridge sites between the Sodo and Konso highway routes. The Morphodynamic analysis of 1984 to 2021 years 1,276 Landsat images of 5 selected scour susceptible bridge sites have been analyzed using RivMap toolbox in MATLAB after rapid assessment of the channel stability of 15 bridges based on the field observation. The 38 years of channel geomorphological changes show that lateral widening of the alluvial valley is dominant in selected bridge sites. This channel widening also aggravates the contraction, local scour potential, and increased flow velocity induced by narrow bridge opening. The HEC RAS result confirms that the three of the selected bridge sites namely Kulfo, Alge, and Wajifo are low risk whereas Sile and Sego Bridge sites are the most scour critical. Moreover, Sego Bridge is severe for 50, 100, and 500-year return period floods due to the extreme narrowing of the valley at the bridge site and continuous upstream channel lateral widening of the floodplain through time. This indicates that this bridge needs a new relief culvert/countermeasure for safe operation. Thus, the design and analysis of the bridge structure in the lower alluvial reaches of the river requires a thorough understanding of the geomorphology and provision of adequate openings for the potential floodplain widening.