Assessment and Upgrading of Scour Depth Estimation Equation for Bridge Piers in Kombolcha-Weldia Highway, Ethiopia

Abstract

Frequent scouring caused by turbulent river flow has significantly impacted bridge stability along the Kombolcha-Weldia highway in Ethiopia’s Amhara region, leading to foundation erosion over successive years. To address this, six established scour depth estimation equations—Colorado State University (CSU), Bruisers, Jain and Fischer, Froehlich, HEC-18/Muller, and Laursen’s empirical equation—were evaluated for their accuracy in predicting bridge pier scour depths. A comprehensive field study was conducted, including topographic surveys, assessment of pier alignment and shape, riverbed material sampling, and direct scour depth measurements at five selected bridges. Peak flood discharge was calculated using the Rational Method for catchments under 50 km² and the Soil Conservation Service (SCS) unit hydrograph method for larger catchments. Total scour depth was determined by considering three key components: long-term degradation, constriction scour, and local scour. Critical variables such as pier width, shape, and flow alignment were found to significantly influence scour magnitude. The predicted scour depths from the selected formulae were compared with field-measured values. Comparative analysis revealed that the equations proposed by Jain and Fischer, Laursen, and Froehlich provided more accurate scour depth estimates than the other methods. Among these, the Jain and Fischer equation was identified as the most suitable for predicting local scour in both sand-bed and gravel-bed rivers within the study area. Based on these findings, it is recommended that the Jain and Fischer equation be prioritized for scouring depth estimation in similar hydrological and geomorphic conditions, ensuring better assessment and mitigation of bridge foundation risks in the region.

Keywords: Bridge Pier Scour; Design return period; Jain and Fischer equation; Kombolcha-Weldia; Scour depth estimation.