Local Scour at Bridge Piers: the Case of Failure of Kulfo River Bridge at Arba Minch, Southern Ethiopia

The scouring of the streambed around bridge piers can be caused by the characteristics of the stream itself, due to the contraction of the flow by the bridge crossing and/or due to other human interference upstream and downstream of the crossing. Kulfo River Bridge failure in October 1997 due to excessive scour under one of the piers after a flood event is one typical case. It was known that excessive sand/gravel was being extracted for construction purposes prior to the failure. To determine the extent and specific cause of the failure, river survey, bed material sampling and simulation of the pier scour under different river bed scour scenarios were conducted. From the bed material sampling, the grain size was found to vary from very fine sand to small cobbles, showing that the bed is in the coarse-grained class – gravel bed alluvial river. Simulation of the pier scour was done using BRI-STARS (Bridge Stream Tube Model for Alluvial River Simulation) model. The model was calibrated in two phases: (i) calibration of hydraulic parameters, and (ii) calibration of sediment transport parameters.Measured water surface elevations and Manning’s roughness coefficient were used as calibration parameters. Among the two gravel/sand mining scenarios (wet-pit mining from the active channel, and bar skimming or scalping above low water level) simulated, the wet-pit mining resulted in maximum scour under the bridge pier when it is followed by a 100-year flood and consequent failure. This result shows that sand/gravel extraction operations within or near the streambed have direct impact on the stream morphology and river structures.