Sills and gabions as countermeasures at bridge pier in presence of debris accumulations

Transverse structures, such as sills, can reduce local scour at bridge piers. However, in the presence of a sill, the scour hole evolves more rapidly with a debris accumulation, yielding a different behaviour of the countermeasure. This research aims to analyse the efficiency of sills and gabions as countermeasures against local scour in the presence of simplified debris geometry. Experiments were conducted under clear-water conditions with the approach flow velocity set at the threshold of the sediment motion. The temporal evolution and the scour morphology in the presence of smooth sills and gabions were investigated. The efficiency of the countermeasures was assessed by referring to a pilot test with an unprotected pier. Although a delay occurs in the scour development, the main findings reveal an insufficient countermeasure efficiency for large scour development in the presence of large debris accumulation, while gabions were able to further delay the scour process mainly due to larger surface roughness when compared with smooth sills.