Use of spur dikes with different permeability levels for protecting bridge abutment against local scour under unsteady flow conditions

Local scour around abutments is one of the most important reasons for a bridge collapse, making it a major concern for hydraulic and river engineers. Available studies on scour control around abutments have been limited to steady flow conditions. In this light, the present experimental study investigates using a single spur dike with different levels of permeability (0%, 35%, 50%, and 65%) to reduce scour around a short vertical-wall abutment (abutment length/flow depth < 1) under unsteady flow conditions. The hydrographs with Gaussian distribution and different base flow times (i.e., duration of 15, 30, 60, and 90 min) were used to simulate the unsteady flow. Abutment scour depth variations with time showed that the final scour depth always appeared after the peak discharge of the hydrograph regardless of whether a spur dike was used. It was shown that the spur dike has successfully reduced local scour around the abutment. The maximum depth of scour hole around abutment was reduced by 47%, 32%, and 11%, when spur dikes with 35%, 50%, and 65% permeability were, respectively, used. Using an impermeable spur dike not only prevented the scour upstream nose of the abutment but also led to some deposition that raised the bed level by nearly 0.47La (where La is the abutment's length) in that area. However, the maximum depth of scour hole around the impermeable spur dike was nearly identical to that occurred around the abutment for the experiments without a spur dike, making it necessary to arrange for scour protection around the spur dike.