Effects of Foundation Geometry on Bridge Pier Scour

An experimental investigation of local scour at a nonuniform cylindrical pier is presented. A nonuniform pier is one for which the cross-sectional dimension varies over the length of the pier. In this study the pier comprised a cylinder of diameter D founded on a larger cylinder of diameter D *, called the foundation. Besides the parameters that affect scour at a uniform pier, the scour depth at a nonuniform pier is dependent on the ratio D /D* and the depth from the bed level to the top of the foundation. The results are used to define three scour zones, namely: zone 1 where the foundation is below the bottom of the scour hole and does not affect the scour, zone 2 where the top of the foundation is within the scour hole and reduces the scour, and zone 3 where the top of the foundation is above bed level and increases the scour compared to that at a uniform pier. Various design relationships are presented. The concept of an effective pier size ( D e ) is introduced, and it is shown that use of D e leads to conservative estimates of scour depth for nonuniform piers.