Dimensionless Effective Flow Work for Estimation of Pier Scour Caused by Flood Waves

The pier scour caused by flood waves is analyzed, introducing the dimensionless effective work by the flow on the sediment bed around the pier, $W^{*}$. The three-parameter exponential function is adopted to relate the normalized scour depth $Z^{*}$ with $W^{*}$. A novel experimental installation able to reproduce any hydrograph with high precision in the laboratory flume is described and used to carry out four series of scour experiments in order to calibrate and validate the proposed relation. The first series consists of experiments with constant discharge until advanced stages of scour. The second and third series of experiments use single flood waves of different shapes and durations, respectively. The fourth series consists of scour experiments caused by more realistic flow hydrographs with multiple peaks. Results show that the relation between $W^{*}$ and $Z^{*}$ is unique and thus $W^{*}$ represents a reliable concept for the prediction of the flood wave scour because it appropriately integrates the effects of the hydrograph properties, duration, peak discharge, and shape, on scour. The proposed relation allows a straightforward prediction of maximum scour depth after a flood wave with high precision. A good agreement between measured and computed scour was observed in all cases.