This paper aims to analyze temporal evolutions of pier scour in live-bed condition.Five steady flow experiments are conductedwith different flow intensitiesratio of mean velocity to critical velocity varying from 1.18 to 2.21. To find out the effects of unsteady flow on scour depth,
different steady flow intensities were combined as four unsteady flow experiments, including: (1)
advanced hydrograph, (2) delayed hydrograph, (3) symmetric hydrograph and (4) symmetric hydrograph
with low peak. A semi-empirical model proposed by Hong et al. (2014) was used topredict thetemporal variation of pier-scour depth. Moreover, alinear-combination hypothesis, i.e.scour depth hydrograph of unsteady flow can be simulated by a linear-combination ofscour depth
hydrographs of steady flows,was proposed to extend Hong et al.’s model inunsteady flow conditions.
Comparison of the experimental and model resultsshowed that the present modeldid not perform perfectly on the variation of dune migration velocity with changing flow intensity is changing. Simulation and experiment resultsrevealed a phase difference because of the effect of dune migration velocity.However,the error betweenthe simulatedand experimentalscour depths ranged from 8% to10% , including that the present modelcan estimate the temporal variationof scour depth for cyliderical piersunder unsteady flow conditionsreasonably well.
Key Words : Live-bed, unsteady flow,local scour,pier.