Scale effect occurs when a prototype hydraulic process is simulated at a laboratory scale due to dissatisfaction of similarity laws. It might lead to considerable deviation when the model scour depth is extrapolated to prototype value. Three popular experimental approaches including predictionequation-targeted flume test, series-model test and similitude-model test are reviewed with emphasis upon their merits and limitations in reducing or alleviating scale effects. Scale laws guiding scour physical-model design are discussed for performing cost-effective model tests. An empirical equation is further derived from data of clear-water pier scour experiments to examine the test results in up-scale extrapolating. It is suggested that scale effect in a scour physical model test could be efficiently reduced if both the mobility similarity of bed particles and Froude number similarity are satisfied simultaneously.

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