Laboratory experiment is conducted to investigate the long waves generated by time-varying breakpoint. We focus our attention on the variation of plunging point as well as the variation of breaking point. The results show that free long waves (BFLW) are really generated by the variation of the starting point of wave set-up which coincide with the plunging point and that the BFLW observed can be quantitatively evaluated by Mizuguchi(1995) model, which is more direct and realistic model. The generation mechanisms of long waves of other origins, such as bound long waves to wave groups and free long waves generated during shoaling process of wave groups, are also discussed.

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