Wave Transformations on Coral Reefs

Most of the Ryukyu islands are surrounded by coral reefs. Transformations of the waves approaching the island's reef coasts have been studied by many researchers through field observations and model experiments. An estimating formula for wave height on reef seas was suggested by Takayama et al., and is now used in practical applications. However, it seems that the tests of this formula by past field observations are not sufficient. This is because the conditions of the experiment from which the formula was derived, were different from field conditions, especially in the case of coral reef seas, when the incident wave height is so great as to be accompanied by surf beats. Long period components of incident waves cannot be accurately reproduced in models for effects of multi-reflecting waves in wave channels. Furthermore, previous field observation data used in the testing, could not adequately cover the whole range of periods of waves consisting of incidents wave groups, because not enough wave meters were used. In our study, nine wave meters were used in field observations simultaneously, to observe the incident waves and transmitted waves on reef flats. We succeeded in obtaining field data corresponding to very large incident waves, having significant wave heights of over 10m. Based on our field observations, we have tried to clarify the characteristics of wave transformations and test the coefficients contained in the estimating formula for wave height as mentioned above.