Grand Circulation Process of Beach Cusp and its Seasonal Variation at the Mang-Bang Beach from the Perspective of Trapped Mode Edge Waves as the Driving Mechanism of Beach Cusp Formation

摘要

Using the measured data of waves and shore-line,we reviewed the grand circulation process and seasonal variation of beach cusp at the Mang-Bang beach from the perspective of trapped mode Edge waves known as the driving mechanism of beach cusp.In order to track the temporal and spatial variation trends of beach cusp,we quantify the beach cusp in terms of its wave length and amplitude detected by threshold crossing method.In doing so,we also utilize the spectral analysis method and its associated spectral mean sand wave number.From repeated period of convergence and ensuing splitting of sand waves detected from the yearly time series of spectral mean sand wave number of beach cusp,it is shown that the grand circulation process of beach cusp at Mang-Bang beach are occurring twice from 2017.4.26 to 2018.4.20.For the case of beach area,it increased by 14,142 m2 during this period,and the shore-line advanced by 18 m at the northen and southern parts of the Mang-Bang beach whereas the shore-line advanced by 2.4 m at the central parts of Mang-Bang beach.It is also worthy of note that the beach area rapidly increased by 30,345 m2 from 2017.11.26.to 2017.12.22.which can be attributed to the nature of coming waves.During this period,mild swells of long period were prevailing,and their angle of attack were next to zero.These characteristics of waves imply that the main transport mode of sediment would be the cross-shore.Considering the facts that self-healing capacity of natural beaches is realized via the cross-shore sediment once temporarily eroded.it can be easily deduced that the sediment carried by the boundary layer streaming toward the shore under mild swells which normally incident toward the Mang-Bang beach makes the beach area rapidly increase from 2017.11.26.to 2017.12.22.