| Beach,which is one of the most valuable landforms in the coastal zone,acts as the first barrier against the violent hydrodynamic forces of waves,tides and storm surges.The stability of beach always affects the life and property safety of nearby residents.At present,global changes and increasing extreme storm events are causing serious damage to the beaches all over the world,and the corresponding variations of beach surface sediments are also attracting the attention of researchers.In this context,this research chose Yintan Beach in Beihai City as the research object.Daily and monthly grain size data from 2014 to 2017,and large scale grain size data of beach surface sediments before and after Typhoon Rammasun in 2014 were collected at multi-time scales.Moreover,the high precision point cloud data of Yintan Beach morphology before and after typhoon was captured by a 3D terrestrial laser scanner（RIEGL-VZ4000）and RTK and corresponding hydrodynamic data was collected in China Oceanic Information Network.Based on the analysis of grain size parameters and components of sediments,wavelet analysis and EOF analysis and other mathematical statistical methods,we revealed multi-time scales and spatial differences of sedimentary characteristics in different cross-shore zones in Yintan beach,and the dominant influencing factors were discussed,The results may improve the understanding of beach process,and to provide theoretical support for and beach nourishment and maintenance.The main findings include:（1）There are obvious differences in cross-shore distribution of the sediments in Yintan Beach.Sand content was more than 98% of the surface sediments in Yintan Beach,and in which the fine sand component was the largest,with an average content of 63%,followed by very fine sand,with the content of about 19%,and medium sand content is about 15%.The the sediments in the meso-macro tidal beach is obviously finer than that in the micro tidal beaches.In the cross-shore transect of Yintan beach,it can be divided into five zones in seaward direction,which are foredune,berm,swash zone,surf zone and breaker zone.In general,the mean grain size gradually decreases,and the sorting turns bad in seaward direction.The sorting is the best in swash zone,and the skewness is generally negative.The content of very fine sand gradually increases seaward,up to 44.5%,while the content of fine sand ranges from48% to 74%,and the maximum value is in swash zone.（2）Under the condition of common wave,the daily variation processes in five zones of Yintan Beach are controlled by different dynamic factors.There are 2-4 and15-day periods of the sediments in foredune and berm.The sediments in swash zone mainly has a period of 2-6 days,and that in surf zone has a strong periodic signal of 4days.The sediments in breaker zone shows a 4-6 day cycle.According to the daily variation of sediment subdivision component content and the grey relation analysis of dynamic parameters,it can be seen that the tidal range difference caused by tidal cycle mainly controls the sediment change in the foredune,and the wind action has a decisive influence on the sediment in the grain size component of 0.088-0.15 mm.In berm,sediments variation is mainly affected relative tidal range.However,the surf-scaling parameter has the strongest influence on the sediment grain size change,especially in swash zone and breaker zone.The significant wave height in surf zone has a strong effect on the sediment grain size components of 0.18-0.25 mm,while the wave direction has a significant effect on the sediment components of greater than0.25 mm.（3）There are obvious seasonal and interannual variations of the sediments in Yintan Beach.The boundaries between uprush and backwash process show significant differences in the five zones in winter.The grain size at the boundary in berm is the coarsest,and it gradually becomes thinner seaward to breaker zone.In summer,the boundary in berm disappear,and there is little difference in grain sizes at the boundaries in the other four zones.The grain size at the boundary in surf zone and breaker zone in summer is obviously coarser than that in winter,reflecting stronger wave energy dissipation in summer.The long-term grain size variation of the beach sediments reveals the characteristics of the seaward transport of the very fine sand components in lower swash zone and surf zone,and the significant monthly fluctuation of the contents of sediments components in the five zones.（4）Under the impact of Typhoon Rammasun,the sediments distribution of Yintan Beach showed dramatic changes.The grain size parameters of surface sediments in Yintan Beach after Rammasun generally became coarser and better sorted,and the skewness was more negative than that before Rammasun.The mean grain size in breaker zone decreased from 3.0 Φ to 2.5 Φ.Before Rammasun,the main pattern of sediment components was the very fine sand content of 39% in breaker zone,then after Rammasun,the main pattern shifted to the fine sand content of 76%in swash zone.During Typhoon Rammasun,the strong wave action was the main factor for the shift of grain size distribution pattern of sediment in the surf zone and breaker zone,while the tide level controlled the boundary conditions for the dramatic changes of grain size of sediment in Yintan Beach.The mophological state of the beach mainly affected the grain size distribution in bar and trough in the swash zone.The effect of wind action increased the content of coarse particles in foredune and berm. |
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