Short-term Morphodynamics And Sediment Features Of Beaches

Beach is a sensitive ocean-land interaction zone.The beach sediments are closely related to the evolution of topography,geomorphology and dynamic sediment environment.The particle size and shape analysis is an important means to investigate the sedimentary dynamics and landform evolution of the beach.There have been great research achievements in particle size distribution and hydrodynamic environment of beach sediments and the response of landform evolution.However,study on particle shape of beach sediments is less concentrated,mainly because of the lack in relevant experiment means.Haitang Bay,which is located in the southeast of Sanya City,Hainan Province,is a typical headland-bay beach.Sediments of Haitang Bay mainly include sand and fine gravel and the shoreline of it is basically stable.Combined with dynamic image analysis,this paper studies the particle size and shape characteristics of irregular particles in Haitang Bay beach.The effects of roundness,sphericity,aspect ratio,symmetry and concavity of the sediments on the backshore,berm,beach face,low-tide shoreline and underwater parts of the beach profile and their response mechanism are discussed emphatically.The results of research can provide a scientific basis for coastal resource development and protection.Specific conclusions are as follows:(1)Haitang Bay beach showed a weak erosion trend these years,and different parts of beach profile vary by season and typhoon event.In summer,Tufu Bay and the middle section of Haitang Bay showed weak siltation while Houhai Bay was in erosion state,and there is opposite erosion-accretion trend in winter.Under the influence of typhoons,Tufu Bay and some parts of central Haitang Bay showed weak siltation,and the Houhai Bay was in an eroded state.The period of beach restoration after typhoon,Tufu Bay made the fastest recovery in Haitang Bay,followed by the middle section of Haitang Bay,the slowest was Houhai Bay.Mainly because the longshore sediment transport of Haitang Bay is carried out from south to north in summer.(2)The seasonal variation of sediments in Haitang Bay beach is obvious.The medium sand content of Haitang Bay sediments is the highest,accounting for 96.84%of the total,and it has good sorting feature.The average particle size of beach sand in winter is coarser than that in summer,and beach sand in summer has better sorting feature with more negative bias than that in winter.The reason is mainly that Haitang Bay is in the weak-erosion state in winter.(3)The particle shape distribution of the sediments in Haitang Bay before and after the typhoon has obvious regularity,and the particle shape is sensitive to the typhoon response.Roundness has the highest sensitivity to the dynamic environment,followed by the sphericity and the aspect ratio.The symmetry and the concavity has the lowest sensitivity.Typhoon can shape and sort the beach sand well,which makes the shape of beach sand better.During the beach restoration after typhoon,the shape of beach sand gradually become worse and gradually restored to the dynamic equilibrium state of the grain size and shape distribution before the typhoon.(4)There were significant differences in roundness,sphericity,aspect ratio,symmetry and concavity of sediments in different parts of the beach profile.The shape of particles is the best in berms,followed by the backshore and beach face,the low-tide shoreline is worst.Under different dynamic environment,the particle shape parameters of the berms and beach face are relatively stable,while the low-tide shoreline and underwater parts of beach change greatly.