Study On Multi-scale Morphodynamic Characteristics Of Sandy Coasts Along The Qiongzhou Strait

Sandy coasts is a geomorphic type formed by the accumulation of loose sediment,which is a precious resources.The evolution process of beaches is the result of complex geological and hydrodynamic factors,and is also controlled by social,economic,and other human factors.Conducting research on beach evolution is an important way to reveal the mechanism of sea-land interaction.Simultaneously,it is of great significance for the tourism development of beaches,coastal restoration,and coastal engineering construction.Processes of beach evolution include small-scale(hours,days),mesoscale(months,seasons),long-scale(years,decades)and other scale processes.Restricted by the nonlinearity and uncertainty of the relationship between hydrodynamics,sediment transport,and terrain changes,data-driven models with high-precision spatio-temporal observation data in field are main approaches to explore the beach process inherent characteristics of beach changes.This study took some typical bays along the Qiongzhou Strait as the research object,and carried out the evolution research on different time scales.On the small scale,two-dimensional data,one-dimensional profile point data and wave time series are processed by the Hilbert spectrum analysis and filtering method based on empirical mode decomposition(EMD).Beach change modes are decomposed by empirical orthogonal function(EOF)on the medium scale.The combination of ENVI and Arc GIS is used to extract the shoreline changes of remote sensing images,and to infer the trend of shoreline changes,on the long scale.The following conclusions are drawn:1.Geomorphic evolution characteristics of Qiongzhou Strait on small time scalesThe typhoon Bebinca(No.16 of 2018)is the most unusual and complex storm event that has occurred in this region over the last 10 years.During the storm,the beachface response process occurred in three stages.The first stage was the rapid response stage,wherein the spring tide berm began to erode significantly,and the front edge of the beach berm reacted closely.The two beach sections resisted the harmful energy of the main storm.In the second stage,the beach slope increased after a large sediment loss on the beach berm and its front edge.To adapt to the storm energy,the beach at the low tide line began to erode,and the beach slope decreased.In the third stage,after the storm turned,the wave energy was significantly attenuated,and the beach berm eroded to resist the residual wave energy.The beachface began to oscillate and recover.High-frequency driving factors determine the local characteristics of beach evolution,and low-frequency driving factors determine the beach evolution trend.Moreover,the response of sediment to the storm was not a single sea-transportation,but a single-or two-way conversion driven by factors such as wave energy,swash flow,and secondary wave breaking.2.Geomorphic evolution characteristics of Qiongzhou Strait on the meso-time scalesThe beach profile data(April 2018-March 2019)of Puqian Bay,Haikou Bay and Chengmai Bay in the southern Qiongzhou Strait were analyzed.The main features are siltation.Beaches of Puqian Bay and Haikou Bay showed the seasonal characteristics of silting in the summer to autumn and eroding in the winter to spring.Chengmai Bays beach had the seasonal characteristics of eroding in the summer-autumn and silting in the winter to spring.During the observation period,the impact of the typhoon on the beach was limited.At the same time,the typhoon response of the beaches is closely related to the typhoon intensity,the landing distance,the relative typhoon position and the local terrain shelter degree.Artificial islands in The Puqian Bay and Haikou Bay have increased the bay’s sheltering and promoted the silting of part of the gulf’s beaches.It will be some time before the beach reaches a new balance.3.Long-time scale geomorphic evolution characteristics of Qiongzhou StraitIn Qing’an Bay,north of the Qiongzhou Strait,we have conducted continuous surveys of beach profiles for many years,and these datasets identify erosion/deposition cycle changes in which corrected shoreline changes are consistent with ENSO’s phase shift.Using digital shoreline analysis system(DSAS)and related analysis methods,the evolution process and controlling factors of the month-scale coastline and single-width volume of Qing’an Bay were discussed.An obvious beach cycle was found in Qing’an Bay.Among them,the overall change trend of the Qing’an Bay coastline from 2017 to 2021 shows that the west coast is largely silted,oscillated,and eroded slightly,and the east coast is slightly silted and oscillated.The shoreline is rotated,and the new net sediment transport direction is presumed to be from southwest to northeast.When the strong La Ni(?)a event coincides with the storm surge season,the overall erosion of the west coast of Qing’an Bay is more severe.When the winter coldwave and El Ni(?)o/La Ni(?)a were jointly affected,the coastline of Qing’an Bay retreated slightly,but it produced a large single-width volume increment.Although the Qing’an Bay beach affected by a single winter coldwave also had a recovering single-width volume increase,the coastline moved towards the sea.The results of the coastline changes on the north and south sides of the Qiongzhou Strait in the past 10 years show that,the distribution of coastlines in the Qiongzhou Strait is quite characteristic.The northern coast is dominated by reclamation and sandy coastlines,while the southern coast is dominated by urban industrial coastlines,sandy shorelines and Rock shorelines,and with the development of the economy,the subjectivity of artificial shorelines will be greatly improved.In particular,the economic development of the south bank of the Qiongzhou Strait is relatively fast,the urban industrial coastline has become more regular,and the direction of change is advancing towards the sea.The land area has increased greatly,but the length of the coastline has decreased.