Meso-macro-tidal Beach Morphodynamics Of Beibu Gulf

The coastal zone is the most intense zone of land-sea interaction in the world,and its vulnerability is generally rising in the context of global sea level rise,increased storms and reduced sediments into the sea.Since the coastal zone is the main area of human production and living around the world,it faces heavy environmental carrying capacity while providing important economic and social benefits.Beaches are important part of the coastal zone.In addition to the general vaules of the coastal zone,beaches have high values of tourism and prevention/mitigation of disaster,but the vulnerability of beaches is higher compared to other types of coasts.Therefore,it has a dual important significance that studying beach morphodynamics—the scientific value that deepens the understanding of the changes in the Earth system and the social value that provides the basis for rational coastal management.Due to the lack of high-spatial resolution beach terrain survey in the researches of beach morphodynamics,the rich information contained in the beach morphology lacks excavation.Therefore beach morphodynamics is less involved in the overall beachface and microgeomorphologic structure of beach,especially the lack of profound understanding of the morphodynamic behavior and modes of beach response to typhoon and subsequent short-term recovery.At the same time,there are many different opinions on the mechanism of erosion and accretion in low wave energy beaches,and none research analyzes this topic from the perspective of self-organization.In addition,the rapid economic and social development along the Guangxi Beibu Gulf leads to a significant increase in coastal vulnerability,so detailed research on beach morphodynamics in the region is urgently needed.For theses reasons,this paper takes Beihai Yintan,a typically straight beach in Guangxi Beibu Gulf,as an example.Based on Landsat image,inter-monthly beach profiles,high-spatial beach terrain provided by light detection and ranging and hydrodynamic data,statistical analysis,numerical simulation and conceptual models etc.are used to explore the morphodynamic processes of Yintan beach at different time scales.Besides that,from a self-organization perspective,a new understanding of the mechanism of erosion and accretion is proposed for a low wave energy beach.The main findings include:(1)The Yintan shoreline was propelled seaward driven by human activities,and it retreated landward in the natural environment.The shoreline of the Yintan beach experienced a seaward advance of 50 m/yr in the 1990 s due to high-intensity coastal development,and then the shoreline retreated landward at the rate of 16 m/yr in the first ten years of the 21 st century in the natural state.The shoreline experienced a seaward advance of 12 m/y in the next five years due to another high-strength development of the Yintan.The sections of Yintan,affected by aquaculture,real estate and port construction,had the most significant contributions to the changes of Yintan shoreline.The sections of Yintan were affected by the construction of tourism facilities changed little,while the shoreline of the relatively natural shore(near Baihutou Village on the east side of the Yintan beach)showed an erosion of 3 m/y controlled by the dynamic environment after 2000.Further research on the relatively natural shore of the Yintan beach shows that the shoreline retreated landward at a rate of 3.6 m/yr between 2014 and 2018,and the shoreline changes were controlled by the dynamic environment which is characterized by the relative tidal range.After empirical orthogonal decomposition(hereinafter referred to as EOF)on this shoreline,the main and secondary modes of the shoreline evolution were the back and forth oscillation of the shoreline in the cross-shore direction and the accretion of western shoreline and the erosion of eastern shoreline in the alongshore direction.(2)There is significant heterogeneity of the beach profiles in the alongshore direction.The equilibrium profiles are featured with the slightly concave shape on the western beach,while the equilibrium profiles are featured with the convex shape on the middle and eastern beach.This difference comes from the intermediate beach state on the western beach and the reflective beach state on the middle and eastern beach.After EOF analysis,there are four main modes of sediment transport in each profile of the beach,but there are differences in sediment transport patterns in each profile.In the first mode,the sediment is mainly transported in the mid-tidal,low-tidal and subtidal zones in the four profiles of the western beach,and the eight profiles of the middle and eastern beach have simultaneously sediment transport of two directions in the hightidal,mid-tidal and low-tidal zones.In the second mode,the most profiles of the beach are dominated by erosion and accretion on the whole profile.In the third mode,the eight profiles of the middle and western beach have sediment transport of two directions in the high-tidal,mid-tidal and low-tidal zones,and the seven profiles of the middle and eastern beach have sediment transport of three directions in the high-tidal,mid-tidal and low-tidal zones.In the fourth mode,the seven profiles of the middle and eastern beach have sediment transport of two directions in the high-tidal,mid-tidal and lowtidal zones,and the other profiles had unique sediment transport patterns.(3)The beach morphology has seasonal characteristics.The shape of the beach shoreline changes drastically between spring and summer,and then gradually “returns” to the spring form in autumn and winter,thus forming a seasonal cycle.The beach profile presents two seasonal shapes.In winter and spring,the profile has narrower beachface,steeper slope,and significantly convex shape.In summer and autumn,the profile has no significant convex shape,wider beachface and smaller slope.An analysis of the daily morphodynamic process of the beach in the typical season-spring shows that,under the influence of the beach terrain,the strong waves break before propagating into the high-tidal zone,resulting in the behavior of swash bar welding to land in the middle-tidal zone,but the complexity of the beach micro-geomorphology changes little.The weak waves propagate to the high-tidal zone which occupies most part of the beach,resulting in large changes in the complexity of the beach micro-geomorphology,and the swash bar changes little.Through the EOF analysis of the beach,the main and secondary morphodynamic models of the beach in the spring are the changes of the micro-geomorphology of the high-tidal zone dominated by the weak waves and the evolution of macro-landform featured with the swash bar welding to land under the strong waves.(4)Beach response to typhoon actions are different.For different typhoon events,the beach response to typhoon events depends on the extent of the surf zone and the typhoon track.When the typhoon track is far away from the Yintan beach which is characterized by the ultra-dissipative beach state,the typhoon with strong deep-water wave energy is significantly dissipated before reaching the beach due to the significant widening of the surf zone to the sea,so its impact on beach geomorphology is weaker than other typhoons with weaker deep-water wave energy.When the typhoon track is closer to the beach,the typhoon can replenish energy for waves when the waves propagate in the surf zone,that leads to a strong influence on the beach morphology,so the vulnerability of the beach under this kind of typhoons should be paid attention to.There are significant differences in the response of different morphodynamic zones to the single typhoon event.In the cross-shore direction,the wave energy significantly decreases from surf zone-swash zone-backshore.Correspondingly,the typical contours of surf zone-swash zone-backshore pre-and post-typhoon show the varying frequency distributions of contour changes,which change from Gaussian distribution to power law.The novel conceptual model reveals that the frequency distribution of the contour changes tends to be Gaussian distribution when the external force dominates the system,while that tends to be power law when the internal action of sediment dominates the system.(5)The beach evolution shows different stages in the short-term recovery after typhoon impact.The short-term recovery of the beach is not a “gradual recovery” process,and has three distinct stages of “weak stability-oscillation-stability” and two turning points characterized by extreme accretion.The beach is in a weakly stable stage characterized by net erosion within 13 days after the typhoon impact,during which beach landscape changes slightly.Subsequently,the extreme accretion occurres on the beach,then the beach short-term recovery turns into the second stage,which lasts for 9 days and characterized by the oscillation of the micro-geomorphology.During the second stage,the net erosion of the beach is small while the total erosion and accretion of the beach is large,and the shape of sandbar changes drastically.Afterwards,the beach has extreme accretion once again,then turning into a stable stage featured with slow net accretion,and the shape of sandbar becomes similar to that pre-typhoon.The reconstruction of the intertidal sandbar dominates the short-term recovery,while the wide backshore behind the intertidal sandbar only has slight changes in the short-term recovery.In addition,combined with the beach evolution in the spring,the morphological change of beach is likely to have three essential attributes in statistical sense: 1)the frequency distribution of the dominant slope is consistent with the gamma distribution;2)the frequency distribution of the erosion and accretion is consistent with the generalized Gaussian function;3)the net erosion events of beach are more uniform than the net accretion events of beach,and the net accretion of beach is mainly controlled by fewer extreme accretion events.(6)Under normal hydrodynamic environments,there is a self-organized critical state in beach evolution.By analyzing the daily erosion and accretion patterns of the beach,it can be seen that when the daily average relative tidal range(hereinafter referred to as MDRTR)is between 3 and 7,the frequency distribution of the cluster size of the erosion pattern of the beach conforms to the truncated power law distribution(hereinafter referred to as TPL),and the frequency distribution of the cluster size of the erosion pattern is not in accordance with TPL when MDRTR is in other numerical ranges.Meanwhile,all the frequency distributions of the cluster size of the accretion pattern don't conform to the TPL.These results imply that the erosion behavior of the beach comes to the self-organized critical state when MDRTR ranges from 3 to 7.Combined with the novel cellular automaton model,it can be concluded that the selforganized critical state in erosion behavior leads to bistable state of beach erosion,and the occurrence of bistable state leads to the nonlinear relationship between beach erosion and MDRTR.In the spring and summer,the extreme accretion events on the beach are the results of the beach evoloving to the steady-state featured with “none erosion” when MDRTR ranges from 3 to 7,and the initial erosion and accretion condition of the beach decides the beach evoloving to which certain steady-state in this hydrodynamic environments.