Research Of Beach Response To Continuous Storms Using Argus Monitoring Data

70%of coasts are under erosion worldwide,and 49.5%of sandy coasts in China are under erosion.Therefore,how to protect coast from erosion effectively,especially on the sandy coast,has been a hot topic globally.Sandy beaches are usually precious tourism resources and crucial to the whole coastal tourism industry.However,coastal sandy beaches are very scarce in Zhejiang Province,where the sandy coasts are prone to attacked by typhoons,with an average of more than 6 times a year.Under the attacked by large waves caused by typhoons,the erosion and degradation of the beaches are serious,thus the natural recovering processes cannot manage to restore.The conflict between decreasing of beach resources and people's increasing demands has become increasingly remarkable in the recent years.This requires us to make more scientific,rational and sustainable development and utilization of beach resources.To do this,we need to understand the morphodynamic evolution of Zhoushan's beaches system,especially under storm conditions.In this paper,we used the Argus video image data of 2016,the near-shore wave,tide level,wind and other dynamic data of Dongsha beach and the surface sediments of the beach,and then obtained the beach topographic data of Dongsha beach with numerical simulation and Argus monitoring data.Furthermore,the response characteristics of the beach to the continuous storms "Malakas" and "Chaba" were analyzed from the aspects of beach topography,section,intertidal shoreline,beach width(out of water),and surface sediment characteristics.We discussed the factors affecting the storm's changes to the beach from the beach's own factors,external dynamic factors,as well as human activities and other aspects,and finally obtained the following findings:(1)The average volumetric change after the storm "Malakas" was-73.75 m3/m,while the average volumetric change after the storm "Chaba" was-54.56 m3/m.The erosion degree of the beach before and after the continuous storms appeared as follows:straight zone>the upper shadow zone>the lower shadow zone,and the changes of sections all showed the basic features of scouring and silting along the boundary line of the multi-year average sea level in Zhejiang coast.The slope of each section of the beach generally decreased after the storm occurred.(2)Intertidal shoreline of the beach retreated by an average of 14.75 m after storm"Malakas",and the storm "Chaba" caused the intertidal shoreline retreating landwards 10.91 m(compared with the intertidal shoreline before the two storms).The average beach width before the two storms was113.06 m,and it changed to 98.31 m after the first storm,and then changed to 102.15 m after the second storm.(3)As to lateral variation,sediments with finer grain sizes moved with the water to the sea due to the sediments sorting by hydrodynamic forces,making the sediments' grain size at the higher elevation location of beach and the middle elevation situation of beach become coarser and the grain size of the low bank become finer.From the longitudinal point of view,due to the protection of the horns and the effects of sediment transportation along the coast,the sediments at the lower shadow zone were most stable,and the hydrodynamic disturbances were prominently present at the straight zone and the upper shadow zone.(4)The distribution of different sediment size(gravel,sand,silt,etc)in different parts makes the geomorphological characteristics different at along the beach profile.During the storm,the strong hydrodynamic force transported the fine sediment to the sea,and the coarse-grain gravel occurred in the beach surface.A series of changes in the composition and distribution of beach sediments resulted in corresponding changes in the beach geomorphology(like changes of gravel belt).(5)Dongsha beach faces southeast and is covered by horns or islands in the north and south,which makes the southeast direction waves have the greatest impact on the geomorphological change.The sediments were mainly caused by wind and hydrodynamic forces escaping the near-shore bedrock and weathering and transport of the island land rocks.Sediments lost a lot after the storm,but it was difficult to get supplies.The eastward waves during the two storms directly hit the beach,and the wind direction increased water level in the sea area near the research area.The abnormally high water level made the scouring and deposition of the beach section more obvious,and the cumulative effect of sea level rise would also increase the typhoon storm surge and coastal erosion.The establishment of the seawall obstructed the source of sediment and caused the beach to be eroded without supplies from the hinderland.The water discharged from the drainage outlet increased erosion of the beach,and the beach nourishment can help the beach recovering to some extent.