Complicated Response Of Beach Erosion And Restoration To Consecutive Typhoons

Beach landforms show a significant react to typhoon,as an active response unit of sedimentary dynamic topography.Beach usually suffered great erosion and changed a lot on topography and the disrtibution and consist of sediments,under the influence of strong typhoons.Especially,beach shows a complicated erosion and recovery process under impacts of consecutive typhoons.In order to study the erosion and accretion response of beaches to two typhoons,representative beach profiles along northern and northeastern coast,Hainan Island,were observed and measured before and after typhoonRammasun and two months after typhoon Kalmaegi in 2014.In addition,surficial sediments in each transect were collected to study their depositional environment change.The study aims to find the influence factors of the results from consecutive typoons and summarize the law of change,which would rich the cases that beach has various response to different typhoon conditions and provide advice on beach development and recovery.After the study,the following conclusions are obtained:I.Response of beach profile?The responses of the beach profile to the Rammasun typhoonIt caused severe erosion to beaches which eroded on upper and accumulated on lower during the Rammasun typhoon.Beach slope overall slowed down,berm downed or disappeared and new berm moved toward sea.The shore of mean sea level showed various changes.The order of erosion of bay from strong to weak is Mulan Bay,Puqian Bay,Haikou Bay,Baohu Bay and Hainan Bay.The strength of erosion decreased along the coastline gradually in Haikou Bay,Puqian Bay and Mulan Bay,which showed longshore transport.The strength of erosion was almost equal among profiles in Hainan Bay and Baohu Bay,which showed crossshore transport.?The recovery condition of beach profiles after consecutive typhoonsIt mostly performed accumulation on beach which accumulated on upper and eroded on lower after two months of the Kalmaegi typhoon by daily waves and currents.Beach slope overall became steep,berm got high and new berm position moved toward coast.The shore of mean sea level trended to the position before the Rammasun.That help beach to restore.The order of accumulation of bay from strong to weak is Mulan Bay,Puqian Bay,Haikou Bay,Hainan Bay and Baohu Bay.The strength of accumulation decreased along the coastline gradually in Haikou Bay,Puqian Bay and Mulan Bay,which showed longshore transport.The change was almost equal among profiles in Hainan Bay and Baohu Bay,which showed light erosion or accumulation.The stronger erosion suffered during the former typhoon,the more accumulation got during the restoration process.It was summarized as "stronger erosion and better restoration".But in Hainan Bay and Baohu Bay,which influenced by geological features a lot,showed "strong erosion but weak restoration".Compared to the restoration of beach under influence of half year daily wind and waves after the Kalmaegi,the change of beach almost happened in foreshore and backshore changed little.II.Influencing factors of typhoon response?Wind direction versus shoreline directionBecause of shoreline direction,the two typhoons created onshore wind before landing and offshore wind landed.The angles between shorelines and onshore winds,which caused longshore transport when they were small and caused crossshore transport when they were large,were different.The onshore winds strengthened the erosion of the beaches during the consecutive typhoons.The offshore winds decreased sediments transport,they weakened the erosion of the beach during the consecutive typhoons.?Coastal rock reefs,coral reefs and headland landscapesThe beaches of Hainan Bay and Baohu Bay were under the protection of coastal rock reefs,coral reefs and headland topography,which performed decreasing beach erosion and beach restoration during the consecutive typhoons.?The distance between beach and isometric lineThe profiles in the same bay showed more erosion on backshores,which were closer to the same isometric line,and showed less erosion if the distance was larger,during the Rammasun.But during the restoration process,it found that if the distance between profiles and the same isometric line was small,the backshore got more accumulation,and if the distance was large,the backshore accumulated less.III.Response characteristics of surface sediments in the beach?The response of surface sediments to the typhoon RammasunAfter typhoon,the sediments in foreshores became coarse,and the sorting worsed.The sediments in backshores became coarse except the profiles in Mulan Bay and Baohu Bay,of which the sediments became fine,and the soring overall got better.The range of sediments diameter changed greatest in Haikou Bay,and least in Baohu Bay.The range of sorting coefficient changed greatest in Puqian Bay,and poorest in Hainan Bay.The characteristics of surface sediments changed a lot in Haikou Bay,Puqian Bay and Mulan Bay,which suffered more erosion,and changed a little in Hainan Bay and Baohu Bay,which suffered little erosion.? The response of surface sediments during the restoration process After the consecutive typhoons,under the influence of daily winds and waves,the sediments in foreshores became fine,and the sorting got better.But the characteristics of sediments in backshores became variously.The sediments got fine in Haikou Bay and Puqian Bay,and became coarse in Mulan Bay.It showed little change in Hainan Bay and Baohu Bay.The sorting of sediments worsed in Haikou Bay,Puqian Bay and Mulan Bay,and varied little in Hainan Bay and Baohu Bay.The range of sediments diameter changed greatest in Puqian Bay,and least in Baohu Bay.The range of sorting coefficient changed greatest in Mulan Bay,and poorest in Hainan Bay.The characteristics of surface sediments changed a lot in Haikou Bay,Puqian Bay and Mulan Bay,which accumulated more during restoratioin process,and changed a little in Hainan Bay and Baohu Bay,which accumulated fewer.The rule of response of the surface sediments in beaches is "the more severe destruction,the more restoration and the less severe destruction,the less restoration".The profiles,of which the characteristics of sediments changed a lot during the Rammasun,got well restored during the restoration process.? the change rule of the composition of surface sedimentsThe consecutive typhoons mainly led to severe destruction to beach,after the typhoon,the composition of sediments changed a lot,and the curve of frequency distribution deviated from the state before typhoon much.The daily winds and waves mainly caused restoration to beaches,which helped the composition of sediments and the curve of frequency distibution trended to the state before typhoons.The consecutive typhoons made the beaches go through a process,which first eroded and then restored.On the whole,compared to the state before typhoons,the beaches were still in the state of destruction,which showed a great plasticity of typhoon on beaches.The beaches could restore within a period under the effects of daily winds and waves.And the time of restoration was influenced by the level of typhoons,which the beaches suffered.