Recent Evolution Process And Influencing Factors Of Jinshan Trough In Hangzhou Bay

Tidal channel is one of key important geomorphic features in tide-dominated estuaries and coasts.It has attracted intensive human activities associated with harbor construction,shipping and navigation,due to the large water depth and location.Tidal channels,combined with other adjacent geomorphic features,form complicated geomorphic systems such as tidal inlets and shoal-channel-ridge systems.,Interaction of hydrodynamic,sediment transport and topography within these geomorphic systems,becomes a major concern of estuarine and coastal morphodynamic studies.Therefore,it is important both in theory and practice to study tidal channel morphodynamics.However,there are still limitations in this study field due to multiple influencing factors and their non-linear interactions.In particular,both intensified human activities and extreme meteorological events have severely disturbed the natural evolution of tidal channel,and increased complexity of the system in the last few decades.Meanwhile,such changes highlight the relationship of geomorphic systems with human activities and extreme meteorological events,and afford a good research topic.A system of tidal channels develops along the north shore of Hangzhou Bay,with an east boundary of Jinshan Trough.Before 1960,due to limited coastal exploitation and utilization,both Hangzhou Bay and Jinshan Trough were almost in a natural status,and their morphodynamic processes remained relatively stable.Since 1960,the natural process of Jinshan Trough has faced severe challenge with the increase of human activities(i.e.,coastal reclamation,harbour construction,and submarine pipelines project)and extreme meteorological events.To understand the evolutional processes and mechanisms of Hangzhou Bay and Jinshan Trough In last few decades,five periods of remote sensing data and three periods of bathymetric data from 1960 to 2014 in Hangzhou Bay were collected.These data were processed and contrastive analyses were utilized to identify coastline migration and erosion/deposition in Hangzhou Bay.Based on 17 bathymetric charts covering a period of 51 years(1960~2011),morphological evolution process of Jinshan Trough was investigated by contrastive analysis method and empirical orthogonal function(EOF)method.Numerical modeling(based on Mike 21 model)was used to discuss the impacts of coastal reclamations on hydrodynamics in Hangzhou Bay and Jinshan Trough.Additionally,a hydrodynamic-suspended sediment coupled morphodynamic model was developed to investigate the effects of 9711 typhoon on hydrodynamics,suspended sediment concentration and morphological evolution in Jinshan Trough.We studied The main findings of this study are summarized as below:Since 1960,coastline migration in Qiantang Estuary and Hangzhou Bay has been subjected to reclamations.For flood defense,reclamations were initiated in Qiantang Estuary from 1960 to 1995,and total reclamation area was approximately 180 km~2,which resulted in south coastline advancing northward about 10 km.While reclamations in Hangzhou Bay were mainly for demand of land,and total reclamation area was approximately 300 km~2 from 1995 to 2014,which resulted in south coastline advancing northward about 3~5 km.These historical reclamations led to geometrical variations in Hangzhou Bay.The coastline of south bank became more convex towards the north whilst the north bank tended to be straightened.Coastal reclamations were the primary cause of recent morphological evolution in Hangzhou Bay.Siltation became the main evolutional characteristics of Hangzhou Bay over the last 50 years,and this evolution pattern was also supported by the bathymetric data.Siltation mainly happened the shoal along the south bank.With the expansion of the shoal,erosion was suppressed and mainly happened where flood was dominant.The hydrodynamic model results show that reclamations reduced the intertidal area and narrowed Hangzhou Bay,which triggered obvious changes of current velocities and directions.Along the transect profile between Jinshan and Andong,both flood and ebb velocity increased,but the magnitude of flood velocity decreased and that of ebb velocity increased from north to south along the profile.Thus,the flood dominance enhanced in Jinshan Trough and gradually reduced from north to south.From this transect profile to the head of Hangzhou Bay,flood velocity increased while ebb velocity decreased,resulting in the enhancement in flood dominance from east to west.Due to flood dominance,more sediment could be transported into the upper stream of Hangzhou Bay,and silted up on the shoal along the south bank.The evolutions of Jinshan Trough presented evident spatio-temporal pattern.Especially,the west part of the trough developed three sub-channels expanding northward,westward and southward.The north sub-channel had been silting since 1985,till 1998 it almost disappeared.The middle sub-channel expanded continuously westward,and connected with Quangongting Trough in 1988.The south sub-channel merged with the middle one,causing the trough widening and elongating.The evolutions indicated by sediment volume changes could be classified into 3 stages from1960 to 2011.During the first stage(1960~1987),the trough spread in the direction towards west and southwest,which resulted in an obvious increase in the area and the volume of the trough.During the second stage(1987~1996),the volume of the trough maintained relatively stable.However,siltation happened in the northwest,whilst erosion happened in the west part of the trough.During the third stage(1996~2011),the trough iniatially experienced erosion from 1996 to 1998,then it shifted to compensatory siltation from 1998 to 2000 and kept a slow evolution process from 2000to 2011.The first eigenfunction indicated that erosion was the primary evolution process which could be divided into 3 stages as mentioned above.Hydrodynamic model results show that reclamations along the coast of Hangzhou Bay caused current velocity increase and the subsequent erosion in the west of Jinshan Trough.Sequential reclamations conducted along the coast of Jinshan were likely the major factor influencing the trough evolution during the first and second stages.On one hand the trough spread in the direction of west and southwest,due to negative feedback and diversion effect.On the other hand,an eddy in the residual water circulation was inferred in northwest around the headland,which resulted in siltation in the northwest part of the trough.Morphodynamic model results show that the super typhoon“Winnine”led to the increase of maximum ebb current velocity and suspended sediment concentration,and subsequently induced the erosion of channel.This mechanism can qualitatively explain the erosion process from 1996 to 1998.The following compensating deposition process occurred at the beginning of the third stage.As the stratigraphic interface flushed away by storm surge at the bottom of the trough,the scouring holes can not recover to the disconnection status before the typhoon.