Tidal Dynamics Characteristics And Fine-grained Sediment Transport In The West Offshore Area Of The Bohai Bay

Tide is one of the main hydrodynamics in coastal environment.Fine-grained sediment is commonly moved,transported and accumulated by tidal currents.Those processes induce coastal geomorphic evolution.Meanwhile it is increasingly affected by human activities in coastal waters.There is a large area of continually accumulated intertidal flat in the western Bohai Bay on the premise of insufficient sediment supply which was resulted from the Yellow River diversion and the construction of river dams.Since the 1980s,the adjacent waters had been frequently disturbed by human activities.It also suffered from sequential reclamation for more than a decade since the beginning of the 21st century.Thus,the west offshore area of the Bohai Bay is an ideal area to study the impacts of reclamations on tidal dynamics and fine-grained sediment transport.In this thesis,sediment dynamic observations were conducted at 12 sites in both the north and the south parts of the study area in the year of 2009,2014 and 2015.Observed data analysis and 2DH MIKE 21 simulations were applied to investigate the hydrodynamics characteristics change under reclamation condition.At the same time,we collected the grain size of surficial sediment samples collected at the study area over the corresponding years to further discuss the net sediment transport patterns by using the Gao-Collins grain-size trends method.The main results showed below in detail.(1)The hydrodynamics in the study area were characterized by the irregular semi-diurnal tide and low reciprocating flow,but with the current speed during flood stronger than that during ebb.The SSC during flood period is stronger than that during ebb.Also the flood duration is shorter than ebb duration,which was characterized by typical flood dominant tidal asymmetry.There is a significant relationship between the depth-averaged velocity and SSC or the shear stress at the bottom layer during the flood period.(2)Most stations were characterized as a net suspend sediment transport toward the land.The calculation of flux decomposition indicated that the tidal trapping effect,was the main contribution on the suspend sediment transport in the study area.Sediment from the subtidal waters was transported to the shallow intertidal flat and finally accumulated on the upper intertidal area under the micro-tidal dynamic environment,which may explain the reason of a wide intertidal flat growth but with an under-supply of sediment.(3)Observation shows that sequential reclamations was conducted between the year of 2009 and 2015,which resulted in a stronger tidal asymmetry at the north part of the study area with the suspend sediment transport rate increased from 20.15g/m/s to 24.92g/m/s.Inversely,the tidal asymmetry was weakened in the south part with a transport rate decline of 37.75g/m/s to 6.37g/m/s.This kind of divergence may lead to a faster sediment accumulation rate of the north but slower at the south area.(4)Numerical simulation results show that generally the residual current pattern was not changed by the reclamation.The original pattern of residual current was still maintained,i.e.northward transport at the north area and southward transport at the south area.In the nearshore shallow sea,several small residual circulations were observed,especially with the continuous reclamation in the south area between 2009 and 2014.Further,the M4 amplitude increased 1-2 times and then decrease ?M2/M4 in the south area indication decreasing tidal asymmetry in the south shallow sea.In conclusion,the numerical model could provide a more all-around knowledge on the hydrodynamics pattern than the observation method.(5)Types of surface sediment in the study area were different at the north and the south.There was a distribution of both clayey-silt area and sandy-silt area in the north part of the study area.In the south part,sediment was mostly clay-silt type but with a slim long area of sandy,silty-sand and sandy-silt from the nearshore to the offshore.Though reclamations overlaid the majority of intertidal flats,the sediment types remained as before.However,grain size parameters changed in the study area,with a better sorting,smaller absolute value of skewness coefficient,but there was no significant change for the main grain size.(6)The comparison of bedload and suspend sediment transport flux showed a same order of magnitude.But neither the direction nor the value indicated a relevant relationship.Unlike the net sediment transport trend showed in all the stations,the bedload transport behaved longshore transport,showing a parallel trend to the coast on the northern stations and a trend of 45° intersection angle to the coast on the southern stations.The net bedload transport trend appeared opposite directions even at the near stations of the same year.(7)The grain size trends derived by the Gao-Collins model was compared with the residual current pattern,the bedload transport trend and the suspend sediment transport trend in the study area under sequential reclamation.The grain size trends showed similar directions with both the residual current field and the bedload transport trend at the corresponding sites of the years.Comparison with the sediment transport trend indicated no remarkable similarity.