Summery Intra-Tidal Variations of Suspended Sediment Transportation–Topographical Response and Dynamical Mechanism in the Aoshan Bay and Surrounding Area, Shandong Peninsula

WANG Yan, DOU Yanguang, 2), *, ZOU Liang, GAO Fei, SU Dapeng, HU Rui, YUE Baojing, and XUE Biying

Summery Intra-Tidal Variations of Suspended Sediment Transportation–Topographical Response and Dynamical Mechanism in the Aoshan Bay and Surrounding Area, Shandong Peninsula

WANG Yan1), DOU Yanguang1), 2), *, ZOU Liang1), GAO Fei1), SU Dapeng1), HU Rui1), YUE Baojing1), and XUE Biying1)

1),,266071,2),,266071,

As the second largest bay in Qingdao, the Aoshan Bay and its adjacent sea area play an important role in aquaculture deve- lopment and urban land and sea coordination for the eastern gulf type of city in the Qingdao Blue Silicon Valley Core Area (QBSVCA). Based onsedimentary dynamical observation and previous research results, the thermohaline structure, the transportation of suspended sediment and its mechanism, and the coastal geomorphic response were elaborated and analyzed in detail in this paper. The result indicated that the thermohaline and density distribution have obvious intra-tidal characteristics in the QBSVCA and the adjacent waters of the islands, during summer neap tide stage. The development of the bottom high suspended sediment concentration (SSC) layer was slightly enhanced in flood slack at each of the four stations. Suspended sediment transportation near the QBSVCA is related closely with the vertical mixing-stratification mechanism. Combined with previous research results, this study once again showed that submarine topography and the grain size of sea bed sediments would respond to hydrodynamic forces. The medians of the bottomand D50in the Aoshan Bay were the highest, followed by those in the Daguan Island and Xiaoguan Island, and the data in the Laoshan Bay were the lowest. This showed that the capacity of suspended sediment transportation in the bottom water layer of the Aoshan Bay was stronger than that of the adjacent sea area. The re-suspension and migration of fine sediments lead to the strong coarsening of sediments in this area.

suspended sediment transportation; thermohaline structure; dynamical mechanism; topographical response; coastal environment

1 Introduction

The research on the bay and coastal zone is closely related to the urban economic development, so it is necessaryto closely follow the urban planning and the national strategy.The Qingdao Blue Silicon Valley Core Area (QBSVCA) is not only the new business card of Jimo District, but also a national key project in the whole Shandong Peninsula development. The Aoshan Bay is the second largest bay, onlynext to the Jiaozhou Bay, in Qingdao. The eastern gulf type of city relying on the Aoshan Bay Group is an important fulcrum in the ‘Three Bays’ of the gulf-based city pattern of Qingdao.

The Aoshan Bay is a semi-closed bay located in the nor- theast of Aoshanwei sub-district at the eastern part of the QBSVCA. The mouth of the bay faces to the Southeast. There are 9 reefs in the bay, such as the Ganzui Island, the Zhanggong Island and the Beijiao Reef. Along the bank of the Aoshan Bay, there are several small seasonal rivers with small runoff, including the Daren River, the Wenquan River, the Xinsheng River, the Gaoyu River, the Daqiao River, the Wangcun River, and so on. Similarly, the Xiaodao Bay is also a semi-closed bay located in the southeast of Aoshanwei sub-district, which is a part of the Laoshan Bay. There’s no river flow into the Laoshan Bay, within which the Tuzi Island, the Small Island, the Lion Island, the Daguan Island and the Maer Island are scattered (Fig.1). The Aoshan Bay and the Laoshan Bay are dominated by wind and waves with small surges. Both strong and normal wind directions are southeast in these bays, with a frequency of 23% and a wave height of 2.2m. The tidal type in these bays is normal semidiurnal tide, and the mean sea level is low in winter while high in summer. The se- diment sources mainly include river discharging sediment, coastal erosion sediment and offshore transported sediment,., while the river discharging sediment and the erosion sediment near the Aoshan Bay are limited. Suspended sedi- ment in the offshore sea can spread to the Aoshan Bay through water exchange and then deposit on the seabed (Zhao and Chen, 2001; Chen and Zhao, 2004; Li., 2011). Marine environment quality in the inshore water of Jimo District is related to marine fishery, marine nature reserves and reserves of rare and endangered marine life. The ecosystem provincial marine special protection zones of the Daguan Island and the Xiaoguan Island group in Jimo District was established on July 10, 2015, with the approval of Shandong Provincial Government. The biocoenosis of the protection zone was stable and the habitat environment was good (Qingdao Municipal Ocean and Fisheries Administration, 2018). The adjacent waters of Xiaoguan Island and the Aoshan Bay are important sea areas for proliferating and releasing prawns in Qingdao. The Aoshan Bay is also an important culturing area for theraft culturing of scallops and the bottom seeding and conservation of variegated clam in Jimo District. The prolife- ration and release of the Japanese prawns will promote the development of marine fishing industry, aquatic pro- cessing industry, ship maintenance industry, catering industry and other industries, which produced high social benefits. Scallop breeding industry has also created huge economic benefits. The growing and the distribution of aqua-tic products, affected by marine thermohaline environmentand hydrodynamic conditions, also have seasonal distribution characteristics. The breeding industry of aquatic pro- ducts mainly implemented in summer. Thus, the investigation of thermohaline and hydrodynamic environment in summer could provide scientific basis for the development of aquaculture in the Aoshan Bay and the Laoshan Bay near the QBSVCA.

This research focuses on the construction needs of the QBSVCA, in the offshore areas of the Aoshan Bay, the Xiaodao Bay and the Dingzi Bay, around the QBSVCA, carrying out the sedimentary dynamic investigation of coastal geological environment in coastal important engineering areas. Hydrodynamic data and hydrological environment data were collected systematically to reveal the characteristics of sedimentary dynamic environment in this area. Based on the investigation of hydrographic or thermohaline structures and hydrodynamic conditions in the QBSVCA and the adjacent waters of islands, the spatial and temporal distribution of sedimentary dynamic parameters and the thermohaline environment features in typical areas were determined during summer neap tides in Aoshan Bay and Laoshan Bay, which provides a scientific basis for the development of aquaculture related industries in this area.

2 Materials and Methods

2.1 Cruise and Sampling

The observation of sedimentary dynamics was carried out in the survey area during neap tide from June 15 to 21, 2017. A total of 208 suspended sediment samples were collected after 25hcontinuous sedimentary dynamic observation at 4 stations (Fig.1). The ‘used to conduct the sedimentary dynamics survey. The Trimble SPS351 satellite/beacon differential GPS system and the Haida6.0 integrated navigation and positioning system were adopted to achieve navigation and positioning. Thermohaline environment observations wereaccomplished by the SBE19plus CTD of Seabird Company. Current observations adopted a direct reading sea current profiler of Acoustic Doppler Current Profiler (ADCP) of RDI Company (broadband=300kHz and blanking zone=1m), with measurement errors for the velocity and direction of <5mms?2and <1?, respectively. Suspended sediment samples were collected by the ECO-55 automatic water sampler of Seabird Company of the United States.

Fig.1 Study area, observation stations, and previous beach profiles in the offshore areas of the Aoshan Bay, the Xiaodao Bay and the Dingzi Bay, around the QBSVCA. The re- mote sensing base map was obtained from Google Earth.

CTD was dropped once per hour to obtain temperature, salinity and turbidity data, which can reflect the spatial and temporal variation of water thermohaline and turbid conditions within 25h at each of the 4 stations. The current data were stably and continuously measured within 25h, which can reflect the hydrodynamic characteristics of all the stations. The interval of measured layer is set as 1m, and the sampling frequency is set as 10s. Suspended sediment samples were collected once per hour with the dropping of the CTD instruments, obtained from the surface layer (0.5m–2m below the sea surface) and the bottom layer (0.5m–2m above the seabed).

2.2 Calibration of Suspended Sediment Concentration (SSC) and Turbidity

Suspended sediment concentration (SSC) was measured by filtering suspended sediment samples on double-layer filter membranes weighed previously (the weight of the upper membrane and the lower membrane in this link were represented by WandW, respectively), and then drying and weighing the filter membranes (the weight of the upper membrane and the lower membrane in this link were represented by WandW, respectively) (Eq. (1)). In this experiment, the filter process adopted cellulose acetate filter membranes with aperture of 0.45μm, while the weighing process was operated on the electronic balance with precision of 0.00001g. The filter membranes were dried for 24h before and after the filter process with drying temperature about 45℃. The calculation formula ofis as follow,

whererespects the volume of filtered water.

The turbidity values of seawater collected from the layer where suspended sediment samples were filtered were averaged and fitted to SSCs. As shown in Fig.2, four abnormal data accounting for 0.02% of total data were remov- ed and the coefficient of determination (2) was 0.902116, suggesting the validity of calibration to theand turbidity.

Fig.2 Correlation between suspended sediment concentration (SSC) and turbidity.

3 Results

3.1 Temporal-Spatial Distribution of Thermohaline Parameters in the Tidal Cycle

1) The central part of the Aoshan Bay

The thermohaline parameters and the density of water column were vertically homogeneous in flood tide phase at Station CM01 (average depth of 7.2m), which is located at the central part of the Aoshan Bay (Figs.1 and 3a), with salinity<32.1, temperature<21℃,>22.2kgm?3.here represented the density of water column and its calculation formula is as follows,

whererepresents the density of water column. Then, the temperature of the water column was gradually increased from the top layer in flood slack, while the salinity was simultaneously decreased from the top layer or the middle layer, with downward increasing vertical density gradient. In ebb tide, the temperature and the salinity of water co- lumn increased over time, with vertically homogeneous ther- mohaline structure and decreasing vertical density. Until ebb slack, the temperature and the salinity of the water column reached the highest and the density of water co- lumn dropped to the lowest in the tidal cycle. Overall, the salinity of the water column at Station CM01 was in the range of 31.95–32.45, while the temperature range is 20.4–23.5℃, with the characteristics of higher temperature and salinity and lower density in the upper-middle-layer water during ebb slack phase than other phases of the tidal cycle.

2) North of the Laoshan Bay

Compared with that at Station CM01, the vertical gradients of salinity and density in the water column were higher at Station CM02 (average depth of 13.2m), located at the northern part of the Laoshan Bay (Figs.1 and 3b), with the salinity range of 31.5–32.2 and the temperature range of 17.3–18.7℃. Another difference between these two stations is the intra-tidal vertical salinity variation characteristic of the water column, which showed increase trend in ebb tide phase and decrease trend in flood one at CM02 Station, while appeared to be a reverse trend at the other station. In ebb tide, the vertical temperature gradient of the water column is higher than that in flood tide, and the thermohaline environment of high temperature, low salt and low density was vertically gradually developed. In ebb slack, the temperature of the water column tends to be more homogeneous. In flood tide, the vertical temperature gradient of the water column increased, and the water column with high salinity, low temperature and high density developed until flood slack.

3) East of the Maer Island

Similar to that at Station CM02, the variation of thermohaline environment at Station CM03 (average depth of 17.3m) was characterized by high vertical gradient of salinity and density in the water column (Figs.1 and 3c), with the salinity range of 31.2–32.4 and the temperature range of 16.3–18.5℃. At the depth of less than 7m above the bottom of seabed (MAB), the vertical thermohaline and density structures of the water column were homogeneous. In flood tide, along with the homogeneously increase of salinity, the vertical temperature structure at the depth of more than 7MAB was initially homogeneous with lower temperature, and then rapidly occupied by enhancing stratification structure with higher temperature, accompanied by enhancing stratification of vertical sali- nity structure. Until the next flood tide, the vertical temperature structure tended to be homogeneous again.

4) East channel of the Xiaodao Bay

Similar to that at Station CM01, along with the homogeneous decrease of salinity, the temperature of the water column at Station CM04 (average depth of 8.5m) was uni-formly distributed with lower temperature (Figs.1 and 3d), with the salinity range of 31.7–32.2 and the temperature range of 17.8–21.3℃. The variation regularity was different from two other stations with the depth of more than 10m.

Fig.3 Temporal-spatial distribution of thermohaline parameters in the tidal cycle. MAB, meters above bottom.

In summary, the thermohaline and density distribution have obvious intra-tidal characteristics in the QBSVCA and the adjacent waters of islands, during summer neap tide stage. Water salinity here was basically around 32. Temperature decreased from the coast of Aoshanwei sub-district (about 22℃) to the Laoshan Bay (18℃ or so). Temperature in summer is higher in shallower waters, which is unfavorable to biological cultivation (Janet and Dudgeon, 1996; Naylor., 1998; Joos and Spahni, 2008). It can provide scientific basis for the development of aquaculture industry to make clear the characteristics of the thermohaline environment in the QBSVCA and the adjacent waters of islands.

3.2 The Variation Characteristic of Current Velocity

The horizontal current velocity at Station CM01 was 200–800mms?1, and the current velocities of surface water and bottom water were higher than that of middle water. The average current direction of water column during floodtide was NNW, while it was SSE in ebb tide, and the magnitude of horizontal velocity can be more than 410mms?1in surface water and bottom water during flood tide with the direction of SE (Fig.4a). The eastern component of current velocity () tended to the direction of west (the value ofwas negative) in flood slack and ebb slack, while to the east (the value ofwas positive) in other periods. The northern component of current velocity () tended to the north (the value ofwas positive) in flood tide, while to the south (the value of v wasnegative) during ebb tide (Fig.5a).

In the northern area of the Laoshan Bay (Station CM02), the horizontal current velocity was 300–600mms?1, and the vertical distribution of current velocity was more homogeneous than that in the Aoshan Bay (Station CM01), with uniform structures ofand. The average current direction of water column during flood tide was SWW-NWW while it was NE in ebb tide, and the magnitude of horizontal velocity may be more than 500mms?1during ebb tide (Fig.4b). The velocitytended to the east in flood tide, while to the west in ebb tide. The velocitytended to the north in flood tide, while to the south during ebb tide (Fig.5b).

Fig.4 The variation characteristic of current velocity.

Fig.5 The variation characteristic of east, west, and vertical components of current velocity.

In the eastern area of the Maer Island (Station CM03), the horizontal current velocity was 300–750mms?1, and the vertical distribution of current velocity was homogeneous with uniform structure ofand, similar to Station CM02. The average current direction of water column wasSSW-W during flood tide, while it was NE in ebb tide, and the horizontal velocity more than 500mms?1mainly appeared during ebb tide (Fig.4c). The componenttended to the east in flood tide, while to the west in ebb tide. The componenttended to the north in flood tide, while to the south during ebb tide (Fig.5c).

On account of the restriction by topography, the average current direction of water column in the east channel of the Xiaodao Bay (Station CM04) was NNW-NNE in most of the time. And the range of horizontal current velocity was 300–600mms?1, with higher values in surface layer and bottom layer than those in middle layer (Fig.4d). The componenttended to the west in flood tide, while to the east in ebb tide. The componenttended to the north throughout tidal cycle due to the restriction of topography (Fig.5d).

Overall, the regular semidiurnal tide was dominant in the coastal waters of the QBSVCA, the reversing current was dominant in the Aoshan Bay, and the rotary current was dominant outside the bay (Fig.4; Li, 2012). The peak values of the vertical average current velocity in the Ao- shan Bay (Station CM01) and the east channel of the Xiao- dao Bay (Station CM04) were in the range of 225–278mms?1, while those outside the bay (Station CM02 and Station CM03) were 174–621mms?1(Fig.6). In the Ao- shan Bay, the vertical average current velocity showed quickly increasing first then slowly decreasing trend in flood tide, while it showed slowly increasing first then quickly decreasing trend in ebb tide, with the peak values appearing in the middle stage of flood tide and ebb tide, respectively (Figs.4a and 6). Outside the Aoshan Bay, the gradients of the increasing stage of vertical average current velocity were similar to those of the decreasing stage, with the peak values of vertical average current velocity appearing in flood slack and ebb slack (Figs.4b, c, d and 6).

3.3 The Intra-Tidal Distribution of Suspended Sediments

The surface sediments at each station are mainly clay silt (Li., 2011; Li, 2012). Their resuspension and re- transportation affect the water transmittance and the distribution of benthic organisms in this sea area (Steyaert., 2003; Fu., 2012). Sediment sources in the coastal area of the QBSVCA mainly include river-carrying sediments, coastal erosion and offshore sediments,. The occurrence of near-shore coarse sediments is mainly caused by the settlement of fluvial sediments and coastal erosion, while fine-grain sediments in deep-water area was mainly derived from suspended sediment deposition (Li., 2011).

Fig.6 The average current velocity of water column.

Based on the fitting relationship betweenand turbidity (Fig.2), the spatial and temporal distribution of SSCs at each station was obtained, as shown in Fig.7. Basically, the variation of SSCs at the four stations was relevant to the variation of horizontal current velocities and thermohaline structures (Figs.3 and 7). The larger the velocity and density were, the greater thewould be. Therefore, the spatial and temporal distribution of SSCs also had distinct intra-tidal characteristics in and outside the Aoshan Bay. In the Aoshan Bay (Station CM01), the bottomrapidly increased in ebb slack and the middle stage of flood tide with the peak value more than 45mgL?1, while promptly decreased in the earlier stage of flood tide and the stage transiting from flood slack to next ebb slack, with peak value less than 25mgL?1(Fig.7a). In the east channel of the Xiaodao Bay (Station CM04), the trans- portation of bottom suspended sediments was inhibited in ebb tide and the middle stage of flood tide, while it was enhanced in ebb slack and flood slack (Fig.7b). At Station CM02 and Station CM03, located at the northeast and the southeast of the Laoshan Bay, the suspended sediment transportation in the bottom layer of the water column was enhanced in flood slack and ebb slack, while it was inhibited in other stages (Figs.7c and d).

In general, as shown in Fig.7, SSCs in the water column were higher at Station CM01 and Station CM02 than two other stations. The bottom high SSC layers were developed or slightly enhanced in flood slack at each of the four stations. In addition, suspended sediment plumes occur- red at all of the four stations within two meters of surface water layer, which might be formed by river-carrying se- diment transportation or offshore sediment transportation, with the similar order of SSCs to the bottom water layer.

4 Discussion

4.1 The Spatial and Temporal Variation Characteristics of Suspended Sediment Flux (SSF)

SSF were calculated by decomposing instantaneous material transport vector based on relative water depths (Dyer, 1974; Su and Wang, 1986; Wu., 2006). According to the calculation formula proposed by Wu. (2006), the calculation formula of instantaneous SSF () per unit meter width is as follows,

Fig.7 The intra-tidal distribution of suspended sediments.

The calculation results show that the direction ofwas mostly towards the north in flood tides at Station CM01 in the Aoshan Bay, while it was to the direction ranging from the south to the southeast in ebb tides (Fig.8a). The values ofcould reach up to 62.6g(ms)?1 in flood tides, while basically lower than 18.1g(ms)?1 in ebb tides. Thesediment transportation was restrained during flood slacks and ebb slacks, accompanied by the diversion of direction of suspended sediment transportation.

At Station CM02 in the northern area of the Laoshan Bay, the transport direction of suspended sediment was counterclockwise (Fig.8b). The duration of suspended se- diment transporting northward was longer than that southward, and the values ofwere larger during the northward with the maximum of 145g(ms)?1than during the southward.

In the eastern area of the Maer Island (Station CM03), the temporal variation of the direction and value ofwas similar to that in the northern area of the Laoshan Bay (Station CM02) (Figs.8b and 8c). At Station CM03, the values ofexceeded the maximum ofat Station CM02 (145g(ms)?1) for 3h, with the maximum of 252g(ms)?1.

Combined with the temporal and spatial variation ofs and current velocities (Figs.4d and 7d), suspended sediment transport at Station CM04 in the east channel of the Xiaodao Bay was mainly towards NNW direction during flood tide and flood slack, with the maximum of 150g(ms)?1(Fig.8d).

4.2 The Vertical Mixing and Stratification Mechanism of Suspended Sediment Transportation

To reflect the dynamical mechanism of suspended sediment transportation in the water column, Richardson number () is usually calculated, which is closely related to the density effect of vertical stratification and turbulence in the water column (Zhang and Anthes, 1982; Kundu, andBeardsley, 1991; Wang., 2010; Wang., 2014). The formula ofis as follows:

where ρis the bulk density of water, u is the horizontal current velocity, z is the vertical dimension, positive upward from seabed. With parameters of water salinity, temperature and pressure, ρ can be calculated from the UN- ESCO equation (Millero et al., 1980). The numeratorpart of the function represents the stratification stability of the water column, determined by density gradient. The denominator part indicates the turbulent intension driven by current shear stress. The high Ri value shows more stable status of the water column. On the contrary, the low value of Ri indicates turbulence intensification of the water column. Generally, Ri<0.25 is a necessary condition for turbulent mixing to overcome stratification structure. It also means when Ri>0.25, stratification structure of the water column is stable, and not easy to be broken by turbulent mixing (Turner, 1973).

Fig.9 The vertical mixing and stratification mechanism of suspended sediment transportation.

The hourly vertical-averaged values ofandat the four stations were shown in Fig.9. At Station CM01 and Station CM04, the increases ofvalues were strictly in accord with the situation of<0.25, while the decreases ofhappened to coincide with the situation of>0.25 (Figs.9a, d). In the inshore area of the QBSVCA (Station CM01 and Station CM04), dramatic increase inmostly occurred whenwas less than 0.25 and decreasing, while significant reduction inhappened whenwas more than 0.25 and increasing. The above phenomenon suggested that suspended sediment transportation near the QBSVCA were related closely with vertical mixing and stratification mechanism. When turbulent mixing of the water column overcame stratification structure, it could enhance the resuspension of seabed sediments. However, if stratification structure of the water column was stable, the resuspension of seabed sediments was inhibited. Mean- while, it also gave significant evidence that near-shore su- spended sediments of the QBSVCA were mainly derived fromresuspension of seabed sediments (Figs.9a, d). The poor correlation betweenandat Station CM02 and Station CM03 (Figs.9b, c) represented the suspended sediments beyond the Laoshan Bay originated from more complicated sources, which could be explained by future observational studies.

4.3 The Topographical Response of Suspended Sediment Transportation

For the project ‘Comprehensive survey and potential eva- luation of mineral resources in Shandong Peninsula coastal zone’, which this paper relies on, surface sediments were sampled in the Aoshan Bay, the Laoshan Bay, and the ocean area surrounding the Daguan Island and the Xiao- guan Island in May 2017. These surface sediment samples were analyzed by particle size compositions (Qingdao Institute of Marine Geology, 2018). In summer and winter of the year 2017, the project also monitored two beach topographic profiles along the west side of the Ao- shan Bay (beach profiles C1 and C2 in Fig.1).

Combined with the previous observations of beach pro- files (QAM4, QAM5 and QAM6 in Fig.1, Bi., 2015), there are obvious seasonal differences in erosion and de- position in the western beach of the Aoshan Bay. The re- sults showed that the western beach of the Aoshan Bay was silted up in winter and eroded away in summer. In terms of particle size of sediments on the west coast of the Ao- shan Bay, it was generally finer in winter than that in sum- mer. The near-shore sediment transportation plays a significant role in coastal erosion and deposition. Thus, the hydrodynamic erosion on the western beach was stronger in summer than that in winter (Bi., 2015; Qingdao Institute of Marine Geology, 2018).

Fig.10 The direction of sediment transportation obtained in this study and previous researches (Li, 2012).

In aspect of topographic and geomorphic responses, the role of bottom suspended sediment transportation is more prominent. For the convenience of discussion, the study area was divided into three regions (Fig.10), namely, the Aoshan Bay, the Daguan Island and Xiaoguan Island, and the Laoshan Bay. The Station CM01 was located in the Aoshan Bay, the Station CM04 was located near the Daguan Island and Xiaoguan Island, the Station CM02 and Station CM03 were located in the Laoshan Bay. Compared with the median grain size (D50) of surface sediments in these three regions (Fig.11), the variation trend of the median of the bottomwas consistent with that of D50. Among the three regions, the medians of the bottomand D50in the Aoshan Bay were the highest, followed by those near the Daguan Island and Xiaoguan Island, and the data in the Laoshan Bay were the lowest. This result showed that the capacity of suspended sediment transportation in the bottom water layer of the Aoshan Bay was stronger than those of two other regions. The re-suspension and transportation of fine particles resulted in the coarsening of sediments in this region. The above conclusion is based on the beach profilesin previous observations and the correlation of the bottomand D50, which can be used to understand the relationship between the suspended sediment transportation and the particle size compositions of the bottom sediments. However, the grain size characteristic is controlled by many factors, and is the result of long-term hydrodynamic modulation. This result was obtained in a short duration. Thus, in order to verify the conclusion in this section, more observations should be carried out in the future.

Fig.11 The bottom E and the median grain size (D50) of surface sediments in the three regions.

Further, the direction of sediment transportation (Fig.10) obtained in this study and one previous research (Li, 2012) provided sedimentary dynamical evidences for the above results. The suspended sediments in the middle and the west of the Aoshan Bay were mostly derived fromresuspended sediments, and the directions of transportation were towards the bay both in neap tide and in spring tide. The suspended sediments in the east of the Aoshan Bay were delivered by coastal currents in spring tide, while they were poured out of the bay in neap tide. It can be seen that most sediments settled down in the Aoshan Bay were mainly delivered by coastal currents from the east of the bay in spring tide. The direction of sediment transportation in the Laoshan Bay in spring tide was almost opposite to that in neap tide.

5 Conclusions

Thesedimentary dynamical observations were conducted at 4 stations in the Aoshan Bay and the adjacent sea area. They showed obvious intra-tidal characte- ristics in thermohaline structure, current velocity and. The regular semidiurnal tide was dominant in the coastal waters of the QBSVCA, the reversing current was dominant in the Aoshan Bay, and the rotary current was dominant outside the bay. In the Aoshan Bay, the peak values of vertical average current velocity appeared in the middle stages of flood tide and ebb tide. SSCs in the water column were higher at two stations in northern part of the study area. The bottom highlayer was developed or enhanced in flood slack at the four stations. The variation ofshowed obvious intra-tidal characteristics, which was affected by the vertical mixing and stratification mechanism of the water column. When turbulent mixing overcame stratification structure in the water column, it could enhance the resuspension of seabed sediments. However, if the stratification structure of the water column was stable, the resuspension of seabed sediments was inhibited. Also, the near-shore suspended sediment transportation of the QBSVCA was derived from theresuspension of seabed sediments. The capacity of suspended sediment transportation in the bottom water layer of the Aoshan bay was stronger than those of the adjacent sea area. The resuspension and transportation of fine sediments resulted in the coarsening of sediments in this region. These results can provide scientific basis for the development of aquaculture industry and urban land and sea coordination in the QBSVCA.

Acknowledgements

This work was jointly supported by the National Natural Science Foundation of China (No. 41606082) and the China Geological Survey (Nos. DD20189230, DD20160 148).

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July 2, 2020;

December 4, 2020;

June 1, 2021

? Ocean University of China, Science Press and Springer-Verlag GmbH Germany 2021

. Tel: 0086-532-85731050

E-mail: douyanguang@gmail.com

(Edited by Chen Wenwen)