Yong Zhang , Bin Chen , Ze Ning , Yun-fan Yang , , Gang Hu , Xiao-o Zhang , Hou-jie Wang

a Qingdao Institute of Marine Geology, China Geology Survey, Ministry of Natural Resources, Qingdao 266071, China

b Laboratory for Marine Mineral Resources, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266071, China

c College of Marine Geo-Science, Ocean University of China, Qingdao 266100, China

ABSTRACT

In recent years, development activities have had a significant impact on the environment of the Jiaozhou Bay, China. To ensure the sustainable economic and social development of the Jiaozhou Bay area, it is necessary to strengthen corresponding control measures. The important prerequisite is to properly understand the environmental conditions laws of natural change, especially the dynamic processes of sediment and the characteristics of landform evolution. Based on the data of continuous observation at 6 stations in Jiaozhou Bay for 25 hours, the Hydrodynamic Eutrophication Model (HEM-3D) was used to simulate the sediment erosion and deposition. The results show that the maximum suspended sediment concentration in the sea area of Jiaozhou Bay is about 40 mg/L, which appears in the northwestern area of the bay top and the Cangkou watercourse area, and the low concentration is located in the area of the central Jiaozhou Bay towards the bay mouth. The suspended sediment is 6-10 mg/L. Affected by a decrease in seawater material, the direction of the prevailing current in the Jiaozhou Bay area is different from that of the sediment transport. The velocity of the flood current is higher than that of the ebb current.However, during flood tide, the flux of resuspended seafloor sediment outside and at the mouth of the bay is limited and cannot contribute significantly to the suspended sediment in the bay. During ebb tide, the resuspended sediment at the shallow-water bay head and the east and west sides spreads toward the bay mouth with the ebb current, although it extends beyond the bay through the bay mouth. The research results can provide scientific support for the Jiaozhou Bay project construction and environmental protection.

Keywords:

HEM-3D model

Numerical modeling

Sediment transport

Coastal geological survey engineering

Jiaozhou Bay

Shandong Province

China

1. Introduction

Jiaozhou Bay is located at the southern end of Jiaodong Peninsula, China. It is a semi-closed shallow bay with a narrow mouth and a wide interior. Its surface area is approximately 350 km2, and the average depth is approximately 8.8 m. The deepest part (64 m) is located at the inner bay mouth (Wang YP et al., 2000; Jia YL et al., 1999).The general characteristic of the Jiaozhou Bay sea floor topography is shallow to the northwest and deep to the southeast. Three channels, Cangkou Channel in the east,Central Channel, and Dagu River Channel in the west,connect the bay to the sea (Bian SH et al., 2006; Yang SL et al., 2004; Tao AF et al., 2017; Lin AJ et al., 2017; Zhao YX et al., 2006; Dong HP et al., 2006; Liu ZJ et al., 2004; Jiang DH et al., 2002). There are no major rivers draining into Jiaozhou Bay and the adjacent seas, and the Dagu River is the largest river, with a length of 179 km (Chen XY et al., 2019). The Yang River has the highest sediment concentration of 4.45 kg/m3. In addition to river sediment, the sediment sources in Jiaozhou Bay include coastal erosion and sea floor erosion.

Studies on sediment movement and the evolution of seabed erosion and deposition in Jiaozhou Bay have mainly been based on measured data. Previous studies based on measurements of suspended sediment concentration from 70 stations in Jiaozhou Bay showed that from the bay mouth to the head, the surface and vertical average of the suspended sediment concentration increases gradually, with greater concentrations at the bottom. Suspended sediment concentrations in the southern and eastern portions of Jiaozhou Bay are less than 6 mg/L, whereas the suspended sediment concentrations in the north and northwest are significant higher, exceeding 30 mg/L at the bottom. The suspended sediment concentrations at the bottom of Jiaozhou Bay are indicative of a strong flood-ebb tidal cycle,fluctuating between 10-30 mg/L. The concentration peak is largely consistent with the current peak velocity, indicating that the near-bottom sediment concentration variation is primarily controlled by tide-driven deposition-resuspension processes. The correlation between the variation in the surface suspended sediment concentration and the flood-ebb tidal cycle is more significant during spring tides than during neap tides. The peak surface sediment concentration usually occurs during the low-velocity phase at the end of the ebb tide and the beginning of the flood tide, indicating that the variation in the surface sediment concentration is mainly controlled by advection from the bay head to the bay mouth (Wu SY et al.,1995; Gao S et al., 2002). Deposition mainly occurs in the Dagu River region (Kong LS et al., 2004; Sun YL et al., 2001;Sun HL et al., 2001). Although river sediment has some effect on Jiaozhou Bay under steady-state conditions, the extent is limited and cannot spread to Huang Island and beyond the bay(Han SZ et al., 2007a, 2007b; Bao XW et al., 1999). The deposition rate in Jiaozhou Bay is 0.7-1.4 mm/a (Wang WH et al., 1986; Zheng W et al., 2012; Hou GB et al., 1986).Zhang MH (2000) reported the distribution and seasonal variations of suspension in Jiaozhou Bay based on measured data and suggested that the horizontal distribution of suspension reflects a significant seasonal variation. The annual seasonal variation of the average suspension concentration shows that the concentration is generally highest during spring, lower during fall and winter and lowest during summer. Wang YP et al. (2000) determined the sediment net transport trend using variations in sediment grain size in Jiaozhou Bay. They reported that sediment accumulated at the ridge of the bay center and the end of the trench extending from the entrance to the open sea, which is consistent with evidence of currents, topography, and heavy mineralogy. Based on three-dimensional multi-component sediment numerical modeling, Shi JH et al. (2010) suggested that the magnitude of erosion and deposition in Jiaozhou Bay is small and that the inner bay is dominated by erosion with local deposition, whereas the outer bay is mainly characterized by deposition (Li FY et al., 2003; Wei JW et al.,2006). Wang YH et al. (2009) suggested that the evolution of the modern Jiaozhou Bay sand ridge-waterway systems depicts well-defined inherited characteristics. Zhao KH et al.(1983) and Li NS et al. (2006) suggested that the major source of minerals is the sediment carried by continental rivers and coastal bedrock erosion products that are transported into Jiaozhou Bay.

This study used the most up-to-date measurements to conduct numerical modeling of sediment transport in Jiaozhou Bay after the completion of the cross-sea bridge. The modeling results provide scientific support for major construction and environmental protection in Jiaozhou Bay.

2. Materials and methods

2.1. Field measurement method

In June 2011, the authors collected 25-hours continuous measurements at 6 stations in Jiaozhou Bay (Fig. 1a). The measurements included conductivity-temperature-depth(CTD) profile measurements, ocean current measurements,and stratified water sampling. The authors obtained data on the suspended sediment concentration and sediment grain size by measurement processing and both analysis and suspension filtration tests.

2.2. Method for measuring suspension concentrations

The filtration tests on the suspension samples were primarily conducted by vacuum filtration in the laboratory.The filter membrane diameter was 47 mm, and the aperture was 0.45 mm. The suspension was weighed using a sartorius electronic balance with a detection limit of 0.01 mg.Suspension concentrations were all corrected using doublefilter membranes. The authors tested and analyzed 572 samples using suspension test protocols; these included 15 samples yielding negative suspension concentrations.Substandard tests accounted for 2.6% of the total samples, and the test pass rate was 97.4%, which was consistent with the data quality requirement.

3. Calibration of the water turbidity and suspended sediment concentrations in Jiaozhou Bay

Comparison between the test data of the suspension samples from Jiaozhou Bay and the optical back scatter(OBS)-turbidity measurements at the corresponding levels showed that the suspension concentration and turbidity were linearly correlated. The residual mean square (RMS) of the sampling data was 8.58, and the standard deviation was 7.68.The coefficient of determination (R2) was 0.822, and the confidence level was 0.95. Thus, their relationship was statistically significant (Fig. 2).

Fig. 1. Location of sampling stations (a) and wave rose diagrams of the vertical average velocity (b-g).

4. Model design and verification

4.1. Model design

This study used the HEM-3D model developed by the Virginia Institute of Marine Science (VIMS). This model is a highly synthesized three-dimensional numerical model that can be applied to coastal embayments, estuaries and tributaries. The model accounts for the effects of sediment deposition and resuspension processes, nucleation and suspended sediment concentrations on the sediment deposition rate for sediment transport at the sea floor boundary. It also considers the time-dependent mass input and output processes at the horizontal boundaries. The HEM-3D model has been widely applied and validated by numerous studies of sediment transport in the Chesapeake Bay, James River, Indian Lagoon, Puget Sound, and Yellow River (Wang HJ et al., 2006; Zhang YX et al., 2019).

The model uses orthogonal curvilinear coordinates with a total of 3855 grids. The smallest grid size is 430 m × 430 m.The model effectively resembles the complex coastal geometries of the Jiaozhou Bay area. The model uses vertical sigma coordinates with eight equal sigma levels. The water depth was based on the depth data measured in 2011 with tide corrections (Fig. 3).

Fig. 2. Relationship between the suspension concentration and CTD turbidity measurements (the red solid line denotes the fitted curve,and the dashed line denotes the 95% confidence interval, Blue dots are outliers).

4.2. Verification of the suspended sediment concentrations

This paper compared the vertical average of the suspended sediment concentrations using the measurements from stations QD07 and QD06 and the model output (Figs. 4a, 4b).The comparison shows that the concentrations from these 2 methods are of the same order of magnitude; both depict periodic variations with the flood-ebb tidal cycle. Because the numerical model ignores the spatial heterogeneity of the sea floor sediment types and applies uniform standards to the initial and depositional conditions, the simulated distribution of the suspended sediment is biased. However, because the transport and vertical exchange of suspended sediment are mainly controlled by the dynamic field, the model results reflect the general temporal and spatial processes of the suspension transport.

5. Results

5.1. Characteristics of the tidal current at observational stations

Based on the measured data, the tidal current in Jiaozhou Bay is dominated by oscillatory flows with regular semidiurnal tidal current characteristics. During flood tide,offshore currents flow into Jiaozhou Bay through the bay mouth and split because of the impact of the inner shoreline and the sea floor topography. One branch flows northwestward along the coast (QD07), another branch flows northwestward toward the Dagu River mouth at the bay head(QD08; Fig. 4c), and the third branch flows northeastward toward the Cangkou Channel (QD09; Fig. 4d). During ebb tide, these branches flow approximately in opposite directions to the outer sea through the bay mouth. The fastest currents inside the bay were observed at stations QD08 and QD07. The highest velocity exceeded 60 cm/s during flood tide. Due to the effect of water restriction, the tidal current is fastest at the Jiaozhou Bay mouth, with a maximum velocity of 90 cm/s.The overall horizontal distribution shows that the flood current is greater at the bay mouth, decreases slightly inside the bay, and diverges at the bay head in conjunction with a rapid decrease in velocity. Based on the measurement data,the distribution of the tidal current field in Jiaozhou Bay is mainly controlled by sea floor topography and the shoreline boundary. With the development of Jiaozhou Bay, the proportion of artificial shoreline increases, leading to rapid responses of the inner bay tidal current field, which further affects the tidal volume and water exchange in Jiaozhou Bay.

Fig. 3. Depth distribution of the modeling region (Cartesian coordinates, unit: m).

Fig. 4. The comparison of the measured and modeled suspended sediment concentrations at station QD06 and QD07.

Fig. 5. The distribution of suspended sediment concentrations in Jiaozhou Bay during different time periods. a-The beginning of the flood tide for spring tides; b-the maximum flood tide for spring tides; c-the high tide for spring tides; d-the beginning of the ebb tide for spring tides; e-the maximum ebb tide for spring tides; f-the low tide for spring tides; g-the maximum flood tide for neap tides; h-the maximum ebb tide for neap tides.

5.2. The concentration of suspended sediment at the observational stations

Station QD06 is close to the bay head, where the suspended sediment concentration was approximately 2-5 mg/L from the surface to the middle and bottom layers. However,the suspended sediment concentration at the bottom layers was as high as 53 mg/L and 71 mg/L during ebb tide, which was an order of magnitude higher than the average suspended sediment concentration. Based on the ebb current direction and velocity (approximately 40 cm/s, i.e., less than the flood current velocity), we infer that the suspended sediment in the shallow-water area at the bay head is transported via advection and reaches the station during ebb tide, leading to a significant increase in the suspended sediment concentration in the bottom layers.

The average suspended sediment concentration at station QD07 was approximately 2 mg/L, with little variation between the surface, middle, and bottom layers. Generally,the surface, middle, and bottom suspended sediment concentrations increased during flood tide, with a maximum of 10 mg/L. During ebb tide, the concentration remained between 2 mg/L and 3 mg/L. The ebb current velocity was relatively low (approximately 40 cm/s), and the flood current velocity was relatively high (65-70 cm/s). Thus, we can infer that the high suspended sediment concentration is caused by local resuspension due to the flood current. During ebb tide,the current velocity decreases, accompanied by deposition of suspended sediment grains, leading to a rapid decrease in the suspended sediment concentration in the surface, middle, and bottom layers.

The suspended sediment concentration at station QD08 was relatively low at the surface, ranging between 3-5 mg/L,and varied little between flood and ebb tides. However, the concentration increased to 7-14 mg/L during flood tide at depths of 5 m and 10 m and along the bottom layers. This station is located at the head of the Jiaozhou Bay mouth channel and has a rapid flood current (up to 70 cm/s). The movement of the near-bottom layer sediment grains is very active. Additionally, the topography adjacent to this station varies significantly, leading to large spatial variation in the current and vigorous exchange between the water and seabed sediment.

Station QD09 showed an increase in the suspended sediment concentration during ebb tide, and the concentration was higher at the surface than that in the middle and bottom layers. The tidal current at this station showed a typical oscillatory flow. The flood current was up to 50 cm/s and flowed northeastward; in contrast, the ebb current was slower at approximately 40 cm/s. Therefore, the high suspended sediment concentration during ebb tide is mainly due to the arrival of resuspended sediment from an area northeast of the Jiaozhou Bay head via adjective transport. In addition, the flood current can also cause the resuspension of sediment from the bottom, thereby increasing the suspended sediment concentration in the bottom layers (approximately 10 mg/L).The concentration generally remained between 3-6 mg/L.

Station QD11 is located at the mouth of Jiaozhou Bay,with a depth of approximately 22 m (Fig. 4e). The flood current velocity was up to 90 cm/s. Therefore, the suspended sediment concentration in the bottom layer increased to 20-30 mg/L during flood tide. This is mainly due to near-bottom sediment resuspension caused by the flood current. The duration of this resuspension was very short. The average suspended sediment concentration at depths of 0 m and 5 m was 4-5 mg/L, with little variation with tidal cycles. The suspended sediment concentration at 10 m showed an increasing trend during ebb tide, varying between 8-10 mg/L.The suspended sediment concentration at station QD11 was higher compared to station QD08 in the inner bay.

5.3. Numerical modeling of sediment transport

In general, the suspended sediment concentration in the Jiaozhou Bay area is relatively low, and its movement is relatively inactive. The sediment is mainly derived from the shallow-water region of the Jiaozhou Bay head. Outside the bay, the suspended sediment concentration is low; thus, the suspended sediment flux into the bay through the bay mouth is very low. The suspended sediment inside Jiaozhou Bay is mainly due to the dynamically driven resuspension of the sea floor sediment in the shallow-water region. There is little sediment input from the rivers surrounding the bay, and this input has a limited effect on the distribution of the suspended sediment in Jiaozhou Bay. Thus, the overall distribution pattern of suspended sediment in Jiaozhou Bay shows a higher concentration inside than outside the bay, and the concentration decreases rapidly from the bay head toward the bay mouth. Due to the deposition of fine sediment delivered by the Dagu River, the sediment concentration of the inner bay is higher in the northwest than in the east.

The numerical modeling results show that the maximum suspended sediment concentration during the flood-ebb tides in the Jiaozhou Bay region is approximately 40 mg/L in a location northwest of the bay head and Cangkou Channel. The suspended sediment concentration from the bay center to the mouth and outside the bay is 6-10 mg/L. The regions with high and low concentrations reflect periodic fluctuations with the tidal cycles. A significant sediment concentration front with a high concentration gradient is formed in Jiaozhou Bay.Although the tidal current velocity is high at the bay mouth,the flux of resuspended seabed sediment is not high. This is due to the rapid decrease in the vertical velocity in deep water, which hinders the formation of effective shear stress at the bottom.

At the beginning of the flood tide, open water with a low suspended sediment concentration flows into Jiaozhou Bay through the bay mouth. The suspended sediment concentration at the head and the inner and outer central regions of the bay mouth channel increases rapidly from 8-10 mg/L to greater than 20 mg/L at the bay head, forming a significant sediment concentration front (Fig. 5a). The high concentrations of suspended sediment mainly dominate the shallow-water region of the bay head. At the maximum flood tide, the open water with a low suspended sediment concentration rapidly spreads toward the inner bay. The area with a high suspended sediment concentration contracts significantly at the bay head, concentrating along the shore region (Fig. 5b). At high tide, the invasion range of the open water with a low suspended sediment concentration reaches a maximum. The suspended sediment concentration along the north coast of Red Island is reduced to approximately 10 mg/L, with a higher concentration of approximately 20 mg/L along its east and west sides (Fig. 5c). At the beginning of the ebb tide, the suspended sediment at the bay head spreads toward the bay center under the effect of the ebb current. At the maximum ebb tide, the suspended sediment from the northwest spreads to the coastal region of Yellow Island, and the suspended sediment from the Cangkou Channel region spreads southward (Figs. 5d, 5e). At low tide, the diffusion range of the northwest near-shore suspended sediment further increases, and the sediment concentration west of the bay mouth increases significantly (Fig. 5f). Overall, during the entire spring tidal cycle, the suspended sediment at the bay head is not transported beyond the bay through the bay mouth. The suspended sediment concentration outside the bay is low except during ebb tide, when it reaches approximately 15 mg/L at Tangdao Bay (Figs. 5g, 5h). During the neap tidal cycle, the suspended sediment concentration at the bay head remains high, although the invasion range of the open water with a low suspended sediment concentration during flood tide is limited. During ebb tide, the diffusion of the inner bay sediment to the bay mouth increases.

6. Conclusions

(i) The major source of sediment inside Jiaozhou Bay is the resuspension of seafloor sediment from the bay head region. The overall distribution pattern shows a high concentration of suspended sediment at the bay head, a low concentration at the bay mouth, high concentrations along the east and west sides, with the west side being higher than the east side, and a low concentration in the central region.

(ii) Limited by the sediment source, the direction of the prevailing current in the Jiaozhou Bay area is different from that of the sediment transport. The velocity of the flood current is higher than that of the ebb current. However, during flood tide, there is little flux of resuspended seafloor sediment outside and at the mouth of the bay, and this source does not contribute significantly to the suspended sediment in the bay.During ebb tide, the resuspended sediment at the shallowwater bay head and the east and west sides spreads toward the bay mouth with the ebb current but does not appreciably extend beyond the bay through the bay mouth.

(iii) The resuspended sediment in the northeast shallowwater region of the bay is transported southward along the eastern channel driven by the ebb tide current. In the central and west regions, the resuspended sediment outside the mouth of the Dagu River is transported to the center driven by the ebb tide current. Affected by the drastic variation of the seafloor topography, a suspended sediment concentration front is formed at the northern end of the Central Channel,leading to sediment deposition in this region. This is consistent with the sediment transport trend revealed by the bed sediment grain size distribution in Jiaozhou Bay.

(iv) In recent years, the overall deposition rate in Jiaozhou Bay has been very low. This is mainly due to the sharp decrease in the terrigenous sediment supply surrounding the Jiaozhou Bay region. Thus, the main sediment source is the dynamically driven resuspension in the shallow-water region of the bay head. The occurrence of seafloor erosion and the tidal-driven diffusion of resuspended sediment toward the center of the inner bay contribute to the weak deposition,although the deposition rate is very low. The direction of the net sediment transport in Jiaozhou Bay is different from that of the prevailing current. At the Jiaozhou Bay mouth,although the velocity of the flood current is high, the bottom sediment is generally coarse-grain material or bed rock, which is difficult to resuspend. The sediment transport flux is limited and has little contribution to the deposition in Jiaozhou Bay.

Acknowledgment

This study was jointed funded by the Marine Geology Survey Project (DD20160137, GZH200900501 and DD2019 0502) and the National Natural Science Foundation of China(41376079 and 41276060). The authors thank the anonymous reviewers and Dr. Yan Yang for their comments and suggestions, which significantly improved the quality of the manuscript.