Hydrodynamic and morphological processes in Yangtze Estuary: State-of-the-art research and its applications by Hohai University

Jin-hai ZHENG*, Yi-xin YAN, Chao-feng TONG, Zhi-yi LEI, Chi ZHANG

1. State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering, Hohai University, Nanjing 210098, P. R. China

2. College of Harbor, Coastal and Offshore Engineering, Hohai University, Nanjing 210098, P. R. China

Hydrodynamic and morphological processes in Yangtze Estuary: State-of-the-art research and its applications by Hohai University

Jin-hai ZHENG*1,2, Yi-xin YAN2, Chao-feng TONG1,2, Zhi-yi LEI2, Chi ZHANG1,2

1. State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering, Hohai University, Nanjing 210098, P. R. China

2. College of Harbor, Coastal and Offshore Engineering, Hohai University, Nanjing 210098, P. R. China

This paper presents a review of the state-of-the-art research and its applications developed at Hohai University relating to the hydrodynamic and morphological processes in the Yangtze Estuary. Longitudinal, lateral, and horizontal flow circulations have been revealed based on the measurements with acoustic Doppler current profilers (ADCP). The hydrodynamic mechanism at diversion points as well as the changing patterns of flow and sediment flux in the Yangtze Estuary has been investigated through long-term data analysis. A field survey has been carried out to detect the saltwater intrusion from the North Branch to South Branch. Different numerical models of flow motion, sediment transport, and saltwater intrusion have been developed to simulate the complicated processes and to evaluate the effects of engineering projects. The morphological processes of wetlands over a time scale of decades have been analyzed with an established database. Ideas for further research on the bio-geomorphological model system and long-term evolution mechanisms are put forward.

Yangtze Estuary; hydrodynamic process; morphological process; saltwater intrusion; Hohai University

1 Introduction

The Yangtze Estuary has three diversion points and four outlets. With a 90-km entrance opening to the East China Sea, it accepts a huge tidal volume from the sea, and transmits tidal waves 642 km upstream to Datong, Anhui, where the tidal limit is located in the dry season, and 153 km upstream to Jiangyin, Jiangsu, where the tidal current limit is located in the flood season (Zhong 1985), as shown in Fig. 1.

The Yangtze Estuary has a multi-annual average precipitation of 1 000 mm to 1 100 mm with quite a large amount of interannual variability. The precipitation in wet years is about1 200 mm, up to 1 400 mm at maximum, and in dry years it varies from 600 to 700 mm.

According to statistical analysis of the measured data at the Datong Station from 1951 to 1988, the multi-annual average sediment concentration was 0.403 kg/m3, the highest annual average concentration was 3.14 kg/m3, and the lowest one was 0.28 kg/m3; the multi-annual average sediment load was 4.60 × 108t, with the largest annual load being 6.72 × 108t in 1964, and the smallest one being 3.11 × 108t in 1986.

Fig. 1Tidal limit and tidal current limit of Yangtze Estuary

According to statistical data from the Datong Hydrological Station from 1951 to 2005, the multi-annual average discharge was 28 226 m3/s; the multi-annual average runoff was 8.853 × 1011m3, with the maximum one being 1.358 9 × 1012m3in 1954, and the minimum one being 6.758 × 1011m3in 1978.

In the Yangtze Estuary, tides are controlled by tidal waves propagating from the sea. The East China Sea’s progressive wave system includes the M2 partial tide as the major tide in this region, and is impacted by rotational tidal waves of the Yellow Sea, with the K1 and O1 tides being most notable. The tides in the estuary are non-formal neritic semidiurnal tides, with obviously different daily tides, especially at high tides. From the spring equinox to the autumnal equinox, night tides generally outdo diurnal tides, while from the autumnal equinox to the spring equinox of the following year, diurnal tides outdo night tides.

Wind waves prevail in the Yangtze Estuary. For mixed waves, the cases in which wind waves prevail and swells prevail account for 77% and 23%, respectively. It is rare for only swells to occur. The dominant wave direction is NNE, with a frequency of 10.25%. The seasonal change of wave directions is significant: NW prevails in winter, SSE prevails in summer, and NE often prevails in spring and autumn.

Since the Yangtze Estuary is characterized by these complicated dynamic processes, it has a significant value to scientific research and engineering studies. In recent decades, relevant studies on the Yangtze Estuary have been continuously conducted by various institutes in China. As one of the top universities in estuarine and coastal engineering, HohaiUniversity has played an important role in the development of fundamental theory and engineering applications in this region. This paper presents a general review of the state-of-the-art research and engineering studies conducted by Hohai University with respect to hydrodynamic and morphological processes, such as hydrodynamic characteristics, saltwater intrusion, and wetland evolution.

2 Hydrodynamic characteristics

2.1 Circulation feature analysis based on ADCP measurements

The location of the Yangtze Estuary and the field measurement sections in the North Passage are shown in Fig. 2. The field data used herein were all primarily measured by acoustic Doppler current profilers (ADCP) in the Yangtze Estuary from September 22 to 30, 2002, and in the North Passage from October 21 to 23, 2002 (Yu 2005; Yan et al. 2007). The first step for circulation feature analysis was extracting the flow velocity in three directions: the west-east, north-south, and vertical directions. The next step was making some necessary transforms and comparisons among the relevant field data. Then, as the main task, the flow regime characteristics of the Yangtze Estuary were examined with a numerical model developed by Hohai University. Typical characteristics of circulation were found to exist in the flow regime of the Yangtze Estuary. The flow circulation also has significant effects on the transport and diffusion of dissolved pollutants in the estuary (Wu and Yan 2010). The possible generation mechanisms and some main factors were analyzed.

Fig. 2Location of Yangtze Estuary and field measurement sections in North Passage

It was found that there are three kinds of circulation in the Yangtze Estuary, shown in Figs. 3 through 5: during the low slack tide, longitudinal circulation with the surface layer seaward and the bottom layer landward (Fig. 3(b)), horizontal circulation with the north side landward and the south side seaward (Fig. 4(b)), and lateral circulation with the surface layersouthward and the bottom layer northward (Fig. 5(b)). Both the lateral circulation and horizontal circulation are mainly driven by Coriolis forces, and these three kinds of circulation always appear during the turning process of tides and move with tidal currents. If encountering a strong tidal current, the circulation will disappear. This indicates that, during the turning process of a tide, water flow moves in a complicated three-dimensional spiral formation under the influence of the interaction between river discharge and tidal currents. These three kinds of circulation have relationships with the density gradient induced by saltwater intrusion.

Fig. 3Longitudinal velocity variations at section #1 of North Passage (The rightward arrowhead orients downstream)

Fig. 4Velocity variations along horizontal lines on section #1 of North Passage (The upward arrowhead orients downstream, and the ADCP lines are arranged from the north side to south side)

Fig. 5Cross-sectional velocity fields of section #1 of North Passage

2.2 Hydrodynamic mechanism at diversion points

Some methods such as theory analysis, laboratory experiments, and numerical simulation were used to study the hydrodynamic features and mechanisms at the diversion points of the Yangtze Estuary. It was shown that the diversion points rise and fall periodically, with the channels flourishing and withering alternatively, and the waterways silting periodically (Yan et al. 2000, 2001a, 2001b, 2001c, 2002, 2003; Tong 2005; Lei et al. 2009).

The diversion point of the North Channel and South Channel is located at the Central Shoal. This is a changing diversion point, which swings between Liuhekou and Wusongkou. The Biandan Shoal and Liuhe Shoal are scoured and move downstream alternatively. The two shoals are cut frequently because they block the upper outlets of the North and South channels. The Biandan Shoal was cut in 1924 and 1978, with the sediment falling into the sea through the North Channel, and the Liuhe Shoal was cut in 1958 and 1963. Since the 1980s, the Yangtze Estuary has tended to be stable with the law of bed development still acting. At the outlets of the North Channel and South Channel, the Xinqiao Channel heading to the North Channel and the Nanshatou Shoal heading to the South Channel form an angle of 84°. The lower section of the Nanshatou Shoal is atrophied. Upstream, the Xinliuhe Shoal was cut and its flow was sped up by the flood in 1998, with the separated section moving downstream and uniting with the Central Shoal. Also, the diversion point of the North Channel and South Channel moved upstream.

The river regime of the North Passage is healthy owing to the good boundary conditions: (1) the joint of the North Passage with the main channel of the South Channel is straight; (2)the diversion angle between the North Passage and South Passage is an acute angle; (3) the hydrodynamic features of the velocity field at the diversion point of the North Passage and South Passage are in favor of the bed sand discharge from the South Passage; and (4) a rotary current, in the Jigu Reef area outside the outlet of the North Passage, is advantageous to the discharge through the North Passage.

In 1963, when the North Channel and South Channel were cut at their diversion point, a tremendous amount of sediment entered the South Channel, forming a central sand ridge in the South Channel during the 1970s. The central sand ridge was divided by the main flow from the South Channel during the 1980s, with its lower section forming the Jiangya’nan Shoal. The Jiangya’nan Shoal extended northward, moved downstream, and connected with the head of the Jiuduan Shoal during the 1990s. Because of this, the diversion point of the North Passage and South Passage moved upward to the head of the Jiangya’nan Shoal. During the 1990s, the Jiangya’nan Shoal was scoured downward by the ebb tide, and was restrained from moving downstream by a fish-mouth dike at the diversion point of the North Passage and South Passage in 1998 due to the deep waterway regulation work. The river regime has now entered a good period, with a diversion angle of 65°.

On the east of the main channel of the South Channel, due to the shoreline deflecting southward, the watercourse becomes wide, and the main stream discharges to the 10-m isobath. The boundary layer may be separated, and an orchid vortex forms. The bed sand is brought downward by lateral circulation through the South Passage. The rotary current is intensive near the area of the Jigu Reef outside of the North Passage, with the circulation ratio of the M2 equinoctial tide being 0.7. Because of the lag of aggradations outside the North Passage, the vertical section of the North Passage maintains a larger gradient, and the flow discharge is expedited. This can be considered as a good boundary condition of the river regime.

2.3 Patterns of changing flow and sediment flux

Shao et al. (2011) carried out in situ measurements of the sediment settling velocity near the Baimao Shoal in the Yangtze Estuary. Using a three-dimensional baroclinic model, the fine silt particle pathline of sediment dredging in the Yangtze Estuary was studied (Xie et al. 2010).

Based on linear regression analysis of the hydrological data at the Datong Station from 1950 to 2006, the discharge from the Yangtze River, including the annual discharge and monthly discharge, was basically stable (Shen 2006; Li 2007). According to the linear regression analysis, the sediment concentration in the downstream of the Yangtze River decreased gradually. The average sediment concentration in the flood season is obviously larger than that in the dry season. The variation coefficient of the maximum sediment concentration in the flood season relative to that in the dry season is 0.393, and that of average sediment concentration is 0.311. The annual sediment transport and sediment concentration have evidently been decreasing with the same trend since 1984 (Fig. 6).

Fig. 6Linear regression analysis of annual average sediment concentration at Datong Station

Like the characteristic flow in flood and dry seasons, the annual extreme flow as well as its cumulative frequency can be used to calculate the tidal wave and abstraction volume in lower reaches of the Yangtze River. Then locationships of tidal limit and tidal current limit can be determined, and their relations with the discharge of extreme flow can be obtained. The effect of abstraction volume on the locations of tidal limit and tidal current limit can be shown as well.

The following was found in Li (2007): (1) With a flood volume of 92 600 m3/s in 1954, the tidal limit of the spring tide appeared near Wuhu City, 395.3 km upstream from Xuliujing, and the tidal current limit appeared near the Qigan River and the Chengtong Reach, 22 km upstream from Xuliujing. When an astronomical tide occurred, the tidal wave traced far upstream even if the discharge was very large, which could threat the warning level and flood embankment. (2) With the flood volume of 4 620 m3/s in 1979, the tidal limit of the spring tide was upward to Anqing City, about 600 km upstream from Xuliujing. The tidal current limit was near the Bamao Hill at Heishazhou, about 450 km upstream from Xuliujing. With the discharge decreasing in the dry reason, the location of the tidal current limit traced upstream. Thus, the duration and intensity of saltwater intrusion increased evidently. At the same time, the to-and-fro current was adverse to sewage discharge and self-purification. (3) Compared with the tidal limit, the tidal current limit has a higher sensitivity. The moving distance of the tidal current limit was larger than that of the tidal limit under the same variation of the discharge. (4) The locations of tidal limit and tidal current limit are to some extent affected by the abstraction volume in the lower reach of the Yangtze River when the Tongcheng Reach is mainly controlled by tides.

2.4 Effects of engineering projects on hydrodynamic characteristics

The effects of the Three Gorges Project (TGP) on the hydrodynamic condition, river regime, waterway stability, and estuary morphology were analyzed based on the sediment and shipping technique research for TGP (Yan 1991; Chen et al. 2008). The following conclusions were drawn: (1) The discharge amplitude diminishes because of the regulation of annualreservoir discharge. After the construction of TGP, the range of the stagnation zone is shortened during the period of reservoir water storage with the water level varying from 150 m to 175 m in October of dry years. (2) A decrease of sediment transport from the stream is advantageous to the channel stability. (3) In October of dry years, the decrease of discharge caused by reservoir regulation enhances the inverse flow from the North Branch into the South Branch, but from January to May the increase of discharge eases up this inverse flow.

The following can be seen from the calculated results of the regulation work of the deep-water channel in the Yangtze Estuary: (1) The change of bed resistance and tidal wave distortion may cause the changes of the average tidal level, high tidal level, and low tidal level. At the diversion point of the North Passage, the average level has run up, the high level has fallen down, and the low level has run up. While for the South Passage, the average level has fallen down, the high level has run up, and the low level has fallen down. (2) Affected by jetties and groins, the discharges of the North Passage and South Passage have redistributed with some ebb currents transferring from the North Passage to South Passage and the diversion ratio decreasing. (3) The regulation work of the deep-water channel supports channel maintenance.

Based on the research on saltwater intrusion in the North Branch and the construction of water source regions using the numerical model of Hangzhou Bay, five plans were investigated with respect to the narrowing of the North Branch, construction of a tidal barrier in the North Branch, and reclamation of the Qingcaosha Reservoir on the South Branch. It can be found that (1) the narrowing of the North Branch has little effect on the South Branch and South Channel; (2) the reclamation of the Qingcaosha Reservoir induces the ebb current to increase at the upstream outlet of the North Branch, and causes the flood current to decrease at the downstream outlet of the North Branch; and (3) the plans for the narrowing of the North Branch and reclamation of the Qingcaosha Reservoir are preferable to that for the construction of the tidal barrier in the North Branch.

Delft3D was used to establish a two-dimensional tidal and salinity numerical model of both the Yangtze Estuary and Hangzhou Bay, with a model domain of 330 km from east to west and 340 km from south to north. The effects of the reclamation project and submerged dike and reservoir construction on hydrodynamics in the dry season were studied using this model, in which the discharge, flow velocity, flow direction, and water level were considered. It can be seen that the engineering works will have effects on the flood-ebb current in the Yangtze Estuary, with a decrease of 17.4% of the flood-ebb current in the dry season and a decrease of 18.8% of the flood-ebb current in the flood season. The diversion ratio will increase in the North Passage, whereas the flood-ebb current of other river sections will diminish. The tidal influx will be reduced under the effects of the narrowed North Passage, with an increase of the flood-ebb current and a decrease of the tidal current at the outlet. The estuary area will be more sensitive to the increasing water, whereas the area inside the estuarywill be less sensitive. The characteristics of water and sediment exchange between the Yangtze Estuary and Hangzhou Bay were also studied by Kong et al. (2007).

3 Saltwater intrusion

3.1 Motion laws of back-flowing saltwater from North Branch to South Branch

Supported by the major program of the National Natural Science Foundation of China (Grant No. 50339010), for the purpose of understanding the motion patterns of saltwater back flowing to the South Branch and the Xuliujing Reach, a tracking test for the back-flowing saltwater was conducted from March 12, 2005 to March 14, 2005 (Yang 2006). Some motion characteristics of the back-flowing saltwater were also studied. The moving time and range of the salt wedge were comparatively small. Being mixed well, the back-flowing saltwater rushed downstream along with the flood-ebb current. Owing to a relatively little difference in salinity, the surface saltwater and bottom saltwater mixed completely at the Qiyakou cross-section, with a distance of 10 km to the diversion point of the North Branch and South Branch. The water source region of Shanghai City lies in the area of the south bank in the South Branch, and is under the disadvantageous effects of the back-flowing saltwater.

3.2 Numerical simulation of saltwater intrusion in Yangtze Estuary

In the early 1980s, research showed that the mixture of saltwater and freshwater in the Yangtze Estuary was basically mild. A one-dimensional salinity numerical model was established using the diffusion equation and conservation equation, and the salinity longitudinal distribution and saltwater intrusion range were calculated. In recent years, the unstable longitudinal analytic solution for the one-dimensional saltwater intrusion has been derived. The field data in the deep-water channel have also been analyzed.

Since the 1990s, a two-dimensional vertical numerical model and a two-dimensional horizontal numerical model have been developed to study the saltwater intrusion in the Yangtze Estuary (Wang 1989; Wang and Zhu 1991). With the volume control method and the power function in the discrete format, the tidal level, flow velocity, and salinity have been validated. In recent years, supported by ELCIRC and Delft3D, a two-dimensional numerical model of saltwater intrusion has been established, and a numerical model combining a one-dimensional tidal numerical model covering the region from the Datong Hydrological Station to the Yangtze Estuary and a two-dimensional numerical model of the Hangzhou Bay has been established as well (Yang 2006).

Since the late 1990s, for simulating freshwater and saltwater mixing in estuaries, a three-dimensional nonlinear baroclinic numerical model has been developed, in which the gradient of horizontal pressure contains the gradient of barotropic pressure arising from the gradient of tidal level and the gradient of baroclinic pressure caused by the gradient of salinity (Zhu et al. 2000; Zheng et al. 2002; Song et al. 2008).

Based on the mode-splitting technique, through the σ-coordinate transformation, the three-dimensional motion can be divided into an external mode and an internal mode. An improved double-sweep-implicit (DSI) method was employed in the external mode. The Eulerian-Lagrangian method was employed to deduce both the momentum equations of tidal motion and the equation of saltwater diffusion so as to improve the computational stability and accuracy. Methods to provide the boundary conditions and the initial conditions were proposed. The calculated salinity fields were presented, as shown in Fig. 7, where the dotted and solid lines represent the depth and salinity contours, respectively. Computational results show that the salinity distribution has a partial mixing pattern, and that the model is suitable for the simulation of freshwater and saltwater mixing in the Yangtze Estuary.

Fig. 7Salinity distribution in flood slack tide in South Passage and North Passage in September 1996

3.3 Effects of engineering projects on saltwater intrusion

Based on the National Key Scientific and Technological Project of the Seventh Five-YearPlan, a monographic study of sediment problems of TGP and their effects on navigation channels and saltwater intrusion induced by TGP were analyzed. In October of both average years and dry years, the average discharge of the Three Gorges Reservoir was 33 000 to 41 400 m3/s, and the water storage of the Three Gorges Reservoir had little effect on saltwater intrusion in the Yangtze Estuary. It had a distinct effect only in very dry years. However, the saltwater intrusion eased up when the freshwater discharge increased from January to May (Li et al. 2005).

In the National Key Scientific and Technological Project of the Eighth Five-Year Plan, a study on the evolution characteristics of sand bars and the deep-water channel regulation schemes in the Yangtze Estuary, a two-dimensional numerical model for lateral-vertical salinity distribution was employed to research the effects of the deep-water channel regulation work on saltwater intrusion. It was found that 0.5% saltwater near the bed traced upward in the dry season, but the regulation work had no effect on the salinity at Wusong. A three-dimensional salinity numerical model was developed to study the influence of the regulation work on saltwater intrusion after stages I, II, and III from 1996 to 2000, and during the future phase. With the field data measured six times at seven survey points, the stability of the numerical model was validated.

With the one-dimensional and two-dimensional salinity numerical models, the effects of some hydro-junctions on trunk streams, such as the South-to-North Water Diversion Eastern Route Project and TGP, were studied as well. From the research, it was found that the intensity of saltwater back flowing to the South Branch grew with the decrease of discharge. The discharge under the combined running of the above two projects was between those under the condition that the two projects ran separately, so was the intensity of saltwater intrusion. With a discharge of 25 000 m3/s at the Datong Station, the salinity of the South Branch did not exceed 0.2%, which was less than half the salinity there in the above three cases of project running.

The effects on saltwater intrusion, induced by regulation work in the Yangtze Estuary, were studied with a two-dimensional salinity numerical model. The result showed that enclosing land can reduce the intensity of saltwater intrusion but has little effect on the saltwater back flowing to the South Branch. Other combined plans recommended can reduce the saltwater back flowing, including the regulation work of the South Branch combined with the work of North Branch narrowing, or the regulation work of the South Branch combined with the work of North Branch narrowing and the construction of a tidal barrier. With these projects, salinity around the Chenhang Reservoir will be reduced, while that near the Qingcaosha Reservoir region will remain basically unchanged.

In order to forecast the salinity variations in the Qingcaosha Reservoir region, a hydrodynamic and saltwater intrusion model covering the whole Yangtze Estuary and Hangzhou Bay was developed with the TELEMAC model (Tong et al. 2010). According to thedifferent characteristic discharges corresponding to different cumulative frequencies, the salinity distribution in the Yangtze Estuary was calculated under different flow rates into the sea. The simulation results showed that the salinity values would decrease with the increase of the discharge. When the upstream discharge is below the average value of 16 700 m3/s in the dry season, the salinity response to the runoff is very sensitive.

4 Wetland evolution

4.1 Database and methodology

Estuarine wetlands exist as transition zones among marine areas, land, and freshwater areas. The zones are characterized by coupling interactions among marine, atmosphere, biology, geology, and human activities, and represent the most dynamic region in the nearshore area. The complex processes occurring in the estuarine wetlands are of fundamental importance to climate change, water conservation, flood control, beach protection, land reclamation, biological diversity, and the ecological balance. The total area of wetlands in the Yangtze Estuary is about 3 052 km2, consisting of 2 506 km2of nearshore wetlands, 478 km2of permanent river wetlands, and 68 km2of permanent freshwater lake wetlands. The sustainable development of wetlands in the Yangtze Estuary is essential for engineering projects and economic growth in this region. Fig. 8 shows the spatial distribution of wetlands. Recent data show that the deposition of most wetlands is slowing down, and some areas are now suffering erosion.

Fig. 8Spatial distribution of wetlands in Yangtze Estuary

The remote sensing technique was applied in the Yangtze Estuary. The topography database was implemented using professional software, including MAPGIS, ArcGIS, and Oracle. Data sets are generally classified into two categories, referred to as the digital elevation data and the remote sensing images. These two types of data were combined to establish a time series of wetland topography in the Yangtze Estuary (Zheng et al. 2010). Using underwater data measured in different years, a digital elevation model (DEM) was developed. In addition, the Landsat ETM color composite images were used to determine alongshore landscape and channel regime. To obtain the best match with DEM, the WGS84 coordinate system and the enhanced ERDAS image format with a ground resolution of 30 m were used. The superposition of the remote sensing images and DEM was implemented on the basis of the raster data format, and the superposition of the buoy data and the channel line was based on the vector data format.

4.2 Morphological processes of wetlands

A numerical morphodynamic model, TIMOR3, which was coupled with a hydrodynamic model and a wave model was applied to simulation of the long-term morphological response to the water and sediment changes in a large area in the Yangtze Estuary. A detailed investigation was made of the South Branch, where the deep-water channel regulation project is under construction (Zhou et al. 2009).

The wetland evolution over a time scale of decades was analyzed (Yang et al. 2011). Results show that on the east beach of the Chongming Island, the sections along the shoreline from the Baozhen Port to Xijia Port and from the Beisixiao Port to Beiliuxiao Port were stable, while the section from the Xijia Port to the Tuanjie Shoal suffered from erosion. Rapid and slow accretions were found in the migratory bird natural reserve and the section from there to the Beiliuxiao Port, respectively. The wetland in the north shoals of the North Channel kept migrating seaward along with the lower reach from the Baozhen Port to the Liuxiao sandy ridge. The outer Tuanjie Shoal kept growing, and the siltation promotion project on the eastern Hengsha Shoal shaped the artificial shoreline. The mouth bar reach of the North Channel developed towards a formed channel. The wetland area of the Jiuduan Shoal has been continuously increasing since the 1950s, and from a forecasting point of view, it will merge with the Jiangya’nan Shoal to form a new wetland region between the South Passage and North Passage of the Yangtze Estuary. After floods in 1980, 1982, 1983, and 1988, and ten continuous wet years from 1990 to 1999, the length, width, and elevation of the Baimao Shoal enhanced, and the number of individual sandy bodies reduced, while the Central Shoal formed. Then, the shoal head receded slowly due to the washout of conflux. During the period from May 1992 to September 2002, the –5-m depth contour of the Baimao Shoal head moved backward by 2 720 m, with an annual backward distance of 272 m. The total volume of scoured sediment during that period was about 1.3 × 106m3, which led to negative impacts onthe lower reach of the South Branch of the Yangtze Estuary. The Central Shoal in the lower reach of the South Branch was unstable. The shoal and inlets transferred frequently and regularly. During the 1980s, the sand ridge of the Momao Shoal joined to the south beach of the Yangtze Estuary beyond the depth contour of –2 m and maintained its embossed sandy body. The sand tail of the Momao Shoal extended downstream, and its direction has not changed till now.

5 Conclusions

A review has been presented of the state-of-the-art research and engineering studies conducted by Hohai University relating to hydrodynamic and morphological processes in the Yangtze Estuary. Longitudinal, lateral, and horizontal flow circulations have been revealed on the basis of ADCP measurements. During the low slack tide, there are longitudinal circulation with the surface layer seaward and the bottom layer landward, lateral circulation with the surface layer southward and the bottom layer northward, and horizontal circulation with the north side landward and the south side seaward. The hydrodynamic mechanism at diversion points and the laws of flow change and sediment flux in the Yangtze Estuary were demonstrated through long-term data analysis. It was shown that the diversion points rise and fall periodically, with the channels flourishing and withering alternatively, and the waterways silting periodically. A field survey was carried out to detect the saltwater back flowing from the North Branch to South Branch. Different numerical models of flow motion, sediment transport, and saltwater intrusion were developed to simulate complicated processes and to evaluate the effects of engineering projects (e.g., TGP and the deep-water channel regulation project). It was found that the large engineering projects have significant effects on the patterns of flow and sediment flux as well as saltwater intrusion. The morphological processes of wetlands over a time scale of decades were analyzed with an established database, which is considered important for the management and protection of wetlands and the continuous economic growth in the Yangtze Estuary. Future studies should include effects of varying river runoff and sediment load on estuarine dynamics, the micro-scale movement characteristics of fine sediment, the bio-geomorphological model system, the joint impacts of large engineering projects on the long-term morphological evolution, and an integrated framework of environmental impact assessment for construction projects in the Yangtze Estuary.

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(Edited by Ye SHI)

This work was supported by the National Basic Research Program of China (973 Program, Grant No. 2010CB429002), the Fundamental Research Funds for the Central Universities (Grant No. 2012B06514), the Special Fund of the State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering at Hohai University (Grant No. 2009585812), and the Qing Lan Project of Jiangsu Province.

*Corresponding author (e-mail: jhzheng@hhu.edu.cn)

Received Mar. 6, 2012; accepted Aug. 17, 2012