，in which:
Yan Yixin, Xu Fumin and Mao Lihua
Research Institute of Coastal
& Ocean Engineering,
Hohai university, Nanjing, 210098, China. E-mail: WPP@jlonline.com,
Tel: (86)25-3713777-50637, Fax: (86)25-3701905
Abstract: The stage regulation work of deep water channel of the Yangtzs River Estuary and the topographic change of the Lower-Section of the Jiuduan Sandbank after regulation were simply described. Nonlinear three-dimensional numerical model under the s coordinate system was applied in this paper, the model’s numerical scheme, boundary conditions were presented here, and the model was verified with field observations including both the field water surface elevations and field current velocity. Topographic change of the Lower-Section of the Jiuduan Sandbank after and before theⅠstage regulation work (from May 1997 to March 2000) was obtained based on field measurements, problems to be solved in this paper were concluded as: why the Lower-Section of the Jiuduan Sandbank moved towards north direction, where the silted sediment came from, whether it came from Jiuduan Sandbank itself or from other area. To solve the above problems, concepts of tidal discharge and the Euler’s residual flow were introduced to jointly interpret the hydrodynamic mechanics of sediment movement and topographic change for the first time. By using identical boundary conditions and different topography in May 1997 and March 2000, changes of the tidal discharge and the Euler’s residual flow from May 1997 to May 2000 in the North Channel area were numerically simulated based on the nonlinear three-dimensional numerical model. Finally, the effect of he hydrodynamic change mechanics in the North Channel on the topographic change of the Lower-Section of the Jiuduan Sandbank was analyzed comprehensively, the results showed that sediments silted at the northern part of the Lower-Section of the Jiuduan Sandbank were not come from the Juduan Sandbank but from the Grions Section, the northern part along the North Jetty and the vicinity of the East Sandbank Trench, the simulated hydrodynamic mechanics were in good agreement with the topographic change, so the model and the study method used in this paper could not only serve as a useful research method but also provide scientific basis for the decision making of future regulation work.
Keywords: the stage regulation work, the yangtze
river estuary, the jiuduan sandbank, nonlinear three-dimensional numerical
model, the north channel, tidal discharge, euler’s residual flow, the grions
section, the north jetty, east sandbank trench
The regulation of deepwater channel of the Yangtzs River Estuary is a huge and comprehensive work, it is of great significance in the Shanghai’s economical development, and it will greatly promote Shanghai as the international ship center. The Ⅰ stage regulation work of deepwater channel of the Yangtzs River Estuary was started in January 27 of 1998 and achieved in March of 2000, the regulation consisted of diversion jetties, north and south jetties, groins, dredging work and leading protection apron work. After the completion of theⅠstage regulation work, water depth of the navigation channel has been increased to 8.5m and the channel has been put into use, in the meantime, topography in the North Channel area has changed obviously because of the regulation work. The northward extension tendency of the Lower-Section of the Jiuduan Sandbank has become more serious since the Ⅰstage regulation work started, and the channel’s depth in that place is now influenced by the northward extension of the Lower-Section of the Jiuduan Sandbank, so the dredging work in this section has become a difficult tedious burden. To solve this problem, the hydrodynamic mechanics for the topographic change of the Lower-Section of the Jiuduan Sandbank needs to be studied In order to guarantee the normal navigation and to provide scientific basis for the decision making of future regulation work. With rich sediments and mighty tidal current, physical conditions in the Yangtzs River Estuary are too complicated to be interpreted with static, single and isolated hydrodynamic factors. A nonlinear three-dimensional numerical model is applied in this paper to simulate the changes of the tidal discharge and Eular’s residual flow before and after the Ⅰstage regulation work in the North Channel area, and the northward extension tendency of the Lower-Section of the Jiuduan Sandbank is analyzed comprehensively.
The topography of
the Yangtzs River Estuary is too complex to use the absolute layers in vertical
direction. By applying the s coordinate
system, the vertical water depth can be divided into equal layers (grid numbers)
in the whole computer area, and the horizontal land boundary conditions of
different vertical layer’s have the same values. The nonlinear
three-dimensional numerical model (zhu et al., 2000) under the s coordinate
system is described as follows:
The vertical current velocity under the s coordinate system is：
，in which:
D=h+h，thus the vertical current velocity under the Cartesian coordinate system is:
(1)
Three-dimensional numerical model under the s coordinate system consists of the continuity equation (2)，momentum equations (3) and (4), hydrostatic equation (5):
(2)
(3)
(4)
(5)
Mode Splitting Method (zhu et al., 2000, Yan et al., 1999) is used in numerical scheme, the depth-averaged two-dimensional hydrodynamic model, that is, the outer mode, is obtained by integrating the equations (2), (3) and (4) in vertical direction. A modified double-sweep fully implicit finite difference scheme, which is also called as the Double-Sweep-Implicit (DSI) method, is applied to solve the fluctuations of water surface elevation and the depth-averaged current velocity, then the three-dimensional current field is computed according to the inner mode (equations(2)～(5)).
The model’s computing zone covers the area from Xuliujing in the west to the Luhuashan Island in the east, from the northern part of Lianxinggang in the north to the joining line of Luchaogang and Luhuashan Island in the south. Variable grid size system has been used with finite-difference scheme to increase the resolution at areas of interest and also to cover a sufficiently large area without significantly increasing the computing cost. The grid sizes in the x and y directions in the North Channel area are 197.9m and 185.3m respectively, grid number in the vertical direction is 6 layers. Fluctuations of water surface elevation in Xuliujing (the west boundary condition) are obtained according to field measurements, water surface elevations in the east, south and north boundaries are obtained according to the nonlinear two-dimensional spherical coordinate tidal wave mathematical model of East China Sea (Lin et al., 2000).
The model is verified by comparison with fluctuations of water surface elevation in the Beicaozhong and the Jiuduandong stations from 00:00 o’clock 16 March 2000 to 23:00 o’clock 31 March 2000, see Fig. 1(a) and 1(b). Later the model is also verified by comparison with field current velocity and current directions in the Beicaozhong station from 8:00 o’clock 25 March 2000 to 13:00 o’clock 26 March 2000, see Fig. 2(a) and 2(b).
Fig.1(a) Comparison of water surface
elevations in the Beicaozhong station
（dot—Field observations;
solid line—numerical results）
Fig.1(b) Comparison of water surface
elevations in the Jiuduandong station
（dot—Field
observations; solid line—numerical results）
Fig. 2(a) Comparison of current
directions in the Beicaozhong station
（dot—Field
observations; solid line—numerical results）
Fig. 2(b) Comparison of current velocity
in the beicaozhong station
（dot—field
observations; solid line—numerical results）
Since 1990s, the Lower Section of the North Channel has shown a tendency of silting in the south and scouring in the north, that is, the 5m contour line in the northern part of the Lower-Section of Jiuduan Sandbank moves northward, and the 5m contour line in the southern part of the Hengsha Shoal moves northward. The topographic change map (see Fig.3) from May 1997 to March 2000 shows that the motion length of the 5m contour line at the northern part of the Lower-Section of Jiuduan Sandbank is about 600m. It should be noted that the motion velocity has become faster since the Ⅰstage regulation work started, the distance between the 6m contour line in the northern part of the Lower-Section of Jiuduan Sandbank and the channel’s southern line was as short as 250m also in March 2000, thus the Ⅰstage channel is unavoidably influenced by the deposition of the Lower-Section of Jiuduan Sandbank, and the dredging work there Has become a tedious difficult burden. Problems need to be solved are: why the Lower-Section of the Jiuduan Sandbank moves towards north direction, and where the silted sediment comes from, whether the sediment source comes from Jiuduan Sandbank itself or other area, and thus, this is the goal.
Fig.3 Comparison of 2m and 5m contour lines between May 1997
and March 2000
（solid line—March 2000; dashed line—May 1997）
Topographic change in the Lower-Section of the Jiuduan Sandbank relates closely with the sediments movement of the Yangtzs River Estuary especially of the North Channel area, so the hydrodynamic change Characteristics in the channel area should be studied comprehensively.
The tidal discharge is obtained by summing up
the water discharge of unit width in a tidal cycle at every horizontal grid
nodes of the computing zone, it denotes the water momentum, so it can interpret
the hydrodynamic mechanics of sediment movement and topographic change to a
great extent. Fig. 4(a) shows the tidal discharge field of North Channel area
during a spring tide in May 2000; Fig. 4(b) shows the field of tidal discharge
difference value of North Channel area during a spring tide, e.g., the field is
obtained by subtracting the tidal discharge value of May 1997 from that of May
2000 (the boundary conditions are identical, but the topographies are different
in two cases), areas enclosed by short dash lines and solid lines denote the
decrement and increment areas of tidal discharge respectively.
Fig. 4(a) Tidal discharge field of North Channel area during a spring tide in May 2000
Fig. 4(b) Field of difference value by
subtracting tidal discharge value of May 1997
from that of May 2000 in North Channel area during a spring tide
From Fig.4(a), it can be seen that, after theⅠstage regulation work, tidal discharge vectors in Hengsha East Sandbank, Hengsha Shoal and Jiuduan Sandbank point towards north direction, vectors in the upper reach of grions, Grions Section and Middle Section of North Channel point towards east direction, and vectors in the Lower Section of North Channel point towards northeast direction. Compared with tidal discharge value before the Ⅰstage regulation work (May 1997), Fig.4(b) shows that tidal discharges decrease in Hengsha East Sandbank and the flood tidal trench at Hengsha Shoal, increase in East sandbank Trench, increase slightly in other areas of Hengsha Shoal, increase in the Grions Section, decrease in the Middle Section of North Channel except in the northern part near the flood tidal trench’s 2m contour line, vary little in the Jiuduan Sandbank and the Lower Section of North Channel.
The Ⅰstage regulation work causes significant change of water momentum in Hengsha East Sandbank and East Sandbank Trench, thus the sediment movement there has become more active and more easy to be carried into North Channel by ebb current and circumfluent flow along the North Jetty; sediments in the Grions Section are scoured away because of the significant increase of tidal discharge there, the sediments move with the tidal current in the channel and part of them will be silted in the Middle and Lower Sections of North Channel; whereas tidal discharges in the Jiuduan Sandbank vary little. Thus it can be concluded that the northward movement of the Lower-Section of Jiuduan Sandbank is not caused by the sediments in the Jiuduan sandbank but by the sediments from East Sandbank Trench and Grions Section where increased water momentum scours lots of sediments and carries them to the Middle and Lower section of the North Channel.
Eular’s residual flow is obtained by summing
and averaging the current velocity vectors on bed layer in a tidal cycle at
every grid nodes under the Eular coordinate system, it is a permanent flow
caused by the nonlinear processes of tidal motion, it is of great significance
in the long-term sediment transport and topographic development of ocean
environment, especially in coastal and estuarial zones. Fig. 5(a) shows the bed
layer residual flow field during a spring tide in the North Channel area in May
2000, and Fig. 5(b) is obtained by subtracting bed layer residual flow vectors
of May 1997 from that of May 2000 during a spring
tide (the boundary conditions are identical, but the topographies are different
in two cases).
From Fig. 5(a), it can be seen that residual flow vectors in Hengsha East Sandbank and Hengsha Shoal point towards north direction, vectors in the upper reach of grions, Grions Section, Middle and Lower Sections of North Channel point towards east direction, in the lower part of Jiuduan Sandbank, there exists a strong clockwise rotary residual flow field. Compared with residual flow before the Ⅰstage regulation work (May 1997), Fig.(5b) shows that residual flow vectors change obviously in Hengsha East Sandbank and the 2m flood tidal trench of Hengsha Shoal, the increased residual flow vectors point towards east northeast and southeast directions respectively, residual flow vectors change significantly in East Sandbank Trench, the increased vectors point towards north direction. There exists a strong clockwise rotary residual flow increment field under the joining line between the leading protection apron’s head and the north end of East Sandbank Trench. Residual flow varies little in Jiuduan Sandbank. Residual flow vectors’ increment of the Groins Section points towards ebb tidal direction; residual flow vectors’ change in the southern part of the Middle Section of North Channel is not obvious, whereas that in the northern part is relatively large, the vector’s increment there points towards north. Residual flow vectors in the Lower Section of North Channel show little change.
The strong clockwise rotary residual flow field under the joining line between the leading protection apron’s head and the north end of East Sandbank Trench will bring sediments at
Fig. 5(a) The bed layer residual flow field in May 2000
Fig. 5(b) vectors difference by subtracting bed layer
residual flow of May 1997
from that of May 2000 in North Channel area during a spring tide
northern part of North Jetty into the Middle Section of North Channel, the sediments move downstream with the eastward residual flow, and are carried to the Lower Section of North Channel, in addition, the Jiuduan Sandbank has resistant effect on sediments to some extent, some sediments will deposit and be silted in the northern part of the Lower Section of Jiuduan Sandbank due to the clockwise rotary residual flow there.
Topographic change of the Lower-Section of Jiuduan Sandbank from May 1997 to March 2000 is caused by the following reasons: (a) sediments are scoured away from the Grions Section and some will be silted in the Middle and Lower Sections of North Channel because of the Ⅰstage regulation work. (b) Being a flood and ebb trench, the East Sandbank Trench in the eastern part of Hengsha East Sandbank is in a developing state, a lot of sediments there will be scoured off and then brought into the Middle and Lower Sections of North Channel by ebb current in Hengsha Shoal. (c) In the northern part along North Jetty, sediments will be scoured off by the strong circumfluent flow and brought into North Channel afterwards. Under the complicated tidal current conditions in the Middle and Lower Sections of North Channel, some of the above three part of sediments are silted in large area of the Middle and Lower Sections of North Channel, in addition, the Jiuduan Sandbank has resistant effect on sediments, some sediments deposit at the northern part of the Lower-Section of Jiuduan Sandbank, so that leads to the northward moving tendency.
Effect of hydrodynamic factors of the North Channel on the topographic change of the Lower-Section of the Jiuduan sandbank has been studied in several aspects based on the nonlinear three-dimensional numerical model. Analysis shows that the numerical simulated hydrodynamic mechanics are in good agreement with the topographic change phenomena, so the model and study method used in this paper can provide scientific basis for the decision making of future regulation work. It is obvious that not only the hydrodynamic mechanics of tidal current but also that of wind, waves, salinity, etc., should be studied comprehensively to simulate the scour and silting of topography, this would be a goal for a future study.
References
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