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Sediment resuspension and implications for turbidity maximum in the Changjiang Estuary
ELSEVIER
Marine
Geology
148 (1998) 117-124
Sediment resuspension and implications for turbidity maximum in the Changjiang Estuary Li Jiufa a,*, Zhang Chen b a State Key Lab. for Estuarine and Coastal Morphodynamics and Sediment Dynamics, Institute of Estuarine and Coastal Research, East China Normal University, Shanghai 200062, People’s Republic of China b Nanjing Institute of Geography and Limnology, Nanjing 210008, People’s Republic China
qf
Received 3 1 January 1996; accepted 6 December 1997
Abstract A comparative study on the properties and transport of the cohesive sediments in the mouths of the Changjiang Estuary proves that the suspended sediment concentration (SSC) is principally enhanced by the resuspension of bottom sediment in the mouth-bar reach, coupled with the deformed tidal wave and strong tidal current. Salinity intrusion and vertical gravitational circulation lead to the ‘trapping’ of the suspended sediment inflowed from the river and the sea. The turbidity maximum (TM) is therefore delivered by the local resuspension of the accumulated materials and bed erosion. The TM in the Changjiang Estuary is marked not only by its high SSC relative to the adjacent reaches, but also by the high wash-load content. Transport of sediment is very important. Settling velocity of suspended sediment is increased by flocculation. The massive settling during weak current periods often gives rise to the formation of fluid mud. The TM coincides locally with the mouth-bars. The transport of suspended sediment in the TM is analysed by using a splitting method. It is concluded that the contributing factors consist of advective terms and tidal pumping. Advective transport is dominant in the North Channel and tidal pumping predominate in the North Branch and South Passage, and the North Passage is in an intermediate situation. 0 1998 Elsevier Science B.V. All rights reserved. Keywords: Changjiang
Estuary; sediments; turbidity maximum; sediment concentration
1. Introduction The Changjiang Estuary is a mesotidal, partially mixed estuary. It is characterised by the complexity of morphology associated with multi-step bifurcations and wide mouths. The turbidity maximum (TM), as a combined result of various estuarine dynamical processes, exists in all four mouths and * Corresponding author. Tel.: + 86 21 625 47978; Fax: + 86 21 625 46441; E-mail: [email protected]
0025-3227/98/$19.00 0 1998 Elsevier Science B.V. All rights reserved. PII: SOO25-3227(98)00003-6
is marked by a SSC higher than both the up and down reaches. Usually the TM in an estuary is considered to be the results of the gravitational residual circulation (Schubel, 1968; Festa and Hansen, 1978; Wellershaus, 1981) and in others it is focused on the settling and resuspension process of the fine sediment (Meade, 1972; Allen et al., 1980; Eisma, 1986). Most studies of the Changjiang Estuary considered that the settling and lag washing effects may store sediments in the estuary (Li et al., 1993; Shen et al., 1993). The
L. Jiufu, Z. Chen J Marine Geology 148 (1998) 117-124
118
mixing and circulation patterns, the transport of the suspended sediment and the generating mechanisms of the TM in Changjiang Estuary have been extensively investigated (Shen et al., 1982, 1992; Huang et al., 1980; Su and Wang, 1987; Shi and Li, 1993). The present paper outlines a comprehensive analysis on the sediment composition, the settling and flocculation and the impact on the formation and development of the TM, as well as on the sediment transport.
2. Location and sampling The sampling area is located on the north and south passages of the Changjiang Estuary (Fig. 1) where it is characterised by partial mixing for most of the time. Together eight fixed stations marked D 1-D8 were set on both passages. There were eight sampling boats named Shanghai OBB No. 1 to No. 8 for sampling and simultaneously measuring water temperature, salinity, flow, suspended sediment concentration, water and sediments samples. The sampling and measurement were carried out
0
10
Turbidity
20
for 27 consecutive hours during each spring, mean and neap tide in July-August 1988 and December 1988. Water temperature, salinity, flow and suspended sediment concentrations were measured every hour in different water depths during the sampling period. Water samples of about 500 ml were taken using a horizontal sampler. Samples were taken from 0.5 m below the water surface and 0.5 m above the bottom. 200 cm3 water was filtered with 0.45 p pre-washed and pre-weighed cellulose acetate filters to gather suspended sediments for further chemical analysis. Particle size analysis of suspended materials was carried out using the Coulter counter technique. Organic metal concentrations were analysed by ICP.
3. Results and discussion 3.1. Basic properties of the sediment 3.1.1. Composition
The main components of the sediments in the TM zones are almost exclusively silt and clay. The
...
30tLm)
m:lximum
Fig.
I. The Changjiang
Estuary
and sampling
sites.
L. Jiufiz. Z. Chen /
Marine
median size of the bed sediments ranges between 0.056 and 0.010 mm with the mode between 0.063 and 0.008 mm. The median size of the suspended sediment varies between 0.004 and 0.009 mm with a dominating range around 0.007 mm. The size of the bed sediments and suspended is quite similar, implying that there is exchange between them. Seasonal variations in size are significant due to the change in sediment input of the river. For example the median size of the suspended sediment in the South Passage is between 0.004 and 0.006 mm in the summer and becomes about 0.0072 and 0.008 mm in the winter. 3.1.2. Settling velocity andflocculation By assuming a linear distribution of the internal shear stress, the vertical SSC (S,) distribution in the flow can be described by (Chien et al., 1983):
;=(?I+.&) with: w 17=--,u*=-.u
Ku*
vii CS
Geology
148 (1998)
II 7-124
119
high ions contents, and the high SSC are favourable conditions for flocculation to occur. A flocculation test has been carried out in a laboratory flume by using estuarine water and sediment. Results show that flocculation: (a) is inhibited by strong shear stress when flow velocity is over 0.35 m s-l with only the sediment coarser than 0.032 mm settles down under gravity; (b) occurs for sediments with sizes between 0.032 and 0.008 mm when the flow slows down to 0.3 m s-i: and (c) occurs for sediments with sizes between 0.004 and 0.008 mm when velocity is below 0.20 and above 0.10 m s-l. Clay minerals finer than 0.004 mm flocculate significantly only when flow speed decreases to 0.10 m s-‘. The current slows down to less than 0.35 m s-i only around the slack-waters in the North and South Passages in the Changjiang Estuary; hence the settling velocity of the suspended sediment reaches a maximum at that time and the massive settling leads to a distinct SSC stratification with high SSC near the bottom, which may give rise to a 10 to 35 cm thick fluid mud on the bed.
(2)
in which S, denotes the reference concentration, h the water depth and y the height over the bed, a the height of S,, settling velocity, K the von Karman constant, U* the friction velocity, g gravitational acceleration, Cs the drag coefficient, and U the vertically averaged current velocity. Field data are used to plot the values on both sides of Eq. ( 1) for estimating (Fig. 2). The final result shows the influence of the various factors and is called the effective settling velocity. The corresponding diameter of the suspended sediment, including the floccules, is called the equivalent project diameter (Schubel, 1971), ranging from 0.02 to 0.15cm s-l, showing considerable variation along the estuary although the particle size of the suspended sediment remains relatively constant. A maximum occurs at the mouth and reaches a peak in the TM zones. In general it is larger during the ebb tide than the flood tide, and larger in the winter than the summer. Salinity (2 to 20%0), hyperconcentration of clay mineral, organic matter (300 to 10,000 mg mm3),
3.2. Suspended sediment concentration 3.2.1. Wash-load content Here the wash-load content is defined as the concentration of very fine sediments that settle seldom onto the bed. It is estimated by the following equation ( Li, 199 1): fl=A+~.Fr
(3)
in which Fv=(U(~~)-“~), denoting the Froude number. A and B are to be determined by using field data and A is the wash-load concentration. B is a constant relative to the property of the sediments. A is considerably large in the whole estuary, ranging from 0.06 to 0.35 kg mm3 (Fig. 3). It accounts for 10 to 30% of the SSC. Its maximum is also located in the TM zones in both the North and South Passages, reflecting the contribution to the formation of the TM from that portion of the suspended sediment. The seasonal variation of A shows a low value during the summer, especially during the ebb tide (Fig. 3). This is probably
120
L. Jiufa, Z. Chen / Marine Geology I48 ( 1998) I1 7-124
-0.6 -
channel
I -0.4 channel branch
branch
Fig. 2. The effective settling
velocity
of the suspended
sediment.
(b) (J)
0.11 South branch
I
South channel
South passage
Fig. 3. The A value along the South
because of the conditions more favourable for the flocculation occurring during that season. 3.2.2. Spatial and seasonal SSC variation (a) The SSC in the TM zones in the Passages, either vertically averaged or in a single layer, is much greater than the adjacent up and down streams (Table 1). The mean SSC is 3.5 times that in the South Branch and 2 times that in the South Channel, and also 2.7 times the SSC at Datong Hydrographic Station (located 630 km upstream from the mouth) which can be considered as the input concentration of the river (Shen et al., 1992). The instant SSC in the TM zones varies between 2 and 10 kg me3 with a maximum of 30 kg rne3. (b) The TM is better developed in the summer than in the winter. The mean SSC in the summer is double that of the winter. It is higher during the
Turbidity maximum center
Branch
Lr
sea
to South Passage
ebb tide than the flood tide in the summer, with the reverse in the winter. This can be explained by the varying river sediment (Shen et al., 1982). (c) The peak of SSC in the low portion of the water column in the TM zones occurs generally after the reversal of the tidal currents. It is more likely generated by a strong vertical mixing due to the acceleration and velocity gradient (Shi and Li, 1993) than by a simple current-induced scouring that yields a SSC peak in other reaches (Fig. 4).
3.3. Suspended sediment transport 3.3.1. Split of the sediment transport Several major transport mechanisms in an estuary can be investigated quantitatively by splitting the flux equation (Dyer, 1988). The final split
121
L. Jiufir, Z. Chen / Marine Geology 148 ( 1998) 117-I?4 Table 1 Statistics
over years of the mean SSC in the Changjiang
Estuary
(kg mea)
Tidal period:
Flood
Tidal range:
spring
ssc
mean
max
min
mean
max
min
mean
max
min
mean
max
min
0.43 0.21 0.72 0.61 1.43 0.84 1.28 0.59 0.88 0.98
0.92 0.33 0.14 1.23 2.28 1.26 1.65 1.02 1.81 1.01
0.17 0.12 0.49 0.39 0.88 0.39 0.88 0.26 0.34 0.93
0.24 0.07 0.44 0.41 0.60 0.68 0.38 0.39 0.48 0.64
0.39 0.11 0.72 0.66 0.98 1.12 0.46 0.44 0.75 1.04
0.13 0.04 0.19 0.27 0.26 0.32 0.26 0.35 0.30 0.44
0.46 0.21 0.82 0.48 1.19 0.72 1.39 0.69 0.94 0.93
1.16 0.32 2.18 0.76 I .77 1.46 1.72 1.01 1.73 I .06
0.20 0.13 0.44 0.30 0.83 0.35 1.30 0.44 0.31 0.80
0.30 0.07 0.45 0.37 0.47 0.59 0.57 0.58 0.46 0.61
0.46 0.09 0.84 0.53 1.41 0.97 0.85 0.63 0.59 0.96
0.16 0.05 0.22 0.19 0.11 0.22 0.38 0.55 0.31 0.38
South
Branch
South Channel South
Passage
North
Passage
Out of Mouth
S: Summer,
S W S W S W S W S W
Ebb tides
neap tides
spring
tides
neap tides
W: Winter.
Fig. 4. Model of sediment
resuspension.
equation has the following form: F=(G&)=h.u.c.
+F$z,u,). +u-(h&J
__ +(hu,c.) +(hu.c,). = Tl + T, + T, + T4 + T, + T6 + T, + T8
(4)
in which an over-bar denotes depth averaging, a dot tidally averaging, subscript t the tidal fluctuations, F the sediment flux, and c and u the SSC and current velocity, respectively. Every term in the right-hand side of the second equality in Eq. (4) represents a specific transport mechanism. T, is the averaged or residual transport, T2 the Stokes drift-induced transport. T3 and T4 are the so-called tidal pumping, T, the contribution from the gravitational circulation, Ts and T7 the transport from shear effects, and T8 from the discrepancy between the current speed and SSC at
different depths. The results are shown in Table 2. The basic distribution features of the terms can be deduced as follows. (a) The net sediment transports in the North and South Passages are mainly composed of the terms T,,T4 and T2 in their order of magnitude. T,, T, and T, are of very little importance. The tidal pumping predominates over the residual transport in 10 of 17 times in the TM zones, implying the importance of sediment exchange between the bed and the suspended sediment. (b) The two Passages show a certain difference regarding the relative importance of the terms. The residual terms predominate over the others at the majority of the stations in the North Passage, while tidal pumping predominates in the South Passage with 8 out of 11 showing the largest T, and T4, especially T,. In fact the tidal actions are more important and SSC is much higher in the South Passage than in the North Passage, and hence sediment resuspension is stronger. Tidal actions create also a more pronounced upstream net sediment transport in the South Passage than in the North Passage. A reversed situation is found in the North Channel (Shen et al., 1995). (c) A comparison between summer and winter and between spring and neap tides reveals that both the residual and tidal pumping are larger in the summer season than in thewinter season and
122
L. Jiufa, Z. Chen 1 Murine Geology 148 (1998) 117-124
Table 2 Sediment
transport
terms in the North
and South Passages
(kg mm’ SC’)
Location
Date
STS
T,
T,
T,
T,
T5
T,
T,
Total
South
July 29, 1988 Spring Tides Aug. 6, 1988 Neap Tides
x3 X4 X5 x3 x4 X5 Dl D2 D3 D4 D5 D6 D7 D8 D6 D7 D8
2.872 - 1.098 2.245 0.164 -0.071 0.085 0.893 0.792 1.414 0.046 0.060 4.237 3.616 9.470 0.408 0.163 1.056
-2.131 -1.180 -1.087 -0.051 -0.034 - 0.037 -0.416 -0.347 -0.818 - 0.939 -0.566 - 1.765 -2.569 -3.419 -0.165 -0.258 -0.298
- 0.084 0.008 - 0.007 0.008 - 0.004 0.001 -0.005 0.011 0.003 0.000 0.001 0.029 -0.078 -0.200 0.007 0.002 0.003
- 2.946 0.223 1.235 -0.552 - 0.404 -0.291 0.218 - 1.563 - 1.333 -0.606 - 0.706 -0.902 -0.501 1.945 -0.550 -0.642 ~ 0.236
- 0.476 0.109 -0.530 -0.025 0.027 0.056 -0.425 - 0.055 -0.477 -0.022 - 0.040 -0.663 -0.781 -0.937 -0.115 -0.174 -0.260
-0.461 -0.039 - 1.173 0.006 0.016 -0.037 -0.056 0.195 - 0.086 -0.044 - 0.030 0.127 -0.045 -0.312 - 0.028 0.036 - 1.107
0.132 0.232 0.180 0.004 0.004 0.008 0.054 0.019 0.043 0.015 0.020 0.173 0.131 0.194 0.144 0.023 0.022
2.810 - 1.797 0.086 -0.453 -0.471 -0.216 0.266 -0.949 - 1.245 - 1.543 - 1.267 1.317 -0.205 6.788 - 0.427 -0.344 0.191
Passage
Dec. 19, 1988 Mean Tides North
Passage
Notes:
sample STS D2, D4 and D5 are at the same position
July 30, 1988 Spring Tides Dec. 27, 1988 Mean Tides
as X3, X4 and X5, respectively,
larger during spring tide than during neap tide. The values may differ by several times. Such a difference is probably caused by the flow strength which increases significantly during the spring tides and in the summer when the river discharge is during flood. The critical erosion velocity of the bed sediment U, is roughly estimated to be 0.40 to 0.50 m s-i by using Eq. (5) in which D denotes the diameter of sediment particle, Y,and r represent the specific mass of the sediment and water, respectively. The flow during flood and spring tide periods attains evidently more frequently the critical value than during the dry season and the neap tide. Bed siltation has been observed after the flood discharges and during neap tides, which may easily reach 0.5 to 2.5 m. Contrarily bed erosion has been observed during the spring tides with flood discharges which may deepen the channel by up to 2 m (Li, 1991). The TM is also most strongly developed during spring tide without high river discharge (Chien et al., 1983). r,-r
17.60 ~
r
+6.05 x lo-’ -
at spring tide.
In conclusion, the Stokes drift-induced transport and tidal pumping predominate over other terms in the tide-dominated South Passage; hence the main factors are contributing to the formation of the TM. The gravitational circulation-induced sediment transport is however the most important and the leading factor in the formation of the TM in the North Channel dominated by river runoff. The North Passage shows an intermediate condition. 3.4. Suspended sediment trapping process in the TM
The four mouths of the Changjiang Estuary, especially the South Passage, exhibit a tide-dominated sediment transport in the historical data (Shen et al., 1982). Nevertheless the sediment trapping mechanisms are dependent on the dynamical balance between the river discharge and the flood tidal current, residual current and sediment flux. In fact the suspended sediment in the upper reach of the mouths is transported seaward by the predominant runoff-induced flow and that in the down reach landward by the predominant flood flow (Table 2), resulting in an area in the mouths
L. Jiufa. Z. Chen / Marine Geology 148 (1998) 117-124
Al Zn
IO South
Xuliujin!: Fig. 5. Organic estuary.
heavy
metals
content
passage in solution
Se3 along
the
where the net flow and sediment transport equal nearly zero. The suspended sediment is then trapped and accumulated there. The trapped sediment persists chiefly as a newly deposited and pre-consolidated mass and is apt to be resuspended. Resuspension is very strong by the alternative tidal current. The turbid water and the turbidity maximum are then formed. The tidally averaged SSC at the stations in the centre of the TM during the 1988 cruises are as high as 3.5 kg rne3, almost 10 times that at the adjacent stations. As the suspended materials were trapped in this areas the nutrients are accumulated and this causes the high primary production which results in a high organic-related metal elements content (Fig. 5). From Fig. 5, it is easy to see that the organic-related metal concentrations are much lower in both upper reaches in Xuliujing and down-reach to the sea. The TM performs as a filter in this situation for materials transferred from the river to the sea.
4. Summary (a) The settling velocity of the suspended sediment in the TM is significantly higher than in the adjacent reaches due to flocculation. The flocculation rate is enhanced mainly by the salinity intrusion, high concentration of suspended sediment and organic matter. (b) The main transport mechanisms in the TM zones are the residual current, Stokes drift, tidal pumping and gravitational circulation. Stokes drift-induced transport and tidal pumping predo-
123
minate in the South Passage, contributing the most to the formation of the TM. The gravitationalcirculation transport is of more importance in the runoff-current-dominated North Passage. (c) The sediment is trapped in the zone of zero net transport as a combined effect of tidal asymmetry, runoff and density circulation. The resuspension of such sediment leads to the highest SSC in the mouths and to the formation of the TM. The accumulation rate of the sediment is high and the mouth-bars are gradually formed.
References Allen, G.P., Salomon, J.C., Bassoullet, P., Du Penhoat, Y., De Grant Pre, C., 1980. Effects of tides on mixing and suspended sediment transport in macrotidal estuaries. Sediment. Geol. 26, 69-90. Chien, N., Wan, Z.H., 1983. Mechanics of Sediment Transport (in Chinese). Scientific Publisher, pp. 755309. Dyer, K.R., 1988. In: Dronkers, J., Van Leussen, W. (Eds.), Physical Processes in Estuaries. Springer-Verlag. New York, pp. 2955310. Eisma, D., 1986. Flocculation and deflocculation in estuarine bays. Neth. J. Sea Res. 20 (2/3), 183-199. Festa, J.F., Hansen, D.V., 1978. Turbidity maximum in partially mixed estuaries: a two-dimensional numerical model. Estuarine Coastal Mar. Sci. 7, 347-359. Huang, S., Hon, N.B., Zhang, X.J., 1980. Analysis of siltation at mouth bar of the Yangtze River Estuary. lnt. Symp. River Sedimentation, Preprints, Beijing. Li, J.F., 1991. The rule of sediment transport on the Nanhui tidal flat in the Changjiang estuary. Acta Oceanol. Sin. IO. 117-127. Li, J.F., Shi, W.-R., Shen, H.T., 1993. The bedload movement in the Changjiang estuary (in Chinese). China Ocean Eng. 7, 4. 441-450. Meade, R.H., 1972. Transport and deposition of sediments in estuaries. Geol. Sot. Am. Mem. 133, 91-120. Schubel, J.R., 1968. The turbidity maximum in the Upper Chesapeake Bay. Science 161, 1013-1015. Schubel, J.R.. 1971. Estuaries and estuarine sedimentation. Estuarine Environ. 1, l-28. Shen, H.T., Zhu. H.F., Mao, Z.C., 1982. Circulation of the Changjiang Estuary and its effect on the transport of suspended sediments. In: Estuarine Comparisons. Academic Press, London, pp. 677-691. Shen, H.T., Li, J.F., Han, M.B., 1992. Transport of suspended sediment in the Changjiang Estuary. lnt. J. Sediment. Res. 7 (3) 45563. Shen, H.T., He, S.Z., Pan, D.A., Li, J.F., 1993. Study on turbidity maximum in the South and North Passages of the Chang-
124
L. Jiufa, Z. Chen 1 Marine Geology 148
jiang Estuary. Proc. Symp. Physical and Chemical Oceanography of the China Seas, China Ocean Press. Shen, Jian, Shen, H.T., Pan Dingan, Xiao Chenyou, 1995. Analysis of transport mechanism of water and suspended sediment in the turbidity maximum of the Changjiang Estuary. Acta Geogr. Sin. 50 (5). Shi, W.-R., Li, J.F., 1993. Mud transport calculation in Yangtse
(1998)
II 7-124
Estuary and analysis of formation of turbidity maximum (in Chinese). Mar. Sci. Bull. 4, 69-76. Su, J.L., Wang, KS., 1987. The suspended sediment balance in Changjiang Estuary. Acta Oceanol. Sin. 9 (5), 627-637. Wellershaus, S., 1981. Turbidity maximum and mud shoaling in the Weser estuary. Arch. Hydrobiol. 92, 161-189.
Marine
Geology
148 (1998) 117-124
Sediment resuspension and implications for turbidity maximum in the Changjiang Estuary Li Jiufa a,*, Zhang Chen b a State Key Lab. for Estuarine and Coastal Morphodynamics and Sediment Dynamics, Institute of Estuarine and Coastal Research, East China Normal University, Shanghai 200062, People’s Republic of China b Nanjing Institute of Geography and Limnology, Nanjing 210008, People’s Republic China
qf
Received 3 1 January 1996; accepted 6 December 1997
Abstract A comparative study on the properties and transport of the cohesive sediments in the mouths of the Changjiang Estuary proves that the suspended sediment concentration (SSC) is principally enhanced by the resuspension of bottom sediment in the mouth-bar reach, coupled with the deformed tidal wave and strong tidal current. Salinity intrusion and vertical gravitational circulation lead to the ‘trapping’ of the suspended sediment inflowed from the river and the sea. The turbidity maximum (TM) is therefore delivered by the local resuspension of the accumulated materials and bed erosion. The TM in the Changjiang Estuary is marked not only by its high SSC relative to the adjacent reaches, but also by the high wash-load content. Transport of sediment is very important. Settling velocity of suspended sediment is increased by flocculation. The massive settling during weak current periods often gives rise to the formation of fluid mud. The TM coincides locally with the mouth-bars. The transport of suspended sediment in the TM is analysed by using a splitting method. It is concluded that the contributing factors consist of advective terms and tidal pumping. Advective transport is dominant in the North Channel and tidal pumping predominate in the North Branch and South Passage, and the North Passage is in an intermediate situation. 0 1998 Elsevier Science B.V. All rights reserved. Keywords: Changjiang
Estuary; sediments; turbidity maximum; sediment concentration
1. Introduction The Changjiang Estuary is a mesotidal, partially mixed estuary. It is characterised by the complexity of morphology associated with multi-step bifurcations and wide mouths. The turbidity maximum (TM), as a combined result of various estuarine dynamical processes, exists in all four mouths and * Corresponding author. Tel.: + 86 21 625 47978; Fax: + 86 21 625 46441; E-mail: [email protected]
0025-3227/98/$19.00 0 1998 Elsevier Science B.V. All rights reserved. PII: SOO25-3227(98)00003-6
is marked by a SSC higher than both the up and down reaches. Usually the TM in an estuary is considered to be the results of the gravitational residual circulation (Schubel, 1968; Festa and Hansen, 1978; Wellershaus, 1981) and in others it is focused on the settling and resuspension process of the fine sediment (Meade, 1972; Allen et al., 1980; Eisma, 1986). Most studies of the Changjiang Estuary considered that the settling and lag washing effects may store sediments in the estuary (Li et al., 1993; Shen et al., 1993). The
L. Jiufu, Z. Chen J Marine Geology 148 (1998) 117-124
118
mixing and circulation patterns, the transport of the suspended sediment and the generating mechanisms of the TM in Changjiang Estuary have been extensively investigated (Shen et al., 1982, 1992; Huang et al., 1980; Su and Wang, 1987; Shi and Li, 1993). The present paper outlines a comprehensive analysis on the sediment composition, the settling and flocculation and the impact on the formation and development of the TM, as well as on the sediment transport.
2. Location and sampling The sampling area is located on the north and south passages of the Changjiang Estuary (Fig. 1) where it is characterised by partial mixing for most of the time. Together eight fixed stations marked D 1-D8 were set on both passages. There were eight sampling boats named Shanghai OBB No. 1 to No. 8 for sampling and simultaneously measuring water temperature, salinity, flow, suspended sediment concentration, water and sediments samples. The sampling and measurement were carried out
0
10
Turbidity
20
for 27 consecutive hours during each spring, mean and neap tide in July-August 1988 and December 1988. Water temperature, salinity, flow and suspended sediment concentrations were measured every hour in different water depths during the sampling period. Water samples of about 500 ml were taken using a horizontal sampler. Samples were taken from 0.5 m below the water surface and 0.5 m above the bottom. 200 cm3 water was filtered with 0.45 p pre-washed and pre-weighed cellulose acetate filters to gather suspended sediments for further chemical analysis. Particle size analysis of suspended materials was carried out using the Coulter counter technique. Organic metal concentrations were analysed by ICP.
3. Results and discussion 3.1. Basic properties of the sediment 3.1.1. Composition
The main components of the sediments in the TM zones are almost exclusively silt and clay. The
...
30tLm)
m:lximum
Fig.
I. The Changjiang
Estuary
and sampling
sites.
L. Jiufiz. Z. Chen /
Marine
median size of the bed sediments ranges between 0.056 and 0.010 mm with the mode between 0.063 and 0.008 mm. The median size of the suspended sediment varies between 0.004 and 0.009 mm with a dominating range around 0.007 mm. The size of the bed sediments and suspended is quite similar, implying that there is exchange between them. Seasonal variations in size are significant due to the change in sediment input of the river. For example the median size of the suspended sediment in the South Passage is between 0.004 and 0.006 mm in the summer and becomes about 0.0072 and 0.008 mm in the winter. 3.1.2. Settling velocity andflocculation By assuming a linear distribution of the internal shear stress, the vertical SSC (S,) distribution in the flow can be described by (Chien et al., 1983):
;=(?I+.&) with: w 17=--,u*=-.u
Ku*
vii CS
Geology
148 (1998)
II 7-124
119
high ions contents, and the high SSC are favourable conditions for flocculation to occur. A flocculation test has been carried out in a laboratory flume by using estuarine water and sediment. Results show that flocculation: (a) is inhibited by strong shear stress when flow velocity is over 0.35 m s-l with only the sediment coarser than 0.032 mm settles down under gravity; (b) occurs for sediments with sizes between 0.032 and 0.008 mm when the flow slows down to 0.3 m s-i: and (c) occurs for sediments with sizes between 0.004 and 0.008 mm when velocity is below 0.20 and above 0.10 m s-l. Clay minerals finer than 0.004 mm flocculate significantly only when flow speed decreases to 0.10 m s-‘. The current slows down to less than 0.35 m s-i only around the slack-waters in the North and South Passages in the Changjiang Estuary; hence the settling velocity of the suspended sediment reaches a maximum at that time and the massive settling leads to a distinct SSC stratification with high SSC near the bottom, which may give rise to a 10 to 35 cm thick fluid mud on the bed.
(2)
in which S, denotes the reference concentration, h the water depth and y the height over the bed, a the height of S,, settling velocity, K the von Karman constant, U* the friction velocity, g gravitational acceleration, Cs the drag coefficient, and U the vertically averaged current velocity. Field data are used to plot the values on both sides of Eq. ( 1) for estimating (Fig. 2). The final result shows the influence of the various factors and is called the effective settling velocity. The corresponding diameter of the suspended sediment, including the floccules, is called the equivalent project diameter (Schubel, 1971), ranging from 0.02 to 0.15cm s-l, showing considerable variation along the estuary although the particle size of the suspended sediment remains relatively constant. A maximum occurs at the mouth and reaches a peak in the TM zones. In general it is larger during the ebb tide than the flood tide, and larger in the winter than the summer. Salinity (2 to 20%0), hyperconcentration of clay mineral, organic matter (300 to 10,000 mg mm3),
3.2. Suspended sediment concentration 3.2.1. Wash-load content Here the wash-load content is defined as the concentration of very fine sediments that settle seldom onto the bed. It is estimated by the following equation ( Li, 199 1): fl=A+~.Fr
(3)
in which Fv=(U(~~)-“~), denoting the Froude number. A and B are to be determined by using field data and A is the wash-load concentration. B is a constant relative to the property of the sediments. A is considerably large in the whole estuary, ranging from 0.06 to 0.35 kg mm3 (Fig. 3). It accounts for 10 to 30% of the SSC. Its maximum is also located in the TM zones in both the North and South Passages, reflecting the contribution to the formation of the TM from that portion of the suspended sediment. The seasonal variation of A shows a low value during the summer, especially during the ebb tide (Fig. 3). This is probably
120
L. Jiufa, Z. Chen / Marine Geology I48 ( 1998) I1 7-124
-0.6 -
channel
I -0.4 channel branch
branch
Fig. 2. The effective settling
velocity
of the suspended
sediment.
(b) (J)
0.11 South branch
I
South channel
South passage
Fig. 3. The A value along the South
because of the conditions more favourable for the flocculation occurring during that season. 3.2.2. Spatial and seasonal SSC variation (a) The SSC in the TM zones in the Passages, either vertically averaged or in a single layer, is much greater than the adjacent up and down streams (Table 1). The mean SSC is 3.5 times that in the South Branch and 2 times that in the South Channel, and also 2.7 times the SSC at Datong Hydrographic Station (located 630 km upstream from the mouth) which can be considered as the input concentration of the river (Shen et al., 1992). The instant SSC in the TM zones varies between 2 and 10 kg me3 with a maximum of 30 kg rne3. (b) The TM is better developed in the summer than in the winter. The mean SSC in the summer is double that of the winter. It is higher during the
Turbidity maximum center
Branch
Lr
sea
to South Passage
ebb tide than the flood tide in the summer, with the reverse in the winter. This can be explained by the varying river sediment (Shen et al., 1982). (c) The peak of SSC in the low portion of the water column in the TM zones occurs generally after the reversal of the tidal currents. It is more likely generated by a strong vertical mixing due to the acceleration and velocity gradient (Shi and Li, 1993) than by a simple current-induced scouring that yields a SSC peak in other reaches (Fig. 4).
3.3. Suspended sediment transport 3.3.1. Split of the sediment transport Several major transport mechanisms in an estuary can be investigated quantitatively by splitting the flux equation (Dyer, 1988). The final split
121
L. Jiufir, Z. Chen / Marine Geology 148 ( 1998) 117-I?4 Table 1 Statistics
over years of the mean SSC in the Changjiang
Estuary
(kg mea)
Tidal period:
Flood
Tidal range:
spring
ssc
mean
max
min
mean
max
min
mean
max
min
mean
max
min
0.43 0.21 0.72 0.61 1.43 0.84 1.28 0.59 0.88 0.98
0.92 0.33 0.14 1.23 2.28 1.26 1.65 1.02 1.81 1.01
0.17 0.12 0.49 0.39 0.88 0.39 0.88 0.26 0.34 0.93
0.24 0.07 0.44 0.41 0.60 0.68 0.38 0.39 0.48 0.64
0.39 0.11 0.72 0.66 0.98 1.12 0.46 0.44 0.75 1.04
0.13 0.04 0.19 0.27 0.26 0.32 0.26 0.35 0.30 0.44
0.46 0.21 0.82 0.48 1.19 0.72 1.39 0.69 0.94 0.93
1.16 0.32 2.18 0.76 I .77 1.46 1.72 1.01 1.73 I .06
0.20 0.13 0.44 0.30 0.83 0.35 1.30 0.44 0.31 0.80
0.30 0.07 0.45 0.37 0.47 0.59 0.57 0.58 0.46 0.61
0.46 0.09 0.84 0.53 1.41 0.97 0.85 0.63 0.59 0.96
0.16 0.05 0.22 0.19 0.11 0.22 0.38 0.55 0.31 0.38
South
Branch
South Channel South
Passage
North
Passage
Out of Mouth
S: Summer,
S W S W S W S W S W
Ebb tides
neap tides
spring
tides
neap tides
W: Winter.
Fig. 4. Model of sediment
resuspension.
equation has the following form: F=(G&)=h.u.c.
+F$z,u,). +u-(h&J
__ +(hu,c.) +(hu.c,). = Tl + T, + T, + T4 + T, + T6 + T, + T8
(4)
in which an over-bar denotes depth averaging, a dot tidally averaging, subscript t the tidal fluctuations, F the sediment flux, and c and u the SSC and current velocity, respectively. Every term in the right-hand side of the second equality in Eq. (4) represents a specific transport mechanism. T, is the averaged or residual transport, T2 the Stokes drift-induced transport. T3 and T4 are the so-called tidal pumping, T, the contribution from the gravitational circulation, Ts and T7 the transport from shear effects, and T8 from the discrepancy between the current speed and SSC at
different depths. The results are shown in Table 2. The basic distribution features of the terms can be deduced as follows. (a) The net sediment transports in the North and South Passages are mainly composed of the terms T,,T4 and T2 in their order of magnitude. T,, T, and T, are of very little importance. The tidal pumping predominates over the residual transport in 10 of 17 times in the TM zones, implying the importance of sediment exchange between the bed and the suspended sediment. (b) The two Passages show a certain difference regarding the relative importance of the terms. The residual terms predominate over the others at the majority of the stations in the North Passage, while tidal pumping predominates in the South Passage with 8 out of 11 showing the largest T, and T4, especially T,. In fact the tidal actions are more important and SSC is much higher in the South Passage than in the North Passage, and hence sediment resuspension is stronger. Tidal actions create also a more pronounced upstream net sediment transport in the South Passage than in the North Passage. A reversed situation is found in the North Channel (Shen et al., 1995). (c) A comparison between summer and winter and between spring and neap tides reveals that both the residual and tidal pumping are larger in the summer season than in thewinter season and
122
L. Jiufa, Z. Chen 1 Murine Geology 148 (1998) 117-124
Table 2 Sediment
transport
terms in the North
and South Passages
(kg mm’ SC’)
Location
Date
STS
T,
T,
T,
T,
T5
T,
T,
Total
South
July 29, 1988 Spring Tides Aug. 6, 1988 Neap Tides
x3 X4 X5 x3 x4 X5 Dl D2 D3 D4 D5 D6 D7 D8 D6 D7 D8
2.872 - 1.098 2.245 0.164 -0.071 0.085 0.893 0.792 1.414 0.046 0.060 4.237 3.616 9.470 0.408 0.163 1.056
-2.131 -1.180 -1.087 -0.051 -0.034 - 0.037 -0.416 -0.347 -0.818 - 0.939 -0.566 - 1.765 -2.569 -3.419 -0.165 -0.258 -0.298
- 0.084 0.008 - 0.007 0.008 - 0.004 0.001 -0.005 0.011 0.003 0.000 0.001 0.029 -0.078 -0.200 0.007 0.002 0.003
- 2.946 0.223 1.235 -0.552 - 0.404 -0.291 0.218 - 1.563 - 1.333 -0.606 - 0.706 -0.902 -0.501 1.945 -0.550 -0.642 ~ 0.236
- 0.476 0.109 -0.530 -0.025 0.027 0.056 -0.425 - 0.055 -0.477 -0.022 - 0.040 -0.663 -0.781 -0.937 -0.115 -0.174 -0.260
-0.461 -0.039 - 1.173 0.006 0.016 -0.037 -0.056 0.195 - 0.086 -0.044 - 0.030 0.127 -0.045 -0.312 - 0.028 0.036 - 1.107
0.132 0.232 0.180 0.004 0.004 0.008 0.054 0.019 0.043 0.015 0.020 0.173 0.131 0.194 0.144 0.023 0.022
2.810 - 1.797 0.086 -0.453 -0.471 -0.216 0.266 -0.949 - 1.245 - 1.543 - 1.267 1.317 -0.205 6.788 - 0.427 -0.344 0.191
Passage
Dec. 19, 1988 Mean Tides North
Passage
Notes:
sample STS D2, D4 and D5 are at the same position
July 30, 1988 Spring Tides Dec. 27, 1988 Mean Tides
as X3, X4 and X5, respectively,
larger during spring tide than during neap tide. The values may differ by several times. Such a difference is probably caused by the flow strength which increases significantly during the spring tides and in the summer when the river discharge is during flood. The critical erosion velocity of the bed sediment U, is roughly estimated to be 0.40 to 0.50 m s-i by using Eq. (5) in which D denotes the diameter of sediment particle, Y,and r represent the specific mass of the sediment and water, respectively. The flow during flood and spring tide periods attains evidently more frequently the critical value than during the dry season and the neap tide. Bed siltation has been observed after the flood discharges and during neap tides, which may easily reach 0.5 to 2.5 m. Contrarily bed erosion has been observed during the spring tides with flood discharges which may deepen the channel by up to 2 m (Li, 1991). The TM is also most strongly developed during spring tide without high river discharge (Chien et al., 1983). r,-r
17.60 ~
r
+6.05 x lo-’ -
at spring tide.
In conclusion, the Stokes drift-induced transport and tidal pumping predominate over other terms in the tide-dominated South Passage; hence the main factors are contributing to the formation of the TM. The gravitational circulation-induced sediment transport is however the most important and the leading factor in the formation of the TM in the North Channel dominated by river runoff. The North Passage shows an intermediate condition. 3.4. Suspended sediment trapping process in the TM
The four mouths of the Changjiang Estuary, especially the South Passage, exhibit a tide-dominated sediment transport in the historical data (Shen et al., 1982). Nevertheless the sediment trapping mechanisms are dependent on the dynamical balance between the river discharge and the flood tidal current, residual current and sediment flux. In fact the suspended sediment in the upper reach of the mouths is transported seaward by the predominant runoff-induced flow and that in the down reach landward by the predominant flood flow (Table 2), resulting in an area in the mouths
L. Jiufa. Z. Chen / Marine Geology 148 (1998) 117-124
Al Zn
IO South
Xuliujin!: Fig. 5. Organic estuary.
heavy
metals
content
passage in solution
Se3 along
the
where the net flow and sediment transport equal nearly zero. The suspended sediment is then trapped and accumulated there. The trapped sediment persists chiefly as a newly deposited and pre-consolidated mass and is apt to be resuspended. Resuspension is very strong by the alternative tidal current. The turbid water and the turbidity maximum are then formed. The tidally averaged SSC at the stations in the centre of the TM during the 1988 cruises are as high as 3.5 kg rne3, almost 10 times that at the adjacent stations. As the suspended materials were trapped in this areas the nutrients are accumulated and this causes the high primary production which results in a high organic-related metal elements content (Fig. 5). From Fig. 5, it is easy to see that the organic-related metal concentrations are much lower in both upper reaches in Xuliujing and down-reach to the sea. The TM performs as a filter in this situation for materials transferred from the river to the sea.
4. Summary (a) The settling velocity of the suspended sediment in the TM is significantly higher than in the adjacent reaches due to flocculation. The flocculation rate is enhanced mainly by the salinity intrusion, high concentration of suspended sediment and organic matter. (b) The main transport mechanisms in the TM zones are the residual current, Stokes drift, tidal pumping and gravitational circulation. Stokes drift-induced transport and tidal pumping predo-
123
minate in the South Passage, contributing the most to the formation of the TM. The gravitationalcirculation transport is of more importance in the runoff-current-dominated North Passage. (c) The sediment is trapped in the zone of zero net transport as a combined effect of tidal asymmetry, runoff and density circulation. The resuspension of such sediment leads to the highest SSC in the mouths and to the formation of the TM. The accumulation rate of the sediment is high and the mouth-bars are gradually formed.
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