Applied Ocean Research 43 (2013) 148–156

Contents lists available at ScienceDirect

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Applied Ocean Research

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / a p o r

igration of sediment deposition due to the construction of

arge-scale structures in Changjiang Estuary

angfeng Ma

a , b , * , Fengyan Shi c , Shuguang Liu

d , Dingman Qi a

Key Laboratory of Estuarine and Coastal Engineering, Ministry of Transport, Shanghai, China Department of Civil and Environmental Engineering, Old Dominion University, Norfolk, VA, USA Center for Applied Coastal Research, University of Delaware, Newark, DE, USA Department of Hydraulic Engineering, Tongji University, Shanghai, China

r t i c l e i n f o

rticle history:

eceived 24 February 2013

eceived in revised form 8 July 2013

ccepted 7 September 2013

eywords:

arge-scale structure impacts

hangjiang Estuary

ediment transport processes

a b s t r a c t

The paper presents measurements of sediment deposition in the navigation channel of Changjiang Estuary

during the construction of the world largest jetty-spur system. A significant change of sediment deposition

pattern is found after the second stage of the project, which extended the previous 25 km long jetties built

in the first stage to about 50 km. The measurements show that the main deposition region migrated from

the lower reach to upper reach of the navigation channel, with the strongest deposition occurred at the

upper middle reach. The physical mechanisms inducing the migration of the sediment deposition region are

studied numerically using the finite-volume coastal ocean model (FVCOM). Model results reveal that the tidal

currents as well as the sediment processes in the northern passage are greatly changed by the structures.

With the extension of the structures, suspended sediment concentration decreases at the upper reach and

increases at the lower reach, resulting in a seaward migration of turbidity maximum in the northern passage.

The changes of suspended sediment concentration distributions are mainly caused by the adjustments of

tidal currents at ebb. The analysis based on the local momentum balance identifies two mechanisms causing

these adjustments.

Published by Elsevier Ltd.

Fig. 1. The Changjiang Estuary and bathymetry. The estuary is a multichannel system

with five outlets to the East China Sea. The large-scale jetty-spur structures were built

. Introduction

The Changjiang Estuary is a mesotidal, partially mixed estuary [ 1 ]

ith multiple channels connecting to the East China Sea ( Fig. 1 ). It is

haracterized by a triple bifurcation, separating first into northern and

outhern branches by Chongming Island. The southern branch is fur-

her divided into the northern channel and southern channel, which

s subsequently split into northern and southern passages. The hy-

rodynamics and morphological evolution in the estuary are mainly

ontrolled by tide and river inflow. The tide is dominantly semid-

urnal, with an averaged tidal range of 2.84 m [ 2 ]. The Changjiang

iver provides most of freshwater and sediment input in the estu-

ry. The annually averaged runoff and sediment discharge are about

.24 × 10 11 m

3 and 486 Mt [ 2 ], respectively. With abundant sup-

ly of sediment, the estuarine morphology evolves rapidly with the

evelopment of shoals and tidal flats.

Along with the natural processes, human activities have also im-

osed great influences on the morphological evolution of the estuary.

n order to stabilize the deep-water navigation channel and improve

avigation condition, two long jetties and tens of jetty-attached spurs

* Corresponding author at: Department of Civil and Environmental Engineering, Old

ominion University, Norfolk, VA, USA. Tel.: + 1 757 683 4732; fax: + 1 757 683 5354.

E-mail address: gma@odu.edu (G. Ma).

in the northern passage to maintain the navigational channel.

141-1187/ $ - see front matter Published by Elsevier Ltd.

ttp://dx.doi.org/10.1016/j.apor.2013.09.002

were constructed along the northern passage of Changjiang Estuary.

From August 1998 to March 2000 (1st stage), the structures shown in

G. Ma et al. / Applied Ocean Research 43 (2013) 148–156 149

Fig. 2. The northern passage in the Changjiang Estuary and jetty-spur structures con-

structed at the first construction stage. Sediment deposition strength was measured at

unit A–Z in the navigation channel. Section w2–w3 is composed of unit A–K. Section

w3–w4 is composed of unit L–Y.

Fig. 2 were built. The rest of structures were constructed from April

2002 to March 2005 (2nd stage). A complete view of jetty-spur struc-

tures in the northern passage is shown in Fig. 5 as marked by the red

line. The length of the jetty-spur system is about 50 km and the total

length of two jetties is around 100 km, being the longest jetty in the

world. These large-scale coastal structures play an important role in

determining estuarine circulation and saltwater intrusion [ 3 ], subse-

quently affecting sediment transport, deposition and erosion. Because

the northern passage provides the navigation route to Shanghai port,

it is of great economic importance to study these large-scale structure

impacts on sediment transport as well as erosion / deposition distri-

butions.

Sediment transport processes in the Changjiang Estuary are com-

plicated with significant spatial and temporal variabilities. Sediments

in the Changjiang Estuary are exclusively silt and clay [ 1 ]. The median

size of the bed sediments ranges between 0.01 and 0.056 mm, while

the median size of the suspended sediments varies between 0.004

and 0.009 mm with a dominant range around 0.007 mm [ 1 ]. Exten-

sive studies on fine suspended sediment transport in the Changjiang

Estuary have been carried out, including the investigations of turbid-

ity maximum [ 1 , 4 , 5 ] and suspended sediment transport processes in

the near-bed boundary layer [ 6 –11 ]. Some of these studies have been

reviewed by Shi [ 12 ]. These studies have greatly advanced our knowl-

edge on sediment properties and processes in the Estuary. However,

due to the complexity of the problem, there are still a lot of questions

remaining to be answered. For instance, how do sediment transport

processes vary after the construction of the large-scale coastal struc-

tures.

Several investigations have been conducted to study impacts of

regulation projects on estuarine hydrodynamics and sediment trans-

port in the Changjiang Estuary. Zhou et al. [ 13 ] analyzed the change

and distribution of turbidity maximum along the northern passage

before and after the construction of the 1st stage regulation project,

based on the current and suspended sediment measurements. They

did not find marked difference on the location of turbidity maximum

after the 1st stage construction of structures. Liu et al. [ 14 ] used to-

pographical maps to study river bed erosion and deposition in the

southern passage after implementation of the 1st stage regulation

project. They revealed that, after the 1st stage project, river bed in the

upper reach of the southern passage is eroded during flood seasons

and accreted during non-flood seasons. The river bed in the middle

and lower reach exhibited deposition in flood seasons and erosion in

non-flood seasons. Ma et al. [ 15 ] used empirical orthogonal func-

tion (EOF) method to investigate the temporal and spatial variations

of the sediment deposition rate in the northern passage channel af-

ter the 2nd phase construction of the jetty-spur structures. It was

found that the sediment deposition in the northern passage experi-

enced annual and seasonal variations, and is greatly influenced by

human activities. Ma et al. [ 3 ] employed a coastal ocean numerical

model FVCOM to investigate large-scale structure impacts on hydro-

dynamics in the Changjiang Estuary. They revealed that the structures

along the northern passage not only affect the near-field circulations,

but also play an important role in redistributing freshwater runoff

in the multi-channel system, resulting in changes of far-field circula-

tions and salinity distribution. Most of the above studies on sediment

transport are focused on the effects of the structures built in the 1st

stage. After the extension of the jetty-spur structures, it is expected

that the sediment transport and erosion / deposition patterns have

been changed. This paper is intended to study the migration of sed-

iment deposition in the navigational channel with the extension of

the structures and its causes.

In this paper, we focus our attention on the migration of sediment

deposition patterns observed in the northern passage after the 2nd

phase construction of the jetty-spur system. A numerical model FV-

COM is employed to explore the underlying physical mechanisms. The

paper is organized as follows. The sediment deposition measurements

in the northern passage of Changjiang Estuary are analyzed in Section

2 . In Section 3 , we briefly describe the numerical model FVCOM and

setup of the numerical experiments. Model skills of predicting sedi-

ment concentration are also presented. The physical mechanisms that

cause migration of sediment deposition region after the 2nd phase

construction of structures are discussed in Section 4 . The conclusions

are summarized in Section 5 .

2. Sediment deposition measurements

Systematic surveys of sediment deposition in the northern passage

started on March 2000 after the 1st stage construction of jetty-spur

structures ( Fig. 2 ). During measurements, the navigation channel in

the northern passage was divided into 26 units (A–Z) as shown in Fig.

2 . The deposition volume in each unit was measured about every 15

days. The data from March 2000 to October 2005 were collected for

analysis in this paper.

Fig. 3 shows the sediment deposition fraction in different reaches

in different years. From this figure, we can clearly identify the effects

of structures on the sediment deposition. Before the extension of

jetty-spur structures in the lower reach of the passage (before March,

2002), sediments mainly deposited in the lower reach w3–w4 (from

unit L to Y). During the extension project (2002–2005), a large amount

of sediment deposition gradually shifted to the upper reach w2–w3

(from unit A to K). With the completion of the whole structures in

2005, around 70% of sediments deposited in the upper reach of the

navigation channel.

Fig. 4 shows the averaged deposition rates before and after the

extension project. The deposition rate P i of the i th unit is calculated

as follows:

P i =

Q i

�T A i (1)

where Q i is the deposition volume in the i th unit accounting for dredg-

ing, �T is the time interval, and A i is the unit area.

In Fig. 4 , negative value represents deposition, while positive de-

notes erosion. Before the extension project, it is found that sediment

mostly deposits in the lower reach from unit L to unit Z. The strongest

deposition happens at unit O where the rate is around 0.9 cm / day. In

the upper reach (from unit A to unit K), the deposition rate is relatively

small with the maximum rate about 0.5 cm / day.

To show the structure impacts on sediment deposition, we also

plot the deposition rate in 2005 (from October 2004 to October 2005)

when the whole jetty-spur structures were completed (lower panel of

Fig. 4 ). Most of sediment deposition is shifted to the upper reach. The

deposition rates from unit D to unit K are all larger than 0.5 cm / day.

The strongest deposition happens at unit J where the rate is about 1.1

cm / day. In the lower reach, the deposition rates at unit L, M and N are

150 G. Ma et al. / Applied Ocean Research 43 (2013) 148–156

Fig. 3. The distribution of sediment deposition in different reaches. Upper reach: w2–

w3; lower reach: w3–w4.

Fig. 4. The averaged sediment deposition strength in the navigation channel before

(during March 2000 to March 2002, upper panel) and after (during October 2004 to

October 2005, lower panel) the extension of jetty-spur structures.

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Fig. 5. A blow-up view of unstructured grid and jetty-spur structures in the northern

passage. The along-channel section drawn in red is located at the deep-water navigation

channel. The triangles indicate the location of sediment measurement stations. From

left to right, they are marked as S1, S2, S3, S4, respectively. (For interpretation of the

references to color in thisfigure legend, the reader is referred to the web version of the

article.)

till strong up to 0.5 cm / day. In the lower end of the channel (unit Y

nd Z), sediments are eroded.

From Fig. 4 , we can conclude that, with the extension of jetty-

pur structures in the lower reach of the northern passage, the main

eposition region migrated from the lower reach to upper reach of

he channel. It is hypothesized that these changes of sediment depo-

ition distribution are results of hydrodynamic adjustments by the

tructures. To verify this hypothesis, we conducted a numerical study

sing an unstructured-grid, finite-volume coastal ocean model (FV-

OM) [ 16 ], with special focus on the change of hydrodynamics and

ediment concentration distribution in the northern passage by the

arge-scale jetty-spur structures.

. Model description and validation

FVCOM is an unstructured-grid, finite volume, 3D primitive equa-

ion coastal ocean circulation model developed by UMASSD-WHOI

oint efforts. The model solves momentum, continuity, temperature

nd salinity equations in a terrain-following sigma coordinate system.

he spatial momentum flux is discretized by a second-order accu-

ate finite-volume method [ 17 ]. The model has been applied to study

irculation and transport processes in many estuarine and coastal

systems [ 18 –21 ].

In this study, the temperature effects on flow and sediment trans-

port are not considered. The temperature is set to be constant at 10 ◦C.

Thus, the 3D primitive equations consist of continuity, momentum

and salinity transport equations in σ coordinate, which are given by

∂η

∂t +

∂ Du

∂x +

∂ Dv

∂y +

∂ω

∂σ= 0 (2)

∂ Du

∂t +

∂ Duu

∂x +

∂ Duv

∂y +

∂uω

∂σ− fvD = −gD

∂η

∂x

− gD

ρ0

[

∂

∂x

(

D

∫ 0

σ

ρdσ ′ + σρ∂ D

∂x

) ]

+

1

D

∂

∂σ

(K m

∂u

∂σ

)+ D F x

(3)

∂ Dv

∂t +

∂ Duv

∂x +

∂ Dvv

∂y +

∂vω

∂σ+ fuD = −gD

∂η

∂y

− gD

ρ0

[

∂

∂y

(

D

∫ 0

σ

ρdσ ′ + σρ∂ D

∂y

) ]

+

1

D

∂

∂σ

(K m

∂v

∂σ

)+ D F y

(4)

∂ DS

∂t +

∂ DuS

∂x +

∂ DvS

∂y +

∂ωS

∂σ=

1

D

∂

∂σ

(K h

∂S

∂σ

)+ D F s (5)

where η is the surface elevation, D is the total water depth, u , v are

the horizontal velocities, ω is the vertical velocity in the σ coordinate,

S is the salinity, ρ is the water density, f is the Coriolis parameter, K m

,

K h are the vertical eddy viscosity and diffusivity, F x , F y , F s represent

the horizontal diffusion terms for momentum and salinity.

Various studies have revealed that the presence of suspended sed-

iment may damp turbulence in the water column [ 22 –24 ]. This effect

is accounted for by considering the effect of suspended sediment con-

centration on water density. Thus, the equation of state is given by a

bulk density relation [ 25 , 26 ].

ρ = ρw +

(1 − ρw

ρs

)C (6)

where C is the suspended sediment concentration, ρw

is the water

density excluding the effect of the suspended sediment, and ρs is the

sediment density ( = 2650 kg / m

3 ).

The suspended sediment concentration is obtained by solving the

sediment transport equation.

∂ DC

∂t +

∂ DuC

∂x +

∂ DvC

∂y +

∂ ( ω − w s ) C

∂σ=

1

D

∂

∂σ

(K h

∂C

∂σ

)+ D F c (7)

G. Ma et al. / Applied Ocean Research 43 (2013) 148–156 151

Fig. 6. The comparison of depth-averaged suspended sediment concentration for all

4 stations from May 5 to May 6, 2004. The dashed lines show 30% deviation from the

perfect agreement.

where w s is the sediment settling velocity and F c is the horizontal

diffusion of sediment. Note that w s is not affected by the transfor-

mation of coordinates from the Cartesian coordinate system into the

σ coordinate system [ 27 ]. To solve the sediment transport equation,

no-flux boundary condition is used at the free surface.

K h ∂C

∂z + w s C = 0 (8)

At the bottom, the mass exchange of suspended sediment is accounted

for by the erosion and deposition fluxes.

K h ∂C

∂z + w s C = S e − S d (9)

where S e is the sediment flux for erosion and S d is the sediment flux

for deposition. The erosion flux is given by [ 28 ]

S e =

{E ( τ/τce − 1 . 0 ) τ > τce

0 τ ≤ τce (10)

The deposition algorithm follows Krone [ 29 ]

S d =

{C b w s ( 1 . 0 − τ/τcd ) τ < τcd

0 τ ≥ τcd (11)

in which, E is the entrainment rate. τ ce and τ cd are the critical shear

stress for erosion and deposition, respectively. C b is the sediment

concentration near the bed.

Once the sediment fluxes at the bed are calculated, the bed varia-

tion can be determined by

( 1 − εpor ) ρs ∂z b

∂t = S d − S e (12)

in which z b is the bed elevation and εpor = 0.4 is sediment porosity.

The model has been successfully configured for the Changjiang

Estuary [ 3 ]. The computational domain covers Changjiang Estuary,

Hangzhou Bay and a part of the inner shelf of East China Sea. The

upstream river boundary is located at the Datong hydrologic station

where water level is not affected by tide. The open boundary is located

outside of the Changjiang Estuary with water depth of 80–100 m. The

whole domain is discretized into 32,657 cells with grid size (measured

by the shortest line of a triangle) from about 100 m to 2000 m. Fig. 5

presents a blow-up view of unstructured grid in the northern passage

where jetty-spur structures are located. These structures are well

resolved by fitting grid nodes in the jetties and spurs.

The model is driven by river inflow and open ocean tides. At the

open ocean boundary, water level is calculated from eight tidal con-

stituents, including M2, K2, S2, N2, K1, P1, O1 and Q1. At the river

boundary, water discharge is given by the measured time series of

water discharge at Datong hydrological station. The initial salinity

distribution is interpolated from the previously measured salinity

fields, which are located at 6 different water depth 0 m, 10 m, 20 m,

30 m, 50 m 100 m and at the bottom. During simulations, the first

10 days are used for a spin-up and provide dynamic adjustment of

the density field from initial conditions. The default q 2 − q 2 l turbu-

lence closure model [ 30 ] is used to predict vertical eddy viscosity and

diffusivity.

The model skills of predicting water surface elevation, tidal cur-

rent as well as salinity have been presented in Ma et al. [ 3 ]. In this

paper, we will show the model ’ s capability of predicting suspended

sediment concentration. In the simulations, the sediment is assumed

to be composed entirely of uniform fine cohesive particles. The sed-

iment size is chosen as d 50 = 0.008 mm, which has been used in

other numerical studies [ 31 ]. The settling velocity of a floc is w s =0.4 mm / s. The entrainment rate is E = 3.0 × 10 −4 kg / (m

2 s). The

critical shear stress for erosion and deposition are chosen as τ ce =τ cd = 0.4 N / m

2 , respectively. The sediment concentration at the up-

stream river boundary is set to be constant of 0.5 kg / m

3 . The sediment

concentration at the open ocean boundary is set to be zero.

Field measurements conducted from 5:00 a.m. May 5 to 11:00

a.m. May 6, 2004 were collected to validate the model. The measure-

ment stations are shown in Fig. 5 . Station S1 and S2 are located at the

upper reach of the navigation channel, while station S3 and S4 are

located at the lower reach of the navigation channel. The model-data

comparisons are presented as the plot of depth-averaged simulated

sediment concentration against measurement, as shown in Fig. 6 . The

solid line represents perfect agreement. The dashed lines show 30%

deviation from the perfect agreement. We can see that most of the

model-data comparisons are located within the dashed lines. Con-

sidering the complexity and spatial variability of sediment transport

processes in Changjiang Estuary, these comparisons are acceptable.

Fig. 7 shows the comparison of simulated and measured time-

averaged vertical distribution of suspended sediment concentration

at station S2 and S3. The sediment concentrations are averaged over

day May 5, 2004 (from 5:00 a.m. May 5 to 5:00 a.m. May 6). The ver-

tical structures of suspended sediment concentration are reasonably

captured by the model. The sediment concentration near the surface

is slightly overestimated, while the sediment concentration near the

bottom is underestimated, which indicates that the vertical mixing

is over-predicted by the model. This may be partially caused by the

Mellor–Yamada turbulence closure scheme which is well known for

its ”runaway” stratification [ 32 ].

Model validations have shown that the model can reasonably sim-

ulate the sediment transport processes in the northern passage. How-

ever, it is difficult to simulate sediment transport processes and mor-

phological changes in a long / median term at the construction region

due to daily dredging operation. Here, we carry out two numerical ex-

periments in order to identify the physical mechanisms of sediment

deposition migration in the navigation channel with the construction

of jetty-spur structures. In experiment 1, the entire jetty-spur struc-

tures including the 2nd stage extension were included in the sim-

ulation. In experiment 2, simulation was run without the extended

jetty-spur structures in the lower reach of the passage. Both experi-

ments were run with the same tide, river discharge and background

initial salinity conditions. Only local sediment suspension / deposition

was taken into account in the sediment module. In order to examine

the physical mechanisms that cause the migration of sediment de-

position patterns, a momentum balance analysis was conducted for

both experiments.

152 G. Ma et al. / Applied Ocean Research 43 (2013) 148–156

Fig. 7. Comparison of simulated (solid line) and measured (triangles) vertical distri-

butions of time-averaged suspended sediment concentration at station S2 (left panel)

and S3 (right panel). The sediment concentration is averaged over day May 5, 2004.

4

s

l

e

o

c

t

s

c

s

t

T

n

c

s

t

t

s

t

s

s

c

s

b

s

t

i

e

t

h

l

s

f

w

c

t

t

w

t

s

Fig. 8. Tidal and depth averaged suspended sediment concentration distribution in

the northern passage with the extension of the jetty-spur structures. The sediment

concentration is averaged over a spring-neap tidal cycle.

Fig. 9. Tidal and depth averaged suspended sediment concentration distribution in

the northern passage without the extension of the jetty-spur structures. The sediment

concentration is averaged over a spring-neap tidal cycle.

. Results and discussions

Because the northern passage is where the saltwater front located,

uspended sediments in this region are intensively subject to floccu-

ation and tidal pumping, which may be induced by flood-ebb differ-

nce in sediment erosion and deposition, or from the consequences

f flood-ebb asymmetry in stratification and mixing [ 33 ]. These pro-

esses lead to the formation of estuarine turbidity maximum (ETM) in

he northern passage. The construction of the large-scale jetty-spur

tructures may affect the distribution of suspended sediment con-

entration and sediment erosion / deposition patterns. To prove it, we

how the tidal and depth averaged suspended sediment concentra-

ion with ( Fig. 8 ) and without ( Fig. 9 ) the extension of the structures.

he suspended sediment concentration is averaged over a spring-

eap tidal cycle. Clearly, the distributions of suspended sediment con-

entration in the northern passage are significantly changed by the

tructures. Generally, the sediment concentration is higher without

he extended structures, indicating that less sediment is deposited in

he northern passage. At the lower end of the northern passage, the

ediment concentration is slightly increased with the extension of

he structures. In the northern and southern channels, the suspended

ediment concentrations are seldom affected by the structures.

The structure effects on along-channel distribution of suspended

ediment concentration in the navigational channel ( Fig. 5 , red line)

an be more clearly observed in Fig. 10 . The solid line represents the

imulated tide-averaged suspended sediment concentration distri-

ution without the extended jetty-spur structures. The dashed line

hows the simulated tide-averaged suspended sediment concentra-

ion distribution with the extension. Again, in most part of the nav-

gational channel, the sediment concentration is higher without the

xtended structures. The migration of the high sediment concentra-

ion is noticeable. Without the extended jetty-spur structures, the

ighest sediment concentration is located at the upper reach with

ongitude around 122.0 ◦. With the extended structures, the highest

ediment concentration is located at the middle reach with longitude

rom 122.05 ◦ to 122.15 ◦. At the lower end of the navigational channel

ith longitude greater than 122.2 ◦, the sediment concentration in-

reases with the extension of the jetty-spur structures. It is expected

hat the alteration of suspended sediment concentration distribu-

ion would result in different sediment erosion / deposition pattern,

hich can be represented by the bed elevation variation induced by

he extended structures as shown in the lower panel of Fig. 10 . It

hould be noted that the daily dredging processes are not captured in

the numerical experiments. The sediment supply from the upstream

Changjiang river is also not considered in the experiments. Therefore,

the magnitude of the predicted erosion / deposition strength might be

not realistic. However, the effects of the structures on the erosion /

deposition strength can be clearly observed. The extended jetty-spur

structures elevate the bed elevation at the upper reach and reduce the

bed elevation at the lower reach, indicating that more sediments are

deposited in the upper reach and less sediments are deposited in the

lower reach with the extension of the structures. This result is con-

sistent with the observations of the variations of sediment deposition

strength shown in Fig. 4 .

Fig. 11 presents the vertical distributions of suspended sediment

concentration along the navigational channel without (upper panel)

and with (lower panel) the extended jetty-spur structures. As ex-

pected, the suspended sediment concentration has strong stratifica-

tion with the maximum sediment concentration near the bottom. The

suspended sediment concentration is generally larger without the ex-

tended structures, especially near the point of inflection where the

turbidity maximum (TM) is located. The extended structure may in-

duce seaward migration of the turbidity maximum. The center of the

turbidity maximum is moved from 122.0 ◦ to 122.1 ◦ of the longitude.

G. Ma et al. / Applied Ocean Research 43 (2013) 148–156 153

Fig. 10. Upper panel: comparison of tidal and depth averaged suspended sediment

concentration along the navigation channel with (dashed line) and without (solid line)

extended jetty-spur structures. Lower panel: the daily averaged bed elevation change

z b induced by the extended jetty-spur structures. All these quantities are averaged over

a spring-neap tidal cycle.

Fig. 11. Comparisons of tide-averaged sediment concentration along the navigation

channel without (upper panel) and with (lower panel) extended jetty-spur structures.

The sediment concentration is averaged over a spring-neap tidal cycle.

Fig. 12. (a) Comparison of surface elevation distribution along the upper section with

(solid line) and without (dashed line) extended jetty-spur structures at the maximum

ebb and (b) comparison of surface elevation distribution along the lower section with

(solid line) and without (dashed line) extended jetty-spur structures at the maximum

ebb.

To explain why sediment deposition migrated from lower reach

to upper reach of the navigation channel after the extension of jetty-

spur structures, we take a look at flow patterns and the hydrodynamic

adjustments in the northern passage due to the extended structures.

During flood tide, the jetty-spur structures are submerged, weakening

their function as a flow guide. Our previous study [ 3 ] has shown that

the current velocity along the navigation channel does not change

significantly during flood tide by the presence of structures. During

ebb tide, the jetty-spur structures are mostly exposed, having the

strongest function as a flow guide. In addition, the Changjiang river

provides most of the sediment in the estuary. Therefore, it is expected

that the effects of the jetty-spur structures on sediment transport and

deposition are more significant during ebb tide. From these consid-

erations, we particularly take ebb tides for detailed analysis in the

following.

Fig. 12 compares the along-channel distribution of surface eleva-

tion at the maximum ebb with and without the extended jetty-spur

structures. Because the maximum ebb occurs at different time along

the navigation channel (about 1 h phase difference between the upper

and lower reaches), we show the instantaneous surface elevation dis-

tributions at two different sections. The upper section is along the 25

km long structures completed at the first stage, and the lower section

is along the extended construction. Fig. 12 (a) shows the comparison

of surface elevation distribution along the upper section at the time

when the maximum ebb occurs in the middle of the upper section

of the passage, and (b) shows the same comparison along the lower

section. The comparisons in both two sections show significant dif-

ference in surface elevation distribution with and without extended

structures. With the extended jetty-spur structures, surface elevation

generally becomes higher, with the maximum increase about 43.0

cm, which occurs in the middle of the passage. The effects in the up-

per and lower ends of the entire section are less significant, because

these two regions are close to open water (see Fig. 5 ). Clearly, with the

modulation of water level due to the extended structures, the surface

elevation gradient (defined as positive in the seaward direction) in

the upper reach becomes smaller, which decreases the driving force

on flows in the navigational channel, reducing the sediment carry-

ing capacity and suspended sediment concentration as proven in the

upper panel of Fig. 10 . In the lower reach, the gradient of surface

elevation becomes larger, which increases the current velocity and

sediment carrying capacity, resulting in higher suspended sediment

concentration and less sediment deposition.

The flow field adjustment in the northern passage due to the ex-

tended structures is shown in Figs. 13 and 14 , which demonstrate the

distributions of tide-averaged velocities (over a spring-neap tidal cy-

cle) at ebb tide with and without extended jetty-spur structures. The

simulation with extended structures generally predicts a narrower

and stronger current stream in the lower reach along the navigation

channel, compared to that without extended structures. The compar-

ison also indicates that the simulation without extended structures

predicts stronger currents in the upper reach of the northern pas-

sage, while the velocities in the same region predicted by the simu-

lation with extended structures are relatively small. This scenario is

more clearly observed in Fig. 15 , which shows the comparison of tide-

averaged ebb tide velocity along the navigation channel. For conve-

nience, we divide the whole section into two regions. In region (1), the

velocity predicted by the model with extended structures is smaller

than that predicted by the model without extended structures. In re-

gion (2), the velocity becomes larger with the presence of structures

in the lower reach. The maximum increase and decrease of velocity

caused by the extended structures are 0.27 m / s and −0.25 m / s, re-

spectively. The flow field adjustment in the northern passage results

in the migration of the turbidity maximum as well as the sediment

154 G. Ma et al. / Applied Ocean Research 43 (2013) 148–156

Fig. 13. The magnitude of averaged ebb tide flow velocity in the northern passage with

extended jetty-spur structures. The ebb tidal flow velocity is averaged over a spring-

neap tidal cycle. The unit of the colorbar is m / s. (For interpretation of the references to

color in this figure legend, the reader is referred to the web version of the article.)

Fig. 14. The magnitude of averaged ebb tide flow velocity in the northern passage

without extended jetty-spur structures. The ebb tidal flow velocity is averaged over

a spring-neap tidal cycle. The unit of the colorbar is m / s. (For interpretation of the

references to color in this figure legend, the reader is referred to the web version of the

article.)

d

m

tw

tW

t

w

(

t

s

t

r

s

Fig. 15. The comparison of averaged ebb tide flow velocity along the navigation chan-

nel with (solid line) and without (dashed line) extended jetty-spur structures.

Fig. 16. Momentum balance in the deep-water navigation channel at the maximum

ebb with extended jetty-spur structures.

eposition region as discussed above. To examine the physical mechanisms causing flow field adjust-

ent in the northern passage, we analyzed momentum balance along

he deep-water navigation channel at the maximum ebb. The analysis as done by calculating each term in the depth-integrated momen-

um equation, and then projecting into the along-channel direction. ithout wind shear stress at the surface, the along-channel momen-

um balance is generally given as

∂u l

∂t = −∇ h ·

(→

v →

v )

l − g

∂η

∂l − τbl

ρ0 D

+ f u n − g

ρ0

(∂

∂l

∫ η

z

ρ dz

)+ F l + G l (13)

here l represents the component in the along-channel direction

positive in the seaward direction), while the subscript n represents

he component perpendicular to the along-channel direction. The left

ide term of the equation is the acceleration of velocity with respect

o time (denoted as DUDT hereafter). On the right side, the first term

epresents advection of the flow field (ADV), the second term repre-

ents the surface elevation gradient (DPBP), the third term is bottom

friction term (WBF), the fourth term represents Coriolis force (COR)

and the firth term includes the contribution of baroclinic pressure

gradient forcing (DPBC). The remaining two terms F l and G l are hori-

zontal diffusion (VISC) and residual terms (RES) in the along-channel

direction, respectively, which were given by Chen et al. [ 34 ]. →

v is the

velocity vector, u l is the along-channel component, u n is the velocity

component perpendicular to the channel, g is the gravitational accel-

eration, τbl is the bottom shear stress, ρ is the water density, ρ0 is

the reference density, z is the vertical coordinate, and D is the water

depth.

Fig. 16 shows the momentum balance analysis along the naviga-

tion channel at maximum ebb with the extended jetty-spur struc-

tures. It is noted that momentum is mostly balanced by the nonlin-

ear advection, surface elevation gradient forcing and bottom friction,

which is typical for estuarine balance [ 20 , 35 , 36 ]. The Coriolis force

and baroclinic pressure gradient are much smaller than these three

terms. The current velocity is determined by the surface elevation

gradient. The maximum surface elevation gradient forcing is found at

the lower reach, where the maximum current velocity appears ( Fig.

15 ).

The velocity increase / decrease in the navigation channel with the

jetty-spur structures can be introduced by two distinct mechanisms.

G. Ma et al. / Applied Ocean Research 43 (2013) 148–156 155

Fig. 17. Comparisons of surface elevation gradient and bottom friction along the nav-

igation channel with and without extended jetty-spur structures.

The first mechanism is the direct interaction between flow field and

jetty-spur structures. These structures confine flow to the middle of

the passage, and tend to make the flow stream narrower and stronger.

The second mechanism is the indirect interaction between flow field

and structures in a large scale. The construction of structures changes

the distribution of surface elevation in the northern passage, which

subsequently affects the flow field. To explain the velocity change in

the northern passage with the extension of jetty-spur structures in

the lower reach, we compared the surface elevation gradient forc-

ing and bottom friction terms with and without extended structures,

which is shown in Fig. 17 . The jetty-spur structures clearly affect the

along-channel distributions of these two terms. In the upper reach,

surface elevation gradient is decreased with the extension of large-

scale structures in the lower reach of the passage, which decreases

the current velocity in the navigation channel. As a result, the mag-

nitude of bottom friction term becomes smaller in the upper reach.

On the contrast, the surface elevation gradient in the lower reach is

increased with the extended jetty-spur structures, which makes the

flow stream stronger, resulting in larger magnitude of bottom friction

term. The increase of current velocity in the lower reach is also con-

tributed by the direct interaction between flow field and jetty-spur

structures, which confine flow to the middle of the passage and make

the flow stream narrower and stronger.

From the above analysis, it can be concluded that there is a direct

link between the tidal flow and sediment deposition in the northern

passage. Both the tidal flow field and sediment deposition are dra-

matically changed by the extended jetty-spur structures. With the

extension of the jetties, the ebb tidal flow velocity in the upper reach

is greatly reduced, resulting in smaller sediment carrying capacity

and suspended sediment concentration. As a result, more sediments

are deposited in the upper reach. In the lower reach, the ebb tidal

flow velocity is largely increased, resulting in larger sediment carry-

ing capacity and less sediment deposition. The numerical results are

consistent with the measurements of sediment deposition strength

change in the navigation channel after the 2nd stage construction of

the jetty-spur structures.

5. Conclusions

This paper discussed large-scale structure impacts on the mi-

gration of sediment deposition region in the northern passage of

Changjiang Estuary. The analysis of sediment deposition was carried

out using the sediment deposition data collected in the navigation

channel during March 2000 to October 2005. The study has demon-

strated that main deposition region shifted from lower reach to upper

reach of the channel with the extension of jetty-spur structures in the

lower reach of the passage. The strongest deposition occurred in the

upper middle reach of the navigation channel.

The physical mechanisms that cause migration of sediment depo-

sition region with the extended jetty-spur structures were examined

using a numerical model FVCOM. It was found that the change of sed-

iment deposition distribution is closely related to the velocity field

adjustments during ebb tide when the structures are mostly exposed

and have the strongest function as a flow guide. With the extended

jetty-spur structures, current velocity in the upper reach of the chan-

nel is greatly reduced, while the current velocity in the lower reach of

the channel is increased. Momentum balance analysis suggested that

there are two mechanisms inducing current velocity adjustments.

The first mechanism is the direct interaction between flow field and

structures, which confine flows to the middle of the passage and make

the flow stream narrower and stronger. The second mechanism is

the indirect interaction between flow field and structures. The struc-

tures changed surface elevation gradient forcing which subsequently

altered current velocity. In the upper reach of the channel, the sur-

face elevation gradient is decreased with the extended structures,

reducing current velocity as well as sediment carrying capacity, sub-

sequently resulting in more sediment deposition. In the lower reach

of the channel, the surface elevation gradient is increased with the ex-

tended structures, increasing ebb tide flow velocity and creating less

sediment deposition. The numerical simulation of suspended sedi-

ment transport also indicates that the turbidity maximum migrates

seaward after the 2nd stage construction of the jetty-spur structures.

Acknowledgments

The authors are indebted to three anonymous reviewers for con-

structive comments and thorough reviews of the paper. Ma acknowl-

edges the support from the Key Laboratory of Estuarine and Coastal

Engineering, Ministry of Transport, Project KLECE201301 .

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