Responses of water environment to tidal flat reduction in Xiangshan Bay: Part II locally re-suspended sediment dynamics

Accepted Manuscript Responses of water environment to tidal flat reduction in Xiangshan Bay: Part II locally re-suspended sediment dynamics Li Li, Weibing Guan, Zhiguo He, Yanming Yao, Yuezhang Xia PII:

S0272-7714(17)30395-5

DOI:

10.1016/j.ecss.2017.08.042

Reference:

YECSS 5597

To appear in:

Estuarine, Coastal and Shelf Science

Received Date: 11 April 2017 Revised Date:

19 August 2017

Accepted Date: 23 August 2017

Please cite this article as: Li, L., Guan, W., He, Z., Yao, Y., Xia, Y., Responses of water environment to tidal flat reduction in Xiangshan Bay: Part II locally re-suspended sediment dynamics, Estuarine, Coastal and Shelf Science (2017), doi: 10.1016/j.ecss.2017.08.042. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Tidal flat reduction from 1963 to 2010

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Reduced current; increased Eulerian velocity and tidal pumping

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Variation of SSC and sediment fluxes

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SSC; Erosion

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SSC; Erosion

Locally resuspended sediment

ACCEPTED MANUSCRIPT Responses of water environment to tidal flat reduction in Xiangshan Bay: part II locally re-suspended sediment dynamics Li Li1,2* Weibing Guan2,1 Zhiguo He1,2 Yanming Yao1,2 Yuezhang Xia1,2 Ocean College, Zhejiang University, Hangzhou, China

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State Key Laboratory of Satellite Ocean Environment Dynamics (Second Institute of Oceanography,

SOA), Hangzhou, China

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* Corresponding author, Email: [email protected]

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ACCEPTED MANUSCRIPT Abstract:

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Xiangshan Bay is a semi-enclosed bay in China, in which tidal flats have been

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substantially reclaimed to support the development of local economies and society

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over previous decades. The loss of tidal flats has led to changes of tides and locally

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suspended sediment in the bay. The effects of tidal flat reduction on locally suspended

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sediment dynamics was investigated using a numerical model forced by tidal data and

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calibrated by observed tidal elevation and currents. The model satisfactorily

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reproduces observed water levels, currents, and suspended sediment concentration in

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the estuary, and therefore is subsequently applied to analyze the impact of tidal flat

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reclamation on locally suspended sediment transport. After the loss of the tidal flats

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from 1963 to 2010, the suspended sediment concentrations (SSC) at the bottom

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boundary layer were reduced/increased in the outer bay/tidal flat areas due to

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weakened tidal currents. In the inner bay, the SSC values near the bottom level

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increased from 1963 to 2003 due to the narrowed bathymetry, and then decreased

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from 2003 to 2010 because of the reduced tidal prism. The model scenarios suggest

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that: (1) a reduction of tidal flat areas appears to be the main factor for enhancing the

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transport of sediments up-estuary, due to the increased Eulerian velocity and tidal

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pumping; (2) A reduction of tidal flat areas impacts on spatial and temporal SSC

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distribution: reducing the SSC values in the water areas due to the reduced current;

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and (3) a tidal flat reduction influences the net sediment fluxes: lessening the erosion

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and inducing higher/lower landward/seaward sediment transportation.

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Key words: Xiangshan Bay, tidal flat reduction, locally suspended sediment,

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numerical model

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ACCEPTED MANUSCRIPT 1. Introduction

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Anthropogenic activities are making many estuaries worldwide more urbanized,

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requiring continuous channel deepening and larger ports. These activities have caused

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many estuarine problems such as increasing metal contamination and suspended

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sediment concentration (SSC) (Ghosh & Menon 2010). An example of a heavily

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impacted estuary is the Xiangshan Bay, located on the east coast of China. The study

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of locally suspended sediment dynamics in estuaries and bays, especially in the

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bottom boundary layer, is important for a better understanding of the physical

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processes of coastal stability and is essential to aid coastal management (Kirby 2013;

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Kumar et al. 2016). Suspended sediment stratification and sediment re-suspension at

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the bottom level are vitally important for bed erosion and deposition (Martyanov &

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Ryabchenko 2016). Any changes of the estuarine environment by human activity will

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change the suspended sediment dynamics and the evolution of the geometry, such as

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changes of delta in the Pearl River Estuary (Milker et al. 2016). The nature of

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sediment transport gives insights into the morphological behavior along the

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navigational channels of bays. The areas, where significant bathymetry changes were

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measured, coincide with the location of areas with high sediment concentrations.

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The transport of fine suspended sediment is usually controlled by residual flux and

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flood-ebb tidal asymmetry, which ultimately generate a Turbidity Maximum Zone

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(TMZ) (Jay & Dungan Smith 1990). Topographical changes in estuaries impact tides

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and consequently affect sediment transport. This mechanism has occurred in some

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estuaries worldwide, such as Darwin Harbour in Australia (Li et al. 2014; Li et al.

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2012) and the Changjiang Estuary in China (Song & Wang 2013; Song et al. 2013).

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Tidal flat reduction refers to the loss of tidal flat areas in estuaries, which means that

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the tidal flat is reclaimed as land. Reclaiming new land from the sea has become a

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potential way of satisfying the increasing demand for land (Niu & Yu 2008). The tidal

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flats areas around Xiangshan Bay were reduced, substantially due to reclamations in

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recent decades. The reconstructed coastlines have changed the tidal flat areas and

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bottom stresses. According to the data from the charts, about 42km2 of tidal flats were

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reclaimed from the sea near Xiangshan Bay from 1963 to 2003 (the blue color areas

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in Figure 1c), which considerably narrowed the bay. Following that, another 35km2 of

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tidal flats was converted to land by 2010 (the green color areas in Figure 1c). The 2

ACCEPTED MANUSCRIPT bathymetry of the bay changed slightly (within 1m eroded in the outer bay and within

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1m deposited in the inner bay) from 1963 to 2003, and kept almost unchanged from

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2003 to 2010. This study only focuses on the reduction of tidal flat areas from 1963 to

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2010, as the reduction of tidal flat areas directly changes the bathymetry of the tidal

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flats and the coastlines of the bay. The reduction of tidal flat areas changes the

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hydrodynamics and sediment dynamics in the bay, and then changes the bathymetry

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of the bay, consequently. The massive reduction of tidal flats is of interest to

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researchers both in China and overseas. Past research indicates that small changes of

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topography have a very slight impact on sediment dynamics, but accumulated

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reclamations do affect the sediment transport (Li et al. 2014; Zeng et al. 2011). These

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human interactions, such as dredging navigation channels, affect the flow regime and

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sediment transport in the bay. However, the mechanisms responsible for the changes

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of locally re-suspended sediment dynamics are poorly understood.

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Xiangshan Bay is a semi-enclosed narrow bay (Figure 1a), which is 60km long from

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the mouth to the upper head. Its width shrinks from about 18km at the mouth to about

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4km near the mouth of the Xihu inlet. The bay is a deep water shelter with an average

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depth of 10m. It is characterized by the energetic and asymmetric tides, the minimal

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runoffs, the calm winds and the weak waves all around the year, except the episodic

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typhoons and tropical storms during summer (Xu et al. 2013; Xu et al. 2014). The

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annual-averaged wave height is only 0.4m in the outer bay. The bay is characterized

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by large areas of tidal flats (Figure 1a), which cover more than 30% of the water area

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(563km2) of the bay (Dong & Su 1999a). There are three secondary bays named the

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Xihu inlet, Tie inlet and Huangdun inlet. Rivers running into the secondary bays are

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smaller than the tidal prism in these bays. Tides in the bay are semi-diurnal with a

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maximum and average tidal range of about 5.6m and 3.1m (Middleton & Bode 1987).

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The maximum tidal range increases to ~6m when tides propagate towards the head of

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the bay. Compared with other factors, tides in the bay dominate the sedimentation and

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tidal exchange is responsible for the observed net landward movement of suspended

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sediment (Gao et al. 1990). Suspended sediment in the bay was comprised of locally

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re-suspended sediment together with the sediment input from the bay mouth. The

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sediment input from the bay mouth has been reduced significantly since three Gorge

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dam constructions in recent years, as the amount of sediment from Changjiang River

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was reduced by about 90%. The suspended sediment was slightly deposited at the bay

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ACCEPTED MANUSCRIPT mouth and on the tidal flat from 1935 to 2005, according to the depth data from the

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charts. The 0m depth contour line progressed toward the sea at an average speed of

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1.1m/a. The tidal channel with depth larger than 10m was slightly eroded with an

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average speed of 0.09km2/a (Chen et al. 2015). It was demonstrated that the

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deposition of sediment in the bay could be due to construction of engineering

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facilities (Wang et al. 2014; zhang 2005).

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Hence, much work, e.g. the tidal exchange (Azofra et al. 2014; Cai et al. 1985; Chen

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& Su 1999), tidal response time and deformation (Dong & Su 1999a; Dong & Su

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1999b; Ksanfomality et al. 1997; Mishra et al. 2015; Seo et al. 2015), sediment

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dynamics (Chi 2004; Gao et al. 1990; Zeng et al. 2011), has been carried out in the

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bay, and as a result of this some tidal flats have been reconstructed to restore

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previously damaged tidal flats and artificial tidal flats have been created to replace

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lost ones (Lee et al. 1998). Plans at state level have been made to restore the lost tidal

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flats and wetlands of China. Research has been done on the effects of lost tidal flats

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on sediment dynamics, but the way in which changes of tidal flat impact on locally

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re-suspended sediment dynamics in the bottom boundary layer in the narrow

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macro-tidal Xiangshan Bay still needs to be examined to help the biological-

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environmental management of tidal flats.

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The primary aims of this study are firstly to build a suspended-sediment model of

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Xiangshan Bay and to calibrate the model using field data, and secondly to examine

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the impacts of tidal flat reductions on locally re-suspended sediment dynamics in the

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bay. The results in this study will underpin coastal development and harbor

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management in the near future. The methodology is described in section 2, and the

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model calibration is detailed in section 3. The model results are analyzed in section 4,

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with conclusions presented in section 5.

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2. Methodology

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2.1. Model description

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A two-way coupling was applied between the Finite Volume Coastal Ocean Model

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(FVCOM) and the estuarine suspended-sediment model (UNSW-Sed) of Wang (2002).

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This coupled model allows the sediment concentration to affect fluid density, and

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consequently estuarine hydrodynamics. The FVCOM is used to simulate the 4

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complex geometry of Xiangshan Bay. For the governing equation of flow motion, one

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may refer to the publications of Chen et al. (2003) and Chen et al. (2006). The

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UNSW-Sed model focuses on suspended sediment, particularly in the sediment

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bottom boundary layer, and the model has been applied to study the effects of

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anthropogenic activities on suspended sediment dynamics. For the sediment transport

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model, this study focuses on suspended sediment and the bottom boundary layer. Only

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cohesive suspended sediments are dealt with as they constitute the largest part of the

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suspended sediment and extend increasingly on the seabed [Brenon and Le Hir, 1999].

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Sediment processes are parameterized following the model of Wang (2002), which is

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described by

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∂ ( w − ws )C ∂C ∂ (uC ) ∂ (vC ) ∂ ∂C + + + = (Kh ) + Fc , ∂t ∂x ∂y ∂z ∂z ∂z

(1)

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where x, y and z are the east, north and vertical coordinates, respectively, and u, v and

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w the corresponding velocity components. ws is the settling velocity of the sediment,

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and C the suspended-sediment concentration by volume. The vertical eddy diffusivity

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for suspended sediment was set equal to Kh in the momentum equation, which

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employs the Mellor-Yamada level 2.5 turbulence closure scheme for vertical mixing

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(Mellor & Yamada 1982). Fc is the horizontal diffusion term, parameterized

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according to the Smagorinsky diffusion scheme (Smagorinsky 1963).

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The density of seawater ρ is given by a volumetric relationship, when considering the

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contribution of the suspended sediments (Winterwerp 2001),

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

ρ = ρ w + 1 − 

ρw   C, ρs 

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where ρw is the clear seawater density without sediment determined by the equation of

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state and ρs the sediment density. The effect of the salinity and temperature on water

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density is larger than that of suspended sediment concentration. However, the salinity

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and temperature in the bay is vertically well mixed, particularly during the spring

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tides. Hence, the effect of suspended sediment concentration is considered mainly to

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account for the impact of the sediment on stratification in the bottom boundary layer.

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This method is also prepared for the further study on the high turbid water

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environment like Hangzhou Bay, China. 5

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The bottom drag coefficient in a sediment-laden bottom boundary layer is given by  ( h + zb )  1 Cd =  ln  z0   κ / (1 + AR f )

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−2

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where z0 is the bottom roughness length,

zb is the thickness of the bottom layer

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(σ=–0.991 in this study), κ=0.4 is the von Karman constant and h is the water depth.

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The effect of stratification is specified by a stability function, 1+ARf , where A is an

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empirical constant and Rf is the flux Richardson number. Adams and Weatherly

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(1981) determined that A=5.5 for a sediment-laden oceanic bottom boundary layer.

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The vertical sediment flux (E) on the seabed due to erosion/deposition processes is

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determined according to Ariathurai and Krone (1976),

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  τb  τb > τc − 1 ,  E0    τc  E =  ,   τ  b Cb ws  τ − 1 , τ b < τ d  d  

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where E0 is the erosion coefficient, τc and τd the critical stresses for re-suspension and

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deposition, respectively, and Cb the sediment concentration in the model’s bottom

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layer. The continuous bottom exchange of sediment between the seabed and the water

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column through erosion and deposition is a function of shear stress, which varies in

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both space and time. More information on the sediment model is provided in Wang

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(2002), Wang et al. (2005) and Wang and Pinardi (2002).

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(4)

2.2. Model configuration

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2.2.1. Model setup and initial conditions

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This study focuses on Xiangshan Bay (Figure 1a), with the model domain covering

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the entire bay and avoiding the islands near the mouth. The bay geometry and water

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depth related to mean surface level are shown in Figure 1b. The construction of the

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unstructured triangular model grid, consists of about 49,906 elements and 27,081

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nodes in the horizontal plane. Figure 1b shows the model grids for the entire domain

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where the resolution around the islands is especially high. The cell sizes of the

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domain range from 30m near the islands to 2,000m at the ocean open boundary. To

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simulate the vertical profiles of the suspended sediment accurately, we specified 20

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uniform vertical layers in the water column using the σ-coordinate system.

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The oceanic open boundary is located at the bay mouth and separated by the island in

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the bay mouth (Figure 1b). The open-boundary conditions for the water level are

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specified using tidal elevations predicted by the TPXO7.2 global model of ocean tides

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[Egbert et al., 2010]. Hourly tidal elevations, constructed using four diurnal

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components (K1, O1, P1, Q1), four semidiurnal components (M2, S2, N2, K2), three

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shallow-water components (M4, MS4, MN4) and two long-period components (Mf,

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Mm), were applied to the open-ocean boundary. This study focused on the local

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response of suspended sediment dynamics, as the sediment outside the bay mouth was

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significantly reduced after the construction of the Three Gorgeous Dam. Hence, the

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input of suspended sediment from the mouth of the bay was not considered and only

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the responses of local suspended sediment on topographic changes in the bay were

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examined.

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The runoff catchment area of the bay is very small. There are no large rivers running

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into the bay (Gao et al. 1990). The model time is in winter, when the river runoff was

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low and negligible. Hence, the river discharge is not included in the model. Wind is

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also not considered, as the bay is quite narrow and long. Heat flux at the free-surface

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boundary is neglected in our model. The salinity is well mixed in the bay due to

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strong tides and the wind forcing was neglected (Chi 2004; Xu et al. 2013). This was

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also shown by the field data obtained in the spring tidal cycle (10-11th November

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2003). Hence, the model is initialized with constant values for salinity of 25 and for

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temperature of 25°C, which is typical of the bay’s mean temperature and salinity

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during the model time (in winter). The model ran for 60 days from 7th November 2003.

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Its key parameters are summarized in Table 1.

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According to Gao et al. (1990), the sediment in Xiangshan Bay is mainly silty clay

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and clay silt. It is an effective approximation to treat suspended sediments as a single

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group with particle size of 0.002mm, as this group represents most of the fine

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sediments found in the bay (Chi 2004). As referred by Liu et al. (2014), when salinity

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ranges between 5-15, flocs are easily formed. The salinity in Xiangshan Bay is around

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23~30psu, and not in the above range, so flocs are not considered, and uniform

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settling velocity of 0.0001ms-1 was used in this study. The main parameters in the 7

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sediment model are summarized in Table 1.

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2.2.2. Sediment flux decomposition

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According to the method of mass transport flux of Dyer [1997], if neglecting the

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short-period turbulence, the instantaneous sediment flux through a unit width of a

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section perpendicular to the mean flow is given by F = ∫ ucdz = ∫ hucdσ , where

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h is the water depth, z is the vertical coordinate, u is the velocity, c is the

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sediment concentration, and σ is the relative depth from the seabed ( σ = −1 ) to the

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water surface ( σ = 0 ). The net sediment flux over a tidal cycle can be partitioned into

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seven major fluxes as:

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1 T 0 huc dσ dt T ∫0 ∫−1

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= h0 u 0 c0 + c 0 ht u t + u 0 ht ct + h0 u t ct + ht u t ct + h0 uv cv + ht uv cv = T1 + T2 + T3 + T4 + T5 + T6 + T7 ,

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(5)

where the brackets,

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variable, the over bar the vertical averaged value. h = h0 + ht , where h0 and ht are

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the tidally averaged water depth and its deviation, respectively. u = u + uv and

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c = c + cv , where u v and cv are the deviations at any depth from the vertically mean

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values. T is the tidal period, u 0 and c 0 the mean vertically averaged velocity and

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sediment concentration over the tidal cycle, respectively, and u t and ct the

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corresponding deviations of the vertically averaged values from the means.

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, denote the tidally averaged value of a vertically integrated

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In the equation (5), T1 and T2 is the flux due to the Eulerian velocity and the Stokes

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drift, respectively. T3 + T4 + T5 is the tidal pumping terms that are produced by the

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tidal phase differences, where T3 is the correlation term between the tidal level and

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sediment concentration, T4 related to the sediment erosion threshold and lags, which 8

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T6 is due to the estuarine circulation with a high near-bed sediment concentration

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and a low surface sediment concentration, and T7 arises from the changing forms of

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the vertical profiles of velocity and concentration, due mainly to the lags between

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scouring and settling activities (Wai et al. 2004). The flux of sediment in Xiangshan

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Bay is identified using this method.

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2.2.3. Sensitivity tests

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As shown in Figure 1c, tidal flat reduction data were collected. The two years of 1962

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and 2010 were selected to test the effect of tidal flat reduction on locally suspended

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sediment, as these two years were important milestones of tidal flat reclamation. Two

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more sensitivity tests, as listed in Table 2, were designed to examine the impacts of

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tidal flat changes over about 50 years on locally suspended sediment dynamics. Test 2

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used the coastlines of 1963, which shows the narrowing of Xiangshan Bay from 1963

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to 2003, and test 3 used the coastlines of 2010, which show the reclaimed northern

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cusp of the Tie inlet at the head of the Xiangshan Bay in 2010, indicated by ‘F’ in

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Figure 1a. Both test 2 and 3 used the same bathymetry of 2003 to remove the impact

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of bathymetry variations on sediment dynamics.

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3. Model calibrations

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The tidal elevation data were measured by the Key Laboratory of Satellite Ocean

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Environment Dynamics (Second Institute of Oceanography, SOA). The sea-surface

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level were measured hourly in one month from 7th November 2003 at three field

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stations of Xize, Wushashan and Qiangjiao (Figure 1b), which were located at the

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mouth, the middle and the head of the bay, respectively. The tidal current data were

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measured at stations 1 to 3 in one-hour intervals at the bottom level, the middle level

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and the surface level. SSC data at stations 1 and 3 were measured during a spring tidal

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cycle (10-11th November 2003), medium tidal cycle (13-14th November 2003) and a

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neap tidal cycle (16-17th November 2003). SSC data at stations 2 were measured

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during a spring-neap tidal cycle from 7th November 2003.

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The SSC values at the surface, middle and bottom levels of the water volume at the 11

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stations (Figure 1b) are shown in Figure 2. These stations were selected to basically

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cover the entire bay and to monitor the main tidal channel. Stations near tidal flat

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(stations 8 to 11) were selected to measure the variations of suspended sediment on

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tidal flats. Only station 8 is shown in Figure 1b, as station 8 to 11 were located very

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close. The field data were measured hourly at the bottom level, the middle level and

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the surface level during a spring tidal cycle (10-11th November 2003), medium tidal

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cycle (13-14th November 2003) and a neap tidal cycle (16-17th November 2003).

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During spring tides (Figure 2a), the highest SSC values among all 11 stations

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appeared at station 1 and station 4 (about 1kgm-1) near the bay mouth. The SSC

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values (less than 0.15kgm-1) in the middle (station 5) and inner bay (station 11) were

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lower than those near the bay mouth (stations 1 to 4). SSC values at stations 1 to 5

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had similar temporal and vertical distribution patterns during the monitoring period,

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varied temporally with the flooding-ebbing tidal cycles and decreased vertically from

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the bottom to the surface. At stations 6 to 11, the SSC values only slightly varied with

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the flooding-ebbing tidal cycles. During neap tides (Figure 2b), the SSC values at

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stations 1 to 4 (less than 0.2kgm-3) were only 20% of those during spring tides, but

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were still higher than those at stations 5 to 11 (less than 0.1kgm-3) during neap tides.

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The SSC values decreased from bottom to surface level at all the stations.

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The numerical model calibration was conducted using the observed tidal

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elevation, currents and SSC values. As shown in Figure 3 and Figure 4, the model

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reproduced reasonably well the tidal elevation, currents and the SSC values at the

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field stations. The deviation for tidal height was less than 14% of the largest tidal

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range (~5m). The model trends in current velocity and phase were in reasonable

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agreement with the measurements, with the deviation for velocity less than 20% of the

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largest current speed (1.4m/s) during spring tides. The deviation in current directions

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was less than 50 degrees during the spring and neap tide, at all measuring stations. As

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stations 8 to 11 are located quite close to each other, only the comparison of SSC

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values at station 8 is shown here. The modeled SSC values are mostly lower than the

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observed values, as the input of the sediment from the oceanic boundary was not

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considered. In this study, we focused on the local re-suspended sediment in the water

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areas. Hence, the model was designed to analyze the suspended sediment dynamics in

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the bay and to examine the effects of tidal flat changes on sediment transport.

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4. Suspended sediment dynamics in bottom boundary layer in Xiangshan Bay

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4.1. Characteristics of suspended sediment concentration

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According to the results of the reference model for 2003 (Figure 5), the bottom SSC

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values of less than 1kgm-3 during spring tidal cycles, peaked near the bay mouth and

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decreased from the bay mouth to the bay head. The high SSC zone moved into and

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out of the bay along the main tidal channel with the flooding and ebbing tidal cycle,

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respectively, as a result of current advection. This turbidity maxima (TM) zone, which

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was also numerically simulated by Chi (2004), was formed by the locally

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re-suspended sediment due to large current speed. This phenomenon was also evident

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in the field data, where the SSC values at station 1 were much higher than those at

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other stations, as shown in Figure 2. The suspended sediment concentration in the

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inner harbor was high due to relatively strong current speed. Water in the inner harbor

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was clear compared with that at the mouth, as also indicated by the field data.

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During the neap tides, similar pattern happened to the bottom SSC values, but with

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lower SSC values due to less energetic tides. Although the turbid zone in the outer

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harbor still existed during the neap tidal cycle, its SSC was less than 0.03 kgm-3 due to

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relatively strong currents.

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The suspended sediments in the bay tended to be well mixed throughout the water

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column during the spring tides because of the strong currents. At the mouth of the bay

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(cross section I), the SSC values were higher during the peak flooding and the peak

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ebbing tides than those during the slack water, due to bottom re-suspension caused by

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strong currents. The bottom SSC values reached about 0.5kgm-3 and then decreased to

336

about 0.3kgm-3 at the surface level. Similar decreasing pattern of the SSC values also

337

happened along the tidal channel from the mouth to the head of the bay. The SSC

338

values at the cross sections II, III and IV were high at the high water slack during

339

spring tides, as a large patch of suspended sediment was pushed into the bay by the 11

ACCEPTED MANUSCRIPT 340

tides (Figure 6). This was in accordance with the SSC temporal variations shown by

341

the observed data (Figure 2). The vertical profiles of SSC values during neap tides

342

had a similar distribution pattern, but smaller magnitudes than those during spring

343

tides due to relatively weak tidal currents.

344

4.2. Characteristics of net sediment fluxes

346

According to the results of the reference model, the net fluxes of suspended sediment

347

accumulated in one lunar month at the four cross sections are shown in Figure 7a.

348

Positive indicates landward. As can be seen, sections I and III have landward/seaward

349

net sediment fluxes near the northern/southern bank, while section II and IV have

350

seaward/landward net sediment fluxes near the surface/bottom level. Section I has the

351

largest net sediment flux rate while section IV has the smallest net sediment flux rate.

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In order to examine the controlling mechanism of the suspended sediment fluxes, the

354

net sediment fluxes at the six sections are decomposed into seven factors and shown

355

in Figure 7b. Positive indicates landward. Most of the suspended sediments at the four

356

cross sections are transported by the Eulerian velocity and tidal pumping shown by

357

the blue lines and the red lines in Figure 7b. The sediment fluxes at the bay mouth

358

(section I) and the entrance of the Tie inlet (section IV) are landward, while at the

359

entrances of the Xihu inlet and the Huangdun inlet (section II and III) the sediment

360

fluxes are seaward. These mechanisms of sediment transport were in accordance with

361

the tidal-dominated hydrodynamics in the bay, as the tides in the bay were

362

asymmetric.

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The erosion and deposition pattern at the sea bed is shown in Figure 7c. The sediment

365

is basically eroded in the main tidal channel due to the strong currents, and deposit in

366

the shallow water areas. The magnitude of erosion/deposition is less than 0.01m/a,

367

with the largest erosion and deposition appearing near the mouth and the middle of

368

the bay.

369 370

5. Discussions 12

ACCEPTED MANUSCRIPT 5.1. Effects of tidal flat changes on suspended sediment concentration

372

The changes of the suspended sediment concentration at the bottom level from 1963

373

to 2010 are illustrated (Figure 8) to examine the effects of tidal flat reduction on SSC.

374

During spring tides, the bottom SSC values decreased by less than 0.1kgm-3 near the

375

bay mouth, while slightly increased (about 0.02kgm-3) in the tidal flat areas, from

376

1963 to 2003 (Figure 8a). In the inner bay, SSC values slightly decreased (about

377

0.02kgm-3) during a spring tide, due to the decreased tidal currents compared with

378

1963. A similar pattern with a larger decrease was noted for the SSC values from 2003

379

to 2010 (Figure 8b). From 1963 to 2010, the bay was narrowed and the tidal flat

380

reduced at the head of the Tie inlet, hence the tidal prism was reduced, and

381

consequently the tidal current was decreased.

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During neap tides, the SSC values showed similar increase/decrease patterns from

384

1963 to 2003 and from 2003 to 2010, with one order of magnitude less

385

decrease/increase of SSC change.

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The turbidity maxima (TM) zone at the entrance of the bay still existed when tidal

388

flats were reclaimed from 1963 to 2010 (Figure 8e-h). Although the water is still

389

turbid in the TM zone, the magnitudes of the SSC in 2010 were only half of those in

390

1963. The areas of the turbid zone decreased from 1963 to 2010. The core of the TM

391

at the entrance of the bay moved towards the southern coast. This is in accordance

392

with the current dynamics in the bay, as the current magnitudes decreased by about

393

0.2ms-1 from 1963 to 2010.

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5.2. Effects of tidal flat changes on suspended sediment fluxes

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In order to examine the net fluxes in the secondary bays, the suspended sediment

397

fluxes through sections I to IV accumulated through one lunar month are shown in

398

Table 3. Positive indicates landward. As shown in the table, the net sediment fluxes

399

are landward at the bay mouth (section I) and the mouth of the Tie inlet (section IV),

400

and seaward at the other two sections II and III (Figure 1).

401 13

ACCEPTED MANUSCRIPT At the bay mouth (section I), the amounts of the net fluxes of suspended sediment

403

reduced from 1963 to 2003, and then increased from 2003 to 2010, indicating an

404

increased landward sediment flux from 1963 to 2010. At the mouths of the Xihu inlet,

405

the Huangdun inlet (sections III and V) and the main channel (sections II and IV), the

406

seaward fluxes of suspended sediment decreased continuously from 1963 to 2010,

407

showing a reduction of seaward sediment flux from 1963 to 2010. At the mouth of the

408

Tie inlet (section IV), the amount of the landward sediment fluxes increased from

409

1963 to 2003, and then decreased from 2003 to 2010, showing a decreased sediment

410

flux from 1963 to 2010.

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The changes of the net sediment fluxes at the sections show that the total amount of

413

sediment into the bay had increased, while the net sediment fluxes into the Tie inlet

414

had reduced. Hence, there was an increased amount of sediment in the Xihu inlet, the

415

Huangdun inlet and the main channel of the bay.

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To examine the mechanism controlling the sediment fluxes in the whole bay and the

418

three secondary bays, the sediment transport components are illustrated in Figure 9a.

419

Comparing the sediment transport components for 1963, 2003 and 2010, the main

420

components, the Eulerian velocity and tidal pumping, show a similar trend from 1963

421

to 2010, but decreased magnitudes in 2010, at all the entrances to the four secondary

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bays.

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At the mouth of the entire bay and the mouth of the Xihu inlet (section I and II), the

425

Eulerian veloity component brought more sediment seaward in 1963. At the mouth of

426

the Huangdun inlet (section III), the tidal pumping component tended to bring less

427

sediment seaward and more sediment landward in 2010, than that in 1963 and 2003.

428

At the mouth of the Tie inlet (section IV), the landward sediment increased from 1963

429

to 2010. This was due to the reverse effects of tidal pumping. The flood dominant

430

process was amplified after the enclosure of the dam at the northern cusp of the Tie

431

inlet.

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432 433

As can be seen from Table 4, all the landward and seaward sediment flux components

434

were reduced at the bay entrance and the entrances to the Xihu inlet and the

435

Huangdun inlet. This is due to the weakened tidal currents from 1963 to 2010. At the 14

ACCEPTED MANUSCRIPT 436

Tie inlet entrance (section IV), the landward sediment increased from 1963 to 2010.

437

The seaward sediment fluxes had the reverse trend. This is because of the increased

438

flood dominance due to the enclosure of the dam at the northern cusp of the Tie inlet.

439

Due to the reduction of tidal flat, the tidal prism, shoaling effect and bottom friction

441

were all reduced. The M2 and M4 tidal amplitudes and phases were modulated,

442

consequently. Hence, the flood dominance of the bay, indicated by the sea surface

443

level skewness calculated through the M2 and M4 amplitudes and phases according to

444

Song, et al. (2013), was enhanced from 1963 to 2010. Hence, more sediment was

445

transported landward due to the increased tidal pumping effect from 1963 to 2010.

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446

5.3 Relationship between the sediment transport and the tidal flat morphology

448

Comparing the net sediment erosion and deposition at the seabed in one month in

449

1963 with that in 2010 (Figure 9b-c), a similar erosion and deposition pattern

450

occurred, but less sediment was eroded and deposited in 2010. This was because of

451

the weakened tidal currents in 2010.

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Tidal flat areas basically acted as sinks for suspended sediments. The sea bed was

454

eroded at the tips of the Tie inlet and the Huangdun inlet, as shown by the larger

455

deposition and erosion values, respectively. Along the tidal channel, sea bed was

456

largely eroded, and deposition happened at other areas (Figure 9b-c). The deposition

457

and erosion processes were also illustrated by the depth data measured in 1963 and

458

2003, respectively (figure not shown). Similar erosion-deposition pattern from 1935

459

to 2005 was found by Chen et al. (2015), using depth data from the charts.

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It should be noted that the deposition and erosion pattern from the model only

462

considered the locally suspended sediment, without the input of sediments from the

463

bay mouth and the coastal engineering activities like dredging.

464 465

6. Conclusions

466

The response of locally suspended fine sediment to accumulated tidal flat reduction in

467

Xiangshan Bay was studied using a sediment model named UNSW-Sed coupled with

468

FVCOM. The model tidal level, currents and SSC values were in accordance with the

469

observed ones. The SSC values at the bottom level peak at the bay mouth were less 15

ACCEPTED MANUSCRIPT than 1kgm-3 and 0.3kgm-3 during spring and neap tidal cycles, respectively, and then

471

decreased from the bay mouth to the bay head. The high SSC zone moved into and

472

out of the bay channel with the flooding and ebbing tidal cycle. The sediment fluxes

473

at the bay mouth and the entrance of the Tie inlet were landward, while at the

474

entrances of the Xihu inlet and the Huangdun inlet were seaward, controlled mostly

475

by the Eulerian velocity and tidal pumping. Hence, there is accumulation of sediment

476

in the main channel of the bay and the Tie inlet. The Xihu inlet and the Huangdun

477

inlet are scoured.

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478

After reclamation of the tidal flats, the SSC values at the bottom boundary layer were

480

reduced/increased in the outer bay/tidal flat areas continuously from 1963 to 2010,

481

due to weakened tidal currents. Although the turbidity maxima (TM) at the entrance

482

of the bay still existed in 2010, but the core of the TM moved towards the southern

483

coast and magnitude of the SSC in the core was only half of that in 1963. This is in

484

accordance with the current dynamics in the bay, as the current magnitudes decreased

485

by about 0.2ms-1 from 1963 to 2010. All the landward and seaward sediment flux

486

components were reduced at the bay entrance and the entrances to the Xihu inlet and

487

the Huangdun inlet due to weakened flow after the loss of the tidal flats. At the Tie

488

inlet entrance, the landward sediment increased after 2003, and then decreased after

489

2010, due to the increased flood dominance caused by the enclosure of the dam at the

490

northern cusp of the Tie inlet. Less sediment was eroded and deposited after the tidal

491

flat reduction because of the weakened tidal currents.

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479

493

Hence, coastal structures and social facilities need careful plans as these processes

494

would essentially impact sediment dynamics in estuaries. Furthermore, the

495

accumulated erosion or deposition is harmful to a harbor’s ecosystem.

496 497

Acknowledgements

498

This research was supported by the National Natural Science Foundation of China

499

(Grant No. 41606103), Zhejiang Provincial Natural Science Foundation of China

500

(Grant No. Q16D060002, LR16E090001), the State Key Laboratory of Satellite

501

Ocean Environment Dynamics (Second Institute of Oceanography, SOA) (Grant No. 16

ACCEPTED MANUSCRIPT 502

SOED1512), National Key Research and Development Program of China (Grant No.

503

2017YFC1405101).

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ACCEPTED MANUSCRIPT List of tables

Table 1. Configurations of key model parameters Table 2. Descriptions of numerical tests Table 3. Comparison of the residual sediment fluxes in one month through the six sections shown in Figure 2 (kgm-2s-1). Positive indicates landward. Table 4. Sediment transport components (kg/s) at the Cross-sections I to IV in 1963, 2003 and 2010. Positive values indicate landward.

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List of figures

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Figure 1. (a) Model domain and field stations of Xiangshan Bay. Cross sections I to IV are used for examining vertical profiles of suspended sediment concentration. Green colored areas indicate tidal flats. (b) Bathymetry of the domain and the locations of the field stations. Depth contour relates to mean surface level in 2003. Positive indicates below the datum of mean surface level. (c) Model grids in 2010 (black color area), 2003 (black and green color areas) and 1963 (black, green and blue color areas). Figure 2. Observed suspended sediment concentrations at the 11 stations at the surface, middle and bottom level and observed elevation at stations Qiangjiao Wushashan and Xize during (a) spring tide and (b) neap tide. The time is in hours from 00:00 on 7th November 2003. Figure 3. Comparison between observed and modeled (a) elevation at stations Qiangjiao, Wushashan and Xize; (b to d) current speed and direction at stations 1 to 3. The time is in hours from 00:00 on 7th November 2003. Figure 4. Comparison between observed and modeled SSC at stations 1, 2, 3, 4, 5 and 8 as shown in Figure 1b. The stations 6-7 are not used in the model calibration as they are located on the tidal flat, as the suspended sediment concentration there is easily affected by coastal activities. The stations 9-11 are located near the station 8. Hence, only the station 8 is shown in the figure. The time is in hours from 00:00 on 7 November 2003. Figure 5. Suspended sediment concentrations at the bottom level during (a-d) spring tides and (a’-d’) neap tides. Figure 6. Vertical profiles of suspended sediment concentrations at sections I to IV (Figure 1a) during spring tide. Figure 7. (a) Vertical profiles of net sediment fluxes at sections I to IV (Figure 1) accumulated in one month. Positive indicates landward. (b) The mechanism controlling the suspended sediment fluxes at the six cross sections in 2003, the reference model. Positive indicates seaward. T1 Eulerian velocity; T2 Stokes drift; T3, T4 and T5 tidal pumping; T6 gravitation circulation; and T7 changing forms of the vertical profiles of velocity and concentration in the tide. (c) Net sediment erosion and deposition at the sea bed in one month in 2003. Positive indicates erosion. Figure 8. Suspended sediment concentration differences at the bottom level between (a) 2003 and 1963, and (b) 2010 and 2003 during spring tidal cycle. (c, d) are same with (a, b) but for neap tidal cycle. The blue lines indicate the water areas. (e) and (f) are the bottom SSC in 1963 and 2010, respectively during spring tides. (g, h) are same with (e, f) but for neap tidal cycle.

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21

ACCEPTED MANUSCRIPT

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Figure 9. (a) Mechanism controlling the suspended sediment fluxes at the cross sections I, II, III, and IV in 1963, 2003 and 2010. Positive indicates landward. Net sediment erosion and deposition at the seabed in one month in (b) 1963 and (c) 2010. Positive indicates erosion.

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Tables

Table 1. Configurations of key model parameters Model parameter

Value

Bottom friction coefficient

Minimum 0.0025

Horizontal diffusion

Smagorinsky scheme

Vertical eddy viscosity

M-Y 2.5 turbulent closure

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Model time setup

Uniform fine sediment with diameter of 0.002mm 1.0 s

Sediment diameter

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Node, element, vertical layers 49,906 grids, 20 uniform sigma layers Sea surface elevation time series from Open boundary condition TPXO7.2 Erosion rate 0.00015 kgm-2s-1 0.8 kgm-1s-2

Critical erosion stress

0.6 kgm-1s-2

Critical deposition stress

0.0001ms-1

Settling velocity

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Table 2. Descriptions of numerical tests Tests Details 1

Test items

2003 coastlines;

The reference model. This model uses the above

2

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settings and is calibrated by field data.

1963 coastlines;

Effects of tidal flat reduction from 1963 to 2003

3

677

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on suspended sediment dynamics

2010 coastlines;

Effects of tidal flat reduction from 2003 to 2010 on suspended sediment dynamics

Table 3. Comparison of the residual sediment fluxes in one month through the six sections shown in Figure 2 (kgm-2s-1). Positive indicates landward. Experiments

Section I

Section II

Section III

Section IV

1963

0.61

-0.54

-0.47

-0.02

2003

0.46

-0.46

-0.40

0.04

2010

0.94

-0.33

-0.12

0.02

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ACCEPTED MANUSCRIPT 679

Table 4. Sediment transport components (kg/s) at the Cross-sections I to IV in 1963, 2003 and 2010. Positive values indicate landward. 1963

2003

2010

Components 275.6 -29.7

IV

I

-5.2 -0.1 -0.8

II

185.3 -27.8

0.0 -143.8

0.0

-1.6

-0.2

0.0

-3.5

-1.1 -0.1

0.0

-0.4 -0.0

314.9 -30.2

-5.4 -0.3

258.6 -24.4 -80.9

23.5

4.5

-0.5 -0.1

T7

4.1

0.1

-0.0 -0.0

Landward Seaward 680

618.1

5.9

0

0

-16 -1.4

0.0

-0.5

0.0 -60.6

0.1 -0.2

0.0

4.3

-0.1 -0.0

22.4

3.6 -0.1

-0.1

3.4

0.1

0.0 -0.0

2.3

0.11 -0.0

-0.0

240.4 -48.3 -10.0

0.1 180.2 -34.8 -3.9

-0.1

470.4

5.5

0

0.1 339.4

-291.3 -61.7 -13.1 -0.6 -229.9 -53.8

-9.9

0 -159.4

AC C

EP

Figures

326.8 -55.9 -13.0 -0.6

0.1 214.3

23.1

TE D

Total

0.4

-2.1

M AN U

T6

IV

0.4 -0.2

-1.8

-0.8 -0.1

III

0.0 -95.3

-7.3

0.5

II

-0.7

T3

-101

I

0.7

0.8

T5

IV

0.0

-183

-5.2

III

-6.3 -0.0 100.4 -21.9 -1.9

T2

T4

681

III

RI PT

T1

II

SC

I

24

4.21

0

0

-39 -3.9

-0.1

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT Highlights

1. Tidal flat reduction enhanced up-estuary sediment transport 2. Landward sediment enhanced due to increased Eulerian velocity and tidal pumping

AC C

EP

TE D

M AN U

SC

4. Reduction of tidal flat areas lessened the erosion

RI PT

3. Tidal flat reduction impacted spatial and temporal SSC distribution in estuary