Accepted Manuscript
Effects of cluster land reclamation projects on storm surge in Jiaojiang Estuary, China
Zhi-lin Sun, Sen-jun Huang, Jian-ge Jiao, Hui Nie, Mei Lu
PII: S1674-2370(17)30025-X DOI: 10.1016/j.wse.2017.03.003 Reference: WSE 86
To appear in: Water Science and Engineering
Received Date: 11 April 2016
Accepted Date: 22 October 2016
Please cite this article as: Sun, Z.-l., Huang, S.-j., Jiao, J.-g., Nie, H., Lu, M., Effects of cluster land reclamation projects on storm surge in Jiaojiang Estuary, China, Water Science and Engineering (2017), doi: 10.1016/j.wse.2017.03.003.
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. ACCEPTED MANUSCRIPT Effects of cluster land reclamation projects on storm surge in Jiaojiang Estuary, China
Zhi-lin Sun a,*, Sen-jun Huang a, Jian-ge Jiao a, Hui Nie b,*, Mei Lu c
a Ocean College, Zhejiang University, Hangzhou 310058, China b Science and Technology Department, Zhejiang University of Water Resources and Electric Power, Hangzhou 310018, China c Zhejiang Province Ocean and Fisheries Bureau, Hangzhou 310007, China Received 11 April 2016; accepted 22 October 2016 Available online
Abstract Variations in coastline geometry caused by coastal engineering affect tide, storm surge, and storm tide. Three cluster land reclamation projects have been planned to be conducted in the Jiaojiang Estuary during the period from 2011 to 2023, which will cause significant changes in coastline geometry. In this study, a surge-tide coupled model was established based on a three dimensional finite-volume coastal ocean model (FVCOM). A series of numerical experiments were carried out to investigate the effects of variations in coastline geometry on tide, storm surge, and storm tide. This model was calibrated using data observed at the Haimen and Ruian gauge stations and then used to reproduce the tide, storm surge, and storm tide in the Jiaojiang Estuary caused by Typhoon 9711 (Winnie) in 1997. Results show that the high tide level, peak storm surge, and high storm tide level at the Haimen Gauge Station increased along with the completion of reclamation projects, and the maximum increments caused by the Third Project were 0.13 m, 0.50 m, and 0.43 m, respectively. The envelopes with maximum storm tide levels of 7.0 m and 8.0 m inside the river mouth appeared to move seaward, with the latter shifting 1.8 km, 3.3 km, and 4.4 km due to the First Project, Second Project, and Third Project, respectively. The results achieved in this study have a valuable contribution to reducing the effects of, and preventing storm disasters after the land reclamation in the Jiaojiang Estuary.
Keywords: Cluster land reclamation; Coastline geometry variation; Storm surge; Jiaojiang Estuary; Typhoon Winnie
1. Introduction Coastal development coupled with land shortages has MANUSCRIPT recently assumed importance owing to the booming economy in China. Coastline geometry in an estuary is often affected by coastal evolution and development. Coastal development and variations in coastal geometry, and nearshore topography also have an inevitable effect on tide and storm surge (Guo et al., 2009; Tao et al., 2011). Storm surge, characterized as a sudden rise or decrease in water level, could be attributed to exceedingly strong winds and dramatically declined pressure. Additionally, in certain locations, apart from factors such as exact position and timing of a landfall, minimal central pressure and radius to maximal wind, the degree of damage caused by storm surge is also related to coastal geometry and subaqueous topography. Therefore, both storm surge and storm tide levels (the height of the water level) change with geographical position and coastline geometry, which also differ from time and space (Debernard and Røed, 2008; Weisberg and Zheng, 2008). As it is dominated by the effects of inverse barometric pressure (Zheng et al., 2013), storm surge becomes severe in coastal communities, bringing with strong inshore winds and shoaling effects, and becoming dangerous when it advances into an estuary owing to the effects of energy convergence (Goudeau and Conner, 1968). In view of this, regardless of the effects of alterations to the coastline, a considerable amount of research on storm surge simulation in estuarine zones during typhoons has been conducted, such as the research carried out on the Marle-Pamlico Estuary System (Peng et al., 2004), the Tuross EstuaryACCEPTED (Drewry and Newham, 2009), and open/closed estuaries in South Africa (Riddin and Adams, 2010). Research has also been conducted along the coasts of China, such as the Yangtze Estuary (Duan et al., 1998; Hu et al., 2007; Xu et al., 2014), the Yellow River Estuary in Bohai Bay (Li et al., 2011), and Hangzhou Bay (Nie et al., 2012). However, coastal evolution or development has an obvious impact on storm surge as well as storm tide (Xie et al., 2007). Thus, the effects of coastline geometry variations on tide, storm surge, and storm tide in estuarine zones are worth studying. ————————————— This work was supported by the National Nature Science Foundation of China (Grant No. 40776007), and Projects founded by Zhejiang Province Science and Technology Department (Grant No. 2009C03008-1). * Corresponding author. E-mail addresses: [email protected] (Zhi-lin Sun), [email protected] (Hui Nie).
As far as a specific location is concerned,ACCEPTED its local coastline MANUSCRIPT geometry plays an important role in the generation of storm surges. Thus, land reclamation projects in estuarine zones make it difficult to study the characteristics of storm surge and storm tide. Bohai Bay, for instance, is located in the northern part of the Yellow River Estuary, and Laizhou Bay lies in the southern part. One concern is that the growing Yellow River Delta due to evolution and reclamation is decreasing the maximum storm tide level (MSTL) at the top of Bohai Bay with cold-air outbreaks (i.e., intrusions of an extremely cold polar air mass entering the middle-low latitudes bring strong winds); in contrast, the MSTL appears to be enhanced in Laizhou Bay (Zhao and Jiang, 2011). Thus, the effects of coastal geometry variation on the MSTL seem to be diverse even in the same estuary. Even worse, due to land reclamation, variations in coastline geometry can have an negative effect on coastal disaster prevention in terms of flood drainage and storm surge intrusion (Wang and Cheng, 2002; Wang et al., 2014). The central and southern parts of the Zhejiang coast is prone to typhoons and, even worse, the frequency and intensity of these typhoons are causing an increasing probability in landfall coupled with climate change (Sun et al., 2014a). Significant changes in coastline geometry have also taken place due to land reclamation in the central and southern parts of the Zhejiang coast over recent years. Unfortunately, hardly any research could be found to clarify the effects of the variation of coastline geometry on tide, storm surge, and storm tide in this region. In this study, a series of numerical experiments were carried out in order to investigate the distribution of MSTLs in the vicinity of the reclamation area during different stages of various reclamation projects. This study aimed to provide useful references for disaster management, and prevention and mitigation of the effects of storms after reclamations during the typhoon period.
2. Background
Three cluster land reclamation projects in the Jiaojiang Estuary and its adjacent areas have been planned to be conducted for the purpose of increasing economic growth. These reclamation projects would lead to significant changes in the estuary’s coastline geometry and aggravate the effects of destruction. In this study, the effects of the cluster land reclamation projects on tide, storm surge, and storm tide during MANUSCRIPTTyphoon Winnie were investigated. 2.1. Study area
The Jiaojiang Estuary, the second largest estuary in Zhejiang Province, is located at the end of the Jiaojiang River, which flows through Taizhou City. It is a tidal estuary, which creates an interface between the Jiaojiang River and Taizhou Bay. Taizhou City itself has a thriving economy but suffers from a lack of land resources. The best way of easing this problem is to take advantage of the tideland by engaging in land reclamation projects. Numerous islands and a large area of tideland are located outside the entrance of the Jiaojiang Estuary and in the Puba Harbor Basin, creating favorable conditions to land reclamation projects.
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Fig. 1. Schematic representation of track of Typhoon Winnie and study area (yellow line in left figure indicates track of typhoon, and orange dots indicate observed locations). The geometry of the Jiaojiang River is distinctive, opening outwards like the mouth of a horn. Along the Shanjiaopu (SJP, the pink circle in Fig. 1)-Sanjiangkou (SJK, the yellow circle in Fig. 1) section, the width of the river narrows rapidly upstream towards Niutou Necking (NTN, the red circle in Fig. 1). The width is about 5 km at the cross section of SJP but decreases to 0.85 km at NTN. It then expands to 2 km in the NTN-SJK section. The width of the cross section at NTN is the smallest at the SJK-SJP section. Undoubtedly, the features of this terrain make the storm surges and MSTLs more serious and more susceptible to variations in coastline geometry, enhancing the destructive potential of typhoons. Study on the effects of variations of coastline geometry is therefore very necessary. It is hoped that the results will be of great value in order to reduce and prevent storm disasters and will also provide valuable references for the monitoring and forecasting of the development of MSTLs along the coast after land reclamation projects have been conducted and residential areas have been createdMANUSCRIPT in consequence. 2.2. Cluster land reclamation projects Three cluster land reclamation projects are planned to be carried out by combining existing tideland and islands outside the mouth of the Jiaojiang Estuary (Fig. 2) during the period from 2011 to 2023. The First Project includes almost the whole coastal area outside the mouth of the Jiaojiang Estuary and consists of several sub-reclamation regions with a total area of about 130.5 km 2. The Second Project is conducted within a total area of 171.4 km 2 based on the First Project. During a later phase, the Third Project is carried out in a small area inside the estuary entrance of about 5.5 km 2. In this paper, the original coastline geometry before land reclamation is marked as Old. The coastline geometries of the First Project, Second Project, and Third Project are marked as FP, SP, and TP, respectively. Cluster land reclamation of each project is being carried out in many regions simultaneously. Obvious changes in coastline geometry are thus expected to take place, causing corresponding changes in tide, storm surge, and storm tide.
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Fig. 2. Locations of three cluster reclamation projects. 2.3. Topographic data
In this study, the topographic data, which were measured over the period from 2009 to 2012, were provided by the Navigation Guarantee Department of the Chinese Navy Headquarters. The nearshore data were measured over the period from 2011 to 2012. The runoff discharge data during Typhoon Winnie were collected from the Taizhou Water Resources Bureau. The influence of the rise in the sea level on MSTLs has not been considered in this study because the average annual rate of the rise in the sea level was about 2.7 mm per year in coastal areas of Zhejiang Province (Sun et al., 2014a), which can be ignored. 2.4. Typhoon Winnie
Typhoon Winnie was chosen in this study to provide a windMANUSCRIPT and atmospheric pressure gradient field for the whole simulation domain. Typhoon Winnie was a typical tropical cyclone, which made landfall at the central area of the Zhejiang coast. Considered as one of the most destructive marine disasters, Typhoon Winnie originated in the Western Pacific on August 10, 1997. It subsequently moved northwestward with a growing intensity and caused a tremendous loss of life and property in the Philippines, Japan, and Taiwan during its journey. According to the record of China Meteorological Administration (CMA), the maximum wind speed of Typhoon Winnie rose to 60 m/s with the minimal central pressure declining to 920 hPa. It made a landfall in Shitang Town of Wenling City, in Zhejiang Province, on August 18, 1997. The position of the landfall of Typhoon Winnie was about 50 km away from the Haimen Gauge Station. Its maximum wind speed reached 40 m/s and minimum central pressure declined to 944 hPa at the landfall location (Zhong and Zhang, 2006). Though Typhoon Winnie was not the strongest typhoon attacking the Zhejiang coast, its effects were catastrophic. It damaged 776 km of seawall along the Zhejiang coast, causing an economic loss of RMB 26.7 billion yuan. In addition, it produced an MSTL with a return period greater than one thousand years at the Haimen Gauge Station (SOA of China, 1998). 3. Methods ACCEPTED 3.1. Numerical model
The finite volume coastal ocean model (FVCOM), widely known as an unstructured-grid, primitive-equation and three-dimensional model (Chen et al., 2003), was used to reproduce the storm surge for the tide-surge interaction in this study. Its unstructured grid appears to be beneficial to irregular coastline geometry. FVCOM has been widely used in simulations of coastal ocean circulation (Chen et al., 2007; Xue et al., 2009) and storm surge (Aoki and Isobe, 2007; Weisberg and Zheng, 2006; Rego and Li, 2010; Yoon et al., 2014). With Boussinesq and hydrostatic approximations, the primitive equations governing momentum and mass conservation in the sigma coordinate are shown as follows:
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∂uD ∂ uD2 ∂ uvD ∂ u ω A∂CCEPTED(η − η ) MANUSCRIPT + + +−=−fvD gD a − ∂∂∂∂t x y σ ∂ x (1) gD∂0 ∂∂∂ D 1 u D ρ′dσ ′ + σρ ′ +K + DF ∂∫ ∂∂∂m u ρ0 xσ x D σ σ ∂vD ∂ uvD ∂ v2 D ∂ v ω ∂(η − η ) + + + +fuD =− gD a ∂∂∂∂t x y σ ∂ y (2) gD∂0 ∂∂∂ D 1 v −D ρ′dσ ′ ++ σρ ′ K + DF ∂∫ ∂∂∂m v ρ0 yσ y D σ σ ∂∂η uD ∂ vD ∂ ω + + += 0 (3) ∂t ∂ x ∂ y ∂ σ where u , v , and ω are the velocity components in the x , y , and σ directions, respectively; t is time; f is the ′ Coriolis parameter; ρ0 and ρ are reference and perturbation densities, respectively; D is the total water depth and = + D h η , where η and h are the surface elevation and reference depth below mean sea level, respectively; ηa denotes the sea level displacement induced by the ‘inverse barometer effect’; g denotes the gravitational acceleration; ′ σ is the derivative of σ ; Km denotes the vertical eddy viscosity coefficient, which is calculated using a turbulent closure scheme with a background mixing of 0.0001; Fu and Fv denote the horizontal momentum diffusion terms in the x and y directions, respectively; and the horizontal diffusion coefficient is set to be 0.2, and the number of the vertical level is 11. The surface and bottom boundary conditions for u , v , and ω at σ = 0 are written as ∂u ∂ v D ,= ()τ , τ ∂ ∂ sx s y σ σ ρ0 Km (4) ω = 0 The surface and bottom boundary conditions for u , v , and ω at σ = − 1 are written as ∂u ∂ v D , = ()τ , τ ∂σ ∂ σ ρ K bx b y MANUSCRIPT (5) 0 m ω = 0 where (τsx,τ s y ) and (τbx,τ b y ) are components of surface wind and bottom stress and roughness in the x and y directions, respectively. The initial condition for surface elevation and flow is zero, which can be expressed as ξ =u = v = 0 (6) The tidal elevations on the open boundary are calculated by harmonic analysis using the following equation. Eight constituents are considered K 1, O 1, P 1, Q 1, M 2, S 2, N 2, and K 2. P− P =b a + ++−() ξ ∑ fhiiicos Wt UVi G i (7) ρ0 g where i is the number of different constituents; Pb is the atmospheric pressure outside the storm; Pa is the ( + ) atmospheric pressure at the open boundary; Wi is the radian frequency; U V i is the initial phase; fi is the node factor for each constituent; and hi and Gi are the amplitude and phase angle of each tidal constituent, respectively. For the upper boundary,ACCEPTED the observed time series of runoff during Typhoon Winnie is used (Fig. 3). Due to the lack of runoff data, the runoff is given just from 20:00 on August 17 and 10:00 on August 20. The moment of the landfall made by Typhoon Winnie shown in Fig. 3 is 25 h, i.e., 21:00 on August 18.
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Fig. 3. Time series of runoff discharge given at upper boundary. 3.2. Surface and bottom stress
The surface wind stress is calculated using the surface wind field model, the Fujita & Takahashi model (Sun et al., 2015). This model has been proved to be applicable to the simulation of storm surge along the Zhejiang coast (Sun et al., 2015; Sun et al., 2014b). The surface wind stress is then computed from =2 + 2 (τsxy, τ s) C dsaρ w xyxy w( ww , ) (8) where wx and wy are the velocity components in the x and y directions, respectively, which are derived from the Fujita & Takahashi model; ρa is the air density; and Cds is the surface drag coefficient. Wind velocity and pressure are calculated by P− P 1 r 0 =1 − 2R≤ r ≤ ∞ (9) − + PP∞ 0 1 rR / P− P 1 r 0 =1 − 0≤r < 2 R (10) − 2 P∞ P 0 1+ 2 ()r R uuv v v r− R =+ −π − ≤< WCVV1 ()x i y j exp CF2 1 02 rR (11) 4 R uuv v v π r− R MANUSCRIPT W=CVV() i + j exp − − CFRr 2 ≤≤∞ (12) 1 x y 2 2 4 R where P0 is the pressure at the typhoon center; Pr is the pressure away from the typhoon center with a distance of r; P∞ is the pressure far away from the typhoon center,uuv which is considered as a constant; R is the radius to maximal wind (RMW) with a value of 45 km in this study; W is the combined speed; V and V are the components of v v x y wind mobile speed in the x and y directions, respectively; i and j are unit vectors of wind speed in the x and y directions, respectively; C1 and C2 are the coefficients with values of 1.0 and 0.6, respectively; and F1 and F2 are gradient wind fields, which can be calculated as follows:
− 3 2 − 2 2 r r f f2()P∞ P r =−++3 0 +() + F1 102 12 2 Ai B j (13) 2 4 ρa R R
2 − r r f f ()P∞ P F=−++10 3 0 () Ai + B j (14) 2 2 4 ()+ 2 ρa r1 rR R where x0 and y0 are theACCEPTED coordinates of the typhoon center; xr and yr are the coordinates at the r distance to =− +− =− −− typhoon center; and A( x x 0 )sinθ (y y 0 )cos θ and B( x x 0 )cosθ (y y 0 )sin θ , where θ is the gradient angle.
The surface drag coefficient Cds is dependent on the wind velocity Vw and is determined by the following equation (Large and Pond, 1981):
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≤ACCEPTED MANUSCRIPT 0.0012Vw 11.0 =+ ×−3 <≤ Cds()0.49 0.065 V w 10 11.0 V w 25.0 (15) ()+××−3 > 0.49 0.065 25.0 10Vw 25.0 The bottom stress is determined by flow velocity and can be illustrated as =2 + 2 () (τbx, τ b y ) C dbρ 0 u v uv , (16) where the bottom drag coefficient Cdb is determined by matching a logarithmic bottom layer to the model at a height of the first σ level above the bottom, as shown in
2 = k Cdb max , BFRIC (17) ()+ 2 ln 1 σkb− 1D z 0 = where k is the Carmen constant and k 0.4 ; z0 is the bottom roughness parameter, which is 0.001 m in this study;
BFRIC is the minimum value for Cdb , which is 0.0025 in this study; and σkb− 1 is the vertical level next to the bottom. 3.3. Simulation region and nearshore grid
The vertical datum used in this study is the China 1985 National Altitude Datum, which is defined by the mean sea level at the Dagang Tide Station, Qingdao. The simulated domain in this study covers the area of 120.1°E-124.2°E, and 26.4°N-31.0°N, as shown in Fig. 4. The nearshore water depth of most of the area is less than 10 m along the Zhejiang coast and there are shallow areas in the Jiaojiang Estuary with a depth of less than 2 m. The triangular grid size is set to about 300 m nearshore as plotted in Figs. 5(a) and (b), and gradually increases to about 1000 m at the open boundary. The node number in the mesh is 31507 and the element number is 61128. The time step for the simulation is set to 10 seconds.
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Fig. 4. Simulated domain in this study. ACCEPTED
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MANUSCRIPT Fig. 5. Grid structure of simulated domain. 3.4. Storm surge calculation
Storm tide level ( ζ ) is usually defined by the sum of tide level ( ζ T ), pure storm surge ( ζ S , sea level change = + + caused only by atmospheric forcing), and surge-tide interaction ( ζ I ) (Banks, 1974), i.e., ζ ζT ζS ζ I . Thus, the = + =− storm surge considering the tide-surge interaction ( ζ SI ) can be derived from ζ and ζ T as ζSI ζ S ζ I ζ ζ T . ζ and ζ T can be obtained through two control simulations. ζ is generated through simulation considering atmospheric forcing and tidal forcing. ζ T is generated through simulation under the effect of tidal forcing only. 3.5. Model calibration
Two observation stations are chosen to test the performance of the model: the Haimen Gauge Station (O1 in Fig. 4) and the Ruian Gauge Station (O3 in Fig. 4). The storm tide elevations, storm surge, and tide level between 20:00 on August 17 and 10:00 on August 20 are modeled to correspond with the time series runoff discharge. For the purpose of calibrating the numericalACCEPTED model, a comparison between the modeled results and observed data at Haimen and Ruian gauge stations are plotted in Fig. 6 and Fig. 7, respectively. In general, the modeled results agree well with the observed data. Wilmott’s goodness of fit R2 and skill score S (Wilmott, 1981) are introduced to assess the performance of the model, which can be expressed as N ()− 2 ∑ mi o i 2 = − i=1 R 1 N (18) − 2 ∑()mi m i i=1
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N ACCEPTED MANUSCRIPT − 2 ∑ mi o i S =1 − i=1 (19) N 2 − + − ∑()mi o i o i o i i=1 where N is the number of data; oi is the observed data at time step i; oi is the mean value of the observed data; mi is the modeled result at time step i; and mi is the mean value of the modeled result. The goodness of fit value and skill value of 1 indicate a perfect fit between the modeled results and the observed data.
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Fig. 6. Comparison of modeled results and observed data at Haimen Gauge Station between 20:00 on August 17 and 10:00 on August 20.
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Fig. 7. Comparison of modeled results and observed data at Ruian Gauge Station between 20:00 on August 17 and 10:00 on August 20.
The skill score has been widely used in model calibration MANUSCRIPTfor estuarine and coastal zones (Li et al., 2005; Warner et al., 2005; Vaz et al., 2009). At the Haimen Gauge Station, the goodness of fit of tide level, storm surge, and storm tide level are 0.976, 0.653, and 0.972 and the skill scores are 0.991, 0.911, and 0.992, respectively. At the Ruian Gauge Station, the goodness of fit of tide level, storm surge, and storm tide level are 0.992, 0.677, and 0.985 and the skill scores are 0.994 0.921, and 0.990, respectively. The numerical model applied in this study thus performs reasonably.
4. Results 4.1. Effects of coastline geometry variation on tide and storm surge processes
Two observation points are chosen to analyze the effects of coastline geometry variations on tide, storm surge, and storm tide inside and outside the estuary. Haimen is inside the Jiaojiang river mouth (marked as O1) and O2 is outside the river mouth at a depth of 11.5 m (Fig. 4). The original tide, storm surge, and storm tide for case Old and their differences before and after each reclamation project at O1 and O2 are plotted in Figs. 8 and 9. Tide, storm surge, and storm tide processes on the coastline for case Old correspond to the left y axis and time series differences correspond to the right y axis. FP-Old, ACCEPTEDSP-Old, and TP-Old are the differences in tide level, storm surge level, and storm tide level between each reclamation project and Old, respectively; Differences greater than zero indicate that the reclamation project has positive effects on tide and storm surge processes while that lower than zero indicate the negative effects. Through comparison of Figs. 8 and 9, it can be seen that variations in the coastline geometry mainly affect the tide, storm surge, and storm tide inside the river mouth rather than outside. Moreover, coastline geometry variation has positive effects on the high tide level, peak surge, and high storm tide, potentially intensifying the disaster caused by storm surge in the estuary. For the tide process at O1 (Fig. 8(a)), the FP and SP have almost the same effect on the tide level, increasing the high tide level by 0.05 m (29 h). The TP increases the high tide level by 0.13 m. With regard to the storm surge process
10 at O1, it becomes obvious that variationsA CCEPTED in coastline geometry MANUSCRIPT affect the peak storm surge at the moment when Typhoon Winnie makes the landfall (25 h), as shown in Fig. 8(b). The peak storm surge increases by 0.35 m due to the FP, by 0.44 m on the coastline for the SP, and 0.50 m on the coastline for the TP. Storm tide, as a combination of tide and storm surge, is also affected significantly by variations in coastline geometry, especially between 21 h to 27 h when the increments in storm tide levels are greater than 0.20 m, as shown in Fig. 8(c). The FP and SP cause the storm tide level to increase to its maximum by 0.27 m and 0.47 m at 23 h, respectively. The TP causes a maximum increment of 0.43 m at 26 h. Although the maximum increments in storm tide level do not appear along with the high tide level, it is still notable that the FP, SP, and TP cause increments of 0.08 m, 0.14 m, and 0.17 m, respectively, at high tide levels. Variations in coastline geometry have little effect on tide, storm surge, and storm tide levels outside the river mouth, as shown in Fig. 9. With regard to the tides at O2, the changes in tide levels as well as storm tide levels due to FP, SP, and TP are within 0.03 m. In addition, the changes in storm surge level are less than 0.04 m.
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Fig. 8. Differences in tide level, storm surge level, and storm tide level between each reclamation project and Old at O1.
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Fig. 9. Differences in tide level, storm surge level, and storm tide level between each reclamation project and Old at O2.
4.2. Effects of variations in coastline geometry on maximum storm tide level envelope
Generally speaking, the MSTL during typhoon period is a key factor based on which the probability of overtopping of the seawall can be assessed, in order to predict andMANUSCRIPT prevent storm disasters. The MSTL envelopes of the study area corresponding to coastline geometries for Old, FP, SP, and TP are plotted in Fig. 10. As the coastline geometry changes, the envelopes with an MSTL of 7.0 m and 8.0 m move seaward. That is to say, the MSTLs inside the river mouth increased due to the reclamation projects. Specifically, envelopes with an MSTL of 8.0 m shift 1.8 km (FP), 3.3 km (SP), and 4.4 km (TP) seaward compared with that of the Old. The MSTL at the Haimen Gauge Station exceeds 8.0 m due to the TP. Envelopes with an MSTL of 7.0 m shows a distinct seaward movement along the north bank and a slight movement along the south bank, as shown in Figs. 10(c) and (d). Based on the envelope with an MSTL of 7.0 m on the coastline for case Old, it moves 5.8 km along the north bank on the coastline for the SP and 6.0 km on the coastline for the TP.
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Fig. 10. Maximum storm tide level envelopes under different coastline geometries. 5. Discussion MANUSCRIPT The findings presented above are completely new and demonstrate that the effects of the different reclamation projects (i.e., FP, SP, and TP) on tide, storm surge, and storm tide are diverse in the Jiaojiang Estuary. Storm tide levels are at water level height as a result of the nonlinear interaction of tide and storm surge during storms, as has been illustrated in many research papers (Bernier and Thompson, 2007; Antony and Unnikrishnan, 2013). Because of the tide-surge interactions, storm tide levels are not equal to a simple superposition of astronomical tide and pure storm surge. It is therefore reasonable to consider tide-surge interaction in the simulation of storm tide levels in this study. Although storm tide levels are widely considered to be affected by strong circulatory wind, offshore islands, river discharge, and shallow bathymetry during a typhoon period (Roy, 1995; Roy, 1999), they are also affected by variations in coastline geometry as we can see from the results in this study. Tide, storm surge, and storm tide inside the river mouth are all affected by variations in coastline geometry, which can mainly be attributed to the changes in the flow field after the implementation of reclamation projects. The average vertical velocity fields of current for rising tide (26 h) corresponding to the coastline geometries for the Old, FP, SP, and TP are plotted in Fig. 11.ACCEPTED The landward flow along different coastlines is divided into two parts, one entering the estuary and the other moving southward along the coast. However, the flow fields for different coastline geometries due to land reclamation projects have different characteristics. Seen from the coastline geometry of the Old, the main flow enters the mouth along the north bank with a maximum velocity of 2.31 m/s (Fig. 11(a)). After the FP, the main flow moves along the middle of the river at a maximum velocity of about 2.52 m/s (Fig. 11(b)). With regard to the SP, the main flow still moves along the middle of the river mouth, while the velocity has a more uniform distribution than that for the Old and FP, with a maximum value of 2.24 m/s. In addition, the construction of Toumen Harbor has an influence on the nearshore current along the north coast. The TP narrows the mouth width based on the SP, increasing the velocity of flow into the mouth.
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If we divide the cross section of the AriverCCEPTED mouth along MANUSCRIPT the different coastlines into 50 equal sections with a width of ∆l , the flow discharge into the river mouth at the moment of 26 h can be estimated by 50 50 = = ∆ Q∑ Qi ∑ HVl i i (20) i=1 i = 1 where Q is the total discharge into the river mouth, Qi is the discharge of current at section i, Hi is the water depth at section i, and Vi is the vertical average velocity at section i.
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Fig. 11. Average vertical velocity fields of current for different coastline geometries.
Along the coastline for case Old, the quantity of flow discharge into the estuary is about 18000 m3·s–1, while along the coastline for the FP, there is a larger quantity of about 23000 m3·s–1 flowing into the river mouth when compared with that of the Old. After the SP, although the velocity of flow into the river mouth is reduced due to the construction of Toumen Harbor, which widens the river mouth (Fig. 11(c)), the flow discharge still increases to 31000 m3·s–1 with a larger depth and width of river mouth. After the completion of the TP, the river mouth becomes narrower, causing an increase in flow velocity.ACCEPTED As a result, the discharge increases to about 35000 m3·s–1 and the tide, storm surge, and storm tide are all affected by the variations in coastline geometry due to changes in the flow field in different reclamation projects. It should be noted that the results presented in this study were achieved based on the topographical data measured during the period from 2009 to 2012. That is to say, the bathymetry changes nearshore after land reclamation projects have not been considered in this study due to the lack of bathymetry data. Coastal evolution due to coastal dynamic factors will continue. They may raise the seabed elevation in the vicinity of the reclamation area and thus enhance storm surges as well as storm tide levels (Yin et al., 2004; Nicholls et al., 2007). The storm track and intensity can also
14 affect the results. Further study will be A carriedCCEPTED out using MANUSCRIPT the latest bathymetry data from another land reclamation project for different typhoons. 6. Conclusions
In order to clarify the influences of cluster land reclamation projects, which are planned to be constructed in the Jiaojiang Estuary and its adjacent areas, on tide, storm surge, and storm tide, a set of numerical experiments have been carried out to investigate the responses of tide, storm surge, and storm tide to variations in coastline geometry based on FVCOM. The model performs well in terms of reproducing tide, storm surge, and storm tide during the period when Typhoon Winnie affected the Jiaojiang Estuary. The modeled results agree well with the observed data at the Haimen and Ruian gauge stations. Inside the estuary, the high tide level, peak storm surge, and high storm tide level at the Haimen Gauge Station increase due to the reclamation projects, where the maximum increments reach 0.13 m, 0.50 m and 0.43 m. However, there is little difference in terms of tide, storm surge, and storm tide at points observed outside the river mouth before and after the reclamation projects. MSTLs inside the river mouth increase as a result of the reclamation projects, showing an obvious seaward shifting for envelopes with MSTLs of 7.0 m and 8.0 m inside the river mouth. Importantly, the envelope with an MSTL of 8.0 m appears to move 1.8 km, 3.3 km, and 4.4 km due to the FP, SP, and TP, respectively. The reclamation projects have changed the flow field in the estuary, thereby affecting tide, storm surge, and storm tide inside the river mouth itself. The reclamation projects increase the flow velocity and flow discharge into the river mouth. As a result, the high tide level and peak storm surge could be enhanced. This study is based on the same bathymetry for different reclamation projects and the typhoon track of Winnie. Therefore, further study is necessarily to be conducted when new bathymetry after the reclamation projects is available for different typhoons. Acknowledgments
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