PROMOTING SILTATION EFFECTS AND IMPACTS OF HENGSHA EAST SHOAL ON THE YANGTZE RIVER ESTUARY*

XIE Jun

College of Harbor, Coastal and Offshore Engineering, Hohai University, Nanjing 210098, China

Shanghai Waterway Engineering Design and Consulting Co., Ltd, Shanghai 200120 China, E-mail: xiejunyh@163.com

YAN Yi-xin

College of Harbor, Coastal and Offshore Engineering, Hohai University, Nanjing 210098, China

PROMOTING SILTATION EFFECTS AND IMPACTS OF HENGSHA EAST SHOAL ON THE YANGTZE RIVER ESTUARY*

XIE Jun

College of Harbor, Coastal and Offshore Engineering, Hohai University, Nanjing 210098, China

Shanghai Waterway Engineering Design and Consulting Co., Ltd, Shanghai 200120 China, E-mail: xiejunyh@163.com

YAN Yi-xin

College of Harbor, Coastal and Offshore Engineering, Hohai University, Nanjing 210098, China

For the Hengsha East Shoal Promoting Siltation Project in the Yangtze River Estuary, this work developed a formula for calculating sediment carrying capacity based on variable coefficients and amathematicalmodel for suspended sedimentation using variable saturation recovery coefficients. Not only does themodel yield good verification, preferably give the hydrodynam ics and the sediment concentration, but also reach a good agreement between the simulation results and themeasured topographical changes in the promoting siltation zone of the Hengsha East Shoal. Moreover, this article proposes amethod for exam ing the net sediment transportation flux by tide for studying the effects of the project of promoting siltation and comparatively analyzes the current siltation of the Hengsha East Shoal and siltationmouth layouts, entrance w idths, entrance bottom elevations, and the impact of the South Main Dike on promoting siltation through calculating the net sediment transportation flux by tide over the fixed bed and the directmovable bed numerical simulations. The results are in good agreement with each other, indicating that themethod for calculating the net sediment transportation flux by the tide is also useful for assessing the project in promoting siltation. Finally, we use themodeling results to analyze the water and sediment diversion ratios and the changes in silting and scouring in riverbeds and draw the conclusion that the completion of the Hengsha East Shoal Promoting Siltation Project would not adversely affect the Yangtze River Estuary Deepwater Channel Project.

Hengsha East Shoal, promoting siltation effects,mathematical simulation, sediment diversion ratio, net sediment transportation flux

Introduction

Based on a general survey ofmany coastal countries and regions in the world, to address the conflict between the rapid population grow th and economic expansion, and the acute shortage of land, successive reclamation projects have been implemented and, in general, extraordinary achievements have beenmade from the reclamation projects, especially those integrating with such projects as estuary improvement, channel dredging, environmental protection and flood control and disaster reduction, inmost of such countries and regions and such integrating projects furthermore have achieved the coordination and balance between human race and nature, and protection and development[1].

As the estuary of the Yangtze River, the third longest river in the world, the Yangtze River Estuary is located on the east coast of China, near the junction between the East China Sea and the Yellow Sea (see Fig.1). W ithin the Yangtze River Estuary, rich sediment resources create favorable external conditions for the formation and siltation of largemudflats. However natural siltation rates fall far the short of the land needs within the Yangtze River Estuary tidal flats. As a result, the area of the occupied arable lands is twice that of the land reclaimed from the sea[2]. Most of themedium and high level beaches facing the Yangtze River Estuary have been reclaimed. In 2004, Shanghai beach area with the surface elevation above Wusong 0m was 833 km2and that above Wusong –5m was2 943 km2[3]. The focus has now turned to gradually transferring promoting siltation projects to embankments on low beach faces. Thus, improving the results of the low tideland reclamation projects in promoting siltation has become a critical research issue.

Fig.1 Location of the Hengsha East Shoal Promoting Siltation Project

As one of themost important tidelands at the front of the Yangtze River Estuary, the Hengsha East Shoal is located at the eastern end of Hengsha Island and is in a tongue-shaped distribution along the westeast direction. The North Passage of the Yangtze River Estuary is located to its north, while the Yangtze River Estuary Deepwater Channel Project in the North Channel is located to its south. In 1998, before the implementation of the Yangtze River Estuary Deepwater Channel Project, the Hengsha East Shoal was still evolving through natural processes. At that time,most of the beach faces were flat and basically stable with the exception of a few beach faces that developed as alternating north-south trenches and had an elevation constantlymaintaining between –1.0m and +1.0m[4]. These areas are important formedium and large scale promoting siltation and embankments projects planned by the City Government of Shanghai. Since the Yangtze River Estuary Deepwater Channel Project was implemented in 1998, tidal currents over the beach faces of the Hengsha East Shoal have been reduced and, to a certain extent, blocked by a north guiding jetty in such amanner that an environment beneficial to the sediment accumulation has been formed[5]. The north guiding jetty plays a role in branch blocking, sediment trapping, and flow guiding. At present, the promoting siltation and reclamation projects already completed and still in process in the Hengsha East Shoal and the Hengsha Shoal include: the Hengsha East Shoal Promoting Siltation Project Phase I and II, the N23Submerged Embankment Project and the Hengsha East Shoal Reclamation Project Phase III. These projects promote sedimentation over a total of area of 8 170 ha (see Fig.2). The Municipal Government of Shanghai has planned to reclaim the Hengsha East Shoal for another 3 660 ha of lands and another 5 780 ha of sedimentation promoted areas on the basis of the as-built promoting siltation works and to construct the third phase and post-phase promoting siltation zones during the 11th Five-year Plan period. The shoal is one of the key development areas where promoting siltation and reclamation w ill be performed by the City of Shanghai in the future. Due to the difficulties encountered in the development conducted for the implemented projects to guarantee the subsequent developments can proceed smoothly, it is necessary to conduct an in-depth exploration of such issues as plane layout, etc. related to future promoting siltationmeasures.

Fig.2 Current situation of the promoting siltation and reclamation projects of Hengsha East Shoal

The use of empirical formulae,modeling, and a combination of the two are themethodsmost often used to study the implementation of promoting siltation programs inmudflats. For example, Liu Jia-ju and Yu Guo-hua have applied the formula

where t0is the time required for promoting siltation, 0γ is the deposits dry bulk density, H1and H2represent the beach face depths before and after promoting siltation, respectively, α is the sediment settling probability, ω is the sediment flocculation settling velocity, S1and S2are the sediment concentrations during a certain interval before and after promoting siltation, respectively, as indicated by S= 0.0273γs/ gH . Here γsis the bulk density of sediment particles, Vbtrepresents themean tidal current velocities during a certain interval, and Vwis themean vibration velocity of fluctuating water particles. In studying combined empirical formulae andmodelingmethods, tidal current and wavemodels are used to calculate dynam ic field variations within the promoting siltation zones. The promoting siltation capacity is then estimated using an empirical formula[6]. In contrast, a great deal of numerical simulations of hydrodynamics and sedimentmovement have been applied in the studies of various coastal and estuarine problems[7,8], which have also been involved in studying the effect of promoting deposit[9]. Themodelingmethods directly applymathematical or physicalmodels formovable beds in order to examine promoting siltation. In order to apply empirical formulae,multiple factors have to be determined. In particular, the artificial estimation of the dynam ic changes within the promoting siltation zonesmust be determined empirically[10]. Empirical formulae, which typically work under the assumption that beach faces are stable under natural conditions, cannot be used to verify natural erosion and deposition changes in such beach faces when no projects are underway. The combination of empirical formulae andmodeling offers several advantages, including the ability to predict dynamic changes undermultiple scenarios. However, themethod still cannot be used to verify natural erosion and deposition changes in beach faces without considering the impact of flow state variation and dynamic distribution on sediment concentration. In contrast, themodelingmethod can only estimate the extent of promoting siltation based on the promoting siltation capacity ofmultiple plane layouts and typically requiresmore effort and time. In this study, we develop amathematicalmodel for suspended sediment based on the variable sediment carrying capacity index and related coefficient reflecting the characteristics of various portions of the Yangtze River Estuary. W ith these results, we present amethod for calculating the net sediment transportation flux by the tide. Thismethod can be used to determ ine the extent to which siltation has been promoted. A good agreement is reached between the results of thismethod and those ofmovable bedmodels. In addition, the water and sediment diversion ratio can be used to assess how promoting siltation in the Hengsha East Shoal affects the surrounding area.

1. Modeling

1.1 Area ofmodeling and governing equations

In order to preferably fit the layouts of the promoting siltation zones of the Hengsha East Shoal and of the Yangtze River Estuary Deepwater Channel Project, themodel employs an arbitrary body-fitted coordinate system[11]. W ithin themodeled area, the upstream area is extended to Xuliujing, Jiangsu Province, while the downstream area is extended to the Lvhua Mountain, with a total distance of 180 km. The northsouth part of themodeling area is extended 95 km from Subeizui, Jiangsu Province to Nanhuizui, Shanghai. The total area included in themodel is 11 200 km2(see Fig.1). The promoting siltation project ismainly intended for natural suspended sediment in water bodies. Thus, our sedimentmodel takes suspended sediments as themain subject of study. Themodel is composed of several equations, including those for tidal current, suspended sedimentmotion and riverbed deformation. The equations for tidal current equations and their boundary conditions could be found in Ref.[11].

1.2 Fundamental equations for sedimentmotion

(1) Transport equation for suspended sediment

where S is the depth-averaged sediment concentration and Fsis the erosion-deposition function, determined by the sediment carrying capacitymethod, given as

where μ is the saturation recovery coefficient andis the settling velocity of suspended sediment.

(2) The riverbed deformation equation is w ritten as

wheresγ is the dry bulk density of suspended sediments andsη is the bed surface elevation.

1.3 Calculations

The finite differencemethod was applied to themathematicalmodel. The equations for tidal currents were solved using the ADI scheme[12], the transport equations for suspended sediment and the riverbed deformation were solved using the explicit difference scheme.

The boundary conditions for tidal level were applied to the upstream and downstream areas in themodel. For the upstream boundary, we used a tidal level process corresponding to the typical annual runoff process at the Datong Station. For the downstream offshore boundary neap, it was assumed thatmean and spring tides cycles of half amonth circulate with each other 24 times and the power generated in this process is equivalent to that of the open sea tide during one year[13].

To determ ine the sediment concentrations of the upstream and north-south boundaries, we used the locallymeasured data under similar tide processes. The sediment concentration in the deepwater area on the boundary adjacent to the east open sea was estimated to be 0.05 kg/m3.

1.4 Parameters and related analysis

1.4.1 Saturation recovery coefficient

As an extremely important factor in the equation for non-equilibrium transport of suspended sediment, the sediment saturation recovery coefficient is a complex factor related to the hydrodynam ic and topographical conditions. Therefore, the coefficient was taken to have different values in different areas and for flood and ebb tides based on the characteristics of the Yangtze River Estuary. In a rising tide, it was assumed that the dispersion of upstream sediments to the open sea was blocked, and as the salt water wedge extended to the upstream, sediments within the rivermouth bar wasmore easily deposited in the waterway. Thus, their saturation recovery coefficient was relatively high. Some river channels, such as the North Branch of the Yangtze River Estuary, have smaller water depths and are not themain runoff discharge channels, in which sediments are easily deposited. Thus, they have slightly higher saturation recovery coefficients. To avoid numericmutations, the saturation recovery coefficients in other waters were obtained using linear interpolation. The values for the saturation recovery coefficients used in themodel ranged from 0.45 to 0.75.

1.4.2 Sediment carrying capacity

The sediment carrying capacity of water current is an important indicator in sediment transport simulation and evolution analysis. In his study on suspended sediment carrying capacity based on Velikanov’s theory of gravity, Zhang Rui-jin collectedmeasured data from the Yangtze and Yellow Rivers and considered the resistance loss of the sink and fluctuating velocity of the experimental results under the turbulence control assumption. He worked out the formula, S= k( V3/ gHω)m, by applying a dimensional ana-lysismethod. In this formula, the sediment carrying capacity index (m) and the coefficient (k) are important parameters related to the sediment carrying capacity. The sediment carrying capacities of various waterways in the Yangtze River Estuary differ from each other to a certain extent due to differences in runoff, tidal current, salinity and sediment characteristics. These indices and coefficients vary with V3/gHω, as was shown by Zhang Rui-jin and Xie Jian-heng. That is, k andm are variables changing with the flow-sediment conditions[14]. Based on the flow-sediment characteristics of the Yangtze River Estuary, we applied Zhang Rui-jin’s formula to calculate the sediment carrying capacity with high-tide and ebb oneway variable sediment carrying capacity coefficients, a variable sediment carrying capacity index, and related coefficients calibrated and verified based on themeasured flow velocity and sediment concentration[15,16]. Calibration data include themeasured data in the neap,mean, and spring tides in the Yangtze River Estuary, which were recorded in February 1998, August 2002, and August 2005, respectively. Based on these data, the sediment carrying capacity coefficients and indices for the Yangtze River Estuary ranged respectively from 0.1 to 0.2 and from 0.4 to 0.8, and were determ ined using linear regression analysis.

1.4.3 Analyticalmethod for calculating water and sediment diversion ratios

Water and sediment diversions control the rise and fall of branching channels and affect estuary evolution. Reclamation and promoting siltation projects have changed the local topographies of beaches and affected the nearby ocean dynam ic conditions, resulting in changes in the transport of nearby sediments, which, in turn, has led to new erosion and deposition tendencies and had an impact on the bay tidal volume[17,18]. In an inland river, the water/sediment diversion ratio is the proportion of the water and the sediment transport flux for each branching channel in that of the overall river channel[19].The flow and sediment discharge of each branching channel vary over time due to the tidal action in the estuary. Therefore, we used the tidal volumes and total sediment transportation volumes of the respective branching channels within one tidal cycle to calculate and analyze their water and sediment diversion ratios.

1.4.4 Analyticalmethod for calculating the net sediment transportation flux within the promoting siltation zone

In a 2-D flow-sediment equation, substituting the riverbed deformation into suspended sediment tran-sport equation and letting the sediment diffusion coefficient Ds=0 result in

Fig.3 Hydrometric points and diversion section distribution, August 2005

Based on Eq.(8), the variation in the net sediment transport of a closed control volume w ill directly lead to seabed evolution in that control volume as (HS)t=( HS)t+ T. When the net sediment transport of the control volume is a net output, the seabed w ill erode. When it is a net input, sediment w ill be deposited on the seabed. The larger it is, the greater the seabed change within the control volume w ill be. Therefore, the extent to which siltation has been promoted can be quantitatively determ ined by calculating the direction andmagnitude of the net sediment transport within the promoting siltation zone. Mutual corroborations and comparative studies can be conducted by integrating themodeling results formovable bed evolution within the promoting siltation zone. 1.5 Model verification

The data for flow velocity, flow direction and sediment concentration obtained from totally 35 vertical lines in the spring tide in August 2005 and those from nine tide stations were used as the hydrologicalmeasurement data formodel verification (see Fig.3).

Fig.4 Verification of tidal level, flow velocity, flow direction and sediment concentration

The data for the tidal level, BG2flow velocity, flow direction and sediment concentration from the Gongqingwei Station are presented in Fig.4.

For topographical verification, we used bathymetrymaps obtained from August 2006 to March 2008 because in the Hengsha East Shoal Promoting Siltation Project Phase II no soils generated in dredging channels were transferred onto the shoal bet-ween 2006 and 2008.

Fig.5(a) Measured erosion and deposition

Fig.5(b) Calculated erosion and deposition

Table 1 Com parison betweenm easured and calculated data on sediment erosion and deposition in the Phase II sedimentation promotion zone

The simulated deposition distribution for the promoting siltation zone given by themodel is shown in Fig.5(b). Themodeled erosion and deposition locations are consistent with the actualmeasurements (see Fig.5(a)). The deposition body within the promoting siltation zone is closer to the south central zone. Erosion occurs near the siltationmouths and the north guiding jetty of the Yangtze River Estuary and at the head of the N23submerged embankment and on its upstream side. The range calculated for the head of the N23submerged embankment and its upstream side is w ider than themeasured range. Thismight be because the points for blow ingmud onto the beach exist within the range, but the effect of blow ingmud generated in the channel dredging process was not considered in themodel. Themaximum calculated erosion depth for the siltationmouth is smaller than themeasured value.

In the presentedmodel, the areas downstream of N2and S2where themain channel of the North Channel were all eroded to a certain degree. In contrast, themeasurements indicated that the area between S5and S9was characterized by deposition. This area is amud casting area, but this factor was not considered in themathematical simulation. W ithin the dam groin of the North Channel, the topographic change determined by bothmodeling andmeasurements are characterized by deposition.

Table 1 summarizes the amount of sediment erosion and deposition and the strength of erosion and deposition within the Phase II promoting siltation zone. There is a good agreement between the calculated andmeasured values.

In light of the above verification, it is concluded that ourmathematicalmodel could simulate the Yangtze River Estuary tidal current field and sediment concentration, as well as changes in the Yangtze River Estuary riverbed. The results of the validationmeet the requirements specified by the relevant regulations. Themodel can be used for assessing what extent of siltation has been promoted in the Hengsha East Shoal.

Fig.6 Mean sedimentation concentration in Hengsha East Shoal during spring tide (kg/m3)

2. Research on effects of promoting siltation of Hengsha East Shoal

2.1 Present situations of Hengsha East Shoal Promoting Siltation Project

To implement the water-sand analysis of the Hengsha East Shoal Promoting Siltation Project, we usedmeasurements taken during the spring tide. The tide difference at the Gongqingwei Station is 3.2m during the spring tide. The distribution of themean sedimentation concentration in the Hengsha East Shoal during the spring tide is shown in Fig.6. In thePhase I and II promoting siltation areas, the slow flow areas of promoting siltation of the submerged breakwater and dam groin field have low sediment concentrations. In the North Passage and the partial construction group area at the head of the dam, the water has stronger ability to carry sediment, and the sediment concentration is therefore higher. Thus, the local high beach has a high sediment concentration and the totalmotion trend of the sediment is spreading to the promoting siltation area. As the construction of the Phase IV or the Post Phase promoting siltation has not yet begun, this phenomenon is not obvious.

Table 2 Net sediment transport and annual amount of deposition-erosion in each promoting siltation area in Hengsha East Shoal

Fig.7 Annual change in erosion-deposition in Hengsha East Shoal under present conditions (m)

The net sediment transferred in a tidal period under present conditions in each promoting siltation area is shown in Table 2. Sediment in Phase I and II areas is net input, and that in Phase VI and Post Phase areas is net output. The total Hengsha East Shoal sediment transfer is net output. The annual deposition-erosion trend is that deposition appears in the promoting siltation area in Phases I and II (see Fig.7). The amount of scour-silt sediment quantity corresponds well with the net sediment transport. The annualmicro-flushing in the Hengsha East Shoal is 0.05m. The Shoal surface is in a quasi-equilibrium state and no further net sedimentation increase w ill occur without further intervention. Therefore, further research regarding promoting siltation is necessary in order to increase siltation in the Hengsha East Shoal.

Table 3 Net sediment transport and deposition-erosion thickness in each sedimentation promotion area

Note: Negative is net output and erosion, while positive is net input and deposition.

Fig.8 Simulated effect (m) of w idening the siltationmouths(“–” for sedimentation and “+” for erosion)

Table 4 Net sediment transport and annual depositionerosion thickness for Phase IV promoting siltation area

2.2 Location of siltationmouths

The east boundary of the Phase I area is actually the dike between the Phase I and II areas and does not contribute to sediment retention in these areas. The siltationmouth w ill be flushed and thus needs protection. A 1 000m-w ide siltationmouth would remain closed until the Wusong elevation was increased by 4m. The entrance and exit of sediment at the siltationmouth of the east dike in the Phase II area is also only the exchange of sediment for all of the Hengsha East Shoal and has no effect on promoting siltation. Thus, the two siltationmouths would remain closed until the Wusong elevation is increased by 1.5m. Meanwhile, the siltationmouth of the north boundary of the Phase I area should be w idened by 1 000m eastwards and the siltationmouths 5 and 6 of north boundary of pro-moting siltation project Phase II should be connected so as to increase the net w idth of the new siltationmouth to 650m.The net sediment input in the Phase I area would then be 23 900 t, and the net sediment input from the north dike in the Phase II area would increase significantly (see Table 3). The net input of sediment transport in the Phase I and II areas would increase by 4 000 t. The net loss of sediment in the Phase IV and Post Phase areas would decrease by 20 600 t.

Table 5 Net sediment transport and annual sedimentation thickness for siltationmouths with different bottom elevations in the Phase IV area

As the number of siltationmouths decreases, the remaining siltationmouths should be w idened accordingly. As a result, water flow would slow down, seabed erosion caused by the siltationmouth would decrease, siltationmouths face to the North Passage, net sediment input from the North Passage would increase, net promoting siltation in the Phase II area would increase by 4.118×106m3, and the annual deposition thickness would be as high as 0.21m (see Fig.8). The net input of sediment would be increased significantly and the promoting siltation effect would be improved by changing the location and direction of the siltationmouths.

2.3 Width of siltationmouths

All siltationmouths should be located on one side of the Northern Passage. The extent of sedimentation promotion depends on the w idth of the siltationmouths. Thus, we considered siltationmouths with w idths of 3 000m, 2 000m, and 1 600m. The net sediment transport for siltationmouths of varying w idths is listed in Table 4.

Fig.9(a) Flood current field and sediment concentration (kg/m3) fields in Hengsha East Shoal Promoting Siltation Project

The w idest siltationmouth w ill not necessarily provide the best results. The net sediment transport for siltationmouths with w idths of 1 600m and 2 000m would be nearly equal. However, 1 600m siltationmouth would require the construction of a 400m promoting siltation dike. The preferred siltationmouth w idth for the Phase IV area is thus 2 000m.

Fig.9(b) Ebb flow field and sediment concentration (kg/m3) fields in Hengsha East Shoal Promoting Siltation Project

Table 6 Net sediment transport and annual depositionerosion thickness for Hengsha Promoting Siltation areas

Note: Negative represents net output and erosion, positive represents net input and deposition.

Fig.10 Distribution of annual sedimentation for entire Hengsha East Shoal (m)

A siltationmouth w idth of 2 000m in the Phase VI area would convert the original sediment net loss of 5.12×104t into a net input of 3.52×104t, allow ingfor the retention of 8.64×104t of extra sediment.

Table 7(a) Flow and sediment diversion ratios for each section under present conditions (%)

Table 7(b) Flow and sediment diversion ratios follow ing com p letion of Hengsha East Shoal Promoting Siltation Project (%)

The w ider opening of the siltationmouth requires higher elevation of the deposition sedimentation body. Themaximum deposition heights for siltationmouths 3 000m, 2 000m and 1 600m w ide are 0.99m, 0.81m and 0.80m, respectively. The siltationmouth of the north boundary would be 3 000m in w idth, and the east end of the siltationmouth would have poor protection and thus would be flushed to a large extent. While siltationmouth would be 2 000m and 1 600m in w idth, the east end of the siltationmouth would be well protected and would be flushed to a smaller extent, so the net sediment transport increases.

In the same way, the ideal w idth for the siltationmouth of the Post Phase area is thus 4 000m.

2.4 Bottom elevation of the siltationmouth

In order to protect the siltationmouth,measures need to be taken on and around it. We simulated the effect of a 2 000m w ide siltationmouth in the Phase IV area with various bottom elevations (see Table 5). The bottom elevation of the siltationmouth is Wusong –1.0m, and the deposition volume and average thickness of sedimentation reach theirmaximum levels because sediment retention during the ebb tide would increase slightly as the bottom elevation of the siltationmouth increases to –1.0m even though the input sediment at high tide decreases slightly. The total increase would be only 1.6% of the original amount, so the effect is not obvious. By further increasing the bottom elevation of the siltationmouth, as the sediment input decreases at high tide, the sedimentation amount would also decrease. Therefore, the bottom elevation of siltationmouth should be proper, that is, neither too high or too low.

2.5 Promoting siltation of the South Main Dike

The elevation should increase to Wusong level 7.5m from the north guiding jetty in the Phase III area to the N23submerged embankment in order for the promoting siltation South Main Dike of the Hengsha East Shoal to be constructed. Sediment exchange in the East Shoal occursmainly through the North Passage and the N23submerged embankments (see Fig.9(a) and Fig.9(b)). Only the N23submerged embankment has net sediment output. The net output of sediment would be reduced by 77%. The other sections would have net sediment input. Overall, sediment input would increase by 1.6×105t (see Table 6). The construction of the South Main Dike would permit net sediment accumulation throughout the Hengsha East Shoal. In one year, the net deposition volume in the promoting siltation area would increase by 1.2×107m3, that is, a 127% increase. The average sedimentation thickness of the whole Hengsha East Shoal would increase by 0.13m. Since themaximum distancebetween the Hengsha East Shoal and the Hengsha Island shore is 23 km, the construction of the South Main Dike would also facilitate transportation of the promoting siltation constructionmaterials. Once the whole promoting siltation project is completed, the average promoting siltation thicknessmay be as high as 0.29m (see Fig.10). The South Main Dike is crucial to the success of the Hengsha East Shoal Promoting Siltation Project.

3. Impact of the Hengsha Promoting Siltation Project on the Yangtze River Estuary Deepwater Channel Regu lation Project

To examine the effect of the Hengsha East Shoal Promoting Siltation Project on the water around the Yangtze River Estuary, we simulated the flow and sediment diversion ratios of the North Channel and South Passage (see Fig.3.) under the present conditions and follow ing the completion of the Hengsha East Shoal Promoting Siltation Project (see Table 7).

After the completion of the Hengsha East Shoal Promoting Siltation Project, the high tide flow diversion ratios of the North Channel and South Passage would increase by 2.7% to 5.0%, the high tide sediment diversion ratio would increase by 1.6% to 5.0%, and the change of sediment diversion ratio/ flow diversion ratio would be –0.049 to 0.016. The flow and sediment diversion ratios of ebb tide in the South Passage w ill remain unchanged. During the ebb tide, flow diversion ratios in the Jiangyanan and Jiuduan Shoals in the North Channel would decrease by 1.2% and 0.4%, respectively, and sediment diversion ratios would decrease by 1.0% and 0.2%, respectively. The sediment diversion ratio/ flow diversion ratio would remain almost the same.

Fig.11 Deposition-erosion effect of Hengsha East Shoal on the Yangtze River Estuary Deepwater Channel Project (m)

Based on the projected effect of the Promoting Siltation Project on the riverbed of the North Channel, the areamost affected lies between the Hengsha Pass and the N8dike. Themain river bed between the south and north dikes of the North Passage would be somewhat flushed. Themaximum range would be 0.3m. The river bed near deep water channel would be lowered, which would also aid in themaintenance of the deep water channel. The deposition in the dike tin groin of the S4-S5and N4-N5Dikes would increase further (see Fig.11).

Based on the above analysis, the completion of the Hengsha Eastern Shoal Promoting Siltation Project would not adversely affect the Yangtze River Estuary Deep Water Channel Regulation Project.

4. Conclusions

In order to preferably fit the layout of the Promoting Siltation Project of the Yangtze River Hengsha East Shoal, non-orthogonal body-fitted curvilinear coordinates were used to design a suspended sedimentmodel.

According to various water-sand features of different waterway sections of the Yangtze River Estuary, the sediment carrying capacity formula proposed by Zhang Rui-jin has been applied, different sediment carrying capacity indices and coefficients and different saturation recovery coefficients have adopted to calculate the distribution of sediment concentration, and the deposition-erosion of the promoting siltation area has been calculated and verified. The rationality of themodel presented in this article has been confirmed through comparison with actualmeasurements. The proposedmethod for calculating the net sediment transport flux and the application of the fixed bedmethod ofmathematical simulation of sediment siltation effects are useful for the Hengsha East Shoal Promoting Siltation Project. Our results indicate that:

(1) Construction of the South Main Dike is essential to the success of the Hengsha East Shoal Promoting Siltation Project. A fter its construction, sediment exchange in the Hengsha East Shoal would be net input and the poor performance of the promoting siltation in the Hengsha East Shoal at present would be improved.

(2) In order to promote siltation in the Phase I and II areas, the eastward and westward siltationmouths should be closed as theymake no net contribution to sediment promotion. The net input of suspended sediment would be increased significantly by w idening the siltationmouths on the north side and decreasing the number of siltationmouths. That is, the promoting siltation effect would be improved by changing the location of the siltationmouths.

(3) W idening or restricting the siltationmouth w ill not necessarily achieve an improved effect. There is an ideal w idth to save the project quantity and enable one to have the preferred promoting siltation effect.

(4) The high bottom elevation of the siltationmouth would prevent sediment beyond the promoting siltation area from flow ing in, and the low bottom elevation wouldmake it easy for the sediment in the promoting siltation area to flow out. The perfect elevationfor the bottom of the siltationmouth is around Wusong –1m .

(5) The completion of the promoting siltation project would increase the flow and sediment diversion ratio at the high tide in North Channel and South Passage by 2% to 5 %, but would have little effect on that at the ebb tide. At the same time, the dam tin groin siltation in the transition area of the North Channel would increase and themain river groove would be flushed slightly. In general, the Yangtze River Estuary Deepwater Channel Regulation Project would not be adversely affected.

In fact, in the estuary near coastal segment, in addition to the power of tidal current, there is the role of waves[20-25]and salinity etc.. Therefore, the coastal sedimentmotion is also influenced by the waves, Salinity and other dynamics to a degree. But in this article only the tidal current siltation effect has been considered, so in our further work, the effects of waves and salinity dynamics on promoting siltation should also be considered.

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[4] SANG Yong-yao, YU Zhi-ying and JIN Liu. Natural evolvement and effect of project in Hengsha East Shoal of Changjiang River estuarine[J]. Donghai Marine Science, 2003, 21(3): 14- 23(in Chinese).

[5] DU Jing-long, YANG Shi-lun and ZHANG Wen-xiang. Study of influence on erosion and accumulation of Jiudansha Tidal Island by Deepwater Channel Project at North Passage of the Yangtze River[J]. The Ocean Engineering, 2005, 23(2): 64-68(in Chinese).

[6] SHEN Zheng-chao, CUI Dong. Sedimentation promotion effect quantitative analysis of Nanhui East Shoal Sedimentation Promotion Project[J]. Shanghai W ater, 2008, 4(3): 45-47(in Chinese).

[7] WANG Xiao-qing, DAI Wen-liang and GUO Jin-song. Study on 3D numericalmodeling of suspended sediment transport in the deep bay, Shenzhen[J]. Journal of Sediment Research, 2009, (4): 39-44(in Chinese).

[8] WANG Hai-long, LI Guo-sheng. Numerical simulation on seasonal transport of suspended sediment from Huanhe (Yellow) River to Bohai Sea[J]. Oceanologia Et Limnologia Sinica, 2009, 40(2): 129-137(in Chinese).

[9] LI Meng-guo. Numerical study on deposit-promoting scheme in the reclamation Project of Wenzhou Shoal[J]. Journal of Sediment Research, 2005, (5): 5-12(in Chinese).

[10] XIONG Shao-long, PAN Cun-hong and ZENG Jian et al. Research on accretion promotionmethod of river regulation and reclamation project in strong-tide estuary[J]. Journal of Sediment Research, 2002, (2): 65-70(in Chinese).

[11] XIE Jun. 2Dmathematicalmodel for closure of dike of the Qingcaosha Reservoir[J]. Hydro-Science and Engineering, 2010, (4): 51-57(in Chinese).

[12] ZHANG Zhuo, SONG Zhi-yao and LV Guo-nian. A new implicit scheme for solving 3-D shallow water flows[J]. Journal of Hydrodynam ics, 2009, 21(6): 790-798.

[13] XIE Dong-feng, GAO Shu and PAN Cun-hong. Advances in long-termmorphodynamics of coastal environment[J]. The Ocean Engineering, 2010, 28(2): 134- 140(in Chinese).

[14] WANG Guang-qian, HU Chun-hong. Developm ent of sedim entation study[M]. Beijing: China Hydraulic Power Press, 2006, 186-190(in Chinese).

[15] ZUO Li-qing, LU Yong-jun. 2-D tidal current and sediment numerical calculation for Feiyun Estuary[J]. Hydro-Science and Engineering, 2006, (4): 21-27(in Chinese).

[16] HE Yong, HUANG Yan. The preliminary study on Lingdingyang beach and passage sediment carrying capacity[J]. Pearl River, 2010, (2): 6-9(in Chinese).

[17] LI Jia-lin, YANG Chao-ping and TONG Yi-qin. Progress on environment effects of tidal flat reclamation[J]. Progress in Geography, 2007, 26(2): 43-51(in Chinese).

[18] LI Meng-guo. The effect of reclamation in areas between islands in a complex tidal estuary on the hydrodynamic sediment environment[J]. Journal of Hydro- dynam ics, 2010, 22(3): 338-350.

[19] YU Xin-ming, TAN Guang-ming and ZHANG Yue. Advances in research of computationalmodels for flow and sediment diversion in river channels[J]. Advances in Science and Technology of W ater Resources, 2006, 26(3): 67-71(in Chinese).

[20] LIANG Bing-chen, LEE Dong-yong and LI Hua-jun et al. Sensitivity of the effects of wave-induced verticalm ixing on vertical exchange processes[J]. Journal of Hydrodynam ics, 2010, 22(3): 410-418.

[21] ZHANG Jing, LIANG Bing-chen and LI Hua-jun. Numerical simulation of wave-induced current with vertically varied radiation stress[J]. Journal of Hydro- dynam ics, 2010, 22(2): 254-259.

[22] LETTMANN K. A., WOLFF J. O. and BADEWIEN T. H. Modeling the impact of w ind and waves on suspended particulatematter fluxes in the East Frisian Wadden Sea (Southern North Sea)[J]. Ocean Dynam ics, 2009, 59(2): 239-262.

[23] TANG H. S., KEEN T. R. and KHANBILVARDI R. Amodel-coupling framework for near-shore waves currents, sediment transport, and seabedmorphology[J]. Communications in Nonlinear Science and Numerical Simulation, 2009, 14(7): 2935-2947.

[24] GODA Y. Examination of the influence of several factors on long-shore current computation with random waves[J]. Coastal Engineering, 2006, 53(2-3): 157- l70.

[25] ZHENG Jin-hai, TANG Yu. Numerical simulation of spatial lag between wave breaking point and location ofmaximum wave-induced current[J]. China Ocean Engineering, 2009, 23(1): 59-71.

December 26, 2010, Revised April 16, 2011)

10.1016/S1001-6058(10)60161-8

* Project supported by the Commonweal Program of Chinese Ministry of Water Resources (Grand No. 200701026).

Biography: XIE Jun (1967-), Male, Ph. D. Candidate, Professor