Simulation of morphological changes induced by large-scale coastal engineering using a 3D wave current interaction model

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Indian Journal of Geo-Marine Science Vol. 45(2), February 2016, pp. 215-223 Simulation of morphological change induced by large-cale coatal engineering uing a 3D wave current interaction model Ting Xu 1,2 & Xueyi You 1* 1 School of Environmental Science and Engineering, Tianjin Univerity, Tianjin 300072, China 2 Key Laboratory of Engineering Sediment of Minitry of Communication, Tianjin Reearch Intitute for Water Tranport Engineering, M.O.T, Tianjin 300456, China * [Email: youxycn123@163.com ] Received 31 March 2014; revied 10 October 2014 Preent tudy ued a three-dimenional numerical model of multi-fraction ediment tranport baed on the coupling of a modified code model of environmental fluid dynamic and a model imulating nearhore wave to predict the etuarine morphological change under the combined action of wind, wave, and current. Wave-current interaction played a remarkable role in increaing the ediment concentration. Ditribution of the ediment concentration i better imulated and agreed with the meaured data when uing the coupled model compared to only conidering the current. Morphological change predicted by the coupled model are baically coincident with the meaured topographic map, and the model error i within plu or minu 10 percentage point. [Keyword: modified environmental fluid dynamic code model; nearhore wave imulation model; wave current interaction; ediment concentration; morphological change; the Oujiang River Etuary] Introduction Prediction of the morphological behavior of etuarie generally follow two approache. The firt i the ue of geological and geomorphological evolution model, which are deigned to imulate morphological evolution over very long period and are ometime referred to a top-down model 1,2&3. Second approach i the ue of proce model baed on two- or three-dimenional (3D) hydrodynamic model combined with ediment tranport and morphodynamic module, known a bottom-up model 4,5&6. Thee model are deigned to imulate the phyical propertie of etuarie in the hort term. However, the numerical prediction of etuary morphological evolution i till in it infancy. Proce-baed prediction model are valuable tool for aeing local, hort-term morphodynamic change in an etuary, particularly thoe caued by large-cale man-made coatal engineering work. Due to inufficient knowledge of ediment-tranport procee and their link to hydrodynamic, uncertaintie in the prediction are amplified by treating ediment with a fixed range of grain ize and coheivene. Furthermore, numerical prediction can exhibit great enitivity to the initial condition. In a more baic context, there are limit to the predictability of morphological variable due to the non-linearity of many coatal ytem, which may induce chaotic behavior. Predicting morphological change effectively depend on the particular mixture of the wave, tide, ediment characteritic, and o on. Interaction between wave and current are complex in etuarine area, and ediment tranport i predominantly controlled by the current and wave motion 7. In typical etuarine area, the ediment i mainly upended by wave and tranported by current. In hallow water, wave breaking alo play an important role in the vertical concentration and ditribution of upended ediment. A better undertanding of wave current interaction-induced ediment tranport i crucially important in etuarine area 7,8&9. Intenive tudie have been conducted in thi field uing both experimental and numerical approache. The aumption of applying a time-invariant eddy vicoity to account for turbulence wa adopted in the pioneer wave current interaction model 10. A range of imilar model of the wave current interaction within the eabed boundary layer have been publihed 11. Steady boundary-layer treaming (Eulerian drift) i alo caued by turbulence aymmetry in the ucceive wave half-cycle beneath aymmetric wave. Thi proce wa

216 INDIAN J MAR SCI VOL 45, NO.2 FEBRUARY 2016 decribed in detail by Scandura 12. Yu et al. 13 and Hu et al. 14 invetigated the ediment tranport beneath aymmetrical wave group uing a two-phae model. Recently, Fuhrman et al. 15 reported that Longuet-Higgin treaming (related to convective effect) i capable of promoting onhore ediment tranport, even for highly kewed wave on fine and. Thi reult i upported by meaurement 16 and i conitent with the finding of Blondeaux et al., who compared the prediction of the ediment-tranport rate for ocillating tunnel with thoe beneath progreive wave 17. Other treaming mechanim till exit, uch a the treaming of patially variable roughne 18 and the treaming of bed lope 19,20,21&22. Depite thee effort, predicting the morphological change induced by large-cale coatal engineering ha not been fully decribed previouly, epecially uing a 3D wave-current interaction model. For the typical ilty etuarie of interet here, the ediment i mainly upended by wave and tranported by current; thu, the combined wave current action i very important. In hallow water, wave breaking alo play an important role in the vertical ditribution of upended ediment. Within thi context, the preent tudy aimed to develop a numerical model to imulate and predict the morphological change induced by coatal engineering, namely a large-cale ea embankment in the Oujiang River Etuary of China, under the combined action of wind, wave, and current. A 3D numerical model of multi-fraction ediment tranport baed on the coupling of a modified environmental fluid dynamic code (EFDC) model and a imulated wave nearhore (SWAN) model wa developed and applied in the proce. Material and Method The Oujiang River Etuary (ORE), 35 km long and 30 km wide at the mouth, i characterized by it bifurcated hape and it two outlet (through the north and outh branche) into the Eat China Sea. The complex etuarine geometry conit of ten of taggered hoal, deep tidal creek, and more than 40 large and mall iland, which are cattered outide the mouth of the ORE. The tide in the ORE i a emidiurnal tide, with an average tidal range of 4.5 m and a maximum value over 7 m. Wenzhou Shoal, lying between Lingkun Iland and Niyu Iland, i a well-developed, large-cale mouth bar in the ORE. Wenzhou Shoal i very hallow in the mot of the region. Large intertidal zone exit at the mouth of the river, which are ubmerged at high tide and expoed at low tide. To meet the increaing land demand of urban expanion and deep-water port contruction, the Chinee government ha built the Ling-Ni Embankment, which connect Lingkun Iland and Niyu Iland. The full length of the embankment i 13 km, and it wa completed in April 2005 (Fig. 1). The preent paper utilized the Ling-Ni Embankment a an example and imulated the morphological change induced by it contruction uing a 3D numerical model baed on a combination of the modified EFDC model and SWAN model. Fig. 1--Remote-ening image in 2005. Modified EFDC model The EFDC model i a public domain urface-water modeling ytem that fully incorporate integrated hydrodynamic. Thi model wa originally propoed by John Hamrick, 23 include a turbulence cloure model, and can imulate wave boundary layer, wave-induced current, and combined wave current bottom hear tre with coheive and non-coheive ediment of multiple ize clae. The EFDC model i coupled with a pectral wave model for wave-induced re-upenion, but it doe not take the wave-breaking effect into conideration. Conidering that wave breaking play an important role in vertical ediment ditribution and ediment tranport in nearhore zone, we modified the EFDC model by incorporating the wave-breaking effect uing a ediment-mixing coefficient.

Xu & You: SIMULATION OF MORPHOLOGICAL CHANGE INDUCED BY LARGE-SCALE COASTAL ENGINEERING USING A 3D-CURRENT INTERACTION MODEL 217 A ediment-diffuion coefficient under the combined action of wave-induced current can be repreented by the non-linear uperpoition of the diffuion coefficient under the current and wave alone: 24 2 2 0.5 cw c w (1) where c i the current-related mixing coefficient (m 2 /), and w i the wave-related mixing coefficient (m 2 /). van Rijn further propoed the calculation of a ediment-diffuion coefficient under the action of breaking wave: 24 z wbed, w,max wbed,,, cw z0.5 h 0.5 h qc w hh 0.035 br, z0.5 h Tp (2) where 0.018 w, bed br w U, r (3) w,max 0.035 brhh S / Tp (4) i the wave-related where w, bed ediment-mixing coefficient near the bed, w,max i the maximum of the wave-related ediment-mixing coefficient near the bed, z i the height above the bed; h i the water depth, i the thickne of the effective near-bed ediment-mixing layer, br i the empirical coefficient related to wave breaking, H i the ignificant wave height, T i the ignificant U wave period,,r i the repreentative near-bed peak orbital velocity baed on the ignificant wave height, w =coefficient= 1 2 / 2 u, w with w 1.5, w i the total velocity of upended and, and u,w i the wave-related bed-hear velocity. Generally, the ediment-diffuion coefficient can be conidered equivalent to the turbulent-diffuion coefficient. 25 According to Style and Glenn, the turbulent-diffuion coefficient of the total water depth under the combined action of wave-induced current p without conidering breaking wave can be repreented a follow: 26 A0 z, z0 z cw Kv A0 w, cw z cw qc A0 qc z, z cw qc (5) where A 0 i a contant, uually 0.4; i the wave-current-related turbulence intenity; qc i the current-related turbulence intenity; and i the von Kármán coefficient, uually 0.4. Baed on thee theorie, we introduced Eq. (5) and (2) into Eq. (1), and we obtained the following equation to calculate the ediment-diffuion coefficient: Aq 0 cw z, z0 zcw Aq 0 cw w, cw z cw qc 0.5 2 2 cw z Aq 0 cz wbed, w,max wbed,, cw z0.5 h 0.5 h qc 2 2 2 hh Aq 0 cz 0.035 br, z0.5 h T p (6) Without breaking wave, current primarily play a upending role for the ediment far away the bed; when wave breaking occur, the wave control the ditribution of the upended ediment in the upper water body. Becaue Eq. (6) can reflect the phenomenon of ediment diffuion well, it make up for the deficiencie of ource program that do not conider wave breaking. In addition, other factor uch a wind hear tre on the urface, drying and wetting in hallow water, bed ediment tranport, bed cour and depoition, ettling velocity, and reference concentration were alo incorporated into thi model. SWAN model SWAN i a third-generation wave model baed on the wave-action balance equation formulated for coatal application 27&28. The effect of wind wave generation, refraction, hoaling, bottom friction diipation, white capping, nonlinear wave wave interaction, and

218 INDIAN J MAR SCI VOL 45, NO.2 FEBRUARY 2016 ambient current on the wave propertie are conidered in SWAN 29. In thi tudy, 2D and time-dependent wave were calculated to imulate wave-induced ediment reupenion. The following tranport equation wa ued to calculate the wave-action denity N (the energy denity divided by the relative frequency): N c c xn y N c N c N Sw t x y (7) where c x and c y are the propagation velocitie in the x and y direction, repectively. SWAN account for hoaling and refraction through the dependent variation in c x and c y. The term S w on the right-hand ide i a ource/ink term repreenting the effect of wind-wave generation, wave breaking, bottom diipation, and nonlinear wave wave interaction. SWAN alo account for the effect of diffraction, partial tranmiion, and reflection. Specific formulae for wind input, bottom tre, white capping, wave wave interaction, etc. were previouly decribed in detai 30. Coupling between the EFDC and SWAN model The SWAN and EFDC model were coupled through data exchange in the regional and coatal domain, repectively: EFDC provided SWAN with time erie of water elevation and current velocity, and SWAN upplied EFDC with array of radiation tre (ee Fig. 2). Fig. 2--Flow diagram of the coupling between the EFDC and SWAN model. Verification In the modified EFDC model, the effect of wave breaking on the ediment-diffuion coefficient wa conidered. The calculation accuracy of thi model wa teted. The imulated ediment concentration profile with and without the breaking effect were compared with the meaured ediment concentration from Han et al. 31 (Fig. 3). A hown in the figure, the modified EFDC model incorporating the wave-breaking effect wa more likely to predict the actual ituation, and the modified EFDC model reliably imulated ediment tranport.

Xu & You: SIMULATION OF MORPHOLOGICAL CHANGE INDUCED BY LARGE-SCALE COASTAL ENGINEERING USING A 3D-CURRENT INTERACTION MODEL 219 Fig. 3--Comparion of the imulated ediment concentration profile and the meaured data. To calibrate the high concentration layer near the eabed, the ediment-concentration profile due to combined wave and current for multi-ized ediment imulated by the modified model are compared with the experimental value reported by van Rijn et al. 32, and the reult are hown in Fig. 4. The imulated ediment concentration match very well with the experimental value from van Rijn et al. 32, which demontrate that the modified EFDC model i able to accurately predict the high concentration layer near the eabed, to a certain extent. grid conit of 12161 grid cell in the horizontal direction with a grid ize from 10 m to 1000 m. Six layer were divided in the vertical direction according to the relative water depth, i.e., the urface, 0.2, 0.4, 0.6, 0.8, and the bottom. The water elevation at the offhore open boundary wa forced by eight tidal contituent, namely Q1, O1, P1, K1, N2, M2, S2, and K2. Thee contituent account for mot of the tidal energy in the Eat China Sea. The time tep for the circulation module wa 5 min, with 1 min a a ub time tep for the tranport module. The Corioli force wa included, and Manning' coefficient were 0.01 0.025 according to the ditribution of the medium diameter of bed material meaured in recent year 33. The Smagorinky contant for the horizontal eddy vicoity coefficient take a typical value of 0.12. The ediment wa divided into five fraction with the particle ize of 0.005, 0.01, 0.03, 0.06, and 0.08 mm and ettling velocitie k\of 0.0004, 0.0006, 0.0011, 0.0064, and 0.0075 m/, repectively 34. The bed ediment poroity wa 0.5, and the bed ediment-pecific weight wa 2.5 35. Fig. 5--Depth of domain and grid. Fig. 4--Comparion of the ediment concentration predicted by the coupled model and the experimental data from van Rijn et al. (1993). Model application The ORE i bounded by a complex horeline and contain many iland and a complex terrain. A curvilinear orthogonal dicrete grid and the depth of domain are hown in Fig. 5. The model The wave field wa computed uing the SWAN model, and the input wind data were obervation from the Dongtou Iland weather tation. Depth-induced wave breaking wa enabled with the default option and parameter. The bottom friction wa computed by the Maden cheme with the default equivalent bottom roughne length cale. The model had 18

220 INDIAN J MAR SCI VOL 45, NO.2 FEBRUARY 2016 uniformly ditributed direction, and the frequency reolution wa determined by f i+1 =1.9f i with f max =1.0Hz and f min = 0.04Hz. The calculation flow of the eabed evolution i ummarized in Fig. 2. Firt, the tidal level, current velocitie, and current direction in the contruction area at different time were calculated with the hydrodynamic force module of EFCD. The reult were then input into the SWAN wave model, which allowed the effect of the dynamic force of tidal current on the wave field to be conidered during the calculation. Finally, the wave height, direction, and period obtained baed on the SWAN model were input into the ediment module of the EFDC. The coupling of two-way wave and current wa realized. To undertand the change in the topography of the ea bottom near the Ling-Ni Embankment after contruction wa completed, the Eat China Sea Invetigation and Deign Intitution of the State Bureau of Oceanic Adminitration and the Zhejiang Intitute of Hydraulic & Etuary mapped 1:10000-1:25000 bathygram of thi area in October 1999 (before the contruction) and May 2007 (after the contruction), a directed by the Chinee government. Thi tudy took the Ling-Ni Embankment contruction a an example and predicted the change to the topography of thi area after contruction, with the imulation panning from October 1999 to May 2007. The reult baed on the imulation and the field meaurement data were compared to tet the preciion of the calculation from the coupling model etablihed in thi tudy. Wave imulation We ued the SWAN model to compute the wave field (Fig. 7). A hown in Fig. 7, the imulated wave procee are conitent with the meaured wave height (Fig. 1) from October 10th to 12th, 1999. Fig. 7--Verification of wave height. Sediment imulation We ued the EFDC ediment module for the computation of ediment tranport. Fig. 8 how the comparion of the upended-ediment ditribution with and without a wave effect at the urface layer and the bottom layer. The impact of wave on upended ediment ditribution i remarkable and can greatly increae ediment concentration (Fig. 8). The vertical ditribution of ediment concentration of the propoed coupled model and the current model at the time of high and low tidal level were compared (Fig. 9). The imulated ediment concentration of the propoed coupled model how a better match with the meaured value than thoe from the current model. Reult and Dicuion Hydrodynamic imulation We ued the EFDC hydrodynamic module to compute the current field (Fig. 6). A hown in Fig. 6, the calculated tidal level and phae were conitent with thoe meaured (Fig. 1), which were ampled from October 10 to 12, 1999. The abolute error of the tidal level i generally le than 0.1 m. Fig. 8--Verification of time-averaged ediment concentration. (a), urface layer. (b), bottom layer. Fig. 6--Verification of tidal level.

Xu & You: SIMULATION OF MORPHOLOGICAL CHANGE INDUCED BY LARGE-SCALE COASTAL ENGINEERING USING A 3D-CURRENT INTERACTION MODEL 221 the imulated topographic map. The imulated topographic map predicted by the coupled model agree well with the meaured topographic map. The relative error between the imulated value and the meaured value i within plu or minu 10 percentage point (Table 1). Fig. 9--Verification of vertical ediment concentration. (a), at the time of a high tidal level. (b), at the time of a low tidal level. For the current model, the predicted ediment concentration i very low, and it vertical ditribution i far below the meaured value. The interaction of wave and current increae ediment reupenion and tranport in nearhore water. The ediment concentration i enitive to wave propagation, and the wave effect on the ediment ditribution hould not be neglected. The upended ediment obervation tation i located near Wenzhou Shoal with a water depth of -6 m relative to the mean ea level (Fig. 1), which i in the urf zone. The effect of breaking wave on the ediment ditribution are expected to be large becaue the water column will be mixed by the trong wave-current hear reulting from the wave breaking in hallow water 36&37. Therefore, the 3D EFDC model wa modified by incorporating the wave-breaking effect in nearhore zone, and the modified verion i able to more reaonably predict ediment tranport. Numerou hallow area often exit in the alonghore region of an etuary, which are located within the wave-breaking zone. Wave breaking i accompanied by large wave-energy diipation, which caue trong turbulence of the water body and conequently a great increae in the concentration of upended ediment. Meanwhile, the vertical ditribution of ediment at different depth level alo greatly differ. Thi point ha been proven in numerou tudie 36,38,&39. Therefore, reaonable inverion of the complex alonghore ediment movement under the combined action of wave and current during upended-ediment imulation cannot be achieved without conidering the effect of wave breaking on the ediment-diffuion coefficient. Simulation of morphological change We ued the EFDC ediment module to compute the morphological change induced by the Ling-Ni Embankment. Fig. 10 how the comparion of the meaured topographic map and Table 1--Comparion of the imulated and meaured amount of eroded and depoited ediment induced by the Ling-Ni Embankment Project Poition Meaured Simulated data data Error Wenzhou Shoal 2276.3 2467.1 8.4% Qidutu~Lingkun up -232.6-216.9-6.7% Lingkun up~niyu -420.1-385.5-8.2% Oufei Shoal 695.7 761.5 9.5% - i the amount of eroion ediment; + i the amount of depoition ediment. Fig. 10--Comparion of the imulated and meaured morphology. (a), meaured morphology. (b), imulated morphology. Lu et al. developed a 2D mathematical model for ediment tranport influenced by wave and tidal current 40. Thi model wa ued to tudy the effect of the reclamation cheme for Caofeidian Harbor on the hydrodynamic environment, ediment tranport, and morphological change. The relative error between the calculated data and the obervation on the pattern and magnitude of edimentation and eroion in the related area wa between minu 7 percentage point and 58 percentage point. However, their model ha two major deficiencie, which caue low imulation preciion: 1) The 2D model fail to reflect the vertical ditribution of the ediment; therefore, it cannot imulate the high ediment content of the water near the ea bottom; and 2) although their model conider the effect of wave current coupling, it doe not conider the effect of wave breaking on upended ediment movement near the hore. Concluion In thi tudy, a 3D numerical model for multi-fraction ediment tranport baed on the coupling of a modified EFDC model and a SWAN model wa developed and applied to predict the

222 INDIAN J MAR SCI VOL 45, NO.2 FEBRUARY 2016 morphological change caued by a large-cale ea embankment in the ORE of China under the combined action of wind, wave, and current. The reult howed that our model wa effective and reliable for the current tak. Baed on thee reult, we drew the following concluion: (1) The 3D EFDC model wa modified by incorporating the wave-breaking effect in nearhore zone, and the modified model i able to predict the nearhore ediment tranport more accurately. (2) The model take advantage of the two-way coupling between the EFDC and SWAN model, that i, it conider both the time-to-time effect of the tidal level, current velocitie, and current direction on the wave field output by EFDC and the effect of the wave height, direction, and period on the current field output by SWAN, thereby etablihing a ediment-movement model influenced by the combination of two-way wave and current. 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