Numerical study on tidal currents and seawater exchange in the Benoa Bay, Bali, Indonesia

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1 Act Ocenol. Sin., 214, Vol. 33, o. 3, P. 9 1 DOI: 1.17/s E-mil: hyxe@263.net umericl study on tidl currents nd sewter exchnge in the Beno By, Bli, Indonesi HEDRAWA I Gede 1*, ASAI Koji 2 1 Deprtment of Mrine Sciences, Fculty of Mrine Sciences nd Fisheries, Udyn University, Bdung- Bli 8361, Indonesi 2 Deprtment of Civil nd Environmentl Engineering, Ymguchi University, Ymguchi , Jpn Received 9 My 212; ccepted 6 My 213 The Chinese Society of Ocenogrphy nd Springer-Verlg Berlin Heidelerg 214 Astrct A three-dimensionl (3-D) finite volume costl ocen model (FVCOM) ws used for the study of wter circultion nd sewter exchnge in the Beno By, Bli Islnd. The tidl component ws forced in open oundry nd dischrge from six rivers ws included in the numericl clcultion. The tidl elevtion produced y the FVCOM hs good greement with the oservtion dt. The tidl current is lso successfully clculted under the e tide nd flood tide conditions. The non-liner tidl residul current ws produced y the costline geometry, especilly surrounding the nrrow strit etween the Serngn Islnd nd the Beno Peninsul. The tidl residul current lso generted two smll eddies within the y nd one smll eddy in the y mouth. The slinity distriution influenced y river dischrge could e successfully clculted, where the numericl clcultion nd the oservtion results hve good correltion (r 2 ) of.75. Finlly in order to exmine the sewter exchnge in the Beno By, the Lgrngin prticle trcking method nd clcultion of residence time re pplied. The mechnism of prticle trnsport to the flushing of sewter is depicted clerly y oth methods. Key words: FVCOM, tidl current, residul current, slinity, sewter exchnge Cittion: Hendrwn I Gede, Asi Koji umericl study on tidl currents nd sewter exchnge in the Beno By, Bli, Indonesi. Act Ocenologic Sinic, 33(3): 9 1, doi: 1.17/s Introduction Sewter in the costl re hs n importnt role on the sustinility of the costl ecosystem. For instnce, the costl re is used s nursery region nd feeding ground for lrge numer of mrine species. In the lst decdes, ecuse of the incresed development ctivities exploiting the costl res, the degrdtion of environmentl qulity hs ecome serious issue mong reserchers. Sewter experiences vrious contminnts either directly from pollutnt sources or indirectly from river dischrges. The knowledge of hydrodynmics processes in the costl re hs n essentil role in ny investigtion. It cn e used to investigte the cpility of sewter to ssimilte those vrious pollutnts dischrged into it. The min flow of sewter in the costl region is strongly influenced y tidl system (Imsto et l., 198), in ddition to tht induced y river flow nd wind. Therefore, n ccurte study of the tidl flow in costl wters ecomes one of the most importnt environmentl sciences. In recent yers, the numericl simultion of the tidl current is widely used for costl ocen circultion study. However, until now, ny study concerning costl ocen circultion in the Beno By hs not een found, prticulrly studies tht descrie nd try to mesure the sewter exchnge system. As shown in Fig. 1, the Beno By is semi-enclosed wter situted in the southern prt of Bli Islnd, Indonesi. The lterl dimension of the Beno By is 1 km 5 km in the inner prt of the y. The Beno By is chrcterized y nrrow strit in the y mouth formed y the Serngn Islnd nd the Beno Peninsul. The min vessel hror in the Bli Islnd, nmely Beno Hror, is locted in the inner prt of the y. Since 1996, the Serngn Islnd nd Bli Islnd were connected y ridge, nd very limited wter could pss through under the ridge into the inside of the Beno By. The Beno By is importnt for the Bli Islnd oth from n environmentl s well s economic point of view. From the environmentl point of view, the Beno By is the re for the mngrove ecosystem, ut it is lso used for the disposl of grge. From the economic point of view, the Beno By is used for hror, oil sttion, nd tourism ctivities. Therefore, in order to conserve the mrine environment in the y, understnding of the chrcteristics of the sewter circultion is impertive. Figure 2 shows the thymetry mp tht ws otined from Hydro-Ocenogrphy Division, Indonesin vy (DISHIDROS- TI AL). The Beno By hs shllow wter depth, where the deepest prt in the inner prt of the y is less thn 15 m, nd in the outer prt (towrds the ocen), less thn 5 m. In the inner prt of the y, prticulrly in the nerest costline, the wter depth vries from out 1 to 5 m. The surroundings of the y mouth nd the est prt of Beno Hror hs stepped ottom topogrphy with depth of out 1 to 15 m. In generl, the ocenic circultion within the Beno By is controlled y inflows from the Bdung strit nd the Indin Ocen. The tides nd tidl currents re typicl of wter circultion in the Beno By, which is mostly impcted y sewter from the Bdung Strit. The type of tides in the Beno By is semi-diurnl, in which the is the dominnt tidl compo- Foundtion item: The Besisw Ungguln progrm from Ministry of Eduction nd Culturl Repulic of Indonesi. *Corresponding uthor, E-mil: hendr_mil@yhoo.com

2 HEDRAWA I Gede et l. Act Ocenol. Sin., 214, Vol. 33, o. 3, P S E. S Beno Hror Depth/m Beno Peninsul Mti River Bli Islnd Serngn Bdung River Beno Hror Sm River E Bli Islnd Fig.2. Bthymetry mp. Serngn (efore reclmtion) Fig.1. Beno By mp with scrile lck lines indicting river dischrge. The inset picture is the Indonesi Archipelgo, nd Bli Islnd is indicted with lck dsh circle. nent. During spring tide, the se level in the inner y cn rise to 2.5 m nd drop to out 2.1 m during nep tide. In ddition, the Beno By is under the influence of the six rivers dischrges: the Bdung River, the Mti River, the Sm River, the Telh River, the Lolon River nd Buji River. According to the report of Environmentl Impct Assessment Agency of Bli Province, the fresher wter dischrged into the Telh River Beno Peninsul Buji River Lolon River y is less thn 36 m 3 /h in ech river dischrge. This low slinity of fresher wter runoff hs significnt effect on the wter circultion due to the difference of the slinity with the sewter. It is importnt to understnd the physicl process of the tidl current field in order to discuss the wter qulity nd the ecosystem in the Beno By. Prticulrly, the sewter exchnge is one of the importnt physicl processes. In order to clrify the mechnism of sewter exchnge, we undertke numericl simultion y trcking the prticles relesed in the current driven y the tidl component. Additionlly, the sewter-flushing rte is estimted y clculting the time consumed y the prticles' movement in ox re s function of the initil condition. The finite volume costl ocen model (FVCOM) developed y Chen et l. (23) is used in our study. FVCOM is sed on the Finite Volume Method nd three dimensionl primitive equtions. One unique feture of FVCOM is the use of the unstructured grid. The suitility of n unstructured grid pproch in FVCOM enles us to reproduce costl ocen currents with high resolution in complex costl geometry. FVCOM hs een successfully used y mny reserchers for the investigtion of the costl ocen circultion (Chen et l., 23; Hung et l., 28) nd the physicl mechnism for the offshore detchment (Chen et l., 28). The Lgrngin method ws widely used for exmining prticle trcking nd sewter exchnge (Chen et l., 28; Bilgili et l., 25; Awji nd Kunishi, 198). 2 umericl model The FVCOM used in this reserch is FVCO.7.1 series. The governing three-dimensionl equtions consist of the momentum, continuity, temperture, slinity, nd density equtions s given y Chen et l (26). The verticl eddy viscosity nd the verticl therml diffusion coefficients re otined using modified Mellor-Ymd level 2.5 (MY-2.5) turulence closure model (Glperin et l., 1988). The horizontl diffusion coefficients re determined using Smgorinsky eddy prmeteriztion method (Chen nd Liu, 23). The FVCOM is composed of the externl nd internl modes tht re computed seprtely using two split time steps. The FVCOM sudivides the horizontl numericl computtionl domins into set of non-overlpping unstructured tringulr meshes. An unstructured tringle is composed of three nodes, centroid, nd three sides. The sclr vriles, such s slinity (S), wter elevtion (ζ), nd verticl velocity (w) re plced t the nodes, nd determined y net flux through the section linked to the centroid nd the mid-point of the djcent sides in the surrounding tringles (clled the trcer control element or TCE). The horizontl velocities u nd v re plced t the centroid nd clculted on the sis of the net flux through the three sides of tht tringle (the momentum control element or MCE).The FVCOM uses n exct form of the no flux ottom oundry conditions for temperture nd slinity. The ottom slope nd the grdients of the temperture nd the slinity re clculted using Green s theorem. In this study, the computtionl domin ws configured with unstructured tringulr grids in the horizontl nd σ-level in the verticl (Fig. 3). The grid in the horizontl cses ws designed with different resolutions, out 2 m in the inner of y to 6 m in the outer of y, nd the verticl grid ws divided into ten σ-levels. To consider the wet nd dry conditions during e tide nd

3 92 HEDRAWA I Gede et l. Act Ocenol. Sin., 214, Vol. 33, o. 3, P the University of Tokyo (ORI-Tide) (Mtsumoto et l., 1995). The initil setup conditions re summrized in Tle 1. The investigtion of sewter exchnge in the Beno By ws performed using the Lgrngin prticle trcking method. The Lgrngin prticle trcking solving nonliner system of ordinry differentil eqution (ODE) is s follows (Chen et l., 26): Fig.3. Unstructured tringulr grids. flood tide, wet/dry point tretment method hs een incorported into the clcultion. The verticl column thickness is less thn.5 m in the cell during e tide nd will e designed s dry cell nd its velocity is set to m/s. During flood tide where the wter level rises, dry cell ecomes wet nd its velocity nd elevtion re computed. In this study, the FVCOM ws forced y four river dischrges within the y nd two river dischrges in the northern prt of Serngn Islnd (Fig. 1). In the open oundry of the computtionl domin, tidl component ws forced for tidl wter circultion, nd the temperture is determined to e constnt. The computtionl domin is lso forced y fresher wter from six rivers which re dischrged into the Beno By. In the surfce oundry, the meteorologicl prmeters re determined to e constnt. The constnt slinity is used s n initil condition since no oservtion dt ws otined in the Beno By. This ssumption would e firly cceptle since the wter depth of the Beno By is shllow. Furthermore, constnt slinity nd meteorologic dt were used in this study ecuse the climtologicl condition in the Beno By does not chnge significntly for short periods of time (tropicl re). The tidl component used in the open oundry ws otined from the tidl model developed y the Ocen Reserch Institute (ORI), dx vxt [ ( ), t] dt =, (1) where x is the prticle position t time t, dx/dt is the rte of chnge of the prticle position in time; nd v[x(t), t)] is the 3-D velocity field generted y the model. The prticles relesed in the model domin re treted using conserved mss method. In this clcultion, the dependence of the velocity field on time hs een eliminted since the velocity field is considered sttionry during the trcking time intervl (Chen et l., 26). In order to provide cler nlysis of the sewter exchnge in the Beno By, the model re ws divided into five regions s shown in Fig. 4. In ech region, the chrcteristics of prticle trnsport will e investigted thoroughly to otin n overll chrcteristic of the sewter exchnge. The sewter exchnge in the model domin ws lso investigted y using the clculted time of prticles in residence in the model re A: southern prt of the y B: centrl prt of the y C: western prt of the y D: estern prt of the y E: ocen Fig.4. Beno By divided into five model regions. Tle 1. The initil set up conditions for the model Items Contents Grid numer of node numer of element Lyers uniform lyer with 1 th sigm lyer Open oundry tide conditions tidl component temperture nd slinity uniform River dischrge Bdung River, Mti River, Sm River, Telh River, Buji River nd Lolon River Meteorologicl condition uniform Time step 1. s

4 HEDRAWA I Gede et l. Act Ocenol. Sin., 214, Vol. 33, o. 3, P Oservtion An oservtionl investigtion ws performed to mesure the sewter slinity. The Center for Remote Sensing nd Ocen Sciences (CReSOS) of Udyn University-Bli nd Kyow Concrete Industries Ltd crried out the field oservtion in My 25 for the slinity using compct CTD. The loctions of the oservtion sttions re shown in the Fig. 5. Seven oservtion sttions were locted in the inner prt of the y nd six sttions in the outside of the y. The slinity prmeter ws mesured t every 2 m depth. Within the y, there is one tidl oservtion sttion tht ws set up y the tionl Coordinting Agency for Surveys nd Mpping (Bkosurtnl) of Indonesi to mesure the tidl elevtion. The mesurement of tidl level ws recorded every hour. In order to vlidte the model clcultion, the mplitude ws filtered using lest squre nlysis method. Unfortuntely there is no mesurement of tidl current ville yet in the Beno By. Therefore, in this reserch, we re unle to vlidte the tidl currents produced y numericl clcultion with field dt. 4 Results nd discussion 4.1 Tidl level vlidtion Figure 6 shows comprison etween the numericl nd oservtion results for the tidl height nd phse lg. The comprison etween the oserved nd the simulted mplitudes nd phse lgs of the tidl component t the tidl mesurement sttion in the inner of the Beno By shows good greement. Tle 2 shows smll discrepncies in mplitude nd phse lg, which re.2 m nd 1.2 respectively. The smll discrepncy etween oservtion nd numericl clcultion indicted tht FVCOM hs good performnce to simulte the sewter level in the model domin. Figure 7 shows the results of co-mplitude chrts for the dominnt semidiurnl tide. The numericl clcultion shows tht the mplitude increses from the y mouth to the inner prt of the y. It is due to the fct tht the inner prt of the y is shllower thn the outer prt. The y mouth shows somewht leveling of mplitude tht is evidently ffected y the nrrow strit etween the Serngn Islnd nd the Beno Peninsul. 4.2 tidl current nd tidl residul current Figures 8 nd 9 show the tidl current t the e tide nd flood tide, respectively. During the e tide, the sewter flows out from the inner y into the ocen. The nrrow strit etween the Serngn Islnd nd the Beno Peninsul cuses Mti River Sm River Bdung River Telh River 31 Buji River Lolon River oservtion point tidl sttion Tle 2. Comprison of FVCOM result nd oservtion for mplitude nd phse lg FVCOM Oservtion Difference Amplitude/m Phse lg/( ) Elevtion/m Fig.5. Slinity oservtion sttion indicted with lck circle nd tidl mesurement sttion indicted with tringle model oservtion error t/h Co-mplitude/m Fig.6. Tidl level verifiction. Fig.7. Co-mplitude of tidl component.

5 94 HEDRAWA I Gede et l. Act Ocenol. Sin., 214, Vol. 33, o. 3, P m/s m/s 36 Fig.8. Tidl current velocity t e tide.. Surfce nd. ner ottom lyer m/s m/s Fig.9. Tidl current velocity t flood tide.. Surfce nd. ner ottom lyer. n increse in tidl current. At the nrrow strit in the y mouth, the tidl current speed reched.46 m/s t the surfce lyer nd decresed due to the incresed wter depth. The mximum of the tidl current ws.29 m/s ner the ottom. In the northern prt of Serngn Islnd s well, the tidl current ws slightly high, which could e cused y the effect of nrrow chnnel formed in tht re. During the flood tide, the sewter flowed into the y nd developed smll eddy in the surrounding of Western prt of the Beno Peninsul. This could hppen due to the geometric condition of the model re. During flood tide, the tidl current speed reched.31 m/s in the surfce lyer nd.19 m/s in the nerest ottom lyer. This tidl current is lower thn tht in e tide condition. The tidl residul current is well defined in numer of ppers (Imsto, 1983; Awji, 198). In this pper the tidl residul current defined y Eq. (2) (Imsto, 1983). T 1 Ur ( x, y) = u( xyt,, )dt T, (2) where U r is the residul current, T is the tidl period, nd u(x,y,t) is the velocity for x nd y direction t time t. The wek tidl residul current ws reveled in the Beno By s shown in Fig. 1. The mximum velocity of the residul current occurring in the nrrow strit of the y mouth ws.97 m/s for the surfce lyer nd.59 m/s for the nerest ottom lyer. Despite the wekness of the tidl residul current, it could ply dominnt role in the distriution of pssive contminnts in the ocen (Ysud, 198). Two smll eddies within the y nd one smll eddy in the y mouth cn e seen. These tidl residul circultions re cused y non-linerity tht hppened due to the costl geometry formed y nrrow strit in the y mouth nd the ottom topogrphy (Yngi, 1976; Zhou et l., 212). Along the centrl region of the y until the y mouth, the tidl residul current ws slightly higher nd hd more complicted strem systems thn other regions in the model domin. It could e the cse tht the centrl region of the y is the crossing re for the three-strem regions: the southern prt, the western prt nd the estern prt of the Beno By. The tidl residul current is lso oviously influenced y the ottom topogrphy, in which

6 HEDRAWA I Gede et l. Act Ocenol. Sin., 214, Vol. 33, o. 3, P m/s.1 m/s 36 Fig.1. Tidl residul current.. Surfce nd. ner ottom lyer. the tidl residul current in the surfce lyer is reltively higher thn in the ottom lyer (Yngi, 1976). The river dischrge into the inner of the Beno By could lso influence the tidl residul current s suggested lso y Zhou (212). 4.3 Slinity distriution Figure 11 shows the slinity distriution fter 6 dys of river dischrges. Low slinity ppered in the inner of the y nd in the northern prt of Serngn Islnd. During e tide, the low slinity wter in the inner y ws distriuted towrd the y mouth. Otherwise during flood tide, the higher slinity from the ocen flowed into the y. It implied tht the slinity in the inner of the y incresed. Figure 12 shows the verticl profile of slinity during e tide long the cross section tht is indicted y stright lck line in horizontl topogrphicl figure in the inset figure. It ws depicted clerly tht the slinity ws not well-mixed verticlly, especilly in the inner of the y, wheres well-mixed verticl distriution ws shown in the outer y. The verticl slinity distriution t the inner of the y showed low concentrtion in the surfce lyer (dsh circle line in the Fig. 12) during the e tide. At the sme time reltively higher of the meridionl current velocity component is indicted t the sme loction (dsh circle line in the Fig. 12c). This meridionl current velocity component led the trnsporttion of fresher wter from the Bdung River nd the Mti River to the inner of the y. As the zonl current velocity component ecome wek nd the meridionl current velocity component ecome strong (dot circle line in the Figs 12 nd c), the slinity concentrtion ecome high nd mixed well in the outer y (dot circle line in the Fig. 12). The wek zonl current velocity component t the outer y (dot circle line in the Fig. 12) led the low fresher wter flowing Slinity Slinity Fig.11. Slinity distriution.. E tide nd. flood tide.

7 96 HEDRAWA I Gede et l. Act Ocenol. Sin., 214, Vol. 33, o. 3, P. 9 1 out from the inner y. Menwhile, the strong meridionl current velocity (dot circle line in the Fig. 12c) induced higher sewter slinity trnsported from the Bdung Strit to the outer of the y, which leds to well mixed verticl distriution. The fluctution of current velocity chnges with the time since the meteorologicl nd fresh wter inflow lso vry with the time (Lu et l., 212; Lu et l., 211). This model indicted tht the river dischrges into the inner y do not hve significnt influence on the sewter slinity in the outer y. The reltionship etween numericl clcultion nd oservtion results for slinity re shown in Fig. 13. The correltion determintion (r 2 ) given y FVCOM is.75. This indictes tht the numericl model hs good ility for slinity clcultion including fresh wter dischrges. The slinity discrepncy etween the oservtion nd the model cused y the sptil slinity distriution in the inner y ws significntly cused only y the rivers dischrge y the model, however the oservtion results re climtologiclly considered. 4.4 Sewter Exchnge In order to investigte the sewter exchnge in the Beno By, the 3-D Lgrngin prticle trcking ws used. The neutrl uoynt prticles were relesed t surfce lyer. Initilly 144 prticles were relesed uniformly t the mximum flood tide fter model clcultion ecome stle. The prticle positions were clculted over the length of model time step nd recorded t 6 min intervl. Furthermore, the frction of prticle ws clculted sed on the prticles numer, nd the percentges of prticle movement re clculted t specific time. The prticles reching the ottom re treted to e trpped y the ottom. The overll sewter exchnge in the Beno By cn e seen in Fig. 14. The prticles were trnsported rpidly to the ocen. Thirty seven percent of prticles were exported to the ocen immeditely fter they were relesed. Furthermore, 5% of the prticles were exported to the ocen fter four tidl cycles nd incresed 7% fter nine tidl cycles. Bsed on exponentil de- Depth/m Slinity Oservtion slinity Depth/m Model slinity Fig.13. Vlidity of Slinity distriution. Depth/m u/m s v/m s 1 Fig.12. Verticl profile of slinity distriution (), zonl current component (u) (), nd meridionl current component (v) (c) long the cross section. The inset picture in the left is the loction of cross section, nd the inset picture in the right is the tidl condition for the cross section. c Frction /% tidl cycle Fig.14. Overll prticle remining inside of the y.

8 HEDRAWA I Gede et l. Act Ocenol. Sin., 214, Vol. 33, o. 3, P cy eqution of prticle movement, more thn 95% of prticles predicted will e exported into the ocen fter 3 tidl cycles. It mens tht the prticles initilly lid down within the y will e nerly completely trnsported into the ocen fter 1 month. Figure 15 depicts the prticles remining t ech model region. As shown in the western prt region (lck dot-line) nd the centrl prt region (green dot-line), the prticles were trnsported rther rpidly to nother region thn in the estern prt region (lue dot-line) nd southern prt region (red dot-line). In the western prt region, more thn 9% of the prticles were trnsported fter four tidl cycles nd fter two tidl cycles for the centrl prt region. However, the prticles in the southern prt region were trnsported slower thn in oth of the previously mentioned regions. More thn 9% of the prticles were exported to nother region fter nine tidl cycles. In the estern prt region, the prticles were trnsported very slowly thn in other regions. It tkes more thn ten tidl cycles to trnsport the prticles into other regions. Figure 16 shows the prticles trnsport for ech model regions. In the western prt region (the upper-left frme), out 4% of the prticles initilly in western prt region were trns- Frction /% estern prt of y centrl prt of y southern prt of y western prt of y tidl cycle Fig.15. Prticle remining in ech model region to the centrl prt region to the ocen to the southern prt region prticle remining to the centrl prt region to the ocen prticle remining Frction/% 5 4 Frction/% tidl cycle tidl cycle to the ocen to the southern prt region to the western prt region to the estern prt region prticle remining c to the centrl prt region to the ocen to the southern prt region prticle remining d 6 6 Frction/% 5 4 Frction/% tidl cycle tidl cycle Fig.16. Prticles trnsport chrcteristics in ech region.. Western prt region,. southern prt region, c. centrl prt region nd d. estern prt region.

9 98 HEDRAWA I Gede et l. Act Ocenol. Sin., 214, Vol. 33, o. 3, P. 9 1 ported into the ocen fter nine tidl cycles, while more thn 4% of the prticles were trnsported to the centrl region just fter prticles were relesed, however, fter three tidl cycles it decresed to out 2% nd incresed to 4% fter nine tidl cycles. The prticles from the western prt region were lso connected with those in the southern prt region. The southern prt region received prticles from the western prt region fter one tidl cycle nd incresed to 4% fter four tidl cycles. Some prticles from the western prt region were trnsported directly to the ocen connected y the centrl region, while some prticles were trnsported to the southern prt region first efore finlly trnsported into the ocen. As shown in the southern prt of the y (the upper-right frme), more thn 8% of the prticles were trnsported to the ocen nd less thn 2% of the prticles remined in the centrl prt of the y during nine tidl cycles. The prticles from the southern prt region were not connected with those in the western region nd the estern region. It seemed tht the prticles were directly trnsported from the southern region into the centrl prt of the y nd trnsported to the ocen (Fig. 17). The prticles in the estern prt region (the lower-right frme), roughly more thn 3%, were trnsported into the ocen immeditely fter they were relesed, nd this mount incresed to nerly 5% fter nine tidl cycles. Prticles from this region were lso relted to those in the southern prt region. More thn 1% of the prticles were trnsported fter four tidl cycles. The estern prt region trpped out 4% of the prticles fter the sixth tidl cycle. This gives n impression tht the prticles cnnot e trnsported to nother region in the model domin. This cn e known y investigting the residul current pttern, suggesting tht the residul current in the estern prt region went north. Additionlly, the tidl current in the y mouth strongly flowed to the west-est direction. Therefore, the sewter in this region will e difficult to exchnge to c d Fig.17. Prticle trcking, color indicted the different wy distriution.. Western prt region,. southern prt region, c. centrl prt region nd d. estern prt region.

10 HEDRAWA I Gede et l. Act Ocenol. Sin., 214, Vol. 33, o. 3, P the other region. Finlly, more thn 5% of prticles were directly trnsported from the centrl prt of the y (the lower-left frme) into the ocen immeditely fter the prticles were relesed, nd this mount incresed to ove 7% fter eight tidl cycles. Some prticles were lso trnsported into the southern prt region, with more thn 2% trnsported fter two tidl cycles. It is ovious tht the incresing prticles trnsported to the ocen will e ccompnied y decresing prticles trnsported to the southern prt region. A few prticles were lso trnsported to the western nd the estern prt region. Aove results reveled the generl chrcteristics of the sewter exchnge in the Beno By. In generl the frction of prticles hs two wys to find the gte into the ocen. In the first wy, the prticles re directly trnsported into the ocen from ech region, nd in the other wy the prticles trnsported to the southern prt of the y nd then exported to the ocen (Fig.17). However, the prticles in the estern prt of the y re not trnsported into the ocen esily compred with other regions. It could e cused y the residul current (Zhou, 212). The residul circultion ner the open oundry of the estern prt of the y cn e seen in Fig. 1. The prticles in the estern prt would e trpped y this circultion. It would explin why the prticles do not move to the ocen esily. Another wy to investigte the sewter exchnge in the Beno By is y clculting the residence time. The residence time of the prticles could e known y seeing how long the prticles reside in specific re to e flushed. In this study, we used 3 m 3 m moving ox to estimte the residence time (Bilgili et l., 25). Figure 18 shows the time prticles spend in these ox regions s function of their initil conditions. Within the y the prticles spend 1 to 5 h. The figure depicted tht the residence time hs good reltionship with the energy of tidl residul current (Fig. 18). The higher energy of the tidl residul current cused the residence time of the prticles to e short. Wters, surrounding the y mouth such s the south edge of the estern prt region, the est edge of the southern prt region nd few res in the western prt region (red color), hve shorter residence time, which corresponds to reltively high energy of the tidl residul current. On the other hnd, in the rest of the re (green color) with reltively wek energy of the tidl residul current, wters hve longer residence time. 5 Concluding remrks FVCOM is successfully pplied to investigte wter circultion in the Beno By-Bli. The nrrow strit formed in the y mouth doesn't result in high discrepncy vlue for the tidl elevtion, mplitude, nd phse lg compred with oservtion results. A wek tidl residul current is reveled in whole model domin. However, the wekness of residul current rings out n essentil effect on the longtime prticles trnsport. Regrding the impct of fresh wter dischrge from river, the clcultion shows tht verticl slinity profiles in the inner y is significntly ffected y fresh wter dischrge. The slinity computtion suggests tht FVCOM cn predict well the slinity distriution in the Beno By. The chrcteristics of the sewter exchnge re clerly depicted y using the Lngrngin prticle method. Ech region in the model domin hs different wy to trnsport the prticles into the ocen. Overll, nerly 7% of the prticles were exported to the ocen fter nine tidl cycles. Furthermore, the residence time of the prticles in model domin is useful to investigte the sewter exchnge in the Beno By. Acknowledgements We would like to thnk Pllv Koilrl nd Azizul Moqsud for their coopertion to check English text Residence time/h Energy of residul current/m 2 s 2 Fig.18. Residence time () nd Energy of residul current ().

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