Fujihara, Masayuki

Izumi, Tomoki

Kanaguchi, Takaaki

English

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Conference Paper, Published VersionIzumi, Tomoki; Kanaguchi, Takaaki; Fujihara, MasayukiNumerical Investigation of Seaweed Expansion byConstructing Grounds on a Filled Sea CaldronZur Verfügung gestellt in Kooperation mit/Provided in Cooperation with:Kuratorium für Forschung im Küsteningenieurwesen (KFKI)Verfügbar unter/Available at: https://hdl.handle.net/20.500.11970/99510Vorgeschlagene Zitierweise/Suggested citation:Izumi, Tomoki; Kanaguchi, Takaaki; Fujihara, Masayuki (2014): Numerical Investigation ofSeaweed Expansion by Constructing Grounds on a Filled Sea Caldron. In: Lehfeldt, Rainer;Kopmann, Rebekka (Hg.): ICHE 2014. Proceedings of the 11th International Conference onHydroscience & Engineering. Karlsruhe: Bundesanstalt für Wasserbau. S. 853-858.Standardnutzungsbedingungen/Terms of Use:Die Dokumente in HENRY stehen unter der Creative Commons Lizenz CC BY 4.0, sofern keine abweichendenNutzungsbedingungen getroffen wurden. Damit ist sowohl die kommerzielle Nutzung als auch das Teilen, dieWeiterbearbeitung und Speicherung erlaubt. Das Verwenden und das Bearbeiten stehen unter der Bedingung derNamensnennung. Im Einzelfall kann eine restriktivere Lizenz gelten; dann gelten abweichend von den obigenNutzungsbedingungen die in der dort genannten Lizenz gewährten Nutzungsrechte.Documents in HENRY are made available under the Creative Commons License CC BY 4.0, if no other license isapplicable. Under CC BY 4.0 commercial use and sharing, remixing, transforming, and building upon the materialof the work is permitted. In some cases a different, more restrictive license may apply; if applicable the terms ofthe restrictive license will be binding. 1 INTRODUCTION The Seto Inland Sea is a representative semi-enclosed coastal sea in Japan, and consists of several basins which are connected by narrow channels or straits. Around channels or straits, there are many sea cal-drons resulting from submarine erosion by tidal currents. In some of them, the accumulation of marine sediments worsen the environment of fishing grounds. For example, fishing activities such as the fixed net fishing may become difficult, and bottom water often becomes oxygen-deficient during summer when thermocline is dominant.  On the other hand, coastal area of the Seto Inland Sea is one of the most industrial areas in Japan. In the 1960s and early 1970s, land reclamation was intensively implemented to build many facilities for heavy industry. Consequently, more than half of the marine forest or seaweed bed that existed in the early 1960s has been lost (Takeoka, 2002). However, seaweed beds have high primary production capability and play a vital role in the aquatic ecosystems (Okuda, 2008). Thus it is very important to conserve sea-weed beds. One of the measures to improve such the environment is to construct seaweed bed by filling up the caldrons. In the present study, to assess a seaweed bed newly constructed on a filled caldron, tidal current is firstly numerically estimated using a multi-level density flow model. Then the possibility for the self-formation of seaweed beds (Sargassum beds) on the filled area is examined based on the estimated reach of the germlings which are released from the natural Sargassum bed and transported by the estimated tid-al current, using the germling particles tracking method. 2 NUMERICAL MODELS A multi-level density flow model and an Euler-Lagrangian transport model (Fujihara et al., 1997; Nakata et al., 2000) are employed for tidal current simulation and germling particles tracking, respectively. Numerical Investigation of Seaweed Expansion by Constructing Grounds on a Filled Sea Caldron T. Izumi & T. Kanaguchi Ehime University, Matsuyama, Japan M. Fujihara Kyoto University, Kyoto, Japan  ABSTRACT: Effects of a fishing ground newly constructed on a filled sea caldron in order to improve the environment of fishing grounds are numerically investigated in terms of the possibility for self-formation of seaweed beds. Tidal current simulations before and after filling up a caldron are firstly car-ried out by use of a multi-level density flow model. Then, Sargassum horneri being selected as a target species, the germlings released from the natural Sargassum bed are tracked based on the tidal current simulations by use of an Euler-Lagrangian transport model. From the results, the flow velocity get fast at the filled area after construction of fishing ground, and the germling particles cover approximately 60% of the filled area by three years under neglecting the survival ratio of germlings. Thus it is concluded that Sargassum bed would expand naturally on the filled area from year to year.  Keywords: Sargassum bed, Sea caldron, Tidal current simulation, Multi-level density flow model, Germling particles tracking, Euler-Lagrangian transport model  ICHE 2014, Hamburg - Lehfeldt & Kopmann (eds) - © 2014 Bundesanstalt für Wasserbau ISBN 978-3-939230-32-88532.1 Tidal current simulation The multi-level density flow model consists of the momentum equation with f-plane and hydrostatic ap-proximations, the continuity equation, the equation of free surface, the diffusion equations of water tem-perature and salinity, and the Knudsen's equation of state. h h h h v( ) 1·( )) ·(wt z zf pzν νρ∂+ + × = − ∇∂ ∂ ∂ + ∇ ∇ +  ∂ ∂ ∂∇  +∂u uuu k u uu   (1) h· 0wz∂∇ + =∂u   (2) zp gdzη ρ= ∫   (3) h· 0H dztηη−∂ + ∇ =∂ ∫ u   (4) h h Th h Tv( )·( ) ·( )wTT KT TT Kt z zz∂ ∂ ∂ + ∇ ∇ +∂+ =  ∂ ∂ ∂∇∂ u   (5) h h ch h cv( )·( ) ·( )Cl ClClwClClz zK Kt z∂ ∂∂+ = ∇ ∂ + ∇ ∇ +  ∂ ∂ ∂∂ u   (6) ( ),T Clρ ρ=   (7) where u is the horizontally two-dimensional current vector, w the vertical current velocity, T the water temperature, Cl the salinity, ρ the water density, g the gravity acceleration, f the Coriolis parameter, η the free surface elevation from the mean sea level (positive upward), H the bottom depth from the mean sea level, ∇h the horizontal differential operation, vh and vv the horizontal and vertical kinematic eddy viscosi-ty coefficients, respectively, KTh and KTv the horizontal and vertical turbulent diffusion coefficients of wa-ter temperature, respectively, Kch and Kcv the horizontal and vertical turbulent diffusion coefficients of sa-linity, respectively, and k the unit vector (positive upward). The shear stress at the bottom is expressed as  2vz Hzν γ=−∂ =∂uu u   (8) where γ 2 is the bottom friction coefficient. These equations are discretized by the finite difference method on the staggered grids. In the discreti-zation, the explicit time-marching scheme is employed for unsteady terms, the second-order upwind scheme for convection terms, and the central difference for diffusion terms. 2.2 Germling particles tracking The Euler-Lagrangian transport model tracks particles of seaweed germlings which are transported in the flow fields computed by the above-mentioned flow model. The governing equations are as follows: ddt=X V   (9) 0(0) =X X   (10) ( 1)* ( ( ))n n tn ++ = ∆FX X   (11) ( ) ( 1)( 1) (*)2nn tnn+ ++ + ∆= FX X F   (12) ( ){ }( ) ( ) ( ) ( )n n n tn+ ⋅∇ ∆=F V V V   (13) where X is the position vector, V the velocity vector in which settling velocity of germlings is taken into consideration, ∇ the differential operation, ∆t the time increment; * denotes the temporary value at the time level, and n the time level. The initial value problem described in Eqs. (8) and (9) is solved using the two-step Runge-Kutta method described in Eqs. (10)-(12). 3 STUDY AREA AND COMPUTATIONAL CONDITIONS The study area is located in the Seto Inland Sea, Japan, and the area with contour of water depth is shown in Figure 1. In the dotted frame, there is a sea caldron whose original maximum water depth is 27 m. An 8544 RESULTS 4.1 Change of tidal current Computed flow fields during four cases at 1 m deep before and after filling the caldron are shown through Figure 2 to 5.  As shown in Figure 2, the main flow direction is south-east at the flood tide. Difference of the flow ve-locities between two flow fields is found at the central part of the strait and the filled area. While the max-imum flow velocity at the filled area before construction becomes approximately 53 cm/s, after construc-tion becomes approximately 64 cm/s. Figure 3 shows that the main flow direction is north-west at the ebb tide. Difference of the flow fields is clearly found at the filled area; the maximum flow velocities at the filled area before and after construction are approximately 66 cm/s and 72 cm/s, respectively. In Figure 4 and 5, flow is very slow because of the slack water. Differences of before and after flow fields are thus invisible.    Figure 2. Computed flow fields before (left) and after (right) filling at maximum flood velocity (Case 1).    Figure 3. Computed flow fields before (left) and after (right) filling at maximum flood velocity (Case 2).    Figure 4. Computed flow fields before (left) and after (right) filling at low tide (Case 3). 50 cm/s40 cm/s30 cm/s20 cm/s10 cm/sLandIsland50 cm/s40 cm/s30 cm/s20 cm/s10 cm/sLandIsland50 cm/s40 cm/s30 cm/s20 cm/s10 cm/s50 cm/s40 cm/s30 cm/s20 cm/s10 cm/s50 cm/s40 cm/s30 cm/s20 cm/s10 cm/s50 cm/s40 cm/s30 cm/s20 cm/s10 cm/s856

Numerical Investigation of Seaweed Expansion by Constructing Grounds on a Filled Sea Caldron

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Numerical Investigation of Seaweed Expansion by Constructing Grounds on a Filled Sea Caldron

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