METHOD OF WATER QUALITY IMPROVEMENT BY USE OF ICE FLOE CONTROL STRUCTURE

Transcription

1 METHOD OF WATER QUALITY IMPROVEMENT BY USE OF ICE FLOE CONTROL STRUCTURE Shunsuke Makita 1 and Hiroshi Saeki 1 Graduate School of Engineering, Hokkaido Universit, Sapporo, Japan Hokkaido Universit, Sapporo, Japan ABSTRACT In Hokkaido, Japan, the aquaculture has been performed in man semi-closed water areas. Ice control structures to prevent the invasion of ice floes had started to be installed at inlets of some of those areas. However, these structures have been used onl during winter. In this paper, the effective use of these structures aiming the improvement of the water qualit of these areas is proposed. B numerical analses, the became clear that it is possible to suppl the seawater to the center part of the area b use of this structure as a training dike at the time of inflow, and in the case that the water area has two or more inlets, it is possible to transfer the seawater b producing the water current in an arbitrar direction b use of this structure as a gorge at one inlet at the time of outflow. KEY WORDS: Ice control structure; Ice boom; semi-closed water area; water echange; training. INTRODUCTION In Hokkaido, more than 3 semi-closed water areas eist, and aquaculture has been performed in man of those water areas. Since semi-closed water areas connected to open sea b a narrow inlet are ver calm water area, such areas are suitable for the aquaculture of scallops, osters, and other marine products. However there is also the risk that the water qualit ma suddenl decrease with change of the surrounding environment. For permanent use for aquaculture, maintenance of the water qualit is needed. As one of the water qualit preservation methods, promotion of water echange has been pointed out. Man of these water areas are located on the coast of the Okhotsk Sea. Ice floes normall reach the coast in winter; tides cause ice floes to pass through inlets into the water area. These ice floes have caused serious damage, such as destruction and wear, to aquaculture facilities. To prevent such damage, ice control structures had started to be installed at inlets of some of those areas. These structures are composed b some piles attached to the seabed and some ice booms installed between these piles. However, ice booms are removed and these structures have not been used for periods other than winter. In this paper, the effective use of these structures aiming the improvement of the water qualit of these areas is proposed

2 METHOD OF ANALYSIS Model of numerical simulation For numerical simulation, a multi-level-model was used (Fujihara, 199). In this model, the object is divided b mesh for horizontal direction, and is divided b laer for vertical direction. The variation of the thickness of laer is considered onl for the surface laer, and the vertical flow components are considered for underling laers. Therefore this model is suitable for analsis of flow conditions in a semi-closed water area where vertical flow such as the lifting of nutrients b upwelling is important. As basic equation, the continuit equation is shown in equation (1) and the momentum equations are shown in equation () and equation (3). divu u v w = + + z (1) M k ζ = ( M kuk) ( M kvk) ( uw) + ( uw) gh k k+ 1 k t Mk Mk 1 k 1, k k, k N + N + ( τ τ ) τ f ρ ρ Nk ζ = ( Nkuk) ( Nkvk) ( vw) + ( vw) gh k k+ 1 k t Nk Nk 1 k 1, k k, k N + N + ( τ τ ) τ f ρ ρ () (3) M k, Nk ; Linear quantit, N, N ; Coefficient of horizontal vorte viscosit, ς ; Water level, hk ; Thickness of laer, ρ ; Water densit, k ; Number of laer, τ, τ ; Frictional force between laers calculated from following equations τ = ργ u u + v, τ = ργ v u + v (4) γ ; Coefficient of friction (Coefficient of friction at water surface; bottom; γ b, Coefficient of internal friction of water; i γ a, Coefficient of friction at γ ), u,v ; Water velocit The terms shown in underlined part of the momentum equations are terms of resistance forces of aquaculture facilities installed in water areas. And these resistance forces are calculated b equation (5). Coefficients of resistance are set based on two coefficients of resistance of aquaculture facilities obtained from the eperiment b authors (Makita, 4). 1 1 τ = ρcu u + v, τ = ρcv u + v (5) C ; Coefficient of resistance -138-

3 Method of evaluation of water echange The water echange was evaluated b performing mass transfer simulation for a diffusion mass initiall diffused to a uniform concentration in the semi-closed water area. Diffusion mass transfer was calculated b the adjective diffusion equation (6). The initial concentration of diffusion mass was 1. in the semi-closed water area, and. in the open sea. Therefore the area in which the concentration of a diffusion mass decreases shows the area into which the water of open sea flowed, and it is possible to evaluate the amount of outflow of water of a lagoon b the residual rate of a diffusion mass. SS SS SS SS = ( uss ) ( vss ) ( wss ) + K + K + K z t z z z (6) SS ; Concentration of diffusion mass, K, K, K ; Coefficient of horizontal diffusion Coefficients Coefficient of friction at water surface; Although the coefficient varies complel according to wind, flow, etc., the following constant is often used. 3 γ a = 1.3 Coefficient of friction at bottom; In the multi-level-model, the following constant obtained b Hasen through tidal current observation is often used. This value was used in this calculation. 3 γ b =.6 Coefficient of internal friction of water; The value that is 1/~3/4 of the coefficient of friction at bottom is commonl used. The following value was used in this calculation. 3 γ i =.5 Coefficient of horizontal diffusion; It is known that this coefficient is proportional to the scale of motion raised to the 4/3 powers, and calculated using the following equation (7). z 4 3 K = K = cl (7) c ; Constant (about.1) and L is the grid size The grid size of this simulation was m, and the following value obtained using equation (7) was used in this calculation. K = K = 4.64 (cm sec) Coefficient of vertical diffusion mass; Although the value calculated from Richardson number, is proposed, in the multi-level-model, the following constant is often used. K = z 1. (cm /sec) -139-

4 Coefficient of horizontal vorte viscosit; Although this coefficient varies according to the state and scale of motion, a value nearl equal to the coefficient of horizontal diffusion is commonl used. The value equal to the coefficient of horizontal diffusion was used in this calculation. N = N = 4.64 (cm sec) Coefficient of resistance of aquaculture facilities; Coefficients of resistance in the equation (5) are set Table 1 Coefficients of resistance based on two kinds of coefficients of resistance of 3 aquaculture facilities, the coefficient in the state where a Parallel C 1 =. flow acts parallel to the main line of the long-line formula cultivating facilit, and the coefficient in the Perpendicular C =. state where a flow acts perpendicular to it. Coefficients of resistance of aquaculture facilities are shown in Table 1. In the state that the flow is diagonal to the main wire, resistance force is calculated from these tow coefficients and the action angle of the flow based on the eperimental result b authors (Makita, 4). Object of simulation The object of this simulation is the A D Saroma Lagoon, which is a tpical semi-closed water area in Hokkaido. The B Inlet No. 1 Inlet No. C lagoon is located on the Coast of the Okhotsk Sea and is used intensivel for aquaculture of shellfish, such as scallops and osters. This lagoon is connected to the open sea b two inlets. Ice control structures have been installed at both km inlets. In this simulation, the control of the flow was made onl b the ice Figure 1 Range of the simulation control structure at the inlet No. 1. It is composed b 14 piles and 13 ice booms installed between them. The range of simulation was the Saroma Lagoon and the area 5km offshore from inlet No. 1. Figure 1 shows the range of the simulation, and the simulation was made b dividing the range into a m-mesh. As the boundar condition, Sommerfeld s open conditions were set for AB, CD and DA, as accesses leading to the open sea. Variations in eternal tide for a 4-hour period and a semi-tide range of.4m were given onl to boundar AD. In the figure, the area of hatching shows the area in which the aquaculture facilities are installed, and the direction of the line shows the direction of main line of facilities. Reliabilit of simulation In the result of the simulation, the vorte flow occurred in a position 3m from inlet No. at the time of inflow. In the local observation, it is reported that the vorte of separation occurs in the almost the same position. This can confirm that the setup of coefficients, such as the coefficient of -14-

5 vorte viscosit, is appropriate. Figure shows the vector diagram of the flow around the inlet No.. The calculated value of the flow velocit at the lagoon inlet is equivalent to the observed value. Figure 3 shows velocities at the inlet. 5 Vorte flow 3m Figure Vector diagram around inlet No. Velocit (cm/sec ) Difference of water level (m) Figure 3 Velocit at inlet Calculated value Observed value Flow control b use of ice control structure Following matters became clear from results of the analsis b authors (Makita, ). At the semi-closed water area, inflows from inlets are linear jet flows and seawater does not easil supplied to the center part of the lagoon. When the amount of outflow at one inlet decreases due to a gorge, such as an accumulation of ice floes, the amount of outflow at another inlet increases, and the water current in a direction from one inlet to another inlet is produced. In this analsis, following possibilities were eamined. The possibilit to suppl the seawater to the center part of the lagoon b use of the ice control structure as a training dike at the time of inflow, and the possibilit to transfer the seawater to the center part b producing the water current in an arbitrar direction b use of this structure as a gorge at one inlet at the time of outflow. Figure 4 shows the location of the ice control structure on the mesh. In addition, m-mesh is Inlet No. 1 nested at the gra area. 13 spans of ice boom were divided into the section A, form the 1 st to the 6 th span, and the section B, from the 7 th to the 13 th span. The calculation processing in each section is summarized in the Table. Pier Section A Section B Ice Booms Figure 4 Location of ice control structure Table Calculation processing Section A Obstruct the flow at both of the time of inflow and the time of outflow. Section B Act the resistance to the flow onl at the time of outflow

6 RESULT OF SIMULATION AND DISCUSSION Suppl of seawater The simulation was made in the state that the flow is and in the state that the flow is. The vector diagrams of the flow, at the time when the velocit of the inflow is the maimum, are shown in Figure 5 and Figure 6. It can be confirmed that the inflow from the inlet No. 1 is trained to the center part of the lagoon in the state that the flow is. The isoconcentration maps of the diffusion mass, after 1 ccle of tide, are shown in Figure 7 and Figure 8. In the state that the flow is, the area where the concentration of the diffusion mass is 1., that is, the area to which the seawater is not supplied, remains at the center part of the lagoon. In the state that the flow is, the area to which the seawater is supplied is reduced but the area where the concentration of the diffusion mass is 1. dose not remain. Figure 5 Vector diagram - Figure 6 Vector diagram Figure 7 Isoconcentration map - Figure 8 Isoconcentration map - The maimum flow velocit in each state is shown in Figure 9. In the figure, the velociti is shown b a positive value in the case of the inflow from the open sea to the lagoon, and is shown b a negative value in the case of outflow. In the state that the flow is, the velocit at the inlet No. 1 decreases and the velocit at the inlet No. increases. Velocit (m/sec) inflow outflow Inlet No. 1 Inlet No. Figure 9 Velocit at each inlet -14-

7 Production the water current The rate of quantit shown in Figure, epresses the role of each inlet in the water echange. The inlet No. 1 plas the role of inflow and the inlet No. plas the role of outflow even in the state that the flow is, both of the inlet pla each role more greatl in the state that the flow is. It is guessed that the amount the water that flows in through the inlet No. 1 and flows out through the inlet No. increases, and the water current in direction form inlet No. 1 to the inlet No. is promoted. Figure 11 shows the residual rate of the diffusion mass. The rate is.87 in the state that the flow is, and.89 in the state the flow is. It is confirmed that the amount of water echanged during 1 ccle of tide decreases. Rate of quantit Residual rate Rate of quantit= 1.5 Inlet No. 1 Inlet No. ontrolled ( quantit of inflow ) + ( quantit of outflow) ( average of the absolute value of quantit) Figure Rate of quantit Figure 11 Residual rate Water echange of ccles period of tide So far, the water echange performed during 1 ccle of tide has been eamined. Henceforth, the water echange performed during ccles is eamined. The isoconcentration maps of the diffusion mass are shown in Figure 1 and Figure 13. In the state that the flow is, the area to which the seawater is supplied is formed in the vicinit of the inlet No. 1. In the state that the flow is, it is formed in the vicinit of the center of the lagoon, and the interval of the contour line is wider in the side of the center of the lagoon than in the side of the inlet No. 1. It is guessed that the seawater is transferred b the water current in direction form inlet No. 1 to the inlet No.... Figure 1 Isoconcentration map - Figure 13 Isoconcentration map

8 The change of the amount of diffusion mass during ccles is shown in Figure 14. It is confirm that the residual rate of the diffusion mass, for ccles of tide, is higher in the state the flow is than that in the state the flow is and the difference of the rate increase at the ccle. Residual rate Ccle of tide From the above-mentioned, it is confirmed that the seawater can be Figure 14 Residual rate supplied to the center part of the lagoon b the flow control using the ice control structure, but the amount of the water echange decrease. Thus, it can be said that the utilization plan, such as limitation at the period of the instillation of the structure, is needed. CONCLUSIONS In conclusion, the possibilit of improvement of the water qualit of the semi-closed water area is summarized as follows. 1) It is possible to suppl the seawater to the center part of the area b use of the ice control structure as a training dike at the time of inflow. ) It is possible to transfer the seawater b producing the water current in an arbitrar direction b use of Ice control structure as a gorge at one inlet at the time of outflow, in the case that the water area has two or more inlets. 3) The total amount of the water echange decrease b the inference of the flow control, the utilization plan, such as limitation at the period of the instillation of the structure, is needed. REFERENCES Fujihara, M., Akada, S. and Takeuchi, T. (199), Development of multi-level densit flow model and its application to the upwelling generated b artificial structures, Tech. rept N.R.I.F.E aquacul. & fish. port 14, pp13-35 Makita, S. and Saeki, H. (4) Influence of the aquaculture facilit to the water echange at the semi-closed water area, Annual Journal of Civil Engineering in the Ocean, JSCE, vol., pp Makita, S., Saeki, H. and Furua, A. (), Effect of ice on water flow at Saroma lagoon, Proceedings of 16 th International Smposium on Ice (IAHR ), Dunedin, New Zealand, pp