Flux of salinity at sea surface Qs, is given as follows:
Qs = S(Pr - Ev) (21)
where
Ev: Evaporation of water at the sea surface
Pr: Precipitation at the sea surface
Boundary conditions for T and S at the open boundary are given by the zero-gradient for outflow and by the observed values for inflow.
3 SIMULATION IN A MODEL BAY
3.1 Numerical conditions
We calculate the tidal and density driven currents around a Mega-Float model(L × B × d = 3km × 1km × 2m) which is located 5km offshore in a rectangular bay 20km x 15km x 20min size as shown in Fig.1. A flat-plate type breakwater(dBW = 5m) apart from the Mega-Float is included in some calculations. A river is assumed at the center of the coast with the discharge rate of R = 500m Tidal oscillation during twelve hours period is given at the open boundary with (0.5m) in tidal amplitude.
Mesh size is 500m in a horizontal direction(Δx = Δy) and five vertical levels are set (Δz = 2to6m).Eddy viscosity and diffusivity are assumed to be the same value, as Am = AC = 100?/sec and KM = KC = 0.001?/sec for simplicity. Time step is set as (Δt = 10sec) and the simulation is conducted for 240 hours(10days). The initial conditions are assumed to be: water temperature(Tw = 20℃), salinity (S = 30‰), atmospheric temperature(Ta = 27℃), relative humidity(f = 80%), cloudiness(n = 0.5), and solar energy at the sea surface (Qs = 70kcal/?) etc.
The following five cases are demonstrated in this study:
Case 1. (Bay, River, Solar Energy)
Case 2. (Case 1) + Breakwater
Case 3. (Case 1) + Mega-Float(3km × 1km)
Case 4. (Case 1) + Breakwater + Mega-Floa(3km × 1km)
Case 5. (Case 1) + Mega-Float(3km × 3km)
3.2 Results and Discussion
Fig.2(a) and (b) show the vertical distributions of temperature and salinity at the center of the bay 240 hours after the initial conditions in Case 1. Temperature distribution is found to be almost horizontal in this case, but the salinity is less near the mouth of the river and the sea surface, which seems reasonable.
Fig.3(a),(b),(c) and (d) show the vertical distributions of temperature, salinity, density and tidal residual vectors in the center of the bay for Case 4. Comparing the temperature distribution with Fig.2(a), the temperature below the Mega-Float is a little bit low due to the obstruction of sunlight in this case, but the difference is surprisingly small between the two cases. This can be explained by the fact that the convection of water is more important than its conduction in this case.
Comparing Fig.3(b) with Fig.2(b), the blocking effects by the breakwater are found to be significant in the distribution of salinity. Fig.3(c) shows the distribution of the density, which is calculated by the temperature and salinity in (9). The difference of the density from Case1 is small, because the temperature is more dominant in the density than the salinity.
From these results, the effect of Mega-Float on the density current is not significant in this case.
Fig.4(a) and (b) show a comparison of the horizontal distribution of the salinity near the sea surface. The diffusion pattern is different near the Mega-Float and the breakwater, but the quantitative difference is not large so that the difference in density is probably minimal.
Fig.5(a), (b) and (c) show the vertical distribution of the temperature, the salinity and the density at the center of the Mega-Float under five combinations of the Mega-Float and a breakwater. Mega-Float(6 × 6) in the legend denotes the Mega-Float(L x B = 3km × 1km) for the comparison. In Fig.5(a), distributions of the temperature are almost the same, except for a slight difference with the breakwater and Mega-Float(6 × 6) combination.In Fig.5(b), the breakwater effects are found to be important in the distribution of the salinity, in Fig.5(c), however, there is only a slight difference among the five cases, because the density is mainly dominated by the temperature.
4 CONCLUSION
The following conclusions were drawn by some of the numerical demonstrations:
1. A multi-level model has been extended to analysis of the tidal flow and the density field around a Mega-Float in a bay. Numerical results in a simplified bay show the validity of the method.
2. The shadowed area by a Mega-Float does not have a significant effect on the temperature or density distributions.
3. The breakwater influences the distribution of the salinity significantly, but the effects on density are small.
REFERENCES
1) Kyozuka,Y.: Ocean environmental change by a huge offshore structure in a bay, International Conf. on Tech. Marine Environ. Preservation, Vol.1(1995), pp.489-496.
2) Inoue, Y., Tabeta, S., Arai, M. and Nakazawa, K.: Feasibility design of a floating airport and investigation into its effects on environment, Journal of the Society of Naval Architects of Japan, Vol.176(1994), pp. Japanese).
3) Hu C. and Kyozuka, Y.: Tidal simulation of a bay with a very large floating structure using a Multi-Level Model, 1996 OMAE, VolI-Part B, Offshore Technology, ASME(1996), pp.23-30.
