Result of suspended solid distribution

Figure 5.2 shows the results of suspended solid (SS) distribution in the surface layer without freshwater input (barotropic condition) when the south wall of reclamation collapses and when the low tide occurs. The SS is released to the bay in four times, that are at neap tide, the transition time from neap to spring tide, spring tide, and at spring to the neap tide. The red color is indicating the high concentration of SS which contained Hg and has concentration around 0.01 kg/m³, this value is set in order to make area of SS distribution more visible. Meanwhile, blue indicates the low concentration of SS. It can be seen that at low tide, the SS is moving toward southwest entering Minamata Bay, and at spring tide (Figure 5.2c), the transport is slightly farther than at neap tide (Figure 5.2a). Meanwhile, at transition times, the transport are in between neap and spring tide cases. These differences can be caused by the differences of current velocity, which is high at spring tide and low at neap tide.

Figure 5.2. SS distribution by the damage of south reclamation wall without freshwater inflow in surface layer at low tide on a) the neap tide, b) the neap to spring tide, c) the spring tide, d) the spring to neap tide.

76

Figure 5.3. SS distribution by the damage of south reclamation wall without freshwater inflow in surface layer at high tide on a) the neap tide, b) the neap to spring tide, c) the spring tide, d) the spring to neap tide.

Meanwhile at high tide (Figure 5.3), the transports of SS are quite similar in each time.

The SS also moves to Minamata Bay, but it is not as far as low tide occur (Figure 5.2).

There is also no significant difference among the spring, neap, and transition tide. The tidal current at high tide which moves SS to the shore is not as fast as at low tide moves SS entering the bay.

In the case with riverine freshwater inflow (baroclinic), the SS distribution results are shown in Figure 5.4 and 5.5. Different from the barotropic case, here the SS transports are seen significant where the red color is spreading almost in the entire area of the bay.

From these results, the baroclinic flow can be expected is more significant for transporting the SS than barotropic flow. The freshwater inflow has an important role of hydrodynamic conditions around the bay. Although not so significant, the difference also can be seen at low tide in spring tide (Figure 5.4c), and the area of SS spreading from the damaged wall to Minamata Bay is larger than in neap tide (Figure 5.4a).

At high tide (Figure 5.5), the areas of distribution in neap and spring tide are quite similar, and it can be seen that the SS moves toward northeast entering the bay, in opposite direction when low tide occurs. This result also indicates that the baroclinic flow is possible to make the magnitude of current in neap and spring tide quite similar and different with the barotropic flow, because of the effect of freshwater inflow and stratification around the bay.

77

Figure 5.4. SS distribution by the damage of south reclamation wall with freshwater inflow in surface layer at low tide on a) the neap tide, b) the neap to spring tide, c) the spring tide, d) the spring to neap tide.

Figure 5.5. SS distribution by the damage of south reclamation wall with freshwater inflow in surface layer at high tide on a) the neap tide, b) the neap to spring tide, c) the spring tide, d) the spring to neap tide.

5.3.1.2 North wall damage

Figure 5.6 shows the SS distributions in the surface layer when the north part of the reclamation wall collapses in barotropic condition and when the low tide occurs.

Meanwhile, Figure 5.7 shows the result of high tide condition. Here, the transport of SS at the neap, spring, and transitions times also do not show significance difference.

However, the transport at spring tide (Figure 5.6c) is slightly larger than at neap tide (Figure 5.6a). Likewise the low tide condition, the high tide condition (Figure 5.7) also shows a similar pattern of transport, but here the northern transport is more visible than at low tide.

78

Figure 5.6. SS distribution by the damage of north reclamation wall without freshwater inflow in surface layer at low tide on a) the neap tide, b) the neap to spring tide, c) the spring tide, d) the spring to neap tide.

Figure 5.7. SS distribution by the damage of north reclamation wall without freshwater inflow in surface layer at high tide on a) the neap tide, b) the neap to spring tide, c) the spring tide, d) the spring to neap tide.

The damage of north part on reclamation wall could move the SS around the reclamation area and Koiji Island, the small island in front of it. Area of distribution is also smaller than the case that the south part of reclamation wall collapses, because the island can block the spreading of SS to Minamata Bay. Here we can see that on the north side of Minamata Bay, the SS transports at high tide (Figure 5.7) is farther than that at low tide (Figure 5.6) due to the position of wall damage.

The influence of freshwater inflow is shown in Figure 5.8 and 5.9 below. Similar to the previous case, we can also see that the baroclinic flow give significance difference to the SS distribution around the bay. At low tide (Figure 5.8), the transport of SS move towards the bay. While, at high tide (Figure 5.9) it is seen moving to the shore and north

79

part from the bay. The distribution of SS in north wall damage seems less spread than the case of south wall collapse as we can see that the red zone with higher SS is not as much as the case of the south wall damage.

Figure 5.8. SS distribution by the damage of north reclamation wall with freshwater inflow in surface layer at low tide on a) the neap tide, b) the neap to spring tide, c) the spring tide, d) the spring to neap tide.

Figure 5.9. SS distribution by the damage of north reclamation wall with freshwater inflow in surface layer at high tide on a) the neap tide, b) the neap to spring tide, c) the spring tide, d) the spring to neap tide.

5.3.1.3 Suspended solid transport after 30 days

After 30 days of SS discharge in spring tide (June 15^th, 2003), the SS movement on each condition in surface and bottom layer are shown in Figure 5.10 and Figure 5.11 below. Figure 5.10a and 5.10b show the SS transport when the south part of reclamation wall collapses and Figure 5.10c and 5.10d when north wall collapse in barotropic condition. Meanwhile, Figures 5.11a-d show results in baroclinic condition when south

80

wall and north wall collapse, respectively. After 30 days, the SS concentration around the bay is reducing and the plots here show that the highest concentration (red color) are around 1x10^-3 kg/m³ of SS concentration.

Figure 5.10. SS distribution after 30 days without freshwater inflow by the damage of south reclamation wall in a) Surface and b) Bottom layer and the damage of north reclamation in c) Surface and d) Bottom layer.

The results of SS transport in the surface (Figure 5.10a and 5.10c) and bottom (Figure 5.10b and 5.10d) on barotropic condition show quite a similar pattern, where the SS mostly transported to Fukuro Bay, a small semi-enclosed bay in Minamata Bay. It is expected to be in the same pattern, because barotropic condition did not calculate stratification effect and resulting in similar pattern and direction of SS transport. The position of the damaged wall also gives different results, which the SS released from south wall damage (Figure 5.10a and 5.10b) has much higher concentration of SS than the one from north wall damage (Figure 5.10c and 5.10d). As explained above, the south wall damage is more exposed to the bay than the north wall damage which is blocked by Koiji Island. That can be the cause of the higher concentration of SS in Fukuro Bay when south wall collapse than when the north wall collapse.

81

Figure 5.11. SS distribution after 30 days with freshwater inflow by the damage of south reclamation wall in a) Surface and b) Bottom layer and the damage of north reclamation in c) Surface and d) Bottom layer.

Meanwhile, the results of the baroclinic case (Figure 5.11) show that the SS concentration after 30 days almost washed away from the bay. Only a small amount of SS concentration is also found in Fukuro Bay. On the surface layer (Figure 5.11a and 5.11c), the SS has been removed from Minamata Bay meanwhile on the bottom layer (Figure 5.11b and 5.11d), a small amount of SS seem still been concentrated in Fukuro Bay. This result indicates that the baroclinic flow in the surface layer is faster than in bottom layer