1 Cormix SIMULATIONS

The effluent from the LNG terminal will be discharged through the outfall located at south of South Soko Island.

The outfall has the following characteristics:

· Discharge via an outfall and diffuser with four nozzles each at a horizontal angle of approximately 30° (upwards). The nozzles have a diameter of 0.75 m. The schematic design is shown below.

The aim of the CORMIX modelling is here to determine the near field mixing characteristics. These characteristics will be used to set the manner in which the discharge is introduced in the 3D hydrodynamic model.

1.2 Conditions around the outfall locations

From the information that was provided is derived that the outfall is located at (809056, 802436) (Hong Kong 1980 coordinate system). To determine the local hydrodynamic conditions that is used for the CORMIX calculations, model output from the indicated grid (in red as shown in the figure below) cell is used. The cell nearest to the outfall is not appropriate because the hydrodynamic conditions in this cell cannot include the alongshore currents. Hence one grid cell further off-shore is used to determine the conditions.

The hydrodynamic conditions were determined for the wet and dry seasons. These conditions were taken from existing baseline computation (Tables 1.1 and 1.2).

Table 1.1 Wet Season Conditions

Notes:

(a) “bot” denotes the bed

(b) “surf” denotes the surface

(d) The model results are shown in Appendix 6B-1.

Table 1.2 Dry Season Conditions

Notes:

(a) “bot” denotes the bed

(b) “surf” denotes the surface

(d) The model results are shown in Appendix 6B-1.

The conditions in the area around and south of the discharge are during the dry season tide related, although there is no significant difference in current speed or direction between spring and neap tides. The current direction is mainly from the west and there is no stratification. During the wet season, stratification is significant. The density difference between the surface and bed is generally between 5 and 15 kg m^-3. Currents are more chaotic than during the dry season. The influence of the tide on currents is small. In general the current is directed towards the west in contrast to the dry season conditions.

The effluent characteristics are based on the vertically averaged ([1]) ambient temperature and salinity with the temperature reduced by 8.5 °C. Note that the CORMIX model results were used in the far-field Delft hydrodynamic model to simulate cooling water discharge of 12.5 °C reduction. This increase of the temperature difference increases the density of the discharge by approximately 0.7-0.8 kg m^-3, compared to a density difference between discharge and ambient of about 2-6 kg m^-3. The CORMIX results (the results for both temperature reduction are enclosed in Appendix 6B – 1) show that the difference due to the change in the discharge density is not significant. The dilution factors are the same to the nearest tenth. There is a small difference in the size of the plume, the thickness of the plume reduces by around 20 cm after a travelling distance of over 3 km, well beyond the near field. At the edge of the near field, there are no significant differences in the CORMIX results. This means that there is no consequence in terms of defining the discharge into the far-field model and that the manner in which the discharge is introduced into the Delft3D model remains correct.

([1]) In considering that the hydrodynamic model has simulated one condition (with averaged forcing), effects of varying wind, stormy conditions etc, are not taken into account in the model results. Hence actual conditions will vary compared with the conditions in the model, and to use the vertically averaged conditions is more likely to represent the characteristics of the water that is drawn in at the intake.