Ocean Current Modelling & Investigation of the Thermohaline Structure of the Canal de la Havannah in New Caledonia for Inco Goro Nickel - Nov 2002

GEMS was contracted to carry out current measurements and plume dispersion modelling in the Canal de la Havannah in New Caledonia for the INCO GORO Nickel Refinery project. The current measurements were required to provide a better understanding of circulation in the vicinity of the proposed outfall pipeline and along the channel where the discharge plume will disperse. Measurements of bathymetry and temperature/density structure were also required.

GEMS was also requested to undertake far field dispersion modelling to investigate the fate of the plume discharged from the outfall diffuser. In particular the likely spatial concentrations of Manganese were to be determined.

The scope of works for the field required that:

1. A course resolution bathymetric survey be carried out along an extension to the existing proposal for the pipeline route. The proposal was to survey a section 2kms long and 400 metres wide extending south east of the proposed diffuser site.

2. Profiles of temperature and conductivity be measured on deployment and retrieval of each current meter mooring. The purpose of this part of this study was to obtain an indication of the stratification (if any) during each measurement period.

3. Current speed and direction profiles be measured at up to 15 sites in the Canal de la Havannah. One mooring was to be placed near to the proposed site of the diffuser for at least 30 days, and another two moorings were to be relocated regularly to measure currents at the remaining sites for a minimum of 2 days.

Physical and chemical parameter profiles of the seawater were measured at one metre intervals with a Hydrolab H2O water quality multiprobe.  Vertical profiles of currents were measured with three Acoustic Doppler Current Profilers (ADCP) mounted in a frame and deployed in a purpose designed mooring array. 

GEMS undertook the discharge plume dispersion study using the GEMS 3D ocean model, GCOM3D, and the GEMS 3D plume model, PLUMETRAK.  GCOM3D was used to model the ocean currents for 12 months using wind data from the Australian Bureau of Meteorology forecast model (LAPS) for 2001.

In order to carry out the discharge plume modelling it was necessary to first establish that GCOM3D was able to represent the ocean currents with sufficient accuracy.  To investigate this three separate verification studies were undertaken:

1. A sample time period (November 24 to December 1, 2002) was chosen from the current meter records at the 30-day mooring site near the proposed outfall diffuser.  Wind data for the period was extracted from the GEMS archive of LAPS wind model output and GCOM3D was run on the 250m bathymetric grid over the region defined in Figure 10.1 driven by winds and tides.

2. The same procedure was followed for the 5-day period of observations at mooring site E1 at the eastern entrance to the channel where the strongest currents were recorded.

3. The client provided data for a number of short-term drifter tracks near the pipeline route and GCOM3D predictions were compared with two tracks selected from these data.

The agreement obtained between model predictions and observations in all three verification studies was excellent giving a high level of confidence that the long-term simulation of currents in the Canal de la Havannah for the discharge plume modelling would represent the circulation patterns very well.

The mixing of the outfall discharge with the ocean environment can be considered in two stages; near-field and far-field.  Near-field mixing occurs as the discharge jet(s) meet the ocean environment in the vicinity of the diffuser and depends on the diffuser configuration and oceanic conditions.