Located
south of Sydney, Port Kembla harbours waters are adjacent to the Pacific
Ocean. The Five islands to the
North and East of the port moderate the effect of the predominant South
Easterly swell but have little effect on harbour waters. There are only
slight differences between the harbour waters and the ocean for most
physical parameters except for water temperature.
This is locally raised by the use of harbour water as a coolant and
the transfer of waster heat to the harbour by BHP Steel.
Tides
and local winds are important mechanisms for water circulation in the
harbour and near-shore regions of less than 25 metres water depth.
In general, little energy can be attributed to the tides.
However, there are times when significant energy occurs at
near-diurnal frequencies at near-shore sites (CWU, 1996).
Current meter studies conducted by Sydney Water
(Water Board, 1991 and 1992; AWACS, 1992) indicated that local semidiurnal
tidal currents were weak. In
Port Kembla harbour, the dominant water movements are wind-generated swell
and tidal transport.
The association between wind directions and currents is difficult to determine since winds in the near-shore environment are significantly affected by the local complex topography. The Illawarra Escarpment protects coastal waters, from westerly winds but large swells generated in the Southern Ocean or from East Coast lows generate very destructive forces along the coast. Drogue studies, which compared wind data with records of currents from various deployments in the region between 1991 and 1992 (AWACS, 1992), showed a marked association between the local winds and surface currents (CWU, 1996).
Contents
Location - describes where the harbour is located and its boundaries.
Oceanography - describes aspects such as the bathymetry, water circulation, tides, winds and waves in Port Kembla Harbour.
Currents - provides an analysis of the current patterns in the harbour.
Port Kembla Harbour is situated about 70 kilometres south of Sydney coastline of Australia and the harbour waters are connected directly to the Pacific Ocean. The harbour is small by international standards and extends only 2 kilometres from the harbour entrance.
Harbour Boundary
The waters of Port Kembla Harbour include all the the harbour waters from the line joining the Eastern and Northern Breakwaters, the Town Drain to Springhill Road Bridge and Allan's Creek to Road Bridge.
The
Port Kembla Harbour Environment Group is primarily concerned with the area
shown.
The Group maintains a significant interest in the three catchments that drain directly into the harbour through the Town Drain, Allan's Creek and the Port Kembla storm water drain that enters the harbour at Red Beach in the Outer Harbour.
Catchment Boundary
The Port Kembla Harbour catchment accepts water from a diverse range of sources. These include the relatively undisturbed slopes of the escarpment with flashy creeks, residential and commercial properties, light and heavy industry, stormwater, sewage treatment plant overflows as well as an extensive road network.
The
most prominent oceanographic feature of the NSW coastline is the East
Australian Current that moves large volumes of warm water from the
equatorial regions close to New Guinea in a southward direction along the
edge of the Great Barrier Reef along the coast lines of Southern
Queensland before breaking away from the NSW coast around the Victorian
border and dissipating in the Tasman Sea off the East coast of Tasmania.
Numerous eddies and whorls are thrown off the main Southward flow
and these may result in local currents travelling in a completely opposite
direction on occasions.
The
main body of the current moves along the 200 metres bathymetry line,
although this will vary according to both the local and distant
meteorological conditions on the day.
Typically it runs parallel to the coast at speeds between two and
three knots, although speeds of up to 5 knots have been recorded along the
Northern part of the NSW coastline.
Coastally
Trapped Waves (CTWs) generated in the Great Australian Bight pass through
Bass Strait and turn Northward generally following the coastline.
They can raise or lower the sea level heights by up to 0.5 metres
although 0.2 metres is more typical.
These can affect water movement around Port Kembla Harbour.
Neither
of these water movements is present all of the time and it is difficult to
accurately predict when they will enter the Port Kembla Harbour area
(Clean Waterways Unit, 1996). Current
meter studies conducted off Port Kembla and Bellambi between 1990 and 1992
(Water Board, 1991 and 1992; Australian Water and Coastal Studies, 1992)
showed the EAC was generally weak inshore of the 30-metre depth contour.
CTWs were only intermittently observed and were inferred as a
probable cause of longshore current reversals.
A strong diurnal current signal was evident at all metered sites
and the principal current direction closely paralleled the bathymetry.
Bathymetry
Bathymetry,
which is the shape of the seabed in relation to water depth, has a major
influence on regional water movement and seabed communities.
The
Continental Shelf along the Illawarra Region coastline is approximately 30
kilometres wide and extends to approximately 200-metre water depth.
The depth of the Continental Slope, seaward of the shelf, increases
steeply to the 5,000 metres contour at 80 kilometres offshore.
The
bathymetry close to the coast has a major influence on local currents.
The promontories along the Illawarra coastline, for example,
Bellambi Point and Red Point, project out into the ocean, blocking or
diverting longshore currents, focusing wave energy and providing a rocky
seabed for marine communities that prefer a hard substrate.
Along the coastline from Bellambi to Port Kembla Harbour, the 5-metre depth contour is typically about 200 metres offshore from a sandy beach, the 10-metre depth contour is generally about 400 metres offshore and the 20-metre depth contour is approximately 1,000 metres offshore. Beyond the 20 metre depth contour, the offshore gradient flattens, so that the 30 metre depth contour is approximately 3 kilometres offshore and the 40 metre depth contour is more than 5 kilometres offshore. The depth contours are irregular where the shoreline or seabed is rocky.
Harbour Circulation
Water circulation within the harbour results from the
interaction of many factors, including creek flow, tidal activity, wind
regime, stormwater, cooling water uptake and discharge and groundwater
movements. Water from
groundwater, raised temperature waste water streams and run-off, tends to
flow seaward over the harbour in a surface layer, while denser sea water
flows up harbour in a bottom layer in a counter current response.
Tidal and wind actions tend to promote mixing of the two layers.
The harbour's physical dimensions determine:
1. the degree of influence from the earth's rotation (Coriolis force) which can be significant in wide estuaries and negligible in narrow ones, and
2.
the degree of tidal mixing, extensive in shallow areas and limited
in deep ones. Generally,
tidal mixing increases with decreased tidal flow and depth and increased
tidal range and width.
Mixing is far more important in Port Kembla Harbour.
A preliminary hydrodynamic model describing the movement of the harbour waters has been prepared in August 2000 shown.
Hydrodynamic model of Port Kembla harbour at full ebb tide
Further development of the model is required before it can be used for predictive purposes.
Secondary Circulation
Secondary
circulation cells in the lee of islands and reefs have the potential to
direct waters onshore, depending on the size and duration of these cells.
These cells have been observed in lee of the islands near Port
Kembla. CWU (1996) details
recording of cells in this region of 4 kilometres in diameter, which
produced current speeds of about 0.1 metres/second (0.36 kilometres/hour)
but rarely lasted longer than several hours.
When stronger ambient currents prevail than was the case during
these recordings, secondary circulation cells are more intense and last
for longer periods of time (CWU, 1996).
The Port Kembla Port Corporation installed a multi-depth current meter about 300 metres offshore from the end of the Eastern Breakwater in 1998. Evidence from these records indicates that currents develop further down the water column to depths greater than 15 metres when wind speed and direction are maintained over more than 12 hours. The maximum current recorded since 1998 is 0.55 metres per second. More typically, currents flow across the harbour entrance in a Southeast or a Northwest direction at speeds between 0.1 and 0.4 metres per second.
Currents in Port Kembla Harbour
Port Kembla Port Corporation installed a current meter about 300 metres off the entrance to Port Kembla Harbour to assist pilots with the movements of large ships approaching the harbour.
The equipment used to monitor these signals is a Doppler shift fixed echo sounder. It measures both the movement and direction of the water directly above the meter at 6 different levels. These levels are described as bin numbers 1 to 6 and two metres apart measuring from the sea floor. So, bin 1 is 2 metres above the sea floor and bin 8 metres above the sea floor. The readings from the equipment are transmitted immediately by electrical cable and radio signals to the Vessel Traffic Centre where real time are displayed.
The graphs shown below indicate the current strength and direction for the 26 May 1999. There are several interesting points to note about the graphs, these are:
- the differences that occur in both strength and direction and different water levels shown through the different bin numbers;
- the abrupt change of direction that occurred between 1500 and 1600;
- there appears to be no obvious connection between current variation and tidal data;
- there are two main directions of current flow about 110 degrees and 310 degrees, more or less at right angles to the main line of approach to Port Kembla Harbour; and
- the strength of the current exceeded 0.27 m/s or 0.5 knots at about 1000.
These results indicate the unpredictability and variability of the movement of water off the port. It is suspected that these water movements are a combination of several different elements including:
- wind stress;
- eddies that are connected with the East Australian Current (EAC);
- tidal flows;
- swell waves and frictional effects from a shallowing seabed; and
- the movement of coastally trapped waves along the coast.
