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Physical Environment
Landscape
The existing Potter Point outfall is
located at the cliff face on the southern side of the Kurnell Peninsula.
The cliffs in this area are made up of Hawkesbury sandstone, which
consists of sandstone and quartz, with some shale (Wollongong Geological
Series Sheet SI 56-9).
Bathymetry &
Substrate
The general seabed
characteristics in the vicinity of the outfall are predominantly rocky near
Kurnell Peninsula out to a water depth of 40 to 50 m, with fine sand and mud
covering the seabed at greater depths. The south-eastern section of the
Kurnell Peninsula, including Potter Point, has large sandstone cliffs at the
waters edge, with a short, wave-cut, intertidal rock platform at the base.
This platform is littered with large boulders and is actively eroded during
storms. The sandstone substrate steps down from the intertidal region to a
depth of 30 m and comprises bedrock covered with boulders and erosion debris.
Beyond the 30 m depth contour, the seabed slopes offshore more gradually and
comprises irregular exposed bedrock with the valleys filled with coarse sandy
sediment. The sandy/silly sediment cover becomes more consistent from about 40
m depth and is about 10 m thick at the 80 m depth contour. Sediments become
finer with distance from shore. To the south-west of Potter Point, the
shoreline changes from cliffs to beach in Bate Bay, with Mercies Reef
extending out into the northern end of Bate Bay (CEE & Woodwind-Clyde
1996).
Currents, Winds and Waves
AWT (1994) studies indicate:
-
coastal currents are the major
influence governing the path and impact of effluent currently discharged.
-
the prevailing southerly flow
generated by the East Australia Current (EAC) transports effluent south
from Potter Point towards the Bate Bay/Cronulla region.
-
occasionally these currents reverse
and flow towards the north.
-
local movements of currents are also
influenced by local wind and wave conditions.
-
Bate Bay is a 4km wide semi-circular
indentation in the coast between Port Hacking and the Kurnell Peninsula.
There is a large reef (Merries Reef) which projects from the northern
shore into the Bay. This reef has a significant influence on the pattern
of circulation in Bate Bay as it reduces currents and water movement in
the north-western sector of the Bay.
-
at times of strong onshore wave
action, an across-reef current is created over Merries Reef, leading to an
anti-clockwise circulation in Bate Bay. The transport of water across
Merries Reef during high wave conditions is the most important mechanism
for moving effluent from the Potter Point outfall to the Cronulla beaches.
- during periods of light wind and small waves, the
waters in Bate Bay inshore of Merries Reef are relatively confined and
there is less mixing with the rest of Bate Bay.
As a result the waters in this area are often less saline than in
other areas in the Bay and frequently have higher levels of faecal
coliforms (attributed to Potter Point effluent) (AWT 1994, CEE &
Woodward-Clyde 1996).
Sediment studie
AWT (1994) studies indicate:
-
concentrations of heavy metals in the
Bate Bay region were generally lower than in other areas offshore of
Sydney and Illawarra.
-
the exception was chromium levels,
which were higher than other areas, but appeared to be coming from Botany
Bay.
-
no widespread accumulation of heavy
metals or organochlorine pesticides was detected within Bate Bay itself or
in adjacent nearshore sediments.
-
copper levels were locally higher at
one site immediately adjacent to the Potter Point outfall (copper was
expected in domestic waste water and the very local nature of the observed
increase indicated that copper was probably being continually discharged,
but was not accumulating widely).
-
concentrations of organochlorine
pesticides in offshore sediments on the continental shelf east of Bate Bay
were generally higher than in sediments from other locations along the
Sydney and Illawarra coastline, however, no widespread accumulation was
detected in Bate Bay.
- several pesticides including aldrin, heptachlor
and heptachlor epoxide were at slightly elevated levels adjacent to Potter
Point outfall and at sites just to the west. This supported the
oceanographic conclusions of effluent transport from Potter Point.
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