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Physical Environment
Landscape
The Illawarra region
lies stratigraphically at the top of the Permian Shoalhaven group of rocks,
which consist of interbedded siltstones, sandstones and shales with various
intrusive igneous rocks of the Gerringong Volcanic Facies. The beaches in the
vicinity of Bellambi Point consist of Quaternary sand dunes (Wollongong
Geological Series Sheet SI 56-9) which are aeolian (wind formed) north-south
oriented dunes 5-15m high. Soils consist of deep calcareous sand on the
beaches, siliceous sands on the foredunes and isolated podzols in the swales
(Patterson Britton & Partners, CEE & AWT 1998). The
Bathymetry &
Substrate
Bellambi Point outfall
is on a shallow rocky reef region formed from Hawkesbury sandstone. The once
high foredunes fronting onto the reef have been drastically lowered by sand
mining and are now approximately 0.5m above sea level in places. The outfall
is located approximately 150m offshore in 5m water depth
(Water Board 1991a). The seabed around the existing Bellambi outfall is
predominantly rock, except for a nearshore strip of sand just to the north and
south of the point. To the north of the outfall are Bellambi Harbour and
Bellambi Beach. To the south of the outfall are Bellambi Reef, Sandspit Point
and Corrimal Beach.
Currents, Winds and Waves
Studies by
AWT (1992a), Patterson, Britton & Partners and CEE & AWT (1998)
indicate:-
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currents
generally flow to the south, but northerly flow is experienced 10-20% of
the time.
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currents
towards the south tended to be stronger in the summer months (February to
March).
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About half
the time there appeared to be significant water movement suggested to be
due to coastally trapped waves.
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current
speeds associated with these waves was about 0.2 to 0.3 m/s to north or
south with a variable period (typically 3-10 days).
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there were
no clear patterns between the winds and water movement.
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wind
directions in the nearshore environment were significantly affected by the
locally complex land topography.
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in
nearshore studies, the east Australian current was rarely observed (less
than 10% of the time).
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there was some evidence of secondary circulation
cells in the lee of the Bellambi Reef, however, the current records from
this region were highly variable and it was not possible to adequately
quantify the intensity and duration of these features (AWT 1992a).
Studies
by Lawson and Treloar (1998 cited by Patterson Britton & Partners, CEE
& AWT 1998) indicate:-
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a net
current to the south of 1cm/s at the proposed deep water outfall site off
Coniston Beach (1km offshore, 20m water depth, current meter deployed at
8m below surface).
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southerly
currents occurred much more frequently than northerly currents, which in
turn occurred more frequently than offshore/onshore currents.
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measurements over a one month period revealed
that currents were weak, with longshore currents averaging 4cm/s and
onshore/offshore currents averaging 2cm/s.
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