Click the photograph for a enlarged view
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Physical Environment
Landscape
First Point is part of the Cape Three
Points system, which extends north from Broken Bay and consists of Gosford
formation sandstone (Sydney Geological Series Sheet S1 56-5). The coastline in
this area consists of broad rocky headlands enclosing sandy beaches. First
Point is the highest of the headlands (115m) and contains three small coves
with boulder beaches and extensive littoral rock platforms. Little Cove (Winney
Bay – also referred to as Winnie Bay in some reports) is a small cove to the
north of First Point where the cliffs drop onto a wide wave cut shore
platform. At the head of the cove the platform is strewn with slabs of
sandstone. The remainder of the platform has no loose boulders. At the seaward
edge of the platform is a 3 to 4 m drop to the ocean.
Bathymetry &
Substrate
At the base of the drop off noted
above is a rock shelf that slopes gently seawards from the shore out into
about 20m water depth. Loose slabs of sandstone cover this underwater rock
shelf. The edges of these rocks are only slightly rounded indicating that
relatively sheltered conditions exist in this cove even during the most severe
storms from the southeast (Laurie, Montgomerie & Pettit 1977d).
Currents, Winds and Waves
Studies by Laurie, Montgomerie
& Pettit (1977d) prior to the commissioning of the outfall and Laurie,
Montgomerie & Pettit (1977d) indicate:-
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surface water movement was controlled
by the East Australian Current (EAC) and local winds in the Cape Three
Points area
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during summer an inshore
counter-current is generated by the EAC and this results in northerly
flows past First Point (the strength of these northerly currents is
governed by the strength of the EAC and its distance offshore).
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on occasions during summer the EAC
flows close to shore producing strong southerly currents.
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during winter, the EAC is usually
absent from the Central Coast and there is a slow northerly movement of
water which is often masked by wind induced surface water movement.
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moderate to strong onshore winds
superimpose surface water movements onto the ocean currents and result in
onshore surface water movement.
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the confined effect of southerly
winds and the northerly counter-current is to induce fast onshore surface
water movement.
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the combined effect of easterly winds
and the northerly counter-current is to induce slower onshore surface
water movement.
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measurement of current speeds at 5m
and 10m water depths indicate that these speeds are generally less than
0.6km/hr.
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in general, the currents in Little
Cove were the same or less than the currents at First Point.
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Average current speed in a northerly
direction was 0.78km/hr and average speed in a southerly direction was
0.38 km/hr.
Further Oceanographic studies by MHL
(MHL526 1988) indicate:-
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the measured initial dilution was 4.4
for a discharge of 250 L/s with the 500mm orifice plate in place.
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a surface field (predicted in the
initial design work) formed in Little Cove and the effluent plume
thickness was between one and two meters (based on the dye experiments).
These dye tests also demonstrated the existence of a shear line between
the local circulation and the longshore current at a distance of about
300m offshore.
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The turbulence at this boundary (on
the day investigated) resulted in a very rapid increase in mixing.
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