FIRST POINT OUTFALL

Click the photograph for a enlarged view

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:-

- surface water movement was controlled by the East Australian Current (EAC) and local winds in the Cape Three Points area

- 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).

- on occasions during summer the EAC flows close to shore producing strong southerly currents.

- 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.

- moderate to strong onshore winds superimpose surface water movements onto the ocean currents and result in onshore surface water movement.

- the confined effect of southerly winds and the northerly counter-current is to induce fast onshore surface water movement.

- the combined effect of easterly winds and the northerly counter-current is to induce slower onshore surface water movement.

- measurement of current speeds at 5m and 10m water depths indicate that these speeds are generally less than 0.6km/hr.

- in general, the currents in Little Cove were the same or less than the currents at First Point.

- 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:-

- the measured initial dilution was 4.4 for a discharge of 250 L/s with the 500mm orifice plate in place.

- 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.

- The turbulence at this boundary (on the day investigated) resulted in a very rapid increase in mixing.

Outfall Site

Outfall Site

First Point

(Firs_5) Last updated June 2000