WONGA OUTFALL

Click the photograph for a enlarged view

Physical Environment

Landscape

Wonga Point consists of the Clifton Sub-group of Narrabeen claystone, sandstone and shale (Sydney Geological Series Sheet S1 56-5). To the north of the outfall is Blue Lagoon and Shelley Beach. To the south of the outfall is Bateau Bay and then rocky cliffs until Crackneck Point.

Bathymetry & Substrate

The water off Wonga Point is shallow (5-10m) and the area of rocky bottom is small, although it dominates the sea bed in the vicinity of the outfall (Laxton & Laxton 1978). The reef area is widest off the center of Wonga Point and breaks up into a series of ridges to the north and south before giving way to sand (Laxton & Laxton 1980).

Currents, Winds and Waves

MHL (MHL249 1978) studies indicate:-

- nearshore current systems are set up in the vicinity of Wonga Point in response to north or south offshore current systems.

- A large proportion of the effluent discharged from Wonga Point moves parallel to the coastline within the offshore current system and not directly onto beaches in the area.

- An anti-clockwise eddy system is established in Bateau Bay under either a north or south offshore current.

- If effluent is ‘blocked’ against the shoreline by waves, wind and currents, it will move into Bateau Bay between the perimeter of the eddy and the shoreline, irrespective of the offshore current direction. This mode of effluent movement occurs frequently.

- Effluent sometimes moves with nearshore waters into Shelley Beach, but this is relatively rare.

- When ocean water levels reach 1.3m above tide datum (70%-80% of all high tides), effluent is pushed up onto the Wonga Point rock platform and moves south-west into Bateau Bay and north-west into Shelley Beach. Most of this effluent moves into Bateau Bay.

Outfall Site

Outfall Site

Outfall Site

Wonga

(Wong_5) Last updated June 2000