Manly Hydraulics Laboratory

NSW Public Works

Manly Hydraulics Laboratory

Water Quality Data Example Site at Narrabeen Lagoon

Narrabeen Lagoon is a shallow lake system fed by short streams which carry irregular flows. The lagoon responds to rainfall, evaporation, and ocean tides and ocean surges when the entrance is open. The main body of the lagoon is connected to the ocean by a long narrow channel. This channel is subject to periods of closure from the sea by littoral movement of sand from Narrabeen Beach into the lagoon entrance.

The lagoon has a catchment of approximately 55 sq km, most of which is undeveloped containing native vegetation. Approximately 20% of the catchment has been developed for urban use.

Manly Hydraulic Laboratory was commissioned to install a telemetered water quality instrument at the pre-existing water level site. This site is approximately 1.7 km from the entrance.

A Typical Rainfall Event

After a period of moderate rainfall starting on the 10 June 1996, the water level shows a small rise. The conductivity in the lagoon drops in response to the fresh water run-off. There is a increase in the dissolved oxygen, and a drop in the pH.

For the following week it is obvious that there is a strong tidal influence in this part of the lagoon, with the measured parameters responding to the tidal peaks and troughs. The water from the incoming tide has a higher pH and conductivity, and lower Dissolved Oxygen values.

By the 23 June 1996, the conditions in this part of the lagoon appears to have returned to back to dry weather conditions.