Water Circulation in Tuggerah Lakes

Circulation and mixing of water

Water circulation and mixing are important for the water quality and ecology of the estuary. The process depends on gravity, tides and wind. The shape and depth of the estuary influences the circulation patterns which in turn respond to variations in wind speed and direction and tidal currents. The water flow through The Entrance channel into Tuggerah Lake is controlled by the tide and makes up about 1 per cent of the system’s water exchange. The incoming tide brings heavier salty water into the estuary which forms a high-density current that flows into deeper parts of the lakes.

 

There was a moderate catchment discharge event on 7 June 2007. A three-dimensional hydrodynamic model was used to calculate the response of Tuggerah Lakes to this discharge event. The movie to the left shows surface salinities as calculated by the hydrodynamic model.

Within a short time, salinity is substantially reduced throughout Tuggerah Lake.

 

 

The 8-month period April-November 2009 was characterized by relatively low catchment discharge --- peak value 40 m3/s. A three-dimensional hydrodynamic model was used to simulate the dilution of a pollutant that had a concentration of 1 at the beginning of April. The concentration of the pollutant was set to zero in the ocean and 1 in catchment discharge.

The movie shows the vertically averaged concentration at 8 hour intervals. Even though the catchment discharge is low, effectively zero for 2 months, the pollutant is not greatly diluted by ocean water --- except near The Entrance.

This is a simulation uses model input corresponding a real low rainfall period. Under wet climatic conditions, the catchment discharge would make concentrations higher!

   

 

 

This movie is constructed using output from the 3D hydrodynamic model. The topography is shown as a three-dimensional surface coloured dark blue at depth and dark red on land. The model domain extended throughout Tuggerah Lake, Budgewoi Lake, and Munmorah Lake. The movie shows only a portion of Tuggerah Lake but includes the locations of major coupling to the ocean and catchment: The Entrance, Wyong River, and Ourimbah Creek.

  • The magenta isosurface shows where salinity is 1 part per thousand higher than the average salinity in the volume plotted. The model was initialized with a uniform salinity of 22 ppt. (Similar to observations at the time.)
  • The movie runs at about 5 frames per second and each frame is separated by 1 hour. Thus the tide comes in and out within a little more than 2 seconds of movie time.

Salt makes water more dense ("heavier"). Water from the ocean falls over the shelf edge and spreads a thin layer of salty water along the bottom of Tuggerah Lake. This salty water has lots of oxygen. Wind-driven currents cause the bottom layer to slop around and mix with the water above.

 

 

Flushing

It takes between 60 to 100 days tides and ocean oscillations to exchange about 1/3 of the material in Tuggerah Lakes with the ocean. But this is only a very simplified way of thinking about what is really a very sophisticated suite of processes.

Impact of sediment

Tuggerah Lakes estuary is one of the slowest infilling estuaries on the coast, and at current rates, would take over 1000 years to fill in completely.  However, increased development in the catchment in recent years has created the potential for increased erosion and infilling of the lakes.