Why are we
silting up?
Over the years there has been a large build up of sand between the entrance walls and
Kalimna. The entrance to Cunningham Arm and Hopetoun Channel have been subject to this
build up of sand.
The cause of this is the natural movement of sand with the tide and is happening at
every estuary around the world that has an abundant supply of sand outside the entrance.
The tide brings sand with it through the entrance which settles out as the tide current
slows down. On the outgoing tide, the current picks up the sand and takes it out of the
entrance. Unfortunately it doesn't take out as much sand as it brings in.
Consider sand travelling in water. As the speed of the water slows down it gets to the
stage where it can no longer support the sand and the sand settles out. This speed is
known as the Settling Velocity.
Settling Velocity
Consider a sandy bed with water flowing over it. As the water speed gets higher it will
get to a stage where it will pull grains of sand from the bottom and put them into
suspension. This is known as the Pick Up Velocity.
Pick Up Velocity
The pick up velocity is always Higher than the settling velocity. This is because
during pick up the water has to dislodge the sand grain from other grains that have packed
in with it.
At Lakes Entrance, the average parameters are:
Sand Diameter: 0.37 mm
Settling Velocity: 0.2 m/s
Pick up Velocity: 0.4 m/s
At the entrance to an estuary, the water comes through a narrow channel and then
spreads out through the estuary. As it spreads out it slows down as it has to cover a
greater area.
In the reverse as the tide goes out, the opposite happens. The closer to the entrance
the higher the current. The accompanying diagram a simple picture shows what is
happening and how it effects the transport of sand.
The water takes the sand into the estuary until it settles out when the speed gets down
to 0.2 m/s. On the outgoing tide sand is only picked up when the water speed gets to 0.4
m/s and so the sand that was left between the 0.2 m/s and the 0.4 m/s speed contours stays
inside the estuary.
The accompanying picture is a very simplified image. The effect of the river outflow
through the estuary helps flush sand out and floods in the Lakes have a major benefit in
cleaning sand out of the system. Tides, winds and barometric pressure also affect the way
sand is left inside the Lakes.
Sand being left inside the estuary is evident from just inside the entrance up to
Fraser Island. From here on the bottom sediments are mainly due to the original material
during the formation of the Lakes and sediment that has been washed in by the rivers.
A study of the sediment build up inside the entrance showed that accretion due to tidal
current action can be in the order of 26,000 cubic metres per year.
