Dredging on a Flood Tide
Why does April Hamer dredge on an incoming tide? Doesn't the sand run into
the Lakes?
This question is often asked when people see April Hamer dredging at Lakes Entrance.
The dredging of the bar on an incoming tide does not cause sand to come into the Lakes
as the system is already running at "full sand transport capacity", and the sand
from the dredge simply falls through to the bottom.
Consider a flow of water over a large bed of sand.
If the water flow is constant then it will dig out a channel.
If no sand is added to the water at the start of the flow
then it will continue to erode sand forming a channel
and the flow velocity will reduce as the channel
cross-section increases.
If sand is added at the start of the water flow then the rate of erosion will slow
down.
If sufficient sand is added then the erosion can be
stopped. The channel will not get
any deeper and the channel cross-section will stay constant. The system will be running at
"full transport capacity".
If we add even more sand then it falls through the water
to the bottom and causes Accretion. The channel will get shallower and the
flow velocity will increase as the channel cross-section
reduces.
Where the accretion occurs is not where the sand is being dropped in, it happens
further downstream. This is due to the time it takes for the sand to fall through the
water and how fast the water is flowing.
In still water 4 metres deep, a grain of sand may take 50 seconds to reach the bottom.
It will land underneath where it was dropped.
If the water is flowing at 2 metres/sec. then the flow will take it:
50 * 2 = 100 metres downstream.
How does this apply to Lakes Entrance?
The entrance channel at Lakes Entrance is in a condition of "full sand transport
capacity". We know this because the bottom of the channel and the
current velocity remain
fairly constant. They do vary with tides and floods, but over the last 100 years there has
been very little variation.
There is an abundant supply of sand both outside and inside the entrance so the channel
flow picks up the maximum amount of sand it can handle. Any other sand introduced into the
flow simply drops through the water to the bottom.
The sand at Lakes Entrance has an average fall velocity through still water of 0.050
metres/sec. The channel is generally 4.3 metres deep and so it takes: 4.3/0.050 =86
seconds for a grain of sand to fall from the water surface to the bottom.
The maximum current speed in the channel is around 3 knots (1.54 metres/sec.) This
means that sand dropped into the water will travel a maximum of:
86 * 1.54 = 133 metres along the channel
before it reaches the bottom.:
The entrance channel is 430 metres long with the Salmon Jetty 120 metres
inside the channel on the western side. April Hamer never dredges closer than 50 metres to
the seaward end of the walls as it uses this naturally deep area as a turning basin.
With a maximum transport distance of 133 metres, sand
discharged (or side cast) by April Hamer cannot travel
through the entrance to the lakes. It was thought that April Hamer was a main contributor
to the sandbank that forms at the Salmon Jetty, however during an absence from port a
large bank formed at this site showing nature is the major contributor.
If April Hamer dredging on an incoming tide is not responsible
for sand coming into the lakes, why is there a large sand buildup between the entrance and
Kalimna?
This is caused by the natural sedimentation action of the
incoming tide and is
happening at all sandy estuaries around the world.
