River of Sand – Longshore Drift
Professor Rodger Tomlinson, Director, Griffith Centre for Coastal Management.
Generally speaking our coast here is completely dominated by longshore transport.
When you look at the wave conditions on the Gold Coast predominately they come from the south.
They come from weather systems that are generated in the Tasman Sea which, based on the wind
direction, force wave action generally from the south to the south east so that creates our major
swells that the surfers like to see.
If we have weather systems that then move or if we have a tropical cyclone develop then that wind
direction can change and for certain periods of time we do get major swells from the north to the
north east which then effectively disrupts the natural flow of sand along our beaches and then we
see some interesting features develop along the coastline.
Erosion might start to appear somewhere where you don’t normally see it. It’s not so much erosion
it’s just a blockage of sand movement and because of the natural way in which our system works we
start to see the beach cutting back. So basically the swell direction is dependent on the weather
systems out in the Coral Sea and the Tasman Sea that generate them.
News Reporter:
In 1970 the Delft Report was completed providing coastal engineers with the first detailed analysis
of what was happening to the beaches, what could be expected in the future and the works needed
to protect the Gold Coast.
Professor Rodger Tomlinson, Director, Griffith Centre for Coastal Management.
When the Delft Report was written back in the 1970s there was a major attempt, with the data that
they had available, to assess the quantity of the longshore drift that was carried along our coastline.
More recently with the implementation of the two major sand bypassing systems on the Gold Coast
we are now able to quantify the amount of sand that is being pumped by those systems and then if
you assess how the beach is moving over time you can make a reasonable estimate of the amount of
sand that is actually naturally moving along the beach. In a sense that sort of validated the original
estimates made in the Delft Report with some variations. And certainly you start to see a picture of a
longshore transport rate that is not steady. It varies very much throughout the year and from year to
year, but on average it comes out about half a million cubic metres per year moving northward.
There are some oceanographic influences that can affect the way that sand moves naturally. For
example, there is what are called internal waves, which are waves that occur on density differences
inside the ocean. They sort of look like surface waves, but they are deep down. When they come
close to the shore they break just like a surface wave will break and create a very energetic flow
motion which is moving sand across the bottom of the ocean. We then have other oceanographic
features, such as coastal cut waves, the east Australian current and a range of oceanographic
influences that cause a high level of movement of sand, but in terms of what we see on the beach
these are very minor compared to the role of waves.
Whether we’ll see a long term change comes into the area of climate change research or climate
change science and at the moment there simply isn’t enough information to make any prediction at
all about the way that climate change will impact on our wave conditions. We do see the influence
of climate variability on the longshore drift, because the longshore drift if purely a function of the
wave energy and the direction of that wave energy.