Shifting Sands I: Disappearing Dunes
This EarthCache will bring you to the Frankston Foreshore, specifically a part of the boardwalk where a sign titled “Shifting Sands” stands from which the inspiration for this EC has been drawn (that said, all of its important information has been carried over to this description).
The tasks span over two waypoints, both of which are accessible either from the boardwalk or paved paths, but do feel free to examine the environment from the beach itself.
To log this EarthCache, you must answer the questions at the bottom of this description and send your answers to the CO.
Basics of Wave Energy
Frankston Beach is a prime example of the erosion that takes place mostly along the eastern shores of Port Phillip Bay.
Prevailing winds along the east coast mean that the cliffs, dunes and headlands in the area are slowly eroded over time and subsequently form sandy beaches that gradually head further inland as erosive processes repeat.
This is in stark contrast to the western shores of the bay which predominantly support mangroves that trap sediments instead.
However, these beaches are prone to another kind of erosion caused by waves.
Waves tend to meet the shore at an angle and move grains of sand along the beach as they do so; how much of this sand is moved and the direction it's moved in is mainly dependent on the wave energy.
Types of Waves
Calmer conditions will bring about constructive waves which have less energy and are generally smaller in height as well.
However, rough conditions such as storms will instead form destructive waves which have more energy and are larger in height.
The diagram below shows the differences between the two types of waves and where their energy is being used.
The differences largely lie in the swash (the wave flowing onto the beach) and the backwash (the wave flowing away from the beach).
Constructive waves have a strong swash to push sand but a weak backwash, meaning that little of it will be pulled back into the sea, resulting in a net gain of sand for the beach.
In contrast, destructive waves have a weak swash and a strong backwash, meaning that the opposite occurs. So as they crash onto the shore, sand is predominantly pulled away with the backwash which causes more erosion to the beach.
Eroding Sand Dunes
Enter Frankston Beach, which is lined with sand dunes that are susceptible to erosion by destructive waves.
A key factor that contributes to the severity of the dune erosion is the steepness of the dunes.
The steeper a dune is, the more vulnerable it becomes to collapsing, as seen in the diagram below.
When a destructive wave crashes into the bottom of a steep dune, its energy is reflected back, allowing for large amounts of sand to be carried with its backwash.
After enough removal of sand from the bottom of the dune, it can eventually reach a point where the top of the dune is no longer adequately supported and it subsequently collapses from the weight of the sand on the top.
Now with fewer dunes to block their path, it means the waves can creep ever-closer towards roads, houses and other structures that are less ideal when underwater.
Dunes that aren’t as steep aren’t necessarily immune from erosion, but it does mean that wave energy is dissipated as it travels up the incline which tends to result in a smaller loss of sand to the dune.
Dune Stabilisation
With this in mind, manmade efforts to halt the erosion of sand dunes have been attempted for centuries to varying degrees of success.
One example used along Frankston’s coastline (alongside many others in Victoria and beyond) is dune stabilisation, where vegetation adapted to coastal environments is planted to allow sand to accumulate and provide some protection against waves and wind.
Historic Use of Marram Grass and Its Effects
A notable instance of this regards the introduction of Ammophila arenaria, more commonly referred to as marram grass.
It’s believed that marram grass was first introduced to Victoria on the advice of the state’s first Government Botanist, Ferdinand von Mueller, who visited southwest Victoria in 1875 as the area had been experiencing noticeable issues with "coastal sand drift" for the past decade.
He’d been providing strategies to mitigate the erosion of the region’s dunes during this time, but it wasn’t until his 1875 trip that he recommended planting marram grass which had been used for similar stabilisation purposes in Europe.
A year later, seed consignments were sent to Warrnambool from the state government and by 1885, the results of the marram grass were looking promising in the south-west region; botanist and eventual director of the Sydney Botanic Gardens Joseph Maiden claimed that “it has proved to be the most effectual sand-stay ever planted”.
There doesn’t seem to be a record of the exact time at which Frankston received its shipments of marram grass seeds, but it was probably in the late 1880s and could likely have even been from the Warrnambool/Port Fairy area who are reported to have “sold it by the ton around Australia and overseas”.
What Mueller and others involved in the introduction of marram grass to Australia failed to realise in time was that marram grass worked so well at stabilising sand dunes that it actually caused them to become steeper and, hence, more prone to erosion which brought about the aforementioned issues associated with it.
It also had another inadvertent effect of dominating the landscape to such a degree that native grasses couldn’t compete with it, many of which actually did a better job at properly stabilising the dunes than marram grass.
Modern Use of Spinifex and Its Effects
Today, marram grass is classed as an environmental weed not least for its efforts in steepening the sand dunes it was planted in centuries ago and subsequently spread across.
Instead, the native Spinifex sericeus is opted for in modern dune stabilisation programs due to the fact that its horizontal stolons (stems that can produce roots and branches which allow the plant’s area to expand) run further along the dune, promoting a gentle slope which will dissipate the energy of any waves that crash against it.
In contrast to this, marram grass forms clumps with its roots and has a tendency to grow taller instead of broader which forms the steeper dunes.
The whole story is an interesting example of the change in human responses to geological processes as times change and more information on the ideal practices becomes accessible.
EarthCache Questions
To log this EarthCache, you must complete the questions below and record your answers.
Proceed to WP1 (posted coordinates) along the boardwalk
- Are destructive or constructive waves more likely to erode sand dunes? Why is this the case?
- What happens when a wave crashes against a steep dune?
- Is there more grass stabilising the dunes to the North or South of WP1?
Proceed to WP2 at an image-taking station
PHOTO: Take a picture using the station and send it along with your answers.
Rest your phone/camera-enabled device in the cradle and zoom all the way out.
The local council recently installed this station as part of a citizen science project to track the changing coastline with an emphasis on the erosion of the dunes.
You needn't participate in the project to find this EarthCache, however.
Use your photo and your observations walking between waypoints to answer the following questions.
PLEASE DO NOT UPLOAD YOUR PHOTO WITH YOUR LOG AS IT WILL CONTAIN SPOILERS.
- Is there any difference between the dunes where the grass is more frequent compared to where it's not?
- Identify at least one other preventative measure in place to reduce erosion of the dunes at Frankston Beach and its location within the image you took (top left, middle right, etc.)
Once you’ve completed the logging requirements, please send your answers ideally to BrainEarthCache@outlook.com (or to Brain through the Message Centre if you find it easier) along with the GC Code of the EarthCache, which is GC9J0P2.
You can log your find right away if you wish, but you must send your answers within 7 days.
Please do not share any answers in your log as to not spoil the cache for others.
References
All of these sources were used in the development of this cache’s description and have been linked roughly in chronological order.
They’re worth checking out if you’re interested in the subject and especially if you want to go into a bit more detail than what’s been explored here.