Shifting Sands II: Restless Ripples
This EarthCache will bring you to the Frankston Foreshore, a short walk away from 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).
Keep in mind that some tasks will require you to head on to the beach, while others will have you observe Kananook Creek. Don’t be afraid to get your feet wet!
To log this EarthCache, you must answer the questions at the bottom of this description and send your answers to the CO.
Recap of Wave Energy
If you haven’t already read and/or completed GC9J0P2: Shifting Sands I, it may benefit you to do so before starting this EarthCache.
Both ECs cover the topic of wave energy as part of geological processes and developing an understanding in that EC will assist you in completing this one.
A quick recap of wave energy is provided here nonetheless.
Waves tend to meet the shore at an angle and move grains of sand along as they do so.
How much of this sand is moved and where it is moved to 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 differences largely lie in the swash (the wave flowing forward) and the backwash (the wave flowing back).
Constructive waves have a strong swash that pushes sand but a weak backwash to pull it, meaning more ends up in the direction of the swash.
Destructive waves have a weak swash and a strong backwash, meaning that the opposite occurs.
Ocean Sand Ripples
Wave energy doesn’t just contribute to the erosion of sand dunes, as was explored in the previous EarthCache, it also moves and deposits sand on the seabed.
The patterns these waves leave in the sand is also a fairly reliable method of determining how wave energy has been used in each scenario.
Formation Process
Similar to how the swash and backwash takes and deposits sand from and to the beach, waves also use this energy to form ripples in the sand it passes over and follow a similar process of shifting the sand for each ripple.
Ripples form over a number of hours and adapt to the conditions of the ocean, slowly migrating in the direction of the swash or backwash dependent on which is stronger.
If you are able to, it's worth taking a moment to watch this video https://www.youtube.com/watch?v=uY2QdZLLRP8 (2:23) as it’s a fantastic and succinct model of ripple formation.
Notice how the relatively calm nature of the generated waves mean that the swash is stronger than the backwash and, in the close-up scenes, more sand travels with the swash as a result.
It’s only an incremental difference between the two, but the first time-lapse scenes show more clearly that the ripples move quicker to the right in the direction of the swash.
Types of Ocean Sand Ripples
There are a few ways to identify characteristics of sand ripples, but we’ll start with the patterns relative to wave energy.
A 1988 Canadian study analysed 18 days of time-lapse film taken of ripples developing on the shoreface where they defined six distinct types of sand ripples.
Take a look at their diagrams below.
Note that the crest of a ripple is its highest point as opposed to the trough which is its lowest point (the same terminology can be used to describe waves as well).
(Also, a Sand Dollar is a type of sea urchin, they’re irrelevant to this EarthCache.)
I’ve included descriptions of all six types paraphrased from the study, but the key ones to note are types 1 and 5.
Type 1 is short-wavelength regular ripples. These ripples have regular straight crests with a short wavelength and rarely branch out. They occurred only during the highest observed energy conditions and exhibited the most rapid migration.
Type 2 is variable bifurcated ripples. Type 2 ripples have a regular pattern but branch out very frequently. Type 2 ripples also migrate, but at slower rates than type 1. These two types appear similar, and type 2 ripples often seem to be type 1 ripples undergoing reorientation.
Type 3 is variable terminated ripples. These ripples have irregular crests and numerous crest singularities. Type 3 ripples occur during falling energy conditions as decay products of either type 1 or type 2. They may also migrate, but only slowly, with rates decreasing soon after the transition from types 1 and 2, and quickly becoming zero.
Type 4 is short-crested ripples. Type 4 ripples are characterized by frequent terminations and are often only one to two wavelengths long. They occupy a transition zone of low energy between types 1 to 3 and type 5. They do not migrate.
Type 5 is long-wavelength regular ripples. These ripples are regular and long-crested. They strongly resemble type 1 ripples, but have a much longer wavelength and show very little migration. Type 5 ripples occurred during persistent low-energy conditions.
Type 6 is chaotic ripples. These ripples have a confused irregular crest pattern and exhibit a wide range of wavelengths. Chaotic ripples typically exist only briefly during dramatic reorganizations of the seabed. Type 6 ripples are associated with the initiation of higher energy wave events.
Type 1, short-wavelength regular ripples, are more likely to occur alongside destructive waves and rough conditions and are wavier in appearance.
Type 5, long-wavelength regular ripples, are more likely to occur alongside constructive waves and calm conditions and are smoother in appearance.
The rest tend to take place during changes between types 1 and 5 when the conditions are less consistent.
Ripple Symmetry
Another method of characterising sand ripples is by looking at their symmetry.
Up to this point, this EarthCache has largely focused on sand ripples in the ocean, but ripples can also be found in rivers and creeks.
One of the key differences between the two environments is whether or not the sand ripples found there are symmetrical which largely has to do with how wave energy acts in each circumstance.
Symmetrical Ripples
Symmetrical ripples are commonly found in the ocean where both a swash and a backwash are present to form ripples.
Even though the forces can be unbalanced and have the ripples slowly migrate in a particular direction, rarely is one ever intense enough to completely overpower the other and form an asymmetrical ripple.
As the diagram shows, the swash and backwash both exert energy onto the sand and create ripples that generally have slopes of equal size.
Asymmetrical Ripples
In contrast to this, asymmetrical ripples are commonly found in rivers and creeks.
In these environments, there is predominantly one direction that the wave energy is pushing the sand in as there is nothing to form a backwash against.
Instead of equal slopes, asymmetrical ripples have one slope that is gentle and one that is steep, with the steeper slope heading downward in the direction of the current.
EarthCache Questions
To log this EarthCache, you must complete the questions below and record your answers.
You can upload your photos with your log if you like, but it's not necessary to do so (provided they've been sent to the CO).
Keep in mind that ripples can be less pronounced in different conditions; do still answer the questions below and note in particular how these factors may have affected the clarity of the ripples.
Proceed to WP1 (posted coordinates) on Frankston Beach
PHOTO A: Take a picture of the ripples in the sand in the ocean close to the beach and send it along with your answers.
The ripples in your image must be from in the water, as this EarthCache focuses on how they're formed by waves.
Take care and don't go any deeper than what you're comfortable with.
- Which of the six types of ocean sand ripples do you think most resembles those in your photo?
- What were the weather conditions like at the time of your photo and how do you think they affected the ripple patterns?
- What were the types of waves you saw at the time of your photo and, from this, do you infer that the swash or backwash had a stronger effect on the sand ripples?
Proceed to WP2 alongside Kananook Creek
PHOTO B: Take a picture of the ripples in the sand in the creek and send it along with your answers.
Just stand next to the creek for this one!
If there is a lot of sediment suspended in the creek preventing vision of the bed, consider walking further along in the direction of the "WP2 Alternative" waypoint. It's often easier to see the ripples the further from the creek mouth you go. If the issues still persist, simply pay particular attention to how the visible water current is affecting the ripples below.
- Are the ripples here symmetrical or asymmetrical; what characteristics distinguish them?
- Was the ripple symmetry different between the two waypoints? Why is this?
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 GC9J1CJ.
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.