Please remember that this is an Earthcache, as such there is no container to find; instead you will need to take the time to make some observations and answer the questions.

The Questions:

From the coordinates you can see an obvious obstruction on the beach sticking out into the bay, go stand on it (Wpt: S 37° 59.567 E 145° 03.930) and have a look around.

1.) Standing on the “obstruction” and facing the water, which side do you think is the deposition side?

2.) On which side of the obstruction is erosion most obvious?

3.) Keeping in mind your answer to the previous questions….which compass direction does the longshore drift move?

A: The updrift (incoming direction of longshore current) moves from the ________

B: The downdrift (the direction in which the longshore current is travelling to) moves to the ________

Then post a photo of you at the location with your log, (please do not show the subject of the questions in your photo). Of course, if you do not want to appear in the photo, a personal item in the photo is enough proof of your presence. You may log the cache as soon as you submit your answers to us via messenger.

Logs without accompanying answers sent or without a photo uploaded may be deleted without notice. Sending the answers is a requirement not a request. Please note, answering the logging tasks is based on your observations at the site and using your understanding of  content of the cache page, there’s no need to do any further research. 

The Lesson:

Longshore (littoral) drift is the movement of sedimentary material along a shoreline by wave action and provides a link between erosion and deposition.

Longshore drift is a continual action;

1. Waves approach the coast at an oblique angle.
2. Swash carries sediment up the beach & deposits
3. Backwash carries sediment down the beach with gravity – at right angles to the beach.
4. This creates a zig-zag movement of sediment along the beach.
5. Eventually the majority of the sediment will be deposited where the waves lose energy.  
6. Net Movement: Over time, this results in a net movement of sediment along the coast, which can cause erosion in some areas and deposition in other areas.

Causes of Longshore Drift:

• Prevailing winds: drive waves toward the shore at an angle.
• Wave direction: influenced by wind patterns & coastline shape.
• Tidal currents: influence direction & strength of sediment movement.
• Storms: Increase wave energy & accelerate the drift process.

Longshore drift causes erosional or depositional effects. This largely depends on weather conditions. When the Bay is calm, with minimal wind, the resulting waves are small and weak; this results in the process of longshore drift having a depositional effect on the beaches and the drift is slow.

However, when prevailing winds are strong, so are the waves and the swash and backwash is often powerful enough to drag a lot of material away from the shore. This can result in coastal erosion.

Obstructing a longshore drift

Every coastal structure along a shoreline, whether it’s natural or engineered, will interrupt the natural movement of Longshore drift. A longshore current loses velocity as it meets a hard shoreline structure and it drops its sediments, thus the structure traps the longshore flow of sand. Deposits accumulate on that side of a coastal structure (like a groyne or breakwater) that is facing the incoming direction of longshore currents, also known as the updrift side. On the updrift side, the beach grows wider, a process known as accretion. Consequently, while accretion builds up the beach on one side of the obstruction, on the downdrift side (the side away from the incoming sediment) of the structure the beach is starved of sand deposition and erosion occurs as the sediment supply is reduced. If the current generally flows in the same direction all year an obstruction “steals” sand that would normally be deposited on the downdrift end of the beach.

Now take a walk on a short jetty. What you are standing on is an example of a hard shoreline structure. Stand at the suggested waypoint and make some observations.

Resources:

https://www.marineandcoasts.vic.gov.au/__data/assets/pdf_file/0023/662036/PPB-VCMP-NourishSitesReport_final_April_2023.pdf