Do You Catch My Drift? (Batu Ferringhi, Penang) EarthCache

Ah, the beach! It’s a great place to unwind, get a tan, go for a nice stroll or have a little dip… but, did you know that it’s also where you can see Nature work through her powerful forces of wind and water? Here, at Penang’s popular beach of Batu Ferringhi, you’ll see evidence of these forces at work if you look closely… and if you can catch my drift ;)

EC Objective: Understanding the nature and effects of Longshore Drift in Coastline Formations

Longshore Drift

Looking out at the sea, it’s really hard to miss the constant pounding of waves on the beach. The waves move towards the shore, carrying with it sediments from the seabed. After that, the wave recedes, pulling back sand into the sea. Looking at this, you’d think that the sand by the seashore would just be pushed back and forth in a never-ending cycle, leaving the beach in place for eternity.

But, nature is rarely so simple. Waves hitting the shore don’t always move directly towards the shore (that is to say, their paths are rarely at right angles to the beach). Due to wind, currents and shape of the surrounding coastline, waves often hit the shore at an angle.

On the other hand, when the water recedes, the path of the water flow is more often than not perpendicular to the coast line. The slope of the beach ensures that this is the case, as the water recedes only due to gravity, independent of whatever the prevailing wind or current direction is.

So, in conclusion, waves (and the stuff it carries) hit the shore at an angle to the shoreline, but recedes perpendicularly to it. The resulting effect is not hard to visualise (see Fig.1 below). Over time, materials like sand and sediment are transported along the shore line, and this leads to the phenomenon of longshore drift. 

Fig 1: The mechanism of Longshore Drift 

Effects on Beaches

Longshore drift plays a huge role in coastal erosion, and it’s not hard to see why. Coastal material is carried along the longshore drift direction, which causes constant erosion on one end of the beach, and accumulation of sediment and sand on the other end. This drift can alter coastlines and the shape of beaches dramatically. Structures built close to the shoreline can either see the shoreline quickly encroaching on their foundations, or find themselves left high and dry in a matter of decades. 

Fig. 2: Coastal Erosion at Tanjung Bungah, Penang

This is a constant battle against nature, and can lead to complete loss of beaches in some areas, and siltation of beaches (i.e. the coast becoming muddy, shallow, and a generally bad place to have a skinny dip in) in others. Humans have built coastal defences to help stem these changes, which can deliberately or inadvertently, change the structure of the coastline forever. But, that’s the topic of another Earthcache.

On Batu Ferringhi Beach     

Fig. 3: Batu Ferringhi Beach

Batu Ferringhi (or Foreigner’s Rock) is a long stretch of beach on the North-western side of the island of Penang. Named for the foreign trading vessels that used to stop there to stock-up on fresh water for their onwards journey, today, the beach still lives up to its name as a popular tourist destination and sea-sports area for visitors to Penang.

The beach is exposed to the open waters of the Straits of Malacca, and as occasionally seen huge waves during the monsoon season and, very rarely, a tsunami due to earthquakes off the nearby Indonesian island of Sumatra.

At the coordinates for this EC, you will be standing on pier by Batu Ferringhi beach. From this point, you will be able to see effects of longshore drift on the beach if you look carefully. The following tasks and questions will guide you in figuring out the direction of the drift along the beach.

1. Standing on the pier, looking out to the sea, first look at the beach to your left. Estimate how far from the landward end of the pier the coastline extends to (that is, the distance between the south end of the pier and the coastline).
2. Now, look on the beach on your right side. Again, estimate how far from the landward end of the pier the coastline on this side extends to.
3. Is there a difference in the two distances from questions 1 and 2? How much is this difference (if any)?
4. Using your understanding of the mechanism of longshore drift, state a possible reason for the difference in distance (if any).
5. From this difference, suggest a possible direction (if any) for the longshore drift along Batu Ferringhi. Is it eastwards or westwards, or is there no evidence of longshore drift?

Once you have completed the tasks, please message or email the answers to me. There is no need to wait for a response from me to log. We will be in touch if I have any reason to doubt the veracity of your answers.

Any logs without an accompanying response will be deleted after a reasonable amount of time, so do state explicitly in your log if you need some more time (I do understand that we all have quite a literal cache backlog after long holidays).