The Tanjung Bunga μ-Tombolo

To complement the FPS island, we propose this EarthCache about tombolos in general and this one in particular. As described by rzinger, this place is an island at high tide but at low tide you can walk from the shore without getting even your feet wet. This type of path linking the island to the shore is usually called a tombolo. After the island has formed, the shoreline advances towards it as sand accumulates in the leeway of the island.

Tombolos

Tombolos link tied islands to the shore at low tide, the island itself is what remains emerged at high tide (see figure below). Tombolos are different from spits which advance into the sea but do not tie to an island. There is also a difference between tied islands (attached at low tide generally on one side) and barrier islands forming a lagoon (restricting the flow of water in and out of the lagoon formed by the barrier).

According to Wikipedia, there are several ways a tombolo can be formed depending on the geographical configuration:

Wave diffraction and refraction:

True tombolos are formed by wave refraction and diffraction. As waves near an island, they are slowed by the shallow water surrounding it. These waves then bend around the island to the opposite side as they approach. The wave pattern created by this water movement causes a convergence of longshore drift on the opposite side of the island. The beach sediments that are moving by lateral transport on the lee side of the island will accumulate there, conforming to the shape of the wave pattern. In other words, the waves sweep sediment together from both sides. Eventually, when enough sediment has built up, the beach shoreline, known as a spit, will connect with an island and form a tombolo.¹

Unidirectional longshore drift:

In the case of longshore drift from one single or a dominant direction, like at Chesil Beach or Spurn Head, the flow of material is along the coast in a movement which is not determined by the now tied island, such as Portland, which it has reached. In this and similar cases, while the strip of beach material connected to the Island may be technically called a tombolo because it links the island to the land, it is better thought of in terms of its formation – as a spit or a bar.

The micro-tombolo at Tanjung Bunga

Here at Tanjung Bunga, on the beach behind the former Four Seasons by Sheraton (FPS), now the Mercure Hotel, you can find a micro-tombolo linking the small rock consolidated island with a few trees and a fishing pondok.

Lower tide: a bar and a tied island                            Higher tide: no bar and an island

 

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Finding the EarthCache

Log this Earth Cache as "found it" and send us the answers to the following questions via our geocaching profile or the geocaching messaging system, we will contact you if a problem arises. Photos are not required but they are always welcome.

1. At the time of your visit, what was the tide like, high or low? How wide was the tombolo? Which way were the waves going between the island and the beach?
2. Does this look like a wave diffraction/refraction or a unidirectional longshore drift? Where does the sand from the emerging bar come from?
3. How natural does this island appear to you? How about the tombolo?

Photos are not required but always a welcome addition to your log.