The Final Resting Place of KRI Nanggala
If you scour the seabed from the Java Sea eastward, once arriving in the waters to the north of Bali mainland, you will come across a very deep giant bowl, which is part of the so-called ancient sea.
If you scour the seabed from the Java Sea eastward, once arriving in the waters to the north of Bali mainland, you will come across a very deep giant bowl, which is part of the so-called ancient sea. It has become the final destination for KRI Nanggala-402 and its 53 crew.
While the Java Sea has an average depth of 20 to 40 meters, the Bali seafloor is characterized by steep elevation with a depth of more than 1,000 meters and the more eastward, the deeper. In fact, the waters to the north of Bali Island is not the deepest sea in Indonesia, but it has a strong and turbulent eddy current.
Under the sea, there is a very active upward fault line, extending farther eastward to the north of Flores Island.
The Bali seabed is a bowl-like basin with changing currents at each depth.
"At a depth of more than 1,000 meters, the Bali Sea runs underneath through the Lombok Strait and the Flores Sea with very strong currents," said Widodo Pranowo, an oceanographic researcher with the Marine Research Center of the Marine Affairs and Fisheries Ministry.
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This giant bowl with a steep slope is bordered by Bali mainland and the Lombok Strait to the south. To the east is the Flores Sea and to the north is the Kangean Islands, the undersea of which converges with the Labani Canal in the Makassar Strait. The western boundaries are the Madura Strait and the Java Sea, which in ancient times used to be the Sunda Shelf, a large landmass that united Java, Kalimantan and Sumatra with Eurasia.
The Java Sea and the eastern waters of Sumatra were formed only after the end of the ice age, when global heat melted the ice on Earth and brought up the sea level around 12,000 to 6,000 years ago, while the Bali Sea was formed as early as ancient times.
The 9th out of 11 marine ecoregions in Indonesia, the Bali Sea spans to the waters of Nusa Tenggara, which, according to geologist Robert Hall (2001), was formed before 15 million to 5 million years ago.
Being part of the Wallacea zone, which holds the highest endemic biodiversity in the archipelago, the ecoregion is made up of the Bali basin, Flores basin, Lombok basin, Sumba basin and Savu Basin, with the deepest slope of up to 7,247 meters.
According to geologists, the waters encroaching the island group from Bali to Nusa Tenggara lie above a very active tectonic zone, in which the Indo-Australian plate converges with Eurasia, making the southern coast of Bali and Nusa Tenggara vulnerable to earthquakes and tsunamis.
The fault line also triggered an earthquake and tsunami that hit northern Bali on November 12, 1815 and July 14, 1976.
On the afternoon of August 19 1977, a large earthquake rocked the Indian Ocean in the southwest of Sumba Island. The shockwave of a large tsunami hit the southern coast of Sumba, Sumbawa, Lombok up to the south of Bali.
The Indo-Australian plate slips so steeply under the Eurasian plate it forms a fault line in the back-arc north of Flores Island to Bali. As a result, the islands of Bali, Lombok and Flores are exposed to tsunami threat from the southern and northern seas.
According to BMKG\'s head of earthquake and tsunami mitigation, Daryono, the fault line up north of Flores, Lombok, to northern Bali is very active, as happened in 1992 when a 7.5-magnitude earthquake followed by a tsunami killed 2,500 people. The epicenter was 35 kilometers from the city of Maumere.
"The fault line also triggered an earthquake and tsunami that hit northern Bali on November 12, 1815 and July 14, 1976," Daryono said.
The 1815 disaster was recorded in Babad [history of] Buleleng and Ratu Panji Sakti. The two chronicles mention about many people and royal families being affected by landslides at that time, which coincided with the Hinduism Saka year of 1737.
More complete records are kept at Puri Ayodya, Singaraja, as told by I Made Kris Adi Astra, an analyst from BMKG Region 2 Denpasar. "On Wednesday, umanis kurantil, Saka 1737 (22 November 1815), a big earthquake rocked. The quake\'s tremors caused the mountain to crack and slide with a roar like thunder.
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“The mountain avalanche hit the capital of Buleleng, Singaraja. Villages were swept out to sea. The disaster killed 10,523 people, including many royal officers. However, Buleleng King I Goesti Angloerah Gde Karang survived. "
Arthur Wichman (1918) compiled a catalog of earthquakes in the Indonesian Archipelago for the 1538-1877 period. He said the 1815 earthquake occurred around 10 pm.
Eddy current
In addition to earthquake and tsunami vulnerability, the geographic condition of Bali and Nusa Tenggara affects the current of the surrounding waters. Widodo said the conditions of the surface and the depth currents of the Bali Sea – or even between sea slopes of varying depths -- could be extremely different.
Current in the sea surface layer is affected by monsoon. At a certain depth, it is more affected by the current masses from the Pacific Ocean to the Indian Ocean through the Lombok Strait, locally known as Arus Lintas Indonesia (Arlindo) [current across Indonesia].
An observation by the ministry’s team and a number of international researchers from 2003 to 2011 found that the Arlindo discharge flowing through the Labani Canal in the Makassar Strait reached 11.6 million cubic meters per second, while the discharge through the Lombok Strait was estimated at around 2.6 million cubic meters per second.
In the opposite direction is the flow of internal waves -- or long waves below sea level -- from the Indian Ocean towards the north. In 2005, Widodo and his research team traveled along these waters on the Baruna Jaya ship to study the patterns of underwater currents.
It was found that the underwater currents in the waters around the Lombok Strait had different directions of flow based on their depth.
The Omni Akuatika journal (2017), by Yogo Pratomo from the Navy Academy together with Widodo and team, reveals that in the Lombok Strait at a depth of 100 meters, there are ocean currents moving to the south. At a depth of 250 meters, there is a current heading north, although it is insignificant.
On the other hand, in 350 meters deep, the currents heading north are stronger than the opposite currents. In 450 meters deep, northward currents strengthens. According to Widodo, the flow to the north is generated by internal waves, while the flow towards the south is by Arlindo.
"This ocean flow pattern makes the current movement under the Bali Sea very complex," he said.
“At a depth of 200 meters, as recorded on April 22, 2021, there was a current flow heading north from the Lombok Strait with its origin in the Indian Ocean. The current was split into two directions, turning west and east, when it hit the wall of the northern Bali Sea basin," Widodo said.
The current from the Lombok Strait that turns west and follows the wall of the Bali Sea basin is in the shape of a bowl with the current rotating against the clock. The current from the Lombok Strait turns east and rotates clockwise.
On April 25, 2021, according to Widodo, the current at a depth of 110 meters in the Bali Sea dominantly came from the Labani Canal in the Makassar Strait, being affected by Arlindo. The current either rotates in the Bali Sea, or moves on towards the Indian Ocean through the Lombok Strait.
No one knows what exactly has happened to KRI Nanggala-402. It has ended up aground under the turbulent depths of the Bali Sea.
This article was translated by Musthofid.