Recurring Tragedy in Lake Toba

Since 1968, at least seven ships have capsized in North Sumatera’s Lake Toba and hundreds of people have been killed or gone missing. In the calm and still waters of the lake, most of the victims descend vertically to its bottom.

The charming Lake Toba conceals the signs of these fatal tragedies. As the largest volcanic lake in the world with its deepest point at 508 meters, the dynamic currents and weather in this region must be heeded, especially in connection with the quality of passenger ferries, which must be improved. Overloaded ferries often capsize in the lake, claiming dozens of lives.

The KM Sinar Bangun ferry capsized in Lake Toba on Monday afternoon (June 18, 2018). The accident reminded us of the tragedy of the KM Peldatari I ferry, which sunk on July 13, 1997 in the waters of Lake Toba off Tomok village, Samosir Island, and killed 84 people.

Like the KM Sinar Bangun, the KM Peldatari I was also overloaded and did not have adequate safety equipment. The KM Peldatari I, with a capacity of 60 passengers, violated maritime regulations by carrying 169 passengers.

A similar tragedy took place when the KM Saor Nauli capsized on Apr. 13, 1987 in Lake Toba off Pangururan district and killed about 30 people.

It was difficult to determine the actual number of victims in each of these three events. Only an estimate was possible, especially as it was difficult to find drowned passengers in the lake.

In the KM Sinar Bangun case, the number of drowned victims could only be estimated because the vessel did not have a passenger manifest. As the Kompas daily reported (Thursday, 20/6/2018), the estimated number of victims was 200, based on reports from the families who had lost their relatives in the tragedy.

The Kompas archives show that a similar accident occurred on June 19, 1975 when the KM Sabar Ma Hita sank in Lake Toba off Tuk Tuk subdistrict, killing five. On March 30, 1968, a ferry had also capsized in Lake Toba, killing about 80 people.

Closed waters

Compared to the high seas, the risk of sailing on Lake Toba is smaller. The currents and waves tend to be calmer because the lake is a closed system. According to geologist Indyo Pratomo of the Geological Museum of Bandung, Lake Toba, which formed from a giant volcanic eruption, is the largest volcanic lake in the world, with an area of about 1,130 square kilometers and about 256.2 cubic kilometers of water.

The maximum depth of the lake is 508 meters – making it the ninth deepest lake in the world – and this point is located in the northern basin; in the southern basin, the maximum depth is 420 meters. The average depth of its waters is 228 meters.

With this large volume of water, Lake Toba is the largest body of fresh water in Indonesia. The sources of the water in the lake are several rivers that mostly supplies the water through waterfalls. Its only outflow channel is the waterfall in the upper Asahan River, which then flows into the Strait of Malacca.

With an outflow capacity of about 100 cubic meters per second, the retention time – the time it takes to drain the entire lake of water – is estimated at about 81 years, which is relatively long compared to other lakes. With this long retention time, the waters of the lake tend to be calm.

"From our study based on simulated climatological conditions over 10 years (1999-2009), the current velocity in Lake Toba averages no more than 2 centimeters (cm) per second. This is actually quite small, so small pollutants such as residual fish feed will accumulate near the coast and often lead to mass mortality of fish," said Hadid Agita Rusmini, a researcher at the Limnology Research Center of the Indonesian Institute of Sciences (LIPI).

In these calm currents, according to Hadid, a ship and its victims will go down vertically to the bottom of the lake in the case of an accident. Moreover, fresh water also reduces buoyancy. This is why the victims drowned in the Lake Toba are difficult to find and recover. According to Hadid, strong currents and waves can be found in the Lake Toba only at Silalahi Beach. "Our research used the scenario of normal weather conditions. There may indeed be extreme conditions. However, the lake’s overall pattern of winds, currents and waves is relatively calm and friendly to sailing vessels, as long as safety standards are maintained," he said.

Anticipating changes

At certain times, the waves on Lake Toba may become strong, but not as high as in open waters. The Meteorology, Climatology and Geophysics Agency (BMKG) records show that the weather in the Lake Toba area when the KM Sinar Bangun capsized was slightly rough. "On June 18, 2018, the average wind velocity around Lake Toba from morning until noon was 2 to 3 meters per second, or relatively weak. However, at around 5:00 in the evening, there was an increase in wind velocity to 6 meters per second. This happened in a matter of 10 minutes, and then it weakened again," said BMKG meteorology deputy chairman Mulyono R Prabowo.

The momentary upsurge in wind velocity caused strong waves. According to Prabowo, when converted, wind speeds of 6 meters per second over open water can cause waves of 0.75-1.25 meters high. "The problem is that Lake Toba is closed water, so there is a possibility of a reverse wave. The wave height remained relatively mid-range and should have been safe for shipping, if the ship was in good condition and not overloaded,” he said.

The BMKG can forecast high waves and bad weather at Lake Toba and Samosir Island. Edison Kurniawan, the head of the Medan Meteorology, Climatology and Geophysics Center, said the center had sent bad weather warnings in the area of the Lake Toba to all North Sumatra branches of the Regional Disaster Management Agency (BPBD). “An early warning that there would be medium to heavy rains and strong winds for the Toba and Samosir region was first sent out at 1:30 p.m., and was later updated at 4:30 p.m. and 6:30 p.m. because the potential was still high,” he said.

“The warning was sent to the BPBD, hoping that all transport operators would be alerted to the rough conditions," said Edison. According to him, the weather conditions in North Sumatra, especially the Lake Toba region, tended to change quickly in May, June and July, during the transition from the rainy season to the dry season.

The wind direction was constantly changing and stronger than usual. "This may be why many accidents occur during those months. But, it depends on the ship’s condition and its load," Edison added.

A tragedy may be unforeseen, but it can be anticipated. Early warnings are given, but previous accidents are often forgotten. Do not let this tragedy reoccur, as in the Batak saying: “Moncak ni Lobutua, dung hona baru jora (in the Lobutua martial art, defense follows experience).”