Don't Blame the Rain If the Jabodetabek Flood Basin is Flooded

Regional leaders up to the President of the Republic of Indonesia may change, but there seems to be something eternal for the Jabodetabek area: flooding. Flooding is a legacy of Dutch colonialism that persists to this day, even worsening with the expansion of development in flood basin zones downstream and the degradation of water catchment areas upstream.

The floods that struck several areas in Greater Jakarta on Monday (3/3/2025) and Tuesday (4/3/2025) were preceded by heavy to extreme rainfall affecting various regions. However, the intensity of this rainfall was actually still within the "normal" category for the average rainy season, even lower compared to previous occurrences.

Siswanto, a climatology researcher at the Meteorology, Climatology, and Geophysics Agency (BMKG), stated that the high rainfall intensity was initially recorded in upstream areas. For instance, on Monday (3/3), rainfall concentration in Cibereum, Puncak, Bogor was recorded at 168 millimeters (mm) per day. In other upstream areas of Bogor, many locations also experienced rainfall intensity exceeding 100 mm per day at that time. On that day, rainfall intensity in the border areas of East Jakarta and Bekasi was not particularly high.

However, on Tuesday (4/3), the rainfall concentration in Katulampa reached 225 mm per day. Similarly, in the upstream areas of Bogor, the average remained above 100 mm per day. Rainfall with an intensity above 100 mm per day also occurred extensively, including in Bekasi and Jakarta City.

"The intensity of rainfall exceeding 150 mm per day falls into the extreme category according to BMKG's classification. However, this time, both the intensity and distribution of the rain are still significantly lower compared to the year 2020," said Siswanto, who is also the Head of the Applied Climate Information Working Group Team at BMKG.

Historical data shows that rainfall with an intensity above 150 mm is actually a common occurrence during the peak of the rainy season in Jakarta and its surrounding areas. For instance, when floods struck Jakarta on January 1, 1892, the rainfall in Jakarta at that time reached a record of 286 mm per day.

BMKG data also indicates that during the major flood that struck Jabodetabek at the turn of the year 2020, rainfall intensity exceeding 150 mm per day occurred at several rain observation stations. In fact, the rainfall intensity at Halim Perdanakusuma, East Jakarta at that time reached 377 mm per day, marking the highest record in 150 years of documentation.

This flood was not solely triggered by the intensity of rainfall. Rather, it was primarily caused by poor environmental management.

Meanwhile, in Taman Mini Indonesia Indah it was recorded to have reached 335.2 mm, in Jatiasih 259.6 mm, in Teluk Pucung 234.6 mm, in Tomang 225.6 mm, in South Tangerang 208.9 mm, in Manggarai 189 mm, in Ciputat 184 mm, in Rorotan 172 mm, and in Ragunan 155 mm.

So, looking at historical data, this flood was not necessarily triggered by the intensity of the rain. However, it was mainly due to environmental management that has not improved, at least since 2020. In fact, it can be said that the environmental carrying capacity in Jakarta is currently deteriorating, along with the widespread damage to water catchment areas upstream.

Looking at the chronology and distribution of rain, it can be seen that this flood started upstream. Head of the Disaster Data and Information Center of the National Disaster Management Agency (BNPB) Abdul Muhari said that flash floods had occurred in Cisarua, Bogor on Sunday (2/3/2025) night.

Heavy rain in the Puncak area was one of the triggers. As a result, the Ciliwung River, which was unable to accommodate the water discharge, overflowed, submerging residents' houses on the riverbanks to the downstream area.

In fact, according to Siswanto, the risk of flooding in Jabodetabek will also continue to increase due to weather factors and climate change trends that change rainfall patterns. Siswanto and his team's research report in the journal Meteorological Society of Japan in 2022 on changes in Jakarta's rainfall patterns between 1900 and 2010 showed that annual rainfall in Jakarta tends to be the same, or even decreasing. However, daily heavy rain with shorter duration tends to increase.

The study found a twofold increase in the number of short-duration rainfall events (1-3 hours) during the rainy season in Jakarta. A significant increase in frequency, around 25 percent, in these short-duration rainfall events, including extreme ones or above 10 mm per hour, occurred mainly in 2001–2010, compared to the early 20th century. In terms of intensity, these short-duration rainfall events increased by 50 percent.

"The magnitude of flooding is influenced by vulnerability and exposure. From my research, Jakarta's vulnerability has increased due to weather factors and climate change. Likewise, exposure has increased due to poor environmental governance," he said.

The floods in Jakarta and its surroundings this time are just a repeat of previous floods.

Only three years after it was built by Jan Pieterszoon Coen, in 1621, Batavia--the old name of Jakarta--was flooded. Floods also occurred in 1654 and have continued to grow since then. The city that Coen originally designed at the mouth of the Ciliwung River as a duplicate of Amsterdam in the Netherlands and nicknamed the Venice of the East because of its canal system was slowly abandoned.

In the late 18th century, there was a massive migration of Batavia residents from the old city at the mouth of the Ciliwung to the higher areas in the south, which were then called Weltevreden, around present-day Central Jakarta. Weltevreden, which was originally forest and swamp, then developed rapidly. Moreover, in 1807, Herman Willem Daendels then built the center of government of the capital of the Dutch colony in Asia there.

Initially, Daendels actually intended to build a government center in Semarang or Surabaya, due to poor environmental support. For cost reasons, he still built it in Weltevreden. In 1830, the capital of the Dutch East Indies officially moved to Weltevreden.

However, flooding became more severe as the city grew further south. Apart from small floods that occurred almost every year, history records a number of major floods that occurred repeatedly in Jakarta, namely in 1654, 1872, 1909, and 1918. The flood in February 1918 was recorded as the largest in the history of floods in the center of the Dutch East Indies government.

The Sin Po newspaper of February 19, 1918, recorded that almost all of Jakarta was flooded. In the areas of Tanah Tinggi, Pinangsia, Glodok, Straat Belandongan, Tambora, Grogol, Petaksinkian, Kali Besar Oost, the average water level was up to the chest of an average adult. Likewise in Angke, Pekojan, Kebun Jeruk, Kapuran, Kampung Jacatra or Kampung Pecah Kulir next to Kali Gunung Sari and Pejambon, the water also submerged people's houses.

Years after that, even after the Dutch East Indies government finished building the water channel that would later be known as the West Flood Canal, from newspaper or magazine clippings at that time it was seen that almost every year Jakarta was flooded. Van Breen, the Dutch engineer who built the West Flood Canal to the Katulampa Dam in Bogor even admitted that the presence of these flood control facilities did not completely guarantee that Jakarta would be free from flooding.

According to him, with the existence of the West Flood Canal and the Manggarai sluice gate, what happened was only a diversion of the flood area. If previously the flood hit the Weltevreden and Menteng areas, with the presence of the canal and sluice gate, the water then flowed to a lower place so that the flood moved to the Manggarai and Jatinegara areas.

As written in the newspaper Siang Po, when a major flood hit Jakarta in 1918, "because Pejambon is the lowest place, almost all the houses were filled with water. Some were chest-high and some were waist-high.…"

After independence, floods became more routine. The denser the population, expanding to Jabodetabek, the wider the floods became. In fact, the 23.5-kilometer East Flood Canal stretching from Cipinang to the Marunda Sea, which was completed in 2010, does not guarantee that Jakarta will be flood-free. Just as Van Breen said, it only moves the flood.

Jakarta's floods cannot be solved with a canal system alone, because the geological conditions of this area are flood basins. The northern area around Ancol and Jakarta Bay is slowly being lifted due to tectonic processes, causing water from 13 rivers that flow into Jakarta Bay to not be able to flow smoothly into the sea and is often trapped in large basins.

So, it can be said that Jakarta is actually like a giant bowl that is covered with flood sediment. This bowl-like geomorphological characteristic causes Jakarta Bay not to form a delta because the material carried through the rivers is mostly redeposited in the "Jakarta flood plain".

This river sediment also caused Jakarta in the past to be mostly swamps. The dry plains during the dry season became water parking areas during floods. So, during the colonial era, the flood overflow areas were used as water parking areas and became agricultural areas and green areas, and only a few could be built.

However, many water parking areas are filled in. For example, the Tebet area which is actually the Ciliwung River flood overflow, the Mampang area which is the Krukut River flood overflow, and Kebayoran Lama which is the Grogol River flood overflow. In fact, the riverbank areas are now densely populated with housing.

The threat of flooding in Jakarta is getting worse as the highland areas surrounding the basin are transformed into new residential areas. Many reservoirs and swamps on the outskirts of Jakarta are now being drained and converted into residential areas. As a result, the water catchment areas are sending more surface water into Jakarta.

From a historical perspective, it seems that flooding has become an inseparable part of Jakarta. When it was still called Batavia with a population of hundreds of thousands of people and when open spaces were still widespread, flooding had already occurred. While in the Puncak area, the land conditions were relatively not as bad as they are now, flooding was also a routine event.

From the perspective of climate and the increasingly massive trend of damage in catchment areas and flood basins, it can be projected that the threat of flooding in Jabodetabek will continue to increase in the future. Moreover, if the peak of the rainy season occurs simultaneously with high tides and tidal flooding from Jakarta Bay.