Entering the New Year, the capital city was severely flooded. The Meteorology, Climatology, and Geophysics Agency (BMKG) reported heavy to very heavy intensity rainfall spread across Greater Jakarta, concentrated in the eastern part of Jakarta with the highest level recorded in the Halim Perdanakusuma area.

It was the highest recorded rainfall since 1866. The BMKG reported that the atmospheric phenomena of the strengthening Asian monsoon flow and the location of the intertropical convergence zone, a low-pressure area located precisely over the north of Java, had caused the heavy downpour of that scale on the northern coast of Java.

This circumstance was observed a few days earlier on the north coast of Java, such as flooding that occurred at the Cikopo-Palimanan (Cipali) toll road in Indramayu. Toward the New Year, this low pressure was stronger in Jakarta, especially in the eastern part of the capital city.

Meanwhile, Australia was experiencing an extreme summer, resulting in very low air pressure over the continent. Hence, the westerly monsoon system that blew from Asia to Australia, carrying water vapor as it passed through the South China Sea and the Indian Ocean, became stronger due to pressure differences. The ocean turned warmer, causing evaporation that created a lot of water vapor.

Arriving in Java, this water vapor condensed rapidly upon reaching the high mountains. This continued to occur so that it generated massive and towering cumulonimbus clouds spreading in the north, as captured by the satellite imagery, which caused strong winds and heavy rains accompanied by lightning in the north of Java and western
Indonesia.

Why was the rain so heavy in East Jakarta? The eastern parts of Jakarta, Bekasi and Cikarang, are home to the industry and massive infrastructure development, resulting in high temperatures and low-pressure areas. Cumulonimbus clouds moved to these areas, and high-intensity rain poured over East Jakarta.

In addition to the extreme weather, the poor drainage system played a part in the flooding. Tons of waste piling up the drainage canals should have been removed through proper maintenance. Due to the poor drainage, the rain created puddles and flooding in Jakarta.

Therefore, a crash program, such as garbage removal aiming to expedite the water flow in the drainage canals, is an urgent necessity to overcome flooding. The Jakarta administration must strengthen the cleaning taskforce and recruit new cleaning agents and utilize neighborhood units and pay them according to the regional minimum wage.

What about the vertical drainage program? Vertical drainage is site-specific. For Jakarta, 60
percent of which has a clay soil texture, recharge wells or so-called vertical drainage is unlikely to be successful.

Horizontal drainage remains the correct solution either through the surface or underground, while further study is required to determine the specific technical needs for these as parts of a proper integrated urban drainage system.

What about the water from upstream? Naturally the water flows from upstream to downstream areas following the toposequence of the watershed. Land use conversion in upstream areas increases the volume of surface runoff and sedimentation, leading to the siltation of reservoirs and river channels so that the river flow capacity decreases and causes the floods.

What distinguishes flood management is a prompt or later response. Immediate response by, for example, clearing all blockages of the flood is necessary. Medium- to long-term responses can take form in normalization of rivers that dissect Jakarta in order to increase their flow capacity when flood volumes dramatically increase.

Currently, the flood control and early warning system in Jakarta is ineffective. Several models of flood control systems have been well-developed, such as the Smart Tunnel in Kuala Lumpur, Malaysia, and a super levy system equipped with underground tunnels and storage in Tokyo, Osaka and Kyoto, Japan.

However, they are not necessarily appropriate to be adopted in Jakarta due to the different local characteristics, both in terms of the terrain and river networks. Each model was developed with its own features. Therefore, optimizing the existing infrastructure, such as the west and east flood canals, polders and improving the drainage system and floodways, as well as making sure the pump system works properly are the priorities.

Simple flood early warning, based on the rainfall and runoff calculations connected to drainage capacity, or at the regional scale by taking into account the land-use conversion in the upstream areas, may work.

However, the fundamental issue is the drainage. As long as garbage clogs the drainage, siltation of reservoirs and rivers will keep occurring and floods will return to Jakarta. Hence, sustainable maintenance of the waterways and the environment from various types of waste, and the normalization of the Jakarta rivers and reservoirs is a simple but smart solution to anticipate future floods.

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Yopi Ilhamsyah is a lecturer of meteorology at Syiah Kuala University Banda Aceh. Hidayat Pawitan is a professor of water resources hydrology, Department of Geophysics and Meteorology, Bogor Agricultural University.

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