Why We Monitor River Flow in Jakarta

1. Where is Jakarta?

Geographical Background

Indonesia's capital and largest city, Jakarta, is located on the northwest coast of Java where the island projects into the Java Sea. Jakarta occupies approximately 661 square kilometers and is a low-lying urban area with elevations varying from sea level to as high as just 10 meters, thus being very susceptible to flood and sea-level rise hazards. South of it are the Bogor highlands, sloping gently upwards, and north the coast is dotted by mangrove swamps, mangrove forest, and a tangled maze of tidal rivers. It is of tropical climate with warm temperatures throughout the year, averaging 28°C, and with 2,000–3,000 mm mean annual rainfall, wherein the greater part precipitates in the rainy season.

Human/Cultural Aspect

A melting pot of cultures brimming with vitality because of centuries of trade, colonization, and immigration. As a port city with rich history, it has been shaped by Malay, Javanese, Chinese, Arab, and Dutch influences. This is also evident in its architecture—anything from the colonial-era Dutch-style buildings of Kota Tua (Old Town) to contemporary modern skyscrapers of Sudirman Central Business District today. The cultural fabric of the city is richly interwoven with celebration like Idul Fitri, Chinese New Year, and Nyepi, and cuisine originating from street food like nasi goreng (fried rice) and satay, an image of the multicultural character of the city. As a densely populated hub of commerce, talent, and creativity with more than 10 million citizens (and 30 million in the greater metropolitan area), Jakarta is nonetheless grounded in its waterways.

Hydrology and River Overview

Jakarta is traversed by a chain of rivers, the most significant of which is the Ciliwung River. Starting in the Puncak highlands south of the city, 119-kilometer-long river flows north through Jakarta's urbanized suburbs like Menteng and Tanah Abang before spilling into Jakarta Bay. The larger rivers, Cisadane, Cakung, and Krukut, form an intricate web of hydrology draining the southern highlands and urbanized part of the city into the Java Sea.

To Jakarta, both of them are both blessing and curse. The Ciliwung and its tributaries bring fresh water to millions of citizens, support small-scale agriculture in suburbs, and in the past linked settlements by transport. But their functions have transformed with increased urbanization—now they also supply stormwater drainpipes, but with sewage and waste typically overloading them. Eco-logically, the estuaries and rivers of Jakarta Bay are crucial to fish and mangrove, despite their massive pollution. The close association of the city with its waterways cannot be helped: it grew along them and continues to power its urban lifestyle, well or ill.

2. What is the River Flow near Jakarta like?

Factors of Influence

Rainfall and Runoff

The two seasons, wet (November–March) and dry (April–October), characterize Jakartas climate. During the wet season, monsoon rains drop over 300 mm of precipitation per month on the city. This creates excessive runoff from the highlands south of the city and the urban landscape, resulting in rapid rising of rivers like the Ciliwung. River water levels rise by as much as 3–5 meters, and flow rates are 1.5–2.5 m/s, flooding the drainage system and overtopping low-lying residential areas. Rain happens only 50–100 mm per month in dry months, and river flows are reduced to a trickle—0.3–0.8 m/s—and riverbeds expose litter, a signal of the city's pollution problem.

Terrain and River Morphology

Jakarta rivers consist of low-lying slopes and meandering courses because the city is lowland. The Ciliwung, for example, flows softly at a mere 0.01% gradient, hence it has slow-moving water with zero drainage at all during rain. The urban section of the river is usually channelized—bound by concrete walls to hold back floods—but this has cut its 50-meter width to 20 meters in sections, making it have less storage capacity. Italic text employed for emphasis phrases. Sedimentation, combined with deforestation that has already occurred on the southern highlands, also reduced the river's length, the average depth of which is now 2–4 meters (previously 5–7 meters, as in the 1980s). They create pools of stagnant water and increase flood risks because the river cannot drain excess water effectively.

Reservoir Operations

Upstream dams, such as the Jatiluhur Dam on the Ciliwung tributary, the Citarum River, are an essential regulator of Jakarta river flows. Jatiluhur, built in the 1960s, distributes water during wet months to reduce flooding downstream in Jakarta and sends it out during dry months to maintain supplies. Its capacity is diminished by sedimentation and increasing demand: in 2020, heavy rains with associated dam releases overflowed the Ciliwung, flooding 20% of Jakarta (Jakarta's Regional Disaster Management Agency). The smaller reservoirs like Situ Gintung, although less important, are also involved in local flow regulation, though their capacity has been lost to siltation.

Historical Hydrological Events

Jakarta has had a long history of disastrous floods. One of the worst was in 2007: 2 weeks of relentless rain overflowed the Ciliwung and the other rivers to their limits, and they broke their banks and flooded 60% of the city with 2–3 meters of water. Over 80 people were killed, 400,000 left homeless, and losses in the economy were over $4.5 billion (based on a World Bank estimate). Roads, power lines, and hospitals were severely affected, proving the weakness of the city.

Later in 2021, the flood struck 170,000 inhabitants, as the Ciliwung was at its record high due to a combination of heavy rainfall and inadequate drainage (Kompas newspaper). Droughts, though relatively less frequent, have also hit the city: river discharges dropped by 70% during the 2019 dry season, prompting water shortages in eastern Jakarta and requiring the importation of water via tankers (per a 2020 Ministry of Public Works study for Indonesia). These occurrences are utilized to emphasize the importance of proper river flow data to be processed to reduce risks.

3. How is River Flow in Jakarta Observed?

Traditional Methods

Surface Drift Buoy Method

Local authorities over the years used improvised equipment like plastic bottles or bamboo floats to estimate surface velocities by observing their movement over designated distances. It was cheap but highly inaccurate, as it ignored subsurface flows—significant in Jakarta's rivers, where pollutants and sediments produce layering currents. Wind and floating vegetation, prevalent in the Ciliwung, also distorted readings, making it impractical for flood forecasting.

Anchored Boat Method

This technique involved mooring a boat along the river and measuring velocity at different depths with a mechanical current meter. While it provided more accurate information, it was impracticable for Jakarta's congested, filthy rivers. Surveys required 6–8 hours per river section, required large teams, and were unsafe—boats would bump into rubbish or toil in strong currents in rainy weather. In 2015, a survey team was compelled to abandon their work because they had their boat wedged in a trash jam, which reflected the limitation of the technique.

ADCP Introduction

Acoustic Doppler Current Profilers (ADCPs) have revolutionized river flow measurements in Jakarta significantly. ADCPs use sound waves to measure velocity across the entire water column, from the surface to the river bottom, in real time. In contrast to traditional techniques, ADCPs can operate in dirty, litter-filled water and measure 3D flow patterns—imperative to deciphering the Ciliwung's complex currents. One ADCP on a small boat can scan a 50-meter section of the Ciliwung in 15 minutes, providing data to forecast flooding and calculate drainage needs. Jakarta's water agency has installed 15 ADCPs on the city's major rivers since 2010, reducing response times to flood incidents by 40%.

4. How Does ADCP Work?

ADCPs employ the Doppler effect: they emit high-frequency acoustic pulses (300–1200 kHz) which bounce off suspended particles in the water, such as sediment or organic matter. When these particles are swept along by the river flow, the frequency of the back-scattered pulses alters—higher when coming toward the device, lower when going away. By computing this change, the ADCP flow meter measures the water's velocity at several depths, forming a full "profile" of flow. Contemporary ADCPs employ four beams for measuring currents in three dimensions and are capable of identifying eddies and rotating flows typical in Jakarta's channelized rivers.

5. What's Needed for High-Quality Measurements in Jakarta?

Equipment Requirements

• Material Durability: Must be resistant to corrosion in Jakarta's contaminated, brackish waters, where high levels of organic material and chemicals accelerate wear. Stainless steel or titanium components are required.
• Lightweight Construction: Light (less than 7kg) for easy deployment from small boats on narrow, crowded rivers like the Ciliwung.
• Economy of Scale: Affordable for Jakarta's budget, with durable batteries (10+ hours of life) to deal with extended surveys during flood levels.

6. Choosing the Right Equipment

Deployment Methods

• Boat-Mounted ADCPs: Most suitable for rapid surveys in flooding, along Jakarta's urban rivers to provide real-time data for emergency response.
• Bottom-Mounted ADCPs: Installed at key points, e.g., under bridges, to provide continuous flow data for long-term monitoring and early warning of flooding.
• Cableway ADCPs: Used at the upper part of the Ciliwung where the river is narrower and deeper to measure flows from elevated installations.

Working Frequency

• 600 kHz ADCPs: Best applied for shallow, 70-meter-wide rivers like the Krukut, which gives high resolution to resolve changes near the riverbed.
• 300 kHz ADCPs: Better for wider, deeper rivers like Cisadane (up to 110 meters), where longer range sensors measure the full water column.

Brand Recommendations

Reputable and high-quality ADCP brands such as Teledyne RDI, Nortek, and SonTek are renowned globally. Chinese brands also include the ADCP supplier’s "China Sonar Panda ADCP," which is cost-effective. It is made of all-titanium alloy, which gives it high durability as well as corrosion resistance, and is capable of weathering the tough environment of Rajasthan's rivers. It is also very cheap and therefore accessible to use in the majority of the monitoring projects in the state. For further information, you can visit their website at https://china-sonar.com/.

Here is a table with some well known ADCP instrument brands and models.