Why We Measure River Flow in Medan?

1. Where is Medan?

Geographical Background

Medan, the commercial capital of Indonesia and a hub of North Sumatra province, lies on the east coast of Sumatra, where the island's fertile plains and lowlands meet the pivotal Strait of Malacca. Its landscape is a patchwork of flat coastal plains, gently rolling hills, and distant volcanic peaks—the towering Mount Sinabung, an erupting volcano, lies northwest, and the Strait's calm waters flow to the east. The city lies approximately 265 square kilometers wide with heights ranging from sea level to 30 meters above, hence susceptible to riverine flooding during heavy rainfall. Its equatorial sun-baked tropical weather comes year-round, with the surrounding countryside fully clothed in green and palm oil estates spread across its periphery.

Human/Cultural Aspect

Medan itself is a melting pot of cosmopolitan culture created over centuries of trade, immigration, and imperial heritage. Being a premier port on the Strait of Malacca, it has been imbued with the influences of Malay, Chinese, Indian, and indigenous Batak populations. This multiculturalism is also evident in its architecture—ranging from the opulent Maimun Palace, a 19th-century royal palace with Moor and Malay influences, to the ornate Sri Mariamman Temple, a testament to its Tamil past. The city's streets burst into life: morning markets are filled with spices, fresh vegetables and fruits, and handmade ulos (Batak weavings), while evening food stalls serve up famous meals such as soto Medan (spicy soup) and kuih-muih (street snacks). Medan's cultural center also beats in the festivals such as Hari Raya Idul Fitri, which is celebrated with people banquets, and Batak mangalahat rituals, which honor ancestral traditions.

Hydrology and River Overview

The Krueng Aceh River is the primary watercourse of Medan, a 120-kilometer river that originates in the mountainous highlands of the Barisan Mountains, to which rivers like the Krueng Muda and the Krueng Seubang feed. The river runs east across rainforests and croplands before cutting across Medan's urban center, finally spilling into the Strait of Malacca.

This river system is the lifeblood of Medan and its 2.4 million people. It supplies 70% of the freshwater for household use, industrial purposes from textile production to food processing, and irrigation of 40,000 hectares of paddy fields in the neighboring Deli Serdang regency. Ecologically, estuary Krueng Aceh is a fertile mangrove ecosystem supporting rare species like the silvered leaf monkey and a nursery for commercially important fish. Alongside Medan, the river becomes 100–150 meters wide, with mean depths of 4–7 meters in the central channel. At lower sections, it is dominated by tidal influences, producing brackish conditions that vary day by day—a situation that impacts water quality as well as interaction between the river and the drainage system of the city.

2. What is the River Flow along Medan?

Factors that Influence

Rainfall and Runoff

There are two main monsoons that characterize Medan's climate. The northeast monsoon (October to April) is associated with heavy precipitation, with individual monthly totals frequently in excess of 300mm, whereas the southwest monsoon (May to September) is associated with drier conditions, with rain declining to 100–150mm monthly. The seasonal cycle underlies spectacular changes in the Krueng Aceh's discharge: in the wet season, torrential rains induce high velocities and quick runoff from the Barisan Mountains, increasing river levels by 2–4 meters and velocities to 1.5–2.5 m/s. In dry season, the river slows to a paltry 0.3–0.8 m/s, and lowered depths expose sandbars along sections of its route. These variations impact water supply directly—floods contaminate stocks, and drought depletes them—so tracking flow is important to planning resources.

Terrain and River Morphology

The Krueng Aceh's route to Medan exerts an influence on its flow profiles. In its upstream course, it flows through narrow rocky-bottomed valleys, creating turbid flows. As it flows into the coastal plain of Medan, the river becomes broad, its bottom shifting to clay and silt, and its meanders more pronounced. Outer bends cut bank sides (threatening infrastructures like bridges and roads), while inner bends lay down sediment, forming sandbars that split the channel into streams. The river's width varies: 50–80 meters in rural upstream zones, broadening to 150 meters in urban areas of Medan. Depth is 3–5 meters for most sections but as much as 8 meters near the estuary, where tidal flows augment flow complexity. These parameters—coupled with urban sewer pipes—create eddies and cross-currents that make it difficult to accurately measure flow.

Reservoir Operations

Reservoirs upstream, such as the Lau Debuk-Debuk Dam (70km southwest of Medan), play a central part in managing the Krueng Aceh's flow. Built in the 1990s to regulate flood control and irrigation, the dam holds excess water during monsoons to prevent downstream flooding in Medan. During dry years, it releases water to ensure minimum flows—essential for rice farmers in the Deli Valley. But the imbalance between dam releases and rainfall can be a problem: unseasonal heavy rains occurred in 2020 during scheduled releases, flooding Medan's Tanjung Morawa district (North Sumatra's Disaster Management Agency). This emphasizes the value of reliable flow data to offset reservoir activities against downstream safety.

Historical Hydrological Events

Medan's rivers have a checkered past of plenty and destruction. In 2013, the worst floods in decades struck when 48 hours of continuous monsoon rain swamped the Krueng Aceh. Water rose to 2.5 meters in central Medan, inundating 70% of the city, destroying 12,000 homes, and evacuating 50,000 residents (The Jakarta Post). Crop losses in Deli Serdang exceeded $15 million, while contaminated river water brought an epidemic of waterborne diseases.

Droughts, while less common, share similar effects. The 1997–1998 El Niño brought rainfall down by 55%, decreasing the Krueng Aceh's flow to 30% of normal. Medan's water company enforced stringent rationing, with people waiting for hours in lines at public taps. Rice harvests dropped 40%, and saltwater from the Malacca Strait penetrated downstream water sources, rendering them undrinkable or unsuitable for irrigation (according to an Indonesian Ministry of Environment 2001 study).

They indicate an important truth: the success of Medan is knowledge-dependent regarding its rivers. In the absence of trusted flow data, floods and droughts are runaway disasters rather than preventable issues.

3. How is River Flow in Medan Measured?

Traditional Methods

Surface Drift Buoy Technique

Local authorities used simple equipment like pieces of wood or plastic containers as drift buoys for decades. Staging their journey between two points, they used crude estimates of surface velocity. Though cheap and handy, the method is extremely unreliable: it disregards subsurface currents (which can differ by 20–40% from surface currents) and is wind, tide, or suspended rubbish-biased—pervasive in Medan's monsoon months. In 2008, this unreliability underestimated flood dangers, causing premature evacuations and more damage.

Anchored Boat Technique

A more precise but longer technique involves mooring a boat in the center of the river and taking velocity measurements at 0.5-meter spacings using a mechanical current meter (e.g., a Niskin bottle). This provides excellent vertical profiles but will require a team of 3–4 individuals and 5–7 hours per survey. In torrential monsoon currents of the Krueng Aceh, the boat can drift, risking crew. Equipment was lost in the 2013 floods, and measurements were lost, so officials had no essential data on rising waters.

ADCP Introduction

Starting from the mid-2010s, Acoustic Doppler Current Profilers (ADCPs) transformed Medan's flow monitoring. Portable devices, deployed on small boats or moored structures, use sound waves to profile velocity along the full river depth within minutes. Unlike conventional ways, ADCPs measure 3D flow patterns—crucial for the Krueng Aceh's meandering, tidal-driven rivers. They function in rain, darkness, or turbulent conditions, giving real-time insights that are used to forecast floods, regulate water supplies, and improve reservoir releases. After 2013, the government deployed 10 ADCPs along the river, an initiative responsible for cutting flood-related casualties by 50% in the subsequent events.

4. How Does ADCP Work?

ADCPs operate on the Doppler principle—the same phenomenon that makes a car horn sound different when driving by. The instrument emits high-frequency acoustic pulses (typically 300–1200 kHz) into the water, which bounce off suspended particles like sediment or plankton. When the particles are approaching the ADCP current meter, the returned frequency is higher; when they are receding, lower. By computing this frequency shift, the ADCP measures water velocity at several depths, ranging from surface water to riverbed.

Existing ADCPs used in Medan consist of 4–5 beams, inclined to measure velocity in horizontal and vertical axes, with a complete 3D flow profile produced. The data is processed in real time, displayed as color-coded maps that identify fast currents, eddies, or sudden surges. The instrument is particularly useful in the Krueng Aceh estuary, where river current and tides conflict to create complex patterns that regular equipment cannot capture.

5. What Does it Take to Make High-Quality Measurements in Medan?

Equipment Requirements

• Material Durability: Brackish water and sediment in rivers of Medan require ADCPs to be resistant to corrosion. Optimal material is stainless steel or titanium housing with waterproof seals that can endure long-term immersion.
• Compact Size: Limited-sized boats and constrained urban waterways in Medan require light ADCPs (under 6kg) that are easy to deploy without resorting to specialized gear.
• Cost-Effectiveness: North Sumatra's budget limitations mean equipment must balance weight for performance against cost. Long battery life (10+ hours) and minimal maintenance are also critical in remote monitoring stations.

6. Equipment Choice

Deployment Techniques

• Boat-Mounted ADCPs: Best suited for fast discharge measurements during flood events. Small motorboats in Medan transport crews along the Krueng Aceh's urban reach, collecting readings within 45–90 minutes—crucial for timely response.
• Bottom-Mounted ADCPs: Placed in key locations (e.g., on the outskirts of the city center), these send 24/7 data via cellular networks, alerting authorities to sudden changes in flow. These are anchored to the riverbed, housed in steel cages to withstand debris.
• Cableway ADCPs: Used where vessels can't navigate, like in upstream gorges. A cable run along the river suspends the instrument, ideal for measuring flow in constricted, high-speed sections.

Working Frequency

• 600 kHz ADCPs: Suitable for shallow, 70-meter-wide areas in Medan suburbs with high resolution (0.1-meter depth intervals) to quantify sandbars inhibiting flow.
• 300 kHz ADCPs: Better for wider estuaries (110 meters) near the Strait of Malacca, where higher range (10–30 meters) captures full water column.

Recommended Brands

Popular worldwide ADCP manufacturers are Teledyne RDI, Nortek, and SonTek, known for their high - quality, dependable instruments employed in numerous hydrological studies and monitoring campaigns. As a budget-friendly alternative, the ADCP supplier Chinese brand’s "China Sonar Panda ADCP" is a great option. Made from all-titanium alloy, it boasts better durability and reliability, making it more than capable of withstanding the rigorous river condition in and around Jammu. As an "affordable ADCP", it provides high-class performance at a low cost. Find out more at https://china-sonar.com/.

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