Why Do We Measure River Flow in Samarinda?

1. Where is Samarinda?

Geographical Background

Samarinda, the provincial capital of Indonesia's East Kalimantan province, is situated on the east coast of Borneo at the confluence of the Mahakam and Karang Mumus Rivers. The city covers approximately 718 square kilometers of a landscape dominated by tropical rainforest environments, vast mangrove swamps, and fertile alluvial plains. Its relief varies from 0 to 30 meters above sea level, and the majority of the urban areas are below 10 meters, making it very susceptible to riverine and coastal flooding.

The equatorial climate has temperatures ranging from 26–29°C throughout the year and annual rainfall of over 3,000 mm, falling mainly during the northwest monsoon period (November–March). To the north lies the Mahakam River delta, an interlaced network of tidal creeks and mangroves, but the south is dominated by the Meratus Mountains, which drain into the city's river systems through its slopes.

Human/Cultural Aspect

Samarinda is a multi-racial city of over 820,000 inhabitants comprising Dayak, Malay, Buginese, and Chinese. The region has long been populated by the Dayak tribes, that is, the Kutai and Paser people, and their practices have been maintained through longhouse villages and ancient traditions like the ngajat dance. The history of the city is traced to the Kutai Sultanate, the oldest Islamic kingdoms of Southeast Asia, whose legacy one can find in such symbols as the 18th-century Kutai Kartanegara Palace.

Modern Samarinda is a thriving center of coal mining and timber trade, but still retains local culture. Markets like Pasar Terapung (Floating Market) are a manifestation of daily life along the Mahakam, where tropical fruits, river fish, and hand-loomed items are sold off boats. Festivals like Hari Santri (Islamic Student Day) and Pesta Kutai (Kutai Festival) are manifestations of the city's multiculturalism, while contestants from throughout Kalimantan attend the annual Mahakam River Race.

Hydrology and River Overview

The longest of Borneo's rivers, the Mahakam River is the lifeblood of Samarinda, flowing 980 kilometers from the Muller Mountains to the Makassar Strait. Throughout its journey through Samarinda, it is joined by the Karang Mumus River and creates a broad waterway that widens to 500 meters within the city center. Significant tributaries like the Belayan and Kedang Kepala Rivers form a vast drainage system draining over 77,000 square kilometers of East Kalimantan's interior.

For Samarinda, the Mahakam system is crucial. It supplies 90% of the city's freshwater, supports fisheries that contribute 35% of local protein consumption, and serves as a vital transportation route—over 60% of goods destined for rural markets are transported by river. Ecologically, the river basin includes endangered fauna such as the Bornean pygmy elephant, proboscis monkey, and the critically endangered Mahakam river dolphin. Mangrove forests along its delta act as natural barriers against coastal erosion, whereas cropland is replenished with nutrient-rich sediment by seasonal flooding. Upstream coal mining and deforestation have increased sedimentation and pollution, threatening both ecosystems and livelihoods.

2. What is the River Flow around Samarinda?

Factors Affecting It

Rainfall and Runoff

Rainfall regimes in Samarinda are governed by the monsoon cycle. Rainy season (November–March) has intense rains, and monthly rainfall is 400–500 mm, causing sudden runoff from the Meratus Mountains. The water level in the Mahakam at Samarinda rises by 4–6 meters, and flow velocities increase to 1.2–1.8 m/s, which overflows into low-lying suburbs like Loa Janan. Rainfall drops to 150–200 mm/month during the dry season (April–October), which decreases flows to 0.5–0.9 m/s, but the river is still navigable all year round.

Terrain and River Morphology

The Mahakam river flows through a flat, deltaic plain in Samarinda with a gradient of merely 0.001%, giving rise to sluggish, sinuous currents. The river bed is composed of sand and silt, and there are mobile sandbars which change the channel's course during floods. Tidal effects of the Makassar Strait stretch 100 kilometers upstream to Samarinda, resulting in daily water level changes of 1–2 meters and tidal reversal of the Karang Mumus River during high tide. The tidal effect creates brackish water conditions in downstream reaches, influencing water quality for irrigation and consumption.

Human Impact on Flow

Large-scale human activities have greatly changed river dynamics. Upstream coal mining operations that produce over 200 million tons annually have introduced sedimentation to the Mahakam—since 2000, the river level in Samarinda has decreased by 2–3 meters, reducing its flood-carrying capacity. Deforestation due to palm oil plantations (1.2 million ha in East Kalimantan) has enhanced runoff, and a WWF study in 2020 concluded that flood peaks in Samarinda today happen 3 days ahead of when they used to happen in the 1990s. Riverbank destabilization through navigation dredging and sand mining have also occurred, with an urban area increase in erosion rates by 40%.

Historical Hydrological Events

Samarinda has been afflicted by successive flood disasters intensified by human activities. The 2019 floods were among the most severe: 60 days of monsoon rains caused the Mahakam to peak 7 meters, flooding 70% of the city, displacing 200,000 people, and leveling 12,000 homes (reported in Antara News). Infrastructure damage was in excess of $100 million, with upstream coal mines hastening sedimentation that funneled floodwater into urban areas.

Droughts, though less frequent, have also affected the region. The 2015 El Niño reduced rainfall by 60%, cutting the Mahakam's flow to 25% normals. This stranded cargo ships in Samarinda port, disrupted coal shipments, and prompted the city's water company to declare water rationing. Fishermen catches decreased by 70%, threatening the livelihoods of 15,000 fishers (from a 2016 WWF report).

3. How is River Flow in Samarinda Monitored?

Traditional Methods

Surface Drift Buoy Method

Years ago, the local government used GPS-equipped wooden rafts to approximate surface velocity from travel between river markers. Inexpensive but inaccurate, the method neglected subsurface currents—essential in the Mahakam, where coal sediment creates layered flows. Floods often rerouted floating material out of course, taking buoys along, and tidal reversals rendered results useless for prediction.

Current Meter Measurements

A more technical approach involved wading or boat travel to mid-channel and lowering mechanical current meters (e.g., Price AA) to the depth range to measure velocity. This provided accurate profiles but was time-consuming: one cross-section survey of the Mahakam required 6 staff and 8 hours. Safety was a risk at high flows, with strong currents capsizing boats in 2013, injuring survey crews.

ADCP Introduction

Acoustic Doppler Current Profilers (ADCPs) have transformed flow measurement in Samarinda since 2016. Fixed aboard specially designed survey vessels, ADCPs measure velocity across the entire water column in real time, even in cloudy, sediment-laden waters. The full 500 meters of the Mahakam can be mapped in 45 minutes, resolving complex patterns like tidal eddies and upstream currents in the confluence at Karang Mumus.

Samarinda's water utility makes use of 7 ADCP machines, positioned strategically along points like the Mahakam Bridge and coal mine outfalls. The readings from these sensors have improved flood predictions by 50% and enabled site-specific dredging to maintain shipping channels (as stated by East Kalimantan's Water Resources Department).

4. How Does ADCP Work?

ADCPs emit high-frequency acoustic pulses (300–1200 kHz) that bounce off suspended matter—coal dust, sediment, and plankton—in water. The Doppler effect shifts the "bounce-back" frequency: upward if approaching the device, downward if moving away. Analyzing these changes, the ADCP calculates velocity at 0.5–1-meter depth intervals, building an entire 3D flow profile from surface to river bed.

Modern ADCPs deployed at Samarinda consist of four 30-degree-angled transducers to monitor horizontal and vertical currents to detect rotating eddies common in the Mahakam's confluence points. Wireless data is transmitted to a central monitor for on-line viewing during flood or mine discharge operations.

5. What Does High-Quality Measurement in Samarinda Require?

Equipment Requirements

• Durability: Must be abrasion-resistant against coal sediment and corrosion in brackish water; titanium-strengthened casings are essential.
• Turbidity Tolerance: Can tolerate high turbidity levels (up to 1,000 NTU), the situation in the Mahakam with mining activities.
• Battery Capacity: Minimum operating time of 16 hours for conducting nocturnal surveys during tidal phases with maximum reversal of flows.
• Portability: Light enough (<12kg) to be deployed from shallow tributary small boats such as the Karang Mumus.

6. Selection of Suitable Equipment

Means of Deployment

• Boat-Mounted ADCPs: General method for cross-river surveys, allowing quick determination of flood and tidal flows in urban areas of Samarinda.
• Fixed ADCPs: Mounted on bridge piers close to mining outlets to observe flow changes due to industrial operations continuously.
• Drifting ADCPs: Used during flooding to monitor water flows in flooded residential areas, which will help in evacuation planning.

Working Frequency

• 600 kHz ADCPs: Ideal for shallow reaches (depth <15 m) like the Karang Mumus River, with good resolution to detect sediment layers.
• 300 kHz ADCPs: Used in the main channel of Mahakam (depth >15 m), where longer acoustic ranges capture full water column data.

Brand Recommendations

Well-established globally ADCP brands are Teledyne RDI, Nortek, and SonTek, which provide high-quality and dependable products that have been used in various hydrological research and monitoring studies. For an affordable option, the ADCP manufacturer Chinese company's "China Sonar Panda ADCP" is the best. Constructed of all - titanium alloy, it is highly durable and reliable, making it perfectly suited to endure the harsh aquatic environment around Kochi. Being an "affordable ADCP", it gives excellent high - quality performance without cost. To know more, check https://china-sonar.com/.

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