1. Where is Makassar?
Background Geography
Makassar, the capital of South Sulawesi province, Indonesia, is situated on the island of Sulawesi's southwest coast overlooking the Makassar Strait. Spanning approximately 175 square kilometers, the terrain of the city is a mix of west-coastal plains (0–50 m a.s.l.) and rolling hills (100–300 m) in the east, with the Walanae River valley cutting through the center. It has a tropical climate with distinctive wet (November–March) and dry (April–October) seasons, with mean temperatures ranging from 26–30°C and mean rainfall ranging from 1,500–2,000 mm per year, concentrated in the wet season.
The city is surrounded by natural boundaries: the Ratangga Mountains in the north, the karst plateaus of Maros in the east, and the Spermonde Archipelago coral reefs in the west. There, the largest city in Sulawesi and the nation's chief seaport, Makassar is a gateway to eastern Indonesia, connecting islands via maritime trade routes.
Human/Cultural Aspect
Makassar is a multi-ethnic city inhabited by over 1.7 million inhabitants (census 2020) and dominated by the Bugis and Makassarese people, with large Javanese, Chinese, and Torajan populations. The city's history as the spice trade hub (16th–19th centuries) is encapsulated in its multi-ethnic heritage, a combination of native culture and Arab, Chinese, and Dutch tradition.
Cultural pursuits revolve around activities like pasola (ritual horse fighting) and gandrang (drumming ceremonies), and landmarks like Fort Rotterdam (a 17th-century Dutch fort) and the Trans Studio Makassar (the largest indoor theme park in Southeast Asia) bear witness to its mix of old and new. Food is famous for coto makassar (beef soup with a kick) and pisang epe (grilled banana), peddled in colorful markets like Pasar Baru. Islam is the dominant religion, with mosques like the 16th-century Makassar Grand Mosque filling the skyline.
Hydrology and River Overview
The Walanae River is Makassar's principal watercourse, a 192-kilometer course from Lake Tempe in mid-Sulawesi to the Makassar Strait. Inside the city, it is augmented by tributaries like the Tallo River—forming a system draining the Maros karst plateau and surrounding fields. The Tallo River, indeed, passes through Makassar's old town, past Fort Rotterdam and the old port.
These rivers are lifeline to Makassar. Walanae system gives the city 70% of fresh water supply, irrigates 25,000 hectares of rice fields in Maros and Pangkep regencies, and supports small-scale fisheries that provide a livelihood to 15,000 families. Environmentally, the river's upper sections in the karst plateau support endemic species like the Sulawesi water monitor, while its estuary in Makassar Bay is a critical area of habitation for dugongs and sea turtles. With the rapid rate of urbanization and industrialization, however, the rivers have been contaminated, with 60% of their urban sections failing national standards of water quality (as reported by the South Sulawesi Environmental Agency in 2021).
2. What is the River Flow around Makassar?
Influencing Factors
Precipitation and Runoff
Rainfall patterns in Makassar follow the monsoon pattern. Heavy rainfall occur during the wet season (November–March) with monthly rainfall of 250–350 mm, leading to quick runoff from the Maros karst. This results in the water level of the Walanae River in Makassar rising 2–4 meters and velocities up to 1.2–2.0 m/s, flooding low-lying areas such as the Tamalanrea district. The dry season (April–October) experiences precipitation to drop to 50–100 mm per month, slowing flows down to 0.3–0.8 m/s, reducing water supply to agriculture and industry.
Terrain and River Morphology
The Walanae River flows across a diverse landscape: the sharp gradients (0.5–1%) of the karst plateau lying to the east of Makassar level out to 0.1–0.2% on entering the urban coastal plain of the city. Within Makassar, the channel of the river is 50–80 meters wide with sandy bottom dotted with eroded limestone boulders from the karst. The Tallo River, narrower (30–50 meters), flows through more urbanized reaches with concrete embankments along 60% of the length.
Tidal influences of Makassar Strait extend 15 kilometers onshore to Makassar's central city, creating 0.5–1.0-meter daily changes in water level and reversing the direction of tidal flow in the Tallo River during high tide. This creates brackish conditions in low river reaches, affecting irrigation water quality.
Human Impact on Flow
Urbanization has totally transformed river dynamics. Since 2000, Makassar's urbanized zone has expanded 55%, with riverbanks converted into housing and industrial zones— reducing the flood capacity of the Walanae by 40%. Wastage from factories in Kappang district has increased pollution, and sand excavation from riverbeds has hollowed out sections 2–3 meters deep, altering flow channels.
A study in 2020 by the Indonesian Institute of Sciences (LIPI) found that expansion of groundwater extraction to support Makassar's population has reduced the base flow of the Walanae by 30% during the dry season, worsening water shortages even more.
Historical Hydrological Events
Makassar also faced serious hydrological hardship. The 2019 wet-season flooding made the Walanae River overflow 5 meters, flooding 30% of the city, including the business center and portions of the airport. Over 20,000 people were displaced, and the economy lost over $50 million (Antara News).
Droughts have also impacted the region: the 2016 El Niño event reduced rainfall by 50%, causing the Walanae to dry up in its middle reaches. This disrupted water supplies for 40% of Makassar’s residents and destroyed 60% of rice crops in Maros regency, leading to food price hikes (per South Sulawesi’s Agriculture Office).
3. How is River Flow in Makassar Observed?
Traditional Methods
Staff Gauge Networks
The South Sulawesi Water Agency maintained 12 staff gauges on the Walanae and Tallo Rivers to record daily stages and estimate flow. The gauges were however continually washed away by floods or covered by industrial refuse, and stage-discharge relations were disrupted by river morphological modifications due to sand mining.
Floating Buoy Surveys
Technicians used bamboo rafts with GPS to measure surface velocity, but this type of method did not account for subsurface currents and tidal reversals and thus led to faulty flood predictions. Buoy readings were found in 2018 to undercount peak velocities by up to 40% compared to modern methods.
ADCP Introduction
Acoustic Doppler Current Profilers (ADCPs) have been employed in Makassar since 2017, revolutionizing flow monitoring. Installed on boats or bridges, the ADCPs measure velocity through the entire water column simultaneously in real time, even in turbid brackish waters. A 200-meter reach of the Walanae can be mapped in 30 minutes, including tidal reversals and eddies missed by older methods.
Makassar’s water authority operates 6 ADCP units, deployed at key locations including the Walanae Bridge and near the Tallo River estuary. Data from these devices has improved flood forecasting accuracy by 55% and guided sand mining regulations to restore river balance (per South Sulawesi’s Water Resources Agency).
4. How Does ADCP Work?
ADCPs emit brief acoustic pulses (300–1200 kHz) that bounce off suspended material, such as sediment and plankton, in the water. The Doppler effect makes frequency of backscattered pulses vary with particle motion: increasing as they approach the device, decreasing as they are moving away. These variations are recorded by the ADCP to calculate velocity at 0.3–0.6-meter depth increments, creating a 3D flow profile from river bottom to top.
In Makassar, ADCPs are installed to account for tidal reversals, utilizing dual-frequency modes to measure upstream and downstream flow. Data is transmitted wirelessly to a central console for observation of real-time readings during floods and high-tide events.
5. What's Required for Makassar High-Quality Measurements?
Equipment Requirements
- Brackish Water Resistance: Must be resistant to saltwater corrosion in tidal areas, with titanium components to combat rust.
- Turbidity Tolerance: Can work in very turbid conditions (up to 800 NTU) caused by industrial wastage and suspended sediments.
- Portability: Lightweight (below 7kg) to be installed on the slender sections of the Tallo River.
- Battery Life: 12-hour minimum runtime to provide complete tidal cycle coverage, with rainproof housing to enable survival during monsoonal rains.
6. Equipment Selection
Deployment Methods
- Boat-Mounted ADCPs: Used for cross-river surveys across the Walanae River, measuring flow fluctuations near sand mining zones.
- Bridge-Mounted ADCPs: Fixed on 4 key bridges, continuously taking measurements of tidal and flood flows.
- Fixed-Bottom ADCPs: Deployed in the Tallo River estuary to monitor saltwater intrusion during dry seasons.
Working Frequency
- 600 kHz ADCPs: Most appropriate for the shallowest parts of the Tallo River (depth <8 meters), offering good resolution to capture tidal reversals.
- 300 kHz ADCPs: Used in the main channel of the Walanae (depth 8–15 meters), offering a compromise between range and resolution to capture industrial discharge plumes.
Brand Recommendations
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Here is a table with some well known ADCP instrument brands and models.
Why Measure River Flow in Makassar?