Why Do We Measure River Flow at Denpasar?

1. Where is Denpasar?

Geographical Background

Denpasar, the provincial capital of Bali in Indonesia, lies on the southern coast of Bali between the volcanic peaks of Mount Agung and Mount Batur to the north and the Indian Ocean to the south. Stretching some 127 square kilometers in area, the city's topography increases from coastal plains (0–20 meters asl) along the south to moderate rolling hills (50–100 meters) towards the north. It boasts a tropical climate with a distinct dry (April–October) and wet (November–March) season, and average temperatures ranging from 25–30°C with yearly precipitation reaching 1,800–2,200 mm, with most of the rain falling in the wet season.

The city is encircled by fertile agricultural lands, like rice fields in northern suburbs, and crossed by small, fast rivers originating from the island's central mountains. It merges with the Nusa Dua peninsula in the east, and on the west, it is connected to the tourist center Kuta—therefore, Denpasar becomes the economic and administrative hub of Bali.

Human/Cultural Aspect

Denpasar is a successful center of Balinese Hindu civilization with over 897,000 inhabitants (2020 census) of mostly Balinese, Javanese, and Chinese ethnicity. The city's character is shaped by ancient customs, as over 1,000 temples (pura) are spread throughout its terrain, among them renowned Pura Besakih (the "Mother Temple") and Pura Maospahit, a 14th-century royal temple.

Cultural life is all about Hindu ceremonies, from morning prayers (canang sari) to elaborate celebrations like Galungan (celebrating victory of good over evil) and Nyepi (Day of Silence). Balinese crafts thrive in the bazaars and workshops of Denpasar in which artisans create intricate wood carvings, paintings, and traditional kecak dance masks. The city's food blends local taste—such as nasi campur (mixed rice) and sate lilit (minced meat satay)—with global influence in recognition of its position as a gateway tourist city.

Hydrology and River Overview

The Wos River and Tukad Badung River are Denpasar's principal rivers, both originating from the Mount Agung slopes and flowing south into the city before emptying into the Indian Ocean. The 32-kilometer-long Tukad Badung flows through the central Denpasar, while the 28-kilometer-long Wos River flows through the outskirts in the east. Smaller tributaries like the Tukad Mati and Tukad Petanu form a network draining southern volcanic plains of Bali.

To Denpasar, these rivers are valuable resources. They supply 60% of the city's freshwater (supplemented by groundwater), feed 15,000 hectares of paddies around the nearby Gianyar and Tabanan regencies, and maintain traditional fish ponds (tambak). Ecologically, the upper river sections are inhabited by endemic species like the Bali myna and the Balinese river frog, while estuaries are crucial habitat for migratory birds. However, population growth and tourism development have exerted pressure on the river systems, with water security under threat from over-draw and pollution.

2. How is the River Flow around Denpasar?

Factors Affecting

Rainfall and Runoff

Denpasar rainfall is monsoon-controlled. Heavy rain (300–400 mm/month) in the wet season (November–March) produces abrupt runoff from volcanic slopes. This elevates Denpasar's Tukad Badung water level by 1–2 meters and increases flow velocities to 1.0–1.8 m/s, sometimes flooding low areas like Sanur district. The dry months (April–October) see rainfall drop to 50–100 mm monthly, and flows are decreased to 0.2–0.6 m/s, straining water resources for agriculture and tourism.

Terrain and River Morphology

Denpasar rivers drain across a volcanic landscape, with high slopes in their headwaters (0.5–1%) that level out to 0.1–0.2% upon entering the city. Denpasar's Tukad Badung channel width is 20–40 meters wide by gravel and sand bed, while the Wos River is narrower (15–25 meters) and shallower with more extreme meanders. Both streams are subject to volcanic ash deposition—Mount Agung's 1963 eruption deposited 5–10 cm of ash in the Tukad Badung, reducing its depth by 1 meter in some areas.

Tidal influences are limited, extending only 2–3 kilometers inland from the shore, creating brackish water in the lower Tukad Badung but not affecting Denpasar's urban zones.

Human Impact on Flow

Tourism and urbanization have significantly altered river dynamics. Since 2000, Denpasar’s built-up area has expanded by 45%, with riverbanks concreted and channels narrowed to make way for roads and hotels—reducing the Tukad Badung’s flood capacity by 30%. Groundwater extraction for hotels and households has also lowered water tables, causing rivers to lose base flow: a 2022 study by Bali’s Water Resources Agency found that the Tukad Badung’s dry-season flow has decreased by 40% since 1990.

Untreated sewage contamination and plastic waste also have damaged the water quality, and 70% of the urban sections of the Tukad Badung are below national water quality levels (based on a 2021 report by Indonesia's Ministry of Environment).

Historical Hydrological Events

Denpasar's worst hydrological problem is brought about by flooding and drought. The floods during the 2018 wet season caused the Tukad Badung to swell 3 meters, inundating 20% of the city, including parts of the central market and residences in West Denpasar. Over 5,000 people were made homeless, and tourism revenue went down by 15% because of road closures (The Jakarta Post).

Droughts have become more frequent: the 2019 El Niño event reduced rainfall by 60%, causing the Tukad Badung to dry up in its middle reaches. Hotels were forced to import water via tankers, while farmers in Gianyar lost 50% of their rice crops, leading to localized food shortages (per Bali’s Agriculture Office).

3. How is River Flow in Denpasar Observed?

Traditional Methods

Staff Gauge Networks

For decades, the Bali Water Agency maintained staff gauges at 10 stations along the Tukad Badung and Wos Rivers, taking daily water levels to estimate flows from historical stage-discharge relations. The gauges were often vandalized or silted up, and the relations degraded as the river morphology changed due to urban development.

Manual Current Meters

Technicians used hand-held current meters (e.g., the Price Type AA) to record fixed-point velocities in the river. This was a dangerous procedure with high-flow cases where it entailed wading in shallow water, and provided only point information with no spatial variations of flow. Manual measurements underestimated peak flood velocities by up to 30% as per a 2015 survey compared to today's methods.

ADCP Introduction

Acoustic Doppler Current Profilers (ADCPs) started being used in Denpasar in 2016, a flow monitoring game changer. They are mounted on a bridge or tiny boat and sense velocity from the bottom up across the water column in real time, even shallow, sediment-laden waters. The Tukad Badung reach can be mapped over 100 meters in 20 minutes, sensing flow structure around bridges and concrete embankments, things that were not detected by traditional approaches.

Denpasar’s water authority operates 4 ADCP units, deployed at key locations including the Badung Bridge and near the city’s main water treatment plant. Data from these devices has improved flood forecasting accuracy by 50% and informed targeted dredging projects to restore river capacity (per Bali’s Disaster Management Agency).

4. How Does ADCP Work?

ADCPs emit high-frequency acoustic pulses (300–1200 kHz) that bounce off suspended particles, such as volcanic ash and sediment, within the water. The Doppler effect causes frequency changes in returned pulses based on particle movement: positive if moving towards the device, negative if moving away. ADCP utilizes these to calculate velocity at 0.2–0.5-meter depth intervals, creating a 3D flow profile from surface to river bottom.

In Denpasar, ADCPs will be calibrated to compensate for shallow depths, using narrow acoustic beams to avoid riverbed interference. Data are transmitted wirelessly to a central database, providing real-time monitoring during flood and drought episodes.

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

Equipment Requirements

• Shallow-Water Performance: Must function in depths as shallow as 0.5 meters, characteristic of the Tukad Badung at dry times.
• Sediment Resistance: Protected from clogging by volcano ash and sand, which may damage transducers.
• Portability: Lightweight (under 6kg) for convenience of setting up in urban sections with limited access.
• Battery Life: Minimum 8-hour working time to finish daily survey schedules, with rain-proof casings.

6. Choosing the Proper Equipment

Deployment Methods

• Bridge-Mounted ADCPs: Fixed on 3 major bridges spanning the Tukad Badung, providing real-time readings on fluctuations in flow.
• Boat-Mounted ADCPs: Used in monthly cross-river surveys to record space variations in flow, particularly close to concrete embankments.
• Handheld ADCPs: Used in narrow tributaries like the Tukad Mati where boats are not allowed.

Working Frequency

• 1200 kHz ADCPs: Suitable for shallow sections of the Wos River (depth <3 meters), with high resolution to detect near-bed sediment movement.
• 600 kHz ADCPs: Used in the main channel of Tukad Badung (3–8 m depth), finding a compromise between range and resolution to capture flow around urban infrastructure.

Brand Recommendations

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