Why Measure the River Flow of Surabaya?

1. Where is Surabaya?

Geographical Background

Surabaya, the second-largest city of Indonesia and capital of East Java, lies on Java's northeast coast where the Brantas River discharges into the Madura Strait. The city proper of approximately 326 square kilometers is dominated by a coastline lowland (0–30 meters above sea level) in the north and gently rolling hills (50–150 meters) in the south. Its northern edge is fringed with mangrove forests and tidal mudflats, while the southern edge blends urbanization with cultivated land in the form of rice paddies and sugarcane farms. Surabaya's tropical climate provides warmth throughout the year, with temperatures ranging around 28°C, and rains of 1,600–2,000 mm, seasonally (November–March).

Human/Cultural Aspect

Surabaya is a vibrant, fast-moving city with a strong heritage as a strategic port and hotbed of Indonesian nationalism. The city, also called the "Hero City" due to its participation in the 1945 Battle of Surabaya, has a blend of Javanese, Madurese, Chinese, and Arab cultures. Its culture can be evidenced through the Ampel Mosque, a 15th-century Islamic monument, and the old Pasar Turi, a bustling marketplace for spices and traditional handicrafts. Javanese traditions thrive in wayang orang (human puppet) performances, while Madurese style dominates hot dishes like soto madura (coconut soup) and satay klopo (coconut satay). Surabaya, with over 3 million inhabitants, is a great industrial and commercial hub, but its character remains still encircled by the waterways that have shaped its growth.

Hydrology and River Overview

The Brantas River is Surabaya's principal watercourse, stretching 320 kilometers from its source in Dieng Plateau of Central Java to its outlet in the Madura Strait. As the Brantas enters Surabaya, it bifurcates into distributaries like the Kali Mas (Golden River), which flows through the city center, and the Kali Wonokromo, which drains southern suburbs. These rivers form a complex network that connects Surabaya with surrounding regencies of Sidoarjo and Gresik.

For Surabaya itself, the Brantas system is crucial. It supplies 60% of the city's fresh water, irrigates 120,000 hectares of rice in Sidoarjo, and facilitates maritime trade at the Port of Tanjung Perak—Indonesia's second-busiest port. Ecologically, the river estuary is a habitat of note for migratory sea birds and sea life, and its mangrove forests along riverbanks act to protect from erosion on the coast. But rapid industrialization has strained the system: factory and urban refuse pollution has polluted the water supplies, so flow measurement is crucial to resource management and environmental protection.

2. What is the River Flow around Surabaya?

Influencing Factors

Precipitation and Runoff

There are two seasons in Surabaya's climate. The wet season (November–March) is heavy, with over 250 mm per month, causing steep runoff on the south-facing slopes. This charges the Brantas and its tributaries, raising river levels in Surabaya by 2–4 meters and velocities of flow to 1.2–2.0 m/s, endangering floods to low-lying areas like Semampir. The dry season (April–October) sees rainfall decrease to 50–100 mm/month, lowering flows to 0.3–0.7 m/s, straining water supplies for industry and agriculture.

Terrain and River Morphology

The Brantas River course in Surabaya is defined by its development from an interior river to an estuarine tidal. In southern Surabaya, upstream, it consists of a long, gently sloping (0.02%) narrow channel and wider out in the city center where the Kali Mas widens to 80–120 meters with an average depth of 5–8 meters. Madura Strait tidal forces induce reversing currents twice daily, and saltwater invades up to 10 kilometers inland at high tide. Urbanization has diverted the river's path: parts of the Kali Mas have been linearized by concrete dikes, reducing its natural capacity to absorb floodwaters.

Reservoir Operations

Upstream reservoirs, such as the Sutami Dam at Malang, regulate the Brantas' outflow. Built in the 1960s, the dam captures water during rainy periods to reduce downstream flooding in Surabaya and releases it downstream during dry spells to maintain the irrigation system. Sedimentation has reduced its capacity by 20% over 60 years. Uncontrolled releases during heavy rain events in 2022 caused Brantas overflows, flooding 15% of Surabaya's northern suburbs (East Java's Disaster Management Agency), illustrating the need for accurate flow data.

Historical Hydrological Events

There has been extreme hydrological distress in Surabaya. The 2002 floods were the worst: monsoon rains overflowed the Brantas and Kali Mas, inundating 40% of the city, displacing 200,000, and costing $200 million (according to Kompas). Still more recently, the 2019 dry season reduced the Brantas' flow by 50%, and mandated water rationing and a 30% reduction in rice yields in Sidoarjo (in a 2020 Indonesian Ministry of Agriculture study). These events show the necessity of river flow observation.

3. How is River Flow in Surabaya Measured?

Traditional Methods

Surface Drift Buoy Method

They used wooden boards to estimate surface speed for decades, but in doing so ignored subsurface currents—crucial in Surabaya's tidal rivers, where currents reverse with tide. Wind and trash also skewed results, making it inappropriate for flood prediction.

Anchored Boat Method

This was accomplished through measuring velocity at different levels with a current meter, taking time: 4 people and 6 hours were required for one Kali Mas survey. Safety was a major risk in wet seasons when there were powerful currents that could be dangerous to boats. In 2016, employees stopped work after colliding with a floating container, the incident underscoring limitation.

ADCP Introduction

Acoustic Doppler Current Profilers (ADCPs) have revolutionized monitoring in Surabaya. The devices use sound waves to measure velocity across the whole water column in real time and capture 3D flow patterns—essential in tidal rivers. It takes 20 minutes to map a 100-meter stretch of the Kali Mas, providing information to predict floods and maximize water resources. Surabaya has installed 12 ADCPs since 2010, shortening flood response time by 35%.

4. How Does ADCP Work?

ADCPs send high-frequency acoustic pulses (300–1200 kHz) that bounce off suspended particles. The Doppler effect—frequency changes in reflected waves—reveals water velocity at several depths. Newer 4-beam designs monitor 3D currents, which are vital for mapping Surabaya's tidal currents and eddies.

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

• Material Reliability: Resistance to corrosion in Surabaya's brackish, dirty waters; stainless steel or titanium components are required.
• Lightweight Design: Portable (less than 6kg) for narrow river use, e.g., Kali Wonokromo.
• Cost-Effectiveness: Economical for Surabaya, using 10+ hour batteries for extended surveys.

6. Choosing the Right Equipment

Deployment Techniques

• Boat-Mounted ADCPs: Ideal for rapid flood surveys along urban reaches.
• Bottom-Mounted ADCPs: Installed under bridges for 24/7 measurement, enabling early warnings.
• Cableway ADCPs: Deployed in upstream narrow gorges where boats cannot travel.

Working Frequency

• 600 kHz ADCPs: Suitable for 70-meter shallow rivers like the Kali Brantas.
• 300 kHz ADCPs: Suitable for wider estuaries (110 meters or less) like the Kali Mas.

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.