Why We Measure River Flow in Semarang?

1. Where is Semarang?

Geographical Background

Semarang, the provincial capital of Central Java Province in Indonesia, lies on the northern coast of the island of Java, where the island meets the Java Sea. It is approximately 373 square kilometers in size, its topography varying from lowland coast (0–50 meters) in the north to hilly terrain (100–300 meters) in the south, just at the foot of the Kendeng Mountains. Its coastal plain has rivers cutting across it and mangrove forests scattered along it, and its southern edges are covered with teak plantations and rice paddies. Semarang's tropical climate means that it is warm throughout the year, with temperatures ranging on average at 27°C, and 1,800–2,200 mm of rainfall annually, during the rainy season.

Human/Cultural Aspect

Semarang is a port city with a rich multicultural past, shaped by centuries of trade and commerce across Java, China, India, and Europe. Its Old Town (Kota Lama) contains Dutch colonial architecture, including 18th-century building Lawang Sewu, a railway office, and Blenduk Church, the oldest Protestant church in Indonesia. The city's Chinese heritage, over 400 years old, has left a lasting mark, as witnessed in the Sam Poo Kong Temple, a temple dedicated to the 15th-century explorer Zheng He. Javanese traditions thrive in these influences, as seen through wayang kulit (shadow puppet) performances and indigenous cuisine like lumpia semarang (spring rolls) and sate klathak (spicy satay). Population of over 1.7 million, Semarang is a working and studying hub, albeit one whose identity remains based on the centuries-old rivers that have given it life.

Hydrology and River Overview

The Brantas River is the most significant watercourse of Semarang, draining from Dieng Plateau south of the city and 320 km north through Central and East Java, eventually emptying into the Java Sea. The Brantas is supplemented by tributaries such as the Bergas and Garang Rivers during its flow through Semarang to form a drainage system draining southern highland and urban areas. Near the city center, the river divides into distributaries like the Kali Semarang, which winds its way through urban residential districts before reaching the coast.

For Semarang, the Brantas system is vital. It supplies 70% of the city's fresh water, waters 150,000 hectares of rice fields in Demak and Grobogan regency, and subsidizes a lucrative fishing industry in its estuaries. Ecologically, the river delta is an important mangrove system that protects the coast from storm surges and erosion. Rapid urbanization has, however, subjected the system to pressure: Semarang's industrial pollution in the form of sewage and factory effluents have poisoned water and sand mining altered river morphology. Seasonal variations in the Brantas flow caused by monsoon rains make it difficult to monitor so that it can effectively regulate water resources and flood risk.

2. What is the River Flow at Semarang?

Factors Affecting the River Flow

Rainfall and Runoff

The Semarang climate can be divided into two seasons: the wet season (November–March) and the dry season (April–October). The Kendeng Mountains receive heavy rainfall during the wet season, with more than 300 mm per month. This results in rapid runoff, which fills up the Brantas and its tributaries. Semarang river stages increase by 3–5 meters, and flow velocities rise to 1.5–2.5 m/s, and overflow low-lying areas like Tanjung Mas. During drought, rainfall drops to 80–120 mm/month, reducing flows to 0.3–0.8 m/s, subjecting water supplies to irrigation and industry to pressure. This yearly cycle puts a razor's edge on demanding careful management of flows.

Terrain and River Morphology

The course of the Brantas River in Semarang is shaped by the transition from hilly to coastal relief. The river runs in steep-valleyed courses with a high gradient (0.3%) in the upstream areas on the southern periphery, forming high-speed, turbulent flows. When it enters the city's flat coastal plain, the gradient reduces to 0.05%, decelerating the river and causing sedimentation. Urbanization has also altered its path: concrete walls have narrowed the Brantas from 80 meters to 50 meters in some sections, reducing its capacity. Sand mining in the riverbed has dropped sections of the channel by 2–3 meters, disrupting natural flow paths and contributing to the risk of erosion.

Reservoir Operations

Upstream reservoirs, such as the Sermo Dam on the Brantas River, are the control over flows. Built during the 1970s, Sermo stores wet-season water to limit downstream flooding of Semarang and releases water during dry periods to maintain irrigation sources full. Sedimentation has nonetheless reduced its storage capacity by 25% since the past 50 years. Heavy rains combined with illegal dam releases in 2021 caused the Brantas to overflow and inundate 10% of Semarang (according to Central Java's Disaster Management Agency). This points towards the necessity of accurate flow data in order to synchronize reservoir operations with city security.

Historical Hydrological Events

Semarang has suffered severe hydrological disasters. Worst were the floods in 2007: rains during the monsoon season caused the Brantas and Kali Semarang to flood, inundating 30% of the city. Over 20 were killed, 50,000 rendered homeless, and damages amounting to over $100 million (The Jakarta Post). Road and power facilities infrastructure were severely damaged.

Droughts have also affected the region: the 2019 dry season saw a 40% decline in rain, reducing the Brantas' flow to 35% of the norm. Semarang's water company turned to rationing, and Demak regency rice crops fell by 25% (based on a 2020 report by Indonesia's Ministry of Agriculture). Such events underline the need for river flow monitoring in risk reduction.

3. How is River Flow in Semarang Monitored?

Traditional Methods

Surface Drift Buoy Method

Local governments used bamboo rafts or plastic bottles to make rough estimates of surface velocity by measuring the time of transit between fixed points for decades. It was cheap but not acceptable since it did not measure subsurface currents—significant in the Brantas, where depth variation creates distinct layers of flow. Wind and floating debris, common in the river, skewed results more, which rendered it unsuitable for forecasting floods.

Anchored Boat Method

It involved mooring a boat and making measurements with a mechanical current meter at different depths. It provided accurate profiles but was labor-intensive: one survey of the Brantas required 3–4 operators and 4–6 hours. Safety risks were high in wet season when strong currents threatened boats. In 2015, activity had to be stopped by a survey team after their boat struck a submerged tree trunk, demonstrating the weakness of the method.

ADCP Introduction

Acoustic Doppler Current Profilers (ADCPs) transformed flow measurement in Semarang. The ADCPs measure sound waves to quantify velocity throughout the entire water column, both in real time and non-invasively. Relative to other conventional methods, ADCPs measure 3D flow patterns, even within the Brantas' complex estuarine reaches. Scanning a 60-meter river reach can be completed in only 15 minutes, providing information vital for flood forecasting and water management. Since 2012, Semarang's water utility installed 8 ADCPs, which reduced flood response time by 25%.

4. How Does ADCP Work?

ADCPs operate through the Doppler effect: they transmit high-frequency acoustic pulses (300–1200 kHz) that bounce off suspended material, such as sediment or plankton. The pulse frequency of the returned signals varies with particle movement—increasing if traveling towards the instrument, reducing if moving away. By calculating these variations, ADCPs derive velocity at multiple depths, creating a complete flow profile. Current 4-beam configurations can measure 3D currents, essential for mapping the Brantas' eddies and tidal influence.

5. What's High-Quality Measurement in Semarang Going to Need?

Equipment Requirements

• Material Reliability: Must be able to resist corrosion in the Brantas' brackish, sediment-laden waters, where industrial effluent accelerates wear. Stainless steel or titanium is essential.
• Compact Design: Light (under 7kg) for deployment in tight, urban rivers like the Kali Semarang.
• Cost-Effectiveness: Affordable for Semarang's budget, with long-life batteries (9+ hours) to allow extended surveys during monsoons.

6. Choosing the Right Equipment

Deployment Methods

• Boat-Mounted ADCPs: Best for quick flood surveys, plying Semarang's urban rivers to gather real-time data to support emergency action.
• Bottom-Mounted ADCPs: Placed at strategic points, for example, underneath bridges, to give continuous data to offer early flood warnings.
• Cableway ADCPs: Employed in the upstream valleys, where the Brantas runs through the narrow gorges, to take flow measurements in steep reach.

Working Frequency

• 600 kHz ADCPs: Suitable for shallow, 70-meter-wide rivers like the Kali Semarang, yielding high resolution to find sediment layers.
• 300 kHz ADCPs: Better for broader sections of the Brantas (up to 110 meters), taking more accurate measures of deeper flows on the coastal plain.

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.