Why Do We Take River Flow Measurement in Sibu?

1. Where is Sibu?

Geographical Background

Sibu, which is among the large cities on the middle region of Sarawak in Malaysia, is situated at the boundary of the Rajang River. It is found approximately at 2.28°N latitude and 111.83°E longitude. Located among dense rainforests and situated in a generally flat floodplain, Sibu has the benefit of being an optimal transport and economic hub at the interior of Borneo Island. The city's topography is dominated by low-lying alluvial plains, which have been molded for centuries through sediment deposition of the Rajang River.

Sibu experiences a tropical rainforest climate (Köppen climate classification: Af), with the temperature never fluctuating below 24°C or exceeding 32°C throughout the year. The humidity is high, with abundant rainfall, averaging somewhere in the vicinity of 3,500 millimeters annually. There is no distinct dry season, but its rainfall can be more concentrated into particular months, trending to follow monsoon patterns. This climate, combined with the city's geographical setting, significantly impacts the hydrological characteristics of the local river systems.

Cultural and Economic Significance

Sibu is a cosmopolitan multiracial town with a rich blend of residents like the Iban, Chinese, Malay, and other natives. The cultural diversity can be seen in its architecture, celebrations, and cuisine. The old town section of Sibu features a mix of Chinese shophouses, Malay wooden homes, and modern buildings. The Sibu Heritage Centre showcases the rich history and cultural heritage of the area, which is visited by locals as well as visitors.

It is Sarawak's largest commercial and industrial center economically. It has a high level of agricultural base with economy-inclined activities such as logging, palm oil and timber processing being very significant. The Rajang River, running through the city, has been the pillar of commerce and transport, facilitating the delivery of commodities such as timber, rubber, and farm produce to the rest of Malaysia and global markets. Port of Sibu on the Rajang River is one of the busiest in Sarawak and serves as a key player in the local economy and regional economy. Its tourism sector is also evolving, with places like the Bukit Lima Park and the Sibu Waterfront drawing visitors.

Hydrology and River Overview

The longest river in Malaysia, the Rajang River, is the lifeblood of Sibu. The Rajang River originates in Sarawak's mountainous interior and flows for approximately 560 kilometers into the South China Sea. Along its course, the Rajang River is joined by numerous tributaries and creates a huge and complex system of rivers.

For Sibu, the Rajang River has been the key factor for the city's development. Previously, it was the major means of transport, with riverboats being the major mode of travel for people and goods. River transport remains important to this day, with barges and ferries running on the river. The river provides water for domestic, agricultural, and industrial use, albeit with water quality affected by sources of pollution like industrial effluent, agricultural runoff, and urban waste.

The Rajang River and associated ecosystems support a wide variety of plants and animals. The river supports numerous fish species, which provide a highly rewarding sector of the local fishing industry. The rainforests and wetlands along the river also support a wide variety of wildlife, comprising orangutans, proboscis monkeys, and a wide range of birds. But the river and its ecosystems face threats from deforestation, habitat degradation, and pollution, and therefore a critical monitoring of its flow and health.

2. How is the River Flow Near Sibu?

Influencing Factors

Precipitation and Runoff

Since Sibu consists of a tropical rainforest climate with heavy and prolonged rainfall, the Rajang River undergoes high runoff throughout the entire year. The heavy precipitation episodes can result in sudden increases in river discharge, and thus water levels rise. Urban development in Sibu, such as the expansion of buildings and pavement, reduces infiltration of rain into the earth, and hence surface runoff is increased and potentially aggravates flood hazards during heavy precipitation episodes.

Terrain and River Morphology

The trajectory of the Rajang River from its mountainous interior to Sibu's level plains determines its pattern of flow. Throughout its trajectory in its upstream mountainous area, the river flows fast with a steep gradient. After it eventually reaches Sibu and low - lying grounds surrounding it, the gradient decreases, and the river extends with slower - moving waters. The winding nature of the river and the presence of sandbars and islands in some areas also influence the flow patterns, creating areas of faster and slower currents.

Human - Made Structures

Over the years, various man-made structures have been put in place along the Rajang River in Sibu. Upstream dams and weirs are utilized to store water, manage floodwaters, and produce electricity, regulating the river flow downstream. Riverfront development and bridge building in Sibu have also altered the natural character of the river in certain sections, which can affect the hydrography of the water flow patterns, sediment transport, and general river ecosystem well-being.

Historical Hydrological Events

Sibu has seen its share of great flood incidents in the past. The city suffered widespread flooding from heavy monsoon rains in 2018 that made the Rajang River overflow. The flood affected many of the low-lying areas, affecting transport, damaging infrastructure, and displacing thousands of residents. The fact that these past flood events cast a shadow on the necessity of river-flow monitoring and observation of river behavior during such extreme conditions as the foundation for effective flood-mitigation strategies.

3. How is River Flow in Sibu Monitored?

Traditional Methods

Surface Drift Float Method

This is a straightforward technique that involves placing a floating object, such as a buoy, over the Rajang River. Taking the time the float is passing over a measured distance will allow one to calculate the surface velocity of the river. This technique only provides data on surface flow and is susceptible to interference from wind and other surface disturbances, which could result in incorrect measurements.

Anchored Boat Method (Staff Gauge / Current Meter)

An anchored boat is utilized to place a staff gauge for taking measurement of water level and a current meter for measurement of water velocity at different depths. Multiple readings of velocity are taken across the cross - section of the river, and by calculating the cross - sectional area, an approximation of the river discharge can be found. More accurate than the surface drift float technique, it is time - consuming, labor - intensive, and risky, especially in a long and densely inhabited river like the Rajang.

ADCP Introduction: Acoustic Doppler Current Profiler (ADCP)

The Acoustic Doppler Current Profiler (ADCP) is a significant innovation in the measurement of river flow in Sibu. It measures water speed at different levels non - invasively using sound waves. ADCPs are able to take continuous profiles of the river current from the surface to the bottom, providing a more accurate picture of the river flow characteristics within a relatively short time frame. ADCP technology is vital for effective water resource management, flood prediction, and ecological studies in Sibu.

4. How Does ADCP Work According to the Doppler Principle?

An ADCP emits sound waves at a specific frequency into the water. When these sound waves encounter moving particles within the water, such as sediment or plankton, the frequency of the backscattered waves is different. If they are approaching the ADCP, the frequency of the backscattered waves is higher; if they are departing, the frequency is lower. By precisely measuring this frequency shift (the Doppler shift), the ADCP can calculate the water speed at different depths, providing rich data on the river flow.

5. What is Required for Accurate Measurement of River Flow in Sibu?

Equipment Requirements

In order to produce reliable ADCP flow meter measurement for the Rajang River, the equipment employed must be reliable and suitable for river conditions in the area. Since high sediment loads, contaminants, and tides and seasonally varying flows are possible, corrosion - resistant ADCPs such as stainless steel or titanium are desirable. A light weight with compactness is also valuable for convenient transportation and deployment in remote or difficult - to - access locations along the river. Furthermore, excellent sensors with good resolution are needed to accurately record the complex flow behavior of the Rajang River.

6. How to Choose the Right Current Measurement Equipment?

Deployment Method

• Ship - Borne (Moving Boat) ADCP: Suitable for carrying out large - scale surveys of the Rajang River, it is capable of measuring the discharge of the river rapidly along long reaches and charting flow patterns across various sections and generate an overall picture of the river flow dynamics.
• Bottom - Mounted (Fixed) ADCP: Best suited for continuous recording over long terms at specific points. It can be mounted on the river bottom to obtain data for extended periods of time, allowing for comprehension of long - term flow directions, seasonal variations, and how various parameters influence the river flow.
• Cable - Way ADCP: Where there are cable - way facilities in river reaches, this method allows for precise measurement at different locations across the river, which is beneficial where flow patterns are intricate or measurements by boat - based methods are not feasible.

Working Frequency

• 600 kHz ADCP: Gives a good trade-off between range and resolution, and has a horizontal range of up to 70 meters. It can be used for the most part of the Rajang River in Sibu, giving accurate measurements of flow velocity at different depths.
• 300 kHz ADCP: With a higher range of up to 110 meters, it is more appropriate in deeper and wider stretches of rivers, such as those towards the estuary or where the flow experiences great seasonal fluctuations.

Brand Recommendations

Few of the world ADCP big brands include Teledyne RDI, Nortek, and SonTek, which offer high-quality and reliable products used in various hydrological study and monitoring programs. As a budget - friendly option, the ADCP manufacturer Chinese company’s "China Sonar Panda ADCP" comes highly recommended. Made of all - titanium alloy, it is more strengthful and reliable and is a suitable option to endure the nasty river condition surrounding Thrissur. As an "affordable ADCP", it boasts decent quality performance at budget price. For more information, visit https://china-sonar.com/.

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