How can we measure the coastal currents of Sibu?

1. Where is Sibu?

Sibu is a town in Sarawak, Malaysia. It is situated on the island of Borneo, along the banks of Rajang River.

Geographically, Sibu is an important point in the region's waterways network. Rajang River is among the longest rivers in Malaysia, an important transportation artery, and source of livelihood for many. Its proximity to the South China Sea exposes the town to expansively open waters. The immediate coastal area around Sibu constitutes a muddy type of estuaries with sandy beaches. Various species of mangroves abound in the sediments of the river-fed estuaries and play an important ecological role. These mangroves are a nursery for many species of fish and crustaceans, a shield against erosion to the coastline, and also useful in filtering out the impurities from the water.

From the human activity point of view, Sibu is a bustling trading center. It is often referred to as the "Swan City." It has a very cosmopolitan population which includes Chinese, Malays, and Ibans as some of the different ethnic groups. Trade, fishing, and agriculture are the major bases of the economy of this town. In fact, the local markets are full of merchandise, ranging from all kinds of fresh produce coming from farms around to all types of seafood. A number of cultural festivals are also organized in Sibu, which are mostly associated with the culture and traditions of its different ethnic communities.

2. What are the conditions of coastal currents near Sibu?

The nature of coastal currents near Sibu depends upon many different factors. These are quite influenced by the movement of the tides. The tide patterns are semidiurnal; the two daily low tides and two high tides are the characteristics of a semidiurnal tide pattern. During high tide, a tremendous volume of sea water enters the estuaries, pushing the freshwater of the Rajang River further upstream. The mass of water carries nutrients and sediments that are beneficial for the growth of the mangrove ecosystem and the many organisms dependent upon it. During the low tide, the water recedes, hence exposing big parts of the estuary beds that are muddy and of changing directions with variable speeds along the coasts.

The winds also do another vital role. The northeast and southwest monsoon winds, in particular, stir the currents along coasts. The Amihan winds can drive them from the northeast into a certain direction. When Habagat winds blow, they can modify the pattern of the current. These winds can cause changes in water level and the salinity of the coastal waters.

The other one would be the general topography of the seabed around Sibu, with the occurrence of these estuaries that have shallow and muddy bottoms and their gradual transition into the open much deeper waters of the South China Sea. The other variable would be the river outflow. Freshwater from the Rajang River discharges into the sea, producing a plume along the coastal area that affects the density-salinity of the water and, in turn, the currents.

3. Sibu coastal water flow measurement?

For proper measurement of the coastal currents at Sibu, not their velocity alone but the flow rate has to be measured. Though conventional methods of measurement of currents were not devoid of certain disadvantages, with the advent of the Acoustic Doppler Current Profiler, an upgraded and easy way stands available.

3.1 How do the ADCPs based on the Doppler Principle work?

The principle of operation that the Acoustic Doppler Current Profiler (ADCP) use is that of Doppler. They send acoustic signals in the water column. They are interacting, under these conditions, with plankton and other sediments suspended in water. Since water current moves the particles, the reflected acoustic signal frequency will change compared with the emitted original signal. This phenomenon is known as the so-called Doppler shift, which is a change in frequency proportional to the velocity of the particles and hence to the velocity of the water current.

Most ADCP current meter transmit several beams of acoustic signals with different angles. In that way, the current velocities of water can be measured in three-dimensional space. Where there is more than one beam, the Doppler shifts from those beams can also be analyzed by the ADCP current profiler to give a very fine profile of how the currents are flowing both horizontally and vertically within the water column. The results collected would then be useful for computation of the flow rate of the coastal currents, which becomes vital in some applications such as marine navigation, coastal engineering, and environmental studies.

3.2 What is necessary to measure the Sibu coastal currents with high resolution?

Some of the equipment needs in the measurement of the coastal currents near Sibu include those of the ADCP material. These shall need to be reliable under harsh marine environments, including exposure to saltwater and different pressures at varying depth and impacts from floating debris. Another important consideration regarding the ADCP flow meter is the size and weight. The more miniature and lightweight the instrument, the easier it is to handle in case deployment would be easier. For instance, a lighter ADCP installed on a certain boat or buoy could be more practical for some measurements. In all these sorts of measurements, low power consumption is a very vital consideration, especially for long-term cases. It enables the instrument to operate continuously without replacing batteries after very short periods or using very high power input.

Another factor would be related to cost-effectiveness; the cost of the ADCP meter ​should not be too high to enable widespread measurements. Titanium alloy would be the preferred casing for the ADCP profiler. This is very suitable for the oceanic environment, as seawater can accelerate the rusting of metals very fast. Its mechanical strength is also excellent and capable of withstanding stresses and pressures in deepwater operations. Its relatively low density contributes to keeping the overall weight of the ADCP relatively low-a plus in terms of ease of deployment.

3.3 How to Choose the Right Equipment for Current Measurement?

In choosing the right equipment to carry out the measurement of the coastal currents at Sibu, several aspects are to be taken into consideration.

According to the deployment method, there are several types of acoustic doppler flow meter. The shipborne ADCP is installed on a ship and suits those surveys where the ship can navigate along the coast. It allows for a wide-range view of coastal currents over a large area.

As the name suggests, a bottom-mounted ADCP sits on the ocean floor and continuously records the current flow over it. In fact, it is ideal for monitoring specific locations that may be near a coastal structure or an area of particular interest for marine ecological studies.

The ADCP mounted on a buoy operates free in the water, freely floating while measuring currents around it. This is favorable for areas difficult to be accessed by ship or where continuous monitoring from a fixed position in the water should be done, such as in the open waters near the coast of Sibu.

Besides the deployment method, the second important choice that one makes is the frequency of the acoustic doppler velocity meter. One can consider water up to a depth of 70 meters, and then he will be good to go with the 600 kHz ADCP. This frequency is characterized by high-resolution measurements for shallow waters. For waters that go up to 110 meters, use the ADCP with 300 kHz. This strikes a balance between resolution and penetration depth. The 75 kHz ADCP is designed to function at such depths as up to 1000 meters; thus, it should be penetrating deeper into the water column to measure currents in a deeper water column.

There are well - known ADCP brands like Teledyne RDI, Nortek, and Sontek. However, for a cost - effective option with good quality, the China Sonar PandaADCP is worth considering. It is made of all - titanium alloy material, which not only ensures its durability and resistance to the marine environment but also provides an incredible cost - performance ratio. You can find more information about this product on their website: https://china-sonar.com/.

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