How are we going to measure Bontang's coastal currents?

1. Where is Bontang?

The city of Bontang is located on the coast, administratively located in the province of East Kalimantan, Indonesia. It borders the Makassar Strait, one of the most essential straits around the region.

The city is set up against a backdrop of great tropical rainforests with high biodiversity. These rainforests host many unique flora and fauna, including numerous endangered species. The local culture of Bontang represents a mixture from different ethnicities such as Bugis, Javanese, and indigenous locals. Their traditions and ways of life are usually linked to the sea and associated natural resources.

It is an excellent stretch of coastline abounding in good sandy beaches and complicated networks of mangrove forests. The mangroves play a very important ecological role: They present a natural barrier against coastal erosion and thus protect the shoreline against the impact of waves and storms. Besides, they provide a vital nursery for innumerable types of marine organisms, such as fish, crabs, and shrimp. The shallow waters here are more nutritionally endowed and hence support a wide variety of shellfish and other invertebrates.

The Makassar Strait is actually close, being a busy shipping lane and also a rich fishing ground. The seabed topography of the strait's waters is characterized by areas of sand, coral reefs, and underwater slopes. The deeper parts of the strait are destined for commercial shipping and large-scale fishing activities, with shallower parts near the coast important for local fishing and navigation by small boats.

2. What is the condition of the coastal currents off Bontang?

Various aspects come into play as far as the coastal currents around Bontang are concerned. Specifically, the action of monsoon winds, which is quite influential, acts to push the surface waters in one direction during the southwest monsoon and often carries along warm nutrient-rich waters from other regions. This can increase the productivity of the coastal marine ecosystem, as nutrients are supplied for the growth of phytoplankton and other primary producers. On the other hand, the northeast monsoon changes the pattern of currents and may introduce cooler waters and different sediment loads.

Seabed topography is another important factor. Deflection, acceleration, or retardation of the currents can be caused by submarine ridges, troughs, and coral reefs. Around a coral reef, for example, the water flow might be deflected to have areas of upwelling and downwelling. These are areas with an important part in the nutrient cycle, as deeper, nutrient-rich waters come to the surface, carrying nutrients which may support a far greater biomass of marine life.

Another important input is the freshwater feeding from the local rivers and streams. Freshwater discharging can change the salinity of the coastal water, produce different densities that may trigger water mass formations, and induce vertical current movements.

3. How to observe the coastal water flow of Bontang?

Surface Drifting Buoy Method: This method prepares buoys on the surface of the water. Such buoys are allowed to drift through the action of the currents while their movements are followed by satellite-based tracking systems. From analysis of the path of the buoys, information is obtained on the direction and speed of the surface currents. This method, however, provides data mainly on the surface layer and does not provide much information on the currents throughout the water column.

Anchored Ship Method: A ship is moored in the coastal waters. Current-measuring instruments are lowered from the ship into the water at various levels. This allows measurement of the current characteristics at various levels. However, the presence of the ship could interfere with the natural flow of the water, and it is not the best method for making long-term continuous measurements.

Acoustic Doppler Current Profiler (ADCP) Technique: ADCP flow meter is a rather modern and handy method of measuring coastal currents. It can provide in detail a current velocity profile right from the surface to a certain depth, hence allowing a better insight into the flow patterns within the water column.

4. How do ADCPs using the Doppler principle work?

In principle, ADCP profiler work based on the Doppler effect. Acoustic signals are emitted into the water. The acoustic pulses interact with moving particles in the water, including sediment particles and small organisms. Due to the motion of the water, the frequency of the reflected pulses is altered by the Doppler effect. This change of frequency is measured by the ADCP current profiler and is used to derive the velocity at different depths of the water. The instrument is equipped with several transducers capable of sending and receiving acoustic signals in various directions, hence enabling one to measure the current three-dimensional velocity structure of the water column.

5. What would be required to achieve high-quality measurement of Bontang coastal currents?

With a view to high-quality measurements of coastal currents around Bontang, the equipment should have materials that are reliable to withstand the chemical corrosion from seawater and the mechanical forces applied by waves and currents. Small dimensions and low weight are advantageous for handling during deployment by several options: on board a boat, attached to a buoy, or on the seabed.

The instrument should consume low power. This is because the measurement may last for a long period, sometimes in areas where access to mains power may not be readily available. It should be cost-effective. In order to perform wide-area measurements, it should not be that expensive. The casing of the ADCP current meter can be made from Titanium alloy. Its corrosion resistance is outstanding, which is a requirement when operating at sea. The material is also sturdy and durable to take the violence of the sea and not lose much of its lightness.

6. How to Choose Appropriate Equipment for Current Measurement?

Usage scenarios will come into play when choosing equipment for the current measurement near Bontang. Ship-based measurements will find a good match with a ship-mounted ADCP. This can be installed on a ship's hull and measure the currents while the ship is sailing or is stationary.

The ADCP current profiler can measure currents near the seabed if it is of the bottom-sitting type. Such a device can sit at the bottom of the seabed and return proper readings for currents in the immediate vicinity of the bottom.

In floating applications, an acoustic doppler velocity meter of buoy-mounted type can suffice. This can be attached to the buoy and measure currents at varied depths as it floats.

The ADCP frequencies range between 75 kHz and 2400 kHz. In relation to the depth of water, a minimum frequency of 600kHz can resolve water as deep as 70m, while frequencies of 300kHz are used for 110m and a frequency of 75 kHz was useful for waters deeper than that, as deep as 1000m, in generating reliable measurements.

There are several well - known ADCP brands such as Teledyne RDI, Nortek, and Sontek. However, a highly recommended Chinese ADCP brand is China Sonar PandaADCP. It is made of all - titanium alloy material, ensuring excellent performance and durability in the marine environment. It also offers an outstanding cost - performance ratio. You can find more information on its website: https://china-sonar.com/.

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