How to measure the Coastal Currents of Jakarta?

1. Where is Jakarta?

Jakarta is the capital of Indonesia, situated on the northwest coast of the island of Java. It is an extended metropolis and is considered one of the biggest cities in the country, with its large population and vivid economy. The city is bordered on one side by the Java Sea, which has a great influence on the climate and other coastal activity there.

Its position makes Jakarta an important hub in trade and transport. Jakarta has a busy port that caters to a great quantity of cargo and sea traffic. In the coastal area, it is diversified by urban growth due to high-rise buildings and industrial zones along the shore, with the presence of beaches and mangrove forests.

In the Java Sea, which lies offshore, there is great biodiversity. Waters host an immense number of fish species, different shellfish, and other organisms. The mangrove forests along the coasts protect the shoreline from erosion and provide a place of habitat for several species, including birds and small marine animals.

2. What is the situation of the coastal currents near Jakarta?

The coastal currents near Jakarta are caused by many different variables. Tidal currents are one of the predominant forces. The tides in the Java Sea are semi - diurnal, meaning that gravitational pull from the moon and the sun causes two high and two low tides every day. Tidal currents can be very strong, in particular near river mouths and narrow coastal passages. These currents are an important factor in the transport of sediments and nutrients.

The monsoon winds also play a significant role. The southwest monsoon, from May to September, is a strong wind that may drive the surface currents in a specific direction. The northeast monsoon takes over from November to March and shifts the wind direction, hence shifting the direction and speed of the currents along the coast. This interaction between tidal and monsoon-driven currents gives rise to complex flow characteristics.

This flow may be deflected and altered by the local bathymetry, such as ridges, channels, and shoals. The discharge into the sea by rivers-like the Ciliwung River and several other smaller rivers in the Jakarta area-will affect current patterns. River discharges provide freshwater to the ocean and can generate density differences resulting in the generation of quite complicated current systems.

3. How to Observe the Coastal Water Flow of Jakarta?

There are several methods for observing the coastal water flow in Jakarta. One of the simple and practical approaches is the surface drifting buoy method. Buoys with GPS or other tracking devices installed on them are allowed to release into the water. The buoys would then be carried by the currents, while their movement over time can be monitored to determine the direction and speed of the surface currents.

The anchored ship method: In this, a ship is anchored at a particular point and, with the help of current-measuring instruments, the flow of water at different depths is calculated. However, there are some flaws in the process: the ship is easily influenced by waves and winds; hence, this method may reflect inaccuracies. It also does not work effectively in covering a bigger area.

The ADCP current meter method is a more advanced and efficient way of measurement of the flow of coastal water. It can measure the velocity profile of water currents over a huge range of depths. They work by emitting sound waves into the water and analyzing the Doppler shift of the reflected waves. It achieves a detailed understanding of the current structure from the surface to the seabed.

4. How do ADCPs using the Doppler principle work?

ADCPs work on the basis of the Doppler principle. They send acoustic pulses-commonly referred to as 'sound waves'-into the water. The sound waves interact with particles in the water, like sediment and plankton or other small particles. When the sound waves bounce off these particles and are reflected back to the ADCP current profiler, the frequency of the reflected waves changes due to the Doppler effect.

If the particles are moving toward the ADCP doppler, then the frequency of the reflected wave is higher than the original emitted frequency. If the particles are moving away from the ADCP, the frequency of the reflected wave is lower. By measuring this frequency shift with high accuracy, the ADCP meter can determine the velocity of the particles. Since the particles are moving with the water current, the velocity of the particles calculated is a measure of the water current's velocity.

Some ADCPs are even capable of sending pulses in multiple orientations, thus enabling them to measure current velocities three-dimensionally and at various depths. For instance, if operated in a vertical profile, they would be able to measure the current velocity from surface to seabed in discrete intervals to give an extensive view of the current structure.

5. What is required for good quality measurement of Jakarta coastal currents?

For high - quality measurement of the coastal currents in Jakarta, the equipment needs to have reliable materials. Given the harsh marine environment, with saltwater corrosion and potential physical impacts from waves and debris, a durable casing is essential.

The size of the equipment should be small enough to be easily deployed, at least in areas with limited access, like in small boats or in shallow coastal waters. Further, it is advantageous if the design is lightweight to allow for easy handling and installation.

Low power consumption is necessary for long -term measurements. Cost-effectiveness is important for allowing large-scale measurements. In the case of ADCPs, the casing will be made from titanium alloy. Titanium alloy exhibits superb corrosion resistance, which is of prime importance in a marine environment like Jakarta. It also resists the corrosive action of salt water for a long time. It is robust, solid, with an excellent strength-to-weight ratio, thereby providing strength with lightness, a perfect combination to house the internal mechanisms of the ADCP flow meter.

6. How to Choose the right equipment for current measurement?

While selecting the right equipment for the measurement of current in Jakarta, the first consideration is usage. In cases of measurements from a moving vessel, a ship-borne ADCP flow meter will do the work. It could deliver current data in real time while the ship is moving.

Bottom-mounted ADCP doppler is a good option for the measurements at a fixed location near the seabed. It can continuously monitor the current at a specific point.

A buoy-type ADCP current profiler is appropriate for measurements that need to cover a wide area and are not restricted to a specific depth.

Regarding the choice of frequency, for water depths up to 70m, a 600kHz ADCP is a good choice. For depths between 70m and 110m, a 300kHz ADCP is more suitable. For very deep waters up to 1000m, a 75kHz ADCP is recommended.

There are well - known ADCP current meter brands such as Teledyne RDI, Nortek, and Sontek. However, a Chinese brand, China Sonar PandaADCP, is also worth considering. It is made of all - titanium alloy material and offers a great cost - performance ratio. You can find more information about it on the website: https://china-sonar.com/.

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