How Do We Measure the Coastal Currents of Bengkulu?

1. Where is Bengkulu?

Bengkulu Province is located in Indonesia on the southwestern coast of Sumatra Island. In fact, it is surrounded by the Indian Ocean, which dictates the large variances in its coastal nature. The region enjoys a rich history wherein it boasts of a mixture of the local cultures and influences from the former colonial powers.

Bengkulu is also the name of its capital city. The place boasts several beautiful beaches, one of which is Pantai Panjang, an extended beach suitable for both local and visiting tourist traffic. It is an indegionous population blended by ethnicity with rich traditions and ways of life.

The nearby Indian Ocean is vast, vibrant, and a huge body of water. The coastal waters of Bengkulu are inhabited by marine life such as colorful fish, various species of crabs, and different types of shellfish. In other areas, coral reefs add up to the biodiversity. On coasts, a row of mangrove forests, very helpful in protecting the shoreline from erosion and serving as homes for numerous organisms.

2. What is the condition of the currents in the coastal areas of Bengkulu?

Several factors drive the currents along the coast off Bengkulu. Monsoon winds of importance include the southwest monsoon, commonly between the months of May and September, with strong winds capable of driving the surface currents northward. This pattern of wind is reversed during the northeast monsoon, from November to March, and generally causes the currents to flow in a southerly direction.

The tidal current is another important consideration in this context. The tides of the Indian Ocean off Bengkulu are, as anywhere else, influenced by the gravitational pull of the moon and the sun. The semi-diurnal tide area holds two high tides and two low tides daily. Tidal currents can get quite strong, especially near river mouths and in narrow coastal passages. Such kinds of currents are very important in transporting sediments and nutrients down the coast.

It is generally modified by the local bathymetry, like the shape of the seabed or the underwater features, such as ridges and channels. Discharge in the sea by rivers might also alter the current patterns. Such density differences can be caused by the input of freshwater from these rivers that complex systems of currents can be formed.

3. How to observe the flow of water in Bengkulu coastal water?

There are several ways of observing the flow of water in the coastal region of Bengkulu. A rather common and feasible way would be through a surface drifting buoy. Buoys are thrown into the water, each equipped with GPS or other tracking devices. The buoys would be carried by the currents, and their position at different times can be tracked, hence showing the direction and speed of the surface currents.

The anchored ship technique involves a ship that is anchored at a known location and, from this position, water flow at different depths is measured using current-measuring instruments. However, there are several drawbacks to this approach: the ship may be affected by waves and winds, thus leading to wrong measurements; it is also not very efficient in covering a large area.

Such an advanced and efficient measurement of coastal water flow is possible through the technique known as the Acoustic Doppler Current Profiler (ADCP). ADCPs can measure the velocity profile of the water currents over a large depth range. They work by emitting sound waves into the water and analyzing the frequency shift of the reflected waves due to the Doppler effect. This gives an excellent insight into the structure of the current right from the surface to the seabed.

4. How do the ADCPs based on the Doppler principle work?

The ADCPs work on the principle of the Doppler. A series of acoustic pulses, or more correctly described, sound waves, is projected into the water. These sound waves would obviously interact with the particles in water-sediments, plankton, and other similar minute particles. When these sound waves bounce off these particles and are reflected back to the ADCP current meter, the frequency of the reflected waves has changed owing to the Doppler effect.

If the particles are moving towards the ADCP current profiler, then the frequency of the reflected wave is higher than originally emitted. Whereas in the case when the particles are moving away from the ADCP flow meter, the reflected wave frequency will be lower. Precise measurement of this frequency shift enables the ADCP profiler to calculate the velocity of the particles. Since the particles are in motion with the water current, the velocity of the particles calculated, measures the velocity of the water current.

The ADCPs can emit sound pulses in multiple directions, hence enabling them to measure the current velocity in three-dimensional space and depth. For example, in a vertical profile, they can measure the current velocity at different intervals ranging from the surface down to the bottom. This would give a very detailed representation of the current structure.

5. What does high-quality measurement of Bengkulu coastal currents require?

Equipment used for measurement should be made with reliable materials that ensure high-quality measurement of the coastal currents in Bengkulu. The casing should be strong and durable enough due to the harsh marine environment, which may involve saltwater corrosion and potential physical impacts from waves and debris.

The size of equipment should be small enough to be easily deployed, especially where access is difficult or in small boats or shallow coastal waters. A lightweight design provides an added advantage in terms of handling and installations that are more convenient.

Long-term measurement is particularly important; low power consumption is therefore essential. Cost-effectiveness is equally important to accommodate large-scale measurement activities. The casing would be made of titanium alloy material. This material is excellent in resisting corrosion, which, in fact, is a necessity in Bengkulu's marine environment to bear the corrosive effects of salty water for a long period of time. Also, it is powerful and has a high strength-to-weight ratio, which will enable a sturdy but lightweight structure that will protect the internal components of the ADCP.

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

First of all, based on the usage, the right choice of equipment for current measurement in Bengkulu would be a ship-borne ADCP in case of measurement from a moving vessel, since it is able to provide real time current data while the ship is underway.

For measurements at a fixed location near the seabed, a bottom - mounted ADCP is a good option. It can continuously monitor the current at a specific point.

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

Frequency selection: For water depths up to 70m, a 600kHz ADCP will be adequate. In the depth range of 70 to 110m, a 300kHz ADCP would be adequate. For as deep as 1000m, a 75kHz ADCP is recommended.

There are well - known ADCP 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.