How to measure the Banda Aceh Coastal Currents?

1. Where is Banda Aceh?

Banda Aceh is located as the capital of Aceh province in Indonesia. It is also called Kutaradja and is located on the northern tip of Sumatra. This city is also well-known for its historical value and proximity to the Indian Ocean.

The beautiful landscape of Banda Aceh can be characterized as both a combination of beautiful seashores and greenery. The local culture is highly influenced by Islamic traditions, with Arabic touches in its architecture and numerous mosques dotting the cityscape. All in all, the people of Banda Aceh have a rich heritage, expressed through unique art forms, handicrafts, and ways of life.

The adjoining Indian Ocean is an immense, powerful body of water. The coasts of Banda Aceh house sandy beaches, coral reefs, and a variety of marine life. For the inhabitants of this area, the sea is not only beautiful but also an economic factor due to fishing and sea trade. There were mangrove forests along the coast that protect the shoreline from erosion and provide a habitat for different species of fish and crabs to live in, and birds of different species to nest in.

2. What is the situation of the coastal currents near Banda Aceh?

Various elements influence the coastal currents around Banda Aceh. Above all, it is the monsoon winds. The southwest monsoon, usually between the months of May and September, carries strong winds that may push the surface currents northward. In contrast, the northeast monsoon, which falls in the months between November and March, alters the pattern of wind and generally drives the currents southward.

Tidal currents are also a strong influence. On the tides in the Indian Ocean off Banda Aceh, gravitational pull by the moon and the sun does have some effect. Semi - diurnal tides, with two high tides and two low tides each day, are experienced in the area. Quite substantial tidal currents, in particular around the mouths of rivers and in narrow passages along coasts, are likely to be developed. These currents are important for the transport of sediments and nutrients along the coast.

These currents can, however, be modified by local bathymetry, such as the form of the ocean bottom and the presence of underwater features like ridges and channels. Discharges into the sea from rivers, such as the Krueng Aceh River, also may have some effects on current patterns. Such freshwater inputs often provide the density contrast necessary for the creation of complex current systems.

3. How to Observe the Flow of Coastal Water in Banda Aceh?

There can be several ways to observe the flow of water at the coast of Banda Aceh: a common and practical method involves placing buoys on the water, fitted with GPS or another tracker device. Once the buoys move with the current of the water, one can monitor their position shift over time and deduce the direction and speed of the surface currents.

In the anchored ship method, the ship is anchored at a particular location, and the current-measuring instruments record the flow of water at various depths. The drawbacks associated with this technique are that the ship may also be subjected to waves and winds that could result in erroneous measurements. It is not very effective for coverage of a large area.

Recently, the Acoustic Doppler Current Profiler (ADCP) method has been considered a more effective and advanced approach to measuring the flow in coastal water. Each ADCP can measure the velocity profile of flowing water over a wide range of depth. They work by emitting sound waves into the water and analyzing the Doppler shift in reflected waves. In this way, detailed knowledge of the current structure from surface to seabed can be obtained.

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

Basically, ADCPs work on the principle of the Doppler effect. They send acoustic pulses, or sound waves, into the water. These sound waves scatter off particles in the water-sediment, plankton, and all those other small particles in the water. If the sound waves bounce off these particles and manage to reflect back to the ADCP current meter, the frequency of the reflected waves changes owing to a thing called the Doppler effect.

If the particles are moving towards the ADCP current profiler, then the frequency of the reflected wave will be higher than the original emitted frequency. But if the particles are moving away from the ADCP flow meter, then the frequency of the reflected wave will be lower. The frequency shift, which is precisely calculated by the ADCP profiler, provides the velocity of the particles. Since the particles are moving with the water current, the calculated velocity of the particles will provide the measure of the water current velocity.

The pulses can be transmitted in several directions, such that the ADCPs can measure the current velocity in three-dimensional space and at different depths. For example, with a vertical profile, they are able to measure the current velocity from the surface interval to the bottom for a comprehensive picture of the structure of the current.

5. What is required for the good-quality measurement of the currents off Banda Aceh coasts?

The equipment should, nevertheless, be made of solid materials that would yield high-quality measurement of coastal currents. In harsh marine conditions, the casings should be hard to resist saltwater corrosion and possible physical impacts from waves and debris.

The size of the equipment should be small enough to be easily deployed in areas where accessibility is minimal, such as in small boats or shallow waters. Light weighted design has its advantages for lighter handling and installation.

On the other hand, low power consumption is very much necessary for long-term measurements. This aside, cost-effectiveness is also an important consideration to realize large-scale measurements. For the ADCPs, the casing is made preferably from Titanium alloy. The Titanium alloy shows very good resistance to corrosion, especially being important to the marine environment at Banda Aceh. It will be able to resist the corrosive attack of salt water for long time. It is also strong and of a high strength-to-weight ratio, hence assured that it will be durable but light to protect the internal components of the acoustic current meter.

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

In the selection of equipment for current measurement in Banda Aceh, the usage is the first consideration. Measurements taken from a moving vessel call for a ship-borne ADCP, which could be able to provide real-time current data while the ship is in motion.

A bottom-mounted ADCP is a good option for fixed-location measurements near the seabed, and it can continuously monitor the current at a point.

A buoy-type ADCP is appropriate when measurements have to be conducted over a larger area and when they are not restricted to a particular depth.

Frequency choice: For water depth up to 70m, ADCP 600kHz is ideal. For a depth range of 70-110m, ADCP 300kHz is more appropriate. For waters as deep as 1000m, the 75kHz ADCP should be used.

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.