How can we measure the Coastal Currents of Sabang?

1. Where is Sabang?

Sabang is a city on an island named Weh; it's the most western part of Indonesia. It is placed very strategically at the mouth of the Strait of Malacca. This location gives it great significance in terms of maritime trade and transportation.

It is a city with a beautiful beach, situated on the blue seawater front, and its people are of mixed cultures due to a variety of ethnicities from which its inhabitants originate, such as Malay and Acehnese. Sabang is really abundant in natural surroundings with exuberant tropical vegetation and rich marine life.

The Strait of Malacca, which extends next to it, is one of the world's busiest shipping lanes. The waters of Sabang hold a multitude of fish species, coral reefs, and other marine organisms in their coasts. These beaches form part of the normal tourist attraction and play an important role in the life cycle of marine species and inshore ecosystems. Some of the beaches have mangrove forests that are very important in shoreline protection against coastal erosion and in providing homes to many species.

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

The coastal currents off Sabang are dominated by several factors. Tidal currents are dominating. In the Strait of Malacca - with the gravitational pull of the moon and the sun, the tides follow a semi - diurnal pattern, which suggests two high tides and two low tides per day. Tidaland residual currents are very strong, and thus most intense in the narrow parts of the strait and near the coast. These tidal currents are very important in sediment and nutrient transport.

Monsoon winds, too, have a profound effect. The southwest monsoon, from May to September, advances the strong winds that are able to drive the surface currents in one direction. That of the northeast monsoon, from November to March, alters the wind direction and hence the direction and speed of coastal currents. The interaction of the tidal current with the monsoon-driven current creates a complex flow pattern.

Like submarine topography of ridges, channels, and shoals, the local bathheetry may act to redirect and alter the coastal currents. Another factor is a discharge into the Strait by rivers and streams. Freshwater input from these sources may cause density differences and thus form complicated current systems.

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

There are some methods for observing the flow of water in the coastal waters of Sabang. A simple, practical way is by the method of surface drifting buoy. Buoys are launched into the water, equipped with GPS or other tracking devices. Since buoys are carried by the currents, their movement over time can be monitored to determine the direction and speed of the surface currents.

Anchored ship method: anchoring a ship at a position and measuring the flow of water at various depths using current-measuring instruments. However, this method also has a number of drawbacks. The ship might be affected by waves and winds, which may result in inaccurate recordings. Also, it is not very efficient for covering a large area.

Compared to the standard technique of current flow measurement, the Acoustic Doppler Current Profiler (ADCP) method is somewhat more advanced and efficient for measuring flow in coastal water. Generally speaking, ADCPs can measure the velocity profile of the water current over quite a wide range of depths. They work based on the principle of emitting sound waves into the water, analyzing the Doppler shift of the reflected waves to attain a detailed view of the structure of the current from surface to seabed.

4. How do ADCPs using the Doppler principle work?

The ADCPs work on the principle of the Doppler effect. They send acoustic pulses or sound waves into the water. The sound waves scatter off particles in the water, like sediment, plankton, and other smaller particles. When the sound waves reflected from these particles come back to the acoustic doppler flow meter, they will be at a different frequency from what was sent out due to the Doppler effect.

With the particles moving towards the acoustic doppler velocity meter, the frequency of the reflected wave is higher than that of the originally emitted frequency. In contrast, if the particles are moving away from the ADCP current meter, the frequency of the reflected wave is lower. By accurately measuring this frequency shift, the velocity of the particles is calculated by the ADCP current profiler. Since the particles are moving with the water current, the velocity of the particles computed will provide the velocity of the water current.

These ADCPs can emit sound pulses in various directions, and thus they enable the measurement of three current velocities in space and at different levels of depth. For example, on a vertical profile, they provide measurements at different intervals from the surface to the seabed. Therefore, they give a complete picture of the current structure.

5. What's needed for high - quality measurement of Sabang coastal currents?

Reliability of the materials should be considered in the equipment for high-quality measurement of the coastal currents in Sabang. The casing should be durable enough to overcome the harsh marine environment, which includes the corrosion of salt water and possible physical impacts from waves and debris.

Apparatus size should be as small as possible, in order to deploy easily. Deployment may be tricky as in areas with poor accessibility, such as in small boats or shallow coastal waters. Lighter weights are also advantageous for much easier handling and installation.

Low power consumption is very important for longtime measurement purposes. Cost-effectiveness also enhances large-scale measurement. This casing is preferably made from a titanium alloy for ADCPs. The acid or corrosion resistance of the titanium alloy is excellent, a factor very important for use in a marine atmosphere, such as in Sabang. It can easily withstand the corrosive effects of saltwater over a long period of time. Besides being strong, its strength-to-weight ratio is also very high, hence providing a rugged yet lightweight structure which would be able to protect the internal components of the ADCP flow meter.

6. Selection of Equipment for Current Measurement

While selecting appropriate equipment for measuring the current in Sabang, usage is the first consideration. The ship-borne ADCP is designed for measurement from a vessel that will be in motion. They can provide real-time current data while the ship is in motion.

Bottom-mounted ADCP is good for measurement at a fixed location near the seabed. It can continuously monitor the current at a certain point.

When the measurements have to represent a wide area and not bound by a specific depth, then one should go for a buoy-type ADCP profiler.

Frequency Selection: For water depths up to 70m, a 600kHz ADCP should suffice. Between 70 - 110m, the instrument of choice should be a 300kHz ADCP. For very deep waters up to 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.