How can we measure the coastal currents of Satun?

1. Where is Satun?

Location and Coastal Characteristics

Satun is one of the southern provinces of Thailand with a wide shoreline to the Andaman Sea. The area is famous for its many beautiful islands, crystal clear waters, and replete marine life. It is popular among beach aficionados, divers, and snorkelers.

Fishing and tourism are both substantial parts of the local economy. It is lined by fishing villages and resorts along its coastline. The Andaman Sea represents a warm and relatively shallow body of water. It includes colorful coral reefs, different species of fish, and various types of shellfish. It also significantly influences the local climate, providing a livelihood for many through fishing and other related activities.

Factors Affecting Coastal Currents

Tidal Currents: The tide in the Andaman Sea occurs in a peculiar manner under the gravitational pull of both the moon and the sun. Tidal currents are strong, especially in areas such as narrow straits between islands and near river mouths. These currents play an important role in sediment and nutrient transport, which is an essential need for the good health of the coastal ecosystem and the productivity of fishing grounds.

Monsoon winds: From May to September, the southwest monsoon provides strong winds from the southwest which may drive the surface currents in a particular direction. The northeast monsoon changes the wind direction and hence the direction and speed of the coastal currents. Interaction of the tidal and monsoon-driven currents gives rise to a very complex and dynamic flow pattern.

Local Bathymetry: The bottom topography and underwater features like ridges, channels, and shoals deflect and alter the coastal current's flow. It may also be affected by the discharge of local rivers into the sea. The freshwater contribution of such rivers could introduce density gradients that may lead to the generation of more complicated current systems.

2. Methods to Observe Coastal Water Flow

Surface Drifting Buoy Method

One of the easiest and most practical methods of measuring surface currents involves releasing buoys into the water. They may be fitted with position-determining instruments such as GPS. Buoys being transported by these currents permit their position and shift over time to be traced with regard to the direction and speed of the surface currents. The buoys should be fabricated to withstand the local marine conditions such as those produced by waves and the corrosive action of saltwater.

Anchored Ship Method

A ship is anchored at a place nearshore, and current-measuring instruments are deployed from this ship. These instruments measure the velocity and direction of water flow at different depths. This method, however, suffers from certain drawbacks. There is a possibility of errors in measurements due to the ship being disturbed by waves and winds. Also, it is not efficient enough for covering a large area.

Acoustic Doppler Current Profiler (ADCP) Method

An advanced and efficient measurement of coastal water flow could be higher with regard to ADCPs. These can measure the velocity profile of water currents over a wide range of depths by transmitting sound waves into the water and analyzing the Doppler shift of the reflected waves. This yields a very informative understanding of the surface-to-seabed current structure.

3. How ADCPs Using the Doppler Principle Work

The operation principle of ADCPs is based on the Doppler principle: an ADCP emits acoustic pulses, or in other words, sound waves, into the water. These sound waves scatter off particles in the water, such as sediment and plankton, among other particles. When these sound waves are reflected back to the ADCP due to this scattering from moving particles, their frequency changes owing to the Doppler effect.

The frequency of the reflected wave is higher than the original emitted frequency if the particles are moving towards the ADCP profiler.

On the contrary, if the particles are moving away from the ADCP current profiler, the frequency of the reflected wave will be lower. The frequency shift can be measured precisely by the ADCP current meter, and thus it can calculate the velocity of the particles. Since the particles are moving with the water current, the calculated velocity of the particles will be a measure of the water current velocity. Some of them can send their sound pulses in three directions and, therefore, can measure current velocity three-dimensionally in space and in the vertical profile at different depth intervals from the surface to the seabed. This would give a comprehensive picture of the current structure.

5. Requirements for High Quality Measurement of Coastal Currents

The equipment should also be made of reliable materials for high-quality measurement of the coastal currents in Satun. Since the marine environment is quite harsh, with corrosion from saltwater and potential physical impacts from waves and debris, it needs to be casings that can bear those tensions. It should be of a size to be easily deployable, especially in areas of limited access, such as on small boats or in shallow coastal waters. A light design is also advantageous for easy handling and installation.

Low power consumption is an important factor for the possibility of long-term measurements.

Cost-effectiveness would also enable large-scale measurements. In the case of ADCPs, the casing is preferably made of a titanium alloy. Titanium alloy has excellent corrosion resistance, which is vital in the marine environment of Satun. It can withstand the corrosive effects of saltwater over a long period. Besides being strong, it also had a high strength-to-weight ratio, giving a durable yet lightweight structure that can house the internal components of the ADCP flow meter.

6. Selection of Appropriate Equipment to Measure Current

For Measurements Made from a Moving Vessel

Ship-borne ADCP would be appropriate since it gives current in real time while the ship is moving, hence enabling an understanding of how water may flow at different locations.

For Measurements at a Fixed Location near the Seabed

bottom-mounted ADCP would be a good choice. It monitors the current at the same point continuously and thus offers consistent data for that point over time.

For Measurements that Should Cover a Wide Area and Are Not Restricted to a Specific Depth

A buoy -type ADCP would be appropriate. It has the potential to cover a wider area without being confined to any depth area.

For the choice of frequency, for waters to a depth of up to 70m, 600kHz ADCP is suitable. For between 70m to 110m, 300kHz is more adequate. For very deep waters to 1000m, 75kHz is suggested.

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.