How to measure coastal currents of Trat?

1. Where is Trat?

Trat is a province in Thailand and is situated on the eastern coast, near the Cambodian border. It is a region of outstanding natural beauty and of strategic importance.

The province is known for its idyllic islands like Koh Chang, Koh Kood, and Koh Mak. These islands are a haven for tourists with their pristine beaches, clear turquoise waters, and lush tropical vegetation. The local economy very much depends on tourism, fishing, and the cultivation of fruits like durian and rambutan.

Trat is as ethnically diverse as it gets, being home to people of Thai, Chinese, and Cambodian origin. This is manifested in the local food, architecture, and even cultural principles. Its coasts are lined with fishing villages that have their livelihoods tied closely to the sea.

The waters surrounding it form part of the Gulf of Thailand. The sea here is shallow in many places and carries a rich variety of marine life. There are extensive coral reefs, which support a huge range of fish species, crustaceans, and other creatures in the water. These reefs also serve as a natural attraction for snorkelers and divers.

2. Current situation of the coastal currents near Trat

The coastal currents near Trat are determined by several driving forces. The tidal currents are one of the most strong driving forces. There are semi-diurnal tides in the Gulf of Thailand, which is induced by gravitational forces of the moon and the sun. The tide rises twice and falls twice each day. While tidal currents are usually strong, those around the coasts and islands are strong enough. These tidal currents are very instrumental in the transportation of sediments and nutrients, which are very vital for the health of the coral reefs and other marine life present within the ecosystem.

Monsoon winds play a major role, too. The southwest monsoon from May through September provides strong winds that can drive the surface currents in one direction, whereas the northeast monsoon from November to February changes the direction of winds and thus the direction and speed of the coastal currents. This interaction between tidal and monsoon-driven currents sets up a very complex and dynamic pattern of flow.

The shape of the seabed and underwater features, like ridges and channels, may alter and deflection the flow of the coastal currents. Another factor is the discharge of rivers and streams into the sea. The freshwater input, induced by these sources, may establish density differences that lead to the creation of a complex current system.

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

There are three methods to observe the coastal water flow in Trat.

The Surface Drifting Buoy Method

This is easy and practical. The buoys of different types are dropped into the water, each one fitted with GPS or other devices for tracking. Carried by the currents, their position and their movement over a period of time can be tracked in order to determine the direction and speed of the surface currents. The buoy design should take into consideration such conditions of that region: wave action, corrosion from saltwater.

The Anchored Ship Method A ship is anchored at a particular location and current-measuring instruments are deployed from the ship. The deployed instruments can measure the velocity as well as the direction of the flow of water at different depths. This method has a number of drawbacks: the ship may be influenced by waves and winds, which result in the inaccurate measurement of water currents. Moreover, this system is not very effective to operate for a large area.

ADCP Method

This is a more sophisticated and finer way to measure the flow of water along coastlines. ADCPs are capable of measuring the velocity profile of water currents over a wide range of depths. The ADCPs emit sound waves into the water and analyze the Doppler shift of the reflected waves. This serves to get a meaningful insight into current structure from the surface to the seabed.

4. How do ADCPs that utilize the Doppler Principle work?

ADCPs make use of the Doppler principle. They transmit acoustic pulses, or simply sound waves, into the water. These sound waves meet and interact with particles in the water-sediment, plankton, and other similar minute particles in the water. The frequency of the reflected waves is altered upon bouncing back to the ADCP by the Doppler effect when the emitted sound waves reflect off these particles.

If the particles are moving towards the ADCP, the reflected wave frequency is higher than the original emitted frequency. In contrast, if the particles are moving away from the ADCP, the reflected wave frequency is lower. By accurately measuring the frequency shift, the ADCP can calculate particle velocity. Since the particles are in motion with the current of water, the velocity calculated for them is a measure of the water current velocity.

ADCPs are able to send pulses in any direction and thus measure current velocity in three-dimensional space and across a range of depths. A profile taken vertically could record the current velocities from the surface to the seabed at intervals, enabling a thorough depiction of the current structure in question.

5. What's required for high-quality measurement in Trat coastal currents?

It requires equipment with reliable materials for the high-quality measurement of the Trat coastal currents. The marine environment is harsh, with saltwater corrosion, and potential impacts from waves and debris; a robust casing is therefore vital.

The size of the equipment should be small enough to be easily deployed, especially in places of limited access, as from small boats and shallow coastal waters. A lightweight design also provides several advantages for handling and installing the equipment.

Low power consumption is necessary for long-term measurements. Low cost will also be important to facilitate large-scale measurements. In the case of ADCPs, titanium alloy is preferably used for casing. Titanium alloy possesses excellent corrosion resistance, an essential feature in the marine environment of Trat. For such purposes, it can withstand the corrosive effects of saltwater over a long period. It is also strong and has a high strength-to-weight ratio to provide durability and light weight in structure to protect the internal components of the ADCP.

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

Usage is the first factor to consider when choosing the right equipment for current measurement in Trat.

For Measurements from a Moving Vessel

A ship-borne ADCP is appropriate. The real-time current data can be obtained while the ship is moving and provides more information about water flows in various locations.

Where Measurements Are to Be Made at a Fixed Location close to the Seabed Bottom-mounted ADCP will be an excellent choice. This can monitor the current at one point continuously and hence can provide data with temporal consistency.

Where Measurements Must Cover a Large Area and It Is Not Essential That It Should Be at A Particular Depth

A buoy - type ADCP will be appropriate. It covers a larger area and is not restricted to any depth.

For the range of water depth up to 70m, a 600kHz ADCP should be used. For water depths ranging from 70m to 110m, a 300kHz ADCP is more suitable. For very deep waters up to 1000m, a 75kHz should be considered.

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.