How can we measure the coastal currents of Krabi?

1. Where is Krabi?

Location and Coastal Features

Krabi is a province in the south of Thailand, located along the coast of the Andaman Sea. It has spectacular limestone cliffs, beautiful islands, and clear turquoise waters. The area is one of the favorite tourist destinations, offering rock climbing, diving, and island-hopping activities.

The local economy is heavily dependent on tourism and fishing. The coastline is dotted with fishing villages and resorts. The Andaman Sea surrounding Krabi is a warm and generally shallow body of water. It is rich in diverse marine life, such as colorful coral reefs, numerous fish species, among other marine organisms. The sea also significantly contributes to the local climate and is a determining factor in the economic well-being of the fishing and tourism industries.

2. Factors Affecting Coastal Currents

Tidal Currents: The tides around the Andaman Sea essentially give form to the currents along its coasts. The gravitational pull from the moon and sun causes the tides to change; where there are narrow passages between islands or off the mouth of rivers, the tidal currents can still be strong. These currents are important for sediment transport and nutrition, which is vital for coastal and marine ecosystems.

Monsoon Winds: The southwest monsoon, from May to September, brings strong winds that have a great influence upon the surface currents. The northeast monsoon, from November to February, changes the direction of the wind and thus the direction and speed of the coastal currents. The interaction between the tidal and monsoon - driven currents creates a complex and dynamic flow pattern.

Local Bathymetry: The bottom topography, with underwater ridges, valleys, and channels, may cause the deflection and change in character of coastal currents. The configuration of the coastline itself - bays, inlets, headlands - controls the flow of water. Where rivers empty into the sea, there the river discharge strongly influences current patterns. A freshwater contribution from these rivers will often lead to density gradients and complex current systems.

3. Methods of Observation of the Flow in Coastal Water

Surface Drifting Buoy Method

This is a straightforward, yet realistic, method. Buoys are released into the water and fitted with GPS or other tracking devices. As the buoys are taken by the currents, their position and movement over time are tracked in order to determine the direction and speed of the surface currents. The buoys must be designed to resist the local marine conditions - powerful waves and the corrosive nature of saltwater.

Anchored Ship Method

A ship is anchored at a particular site near the coast, and from the ship, instruments for measuring the current are deployed. These instruments can measure the velocity of the water flow, as well as its direction at different depths. However, this technique has certain limitations. The ship may be affected by waves and winds; hence, a reasonable amount of possible inaccuracy is involved. It is not very efficient for covering a large area.

Acoustic Doppler Current Profiler (ADCP) Method

ADCPs can record water current velocity profiles within a broad range of measurement depth coverage. In operation, it emits a sound pulse into the water and receives the reflectance of the sound wave, accompanied by a Doppler shift proportional to the target velocity. It gives very specific insight into the structure of the current from the surface down to the seabed.

4. How ADCPs Using the Doppler Principle Work

ADCPs are based on the Doppler principle. They emit acoustic pulses, or sound waves, into the water. These sound waves scatter off of particles within the water, such as sediment, plankton, and other small particles. As the sound waves reflect back to the ADCP flow meter after bouncing off these particles, the frequency of the reflected waves shifts due to the Doppler effect.

If the particles are moving towards the ADCP current profiler, the frequency of the reflected wave is higher than the original emitted frequency. On the contrary, if the particles are moving away from the ADCP current meter, the frequency of the reflected wave is lower. With this method, the frequency shift can be measured precisely by the ADCP to calculate the velocity of the particles. As the particles are moving with the water current, the calculated velocity of the particles measures the water current velocity.

The ADCPs may emit sound pulses in many directions and obtain current velocity in three-dimensional space at different depths; for instance, a vertical profile at an interval from surface to seabed provides an overview of the current structure. The horizontal and vertical resolution of the measurement depends on the sensor principle and its configuration.

5. Requirements for High Quality Measurement of Coastal Currents

For high-quality measurement of the coastal currents in Krabi, it is required that this equipment has reliable materials. Since the environment found within this marine region involves saltwater corrosion and could have physical impacts from waves and debris, it should be durably encased.

The size of the equipment should be small enough to be easily deployed, particularly in areas with very limited access, like in small boats or in shallow coastal waters. Light weight, on the other hand, is beneficial because it leads to more convenient handling and installation.

Low power consumption, on the other hand, facilitates long-term measurements. Cost-effectiveness is also a significant enabling factor for large-scale measurements. In the case of ADCPs, the casing is preferably made from titanium alloy. Titanium alloy is a material that has very good corrosion resistance, which is essential in the marine environment in Krabi. It resists attack for a long period without degradation by saltwater. It is also strong and has a high strength-to-weight ratio, thus providing a strong yet light structure that can protect the internal components of the ADCP meter.

6. Selection of Appropriate Equipment to Measure Current

A. For Measurement on a Moving Vessel Ship - borne ADCP - measurements can be obtained in real time and while the ship is in motion; this allows for a better comparison in different locations about the flow of the water.

B. For Measurements at a Fixed Location near the Seabed It would therefore be appropriate to adopt a bottom-mounted ADCP. It could monitor the current at a single point continuously, hence providing coherent data over time.

For Measurements that Need to Cover a Wide Area and are not Restricted to a Specific Depth

A buoy - type ADCP would be suitable. It can cover a larger area and is not restricted by any depth.

To choose the frequency, if water depth ranges to 70m, the 600kHz ADCP can be applied. For depth between 70m-110m, a 300kHz ADCP can be used. For very deep waters to 1000m, a 75kHz ADCP is to be 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.