How to measure the coastal currents of Phang Nga?

1. Where is Phang Nga？

Location and Coastal Characteristics

Phang Nga is a province in southern Thailand, located along the Andaman Sea coast. It is famous for the beautiful limestone karsts that rise out of the sea and generally comprises a host of beautiful islands. The place is considered a favorite among tourists due to its immense beauty for island-hopping, diving, and snorkeling.

The local economy is maintained by tourism and fishing. The coastline is dotted with fishing villages and resorts. The Andaman Sea is a warm and crystal-clear body of water filled with colored coral reefs, all kinds of fish, and other sea animals. The sea plays a very important role in the local climate and is a very significant factor in the successful industries of fishing and tourism.

2. Factors Affecting Coastal Currents

Tidal Currents: In the Andaman Sea, tidal currents will be affected by the tides themselves. It is to be expected that tidal patterns are complex and the gravitational pull of the moon and sun creates changes in water levels and the movement of water. Tidal current flows stronger in narrow channels between islands or near estuary mouths. These currents are very crucial for carrying nutrients and sediments that help with maintaining healthy coastal and marine ecosystems.

Monsoon Winds: The southwest monsoon, from May to September, provides strong winds that have marked effects on the surface currents. The northeast monsoon, from November to February, changes the wind direction and affects the direction and speed of the coastal currents. The interaction of tidal and monsoon-driven currents generates a very complex and dynamic pattern of flow.

Local Bathymetry: The general topography of the ocean floor, which includes underwater ridges, valleys, and channels, can cause local deflection and modification in coastal currents. The coastline topography with its inlets, bays, and headlands does the same thing to the water flow. In addition, discharge from rivers into the sea might give rise to particular current patterns. The freshwater input due to these rivers can establish proper density differences and lead to complicated current systems.

3. Methods for Observing Flow of Water in Coasts

Surface Drifting Buoy Method

Two buoys are deployed in the water and fitted with Global Positioning System or other types of trackers. The buoys move with the currents, mapping their position and movement over time to deduce the direction and speed of the surface currents. The buoys will be designed to withstand local marine conditions including the high wave energy and saltwater corrosion.

Anchored Ship Method

A ship could be anchored in one place near a coast and current-measuring instruments can be dropped from the ship. These measuring instruments can measure the velocity and direction of the water flow at each depth. This method, however, has its drawbacks. The ship may become subject to waves and winds and may therefore produce faulty measurements. It is also not very effective in covering a large area.

Acoustic Doppler Current Profiler (ADCP) Method

The ADCPs measure the velocity profile of water currents over a wide depth range. They work by emitting sound waves into the water and analyze the Doppler shift of the reflected waves to get an accurate understanding of the current structure from the surface down to the seabed.

4. How ADCPs Using the Doppler Principle Work

ADCPs send out acoustic pulses-simply sound waves-into the water. These sound waves bounce off particles in the water like sediment, plankton, and other small particles. When the sound waves reflect off the particles and back to the acoustic doppler flow meter, the frequency of the reflected waves changes because of the Doppler effect.

If the particles are moving towards the acoustic doppler velocity meter, then the frequency of the reflected wave is higher than the original emitted frequency. If the particles are moving away from the ADCP, then the frequency of the reflected wave is lower. By measuring this frequency shift exactly, the ADCP can work out the particle velocity. Since the particles are moving with the water current, the calculated velocity of the particles is a measure of the water current velocity.

ADCPs can send out pulses in several different directions and thus measure the three - dimensional velocity of the current and at various depths. This allows making a vertical profile of the current velocity at different intervals from the surface down to the seabed, for a complete understanding of the structure of the current.

5. Requirements for High-Quality Measurement of Coastal Currents

For measurement of the coastal currents in Phang Nga of high quality, it is necessary that the equipment be made of reliable materials. In such conditions of the marine environment, when salt-wave corrosion and even physical influence of waves and debris are possible, a durable casing is required.

This is particularly the case when the equipment is to be deployed in areas with difficult access, e.g., from small boats, or in shallow coastal waters. Lightweight design with respect to handling and installation is also advantageous.

Low power consumption is of paramount importance since measurements are to be performed over an extended period of time. Further, cost effectiveness is desirable since large numbers might be acquired for measurements over broad regions. In the case of ADCPs, the casing is preferably made from titanium alloy. The titanium alloy has excellent corrosion resistance, which is very important in the marine environment of Phang Nga. It is resistant to long-term exposure to saltwater. It is also strong and has a high strength-to-weight ratio, giving a durable yet lightweight structure capable of protecting the internal components of the ADCP current meter.

6. Current Measurement Equipment Selection

For Measurements from a Moving Vessel

A ship-borne ADCP will be adequate, providing currents in real time when the ship is in movement to allow a greater understanding of the water flow in different locations.

For Measurements at a Fixed Location near the Seabed

The bottom-mounted ADCP would be a good option, as it can continuously monitor the current at a point and provide consistent data over time.

For measurements which need to cover a wide area and are not restricted to any specific depth, A buoy - type ADCP flow meter is appropriate. It can cover a larger area and is not confined to a particular depth.

Regarding the choice of frequency, for water depths up to 70m, a 600kHz ADCP is appropriate. For depths between 70m and 110m, a 300kHz ADCP is more suitable. 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.