How to measure the Coastal Currents of Rạch Giá?

1. Where is Rạch Giá?

Rạch Giá is a city in the province of Kiên Giang, Vietnam. It is situated on the western part of the Mekong Delta. The area has an extensive network of rivers and canals that connect to the sea.

This city is a significant economic and traffic center in the region. On the local level, the basis of the economy is fishing, agriculture-mostly because of the fertile delta-and trading. The population is ethnically mixed, and the local culture is rich in folk traditions related to fishing and water-based occupations.

The South China Sea influences the sea area that is adjacent to the coast. Nutrients coming from the Mekong River provide a wide variety of marine life in the coastal waters. Different species of fish, shrimp, and crabs are found here. Mangrove forests line the coasts, serving as a natural barrier against erosion and a habitat for many species.

2. What is the situation of the coastal currents near Rạch Giá?

The beach currents off Rạch Giá are influenced by more than one factor. One of the major influences is the tidal current. In this area, the tide is influenced by both the moon and the sun through their gravitational pull. Since the place is near the Mekong Delta, the tidal range could be remarkably big, and tidal currents, therefore, play an important role in the transportation of sediments in and out of the delta.

Monsoon winds also have a great influence. The southwest monsoon, between the months of May and September, brings strong winds that can drive the surface currents in one direction, while the northeast monsoon changes the wind direction and therefore the direction and speed of coastal currents.

The other relevant factor is the discharge of the Mekong River, whose large freshwater volume reaches the ocean and causes differences in density. These differences in density, along with the flow of the river itself, might affect the coastal currents. Local bathymetry, or seabed shape, including the general presence of underwater channels and shoals, may redirect and otherwise alter the flow of the coastal currents.

3. How to observe the coastal water flow of Rạch Giá?

There are several ways to observe the coastal water flow in Rạch Giá.

The Surface Drifting Buoy Method

This is a straightforward and easy method. Buoys are set free in the water, whose location is already fitted with GPS or any other location calculator. When the buoys move with the currents, by determining their position and movement after time, one should be able to tell in which direction the surface currents are moving, and how fast. The buoys must be designed to put up with local marine conditions, specifically waves and salinity.

The Anchored Ship Method

A ship is stationed at a location, and from it, current-measuring instruments are launched. These instruments measure the velocity and direction of flow of water at various depths. However, this method has a few drawbacks. The ship is subjected to waves and winds, which could affect the accuracy of the measured results. It is also quite inefficient in covering a vast area.

ADCP Method: This method belongs to a more advanced and efficient way of measurement of the flow of water in the coast. ADCPs can measure the velocity profile of water currents over a wide range of depths. Basically, they emit sound waves into the water and then analyze the Doppler shift of reflected waves. The result is a fine detalization of the current structure from the surface to the seabed.

4. How do ADCPs using the Doppler principle work?

ADCPs use the so-called Doppler principle. They send acoustic pulses-what is called sound waves-into the water. The sound waves scatter against particles in the water, like sediment and plankton or even smaller particles. Once the scattered sound waves reflect back to the Acoustic Doppler Current Profiler (ADCP), their frequency changes 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 other hand, if the particles are moving away from the ADCP doppler, the frequency of the reflected wave is lower. With a precise measurement of this frequency shift, the ADCP calculates the velocity of the particles. Since the particles are moving with the current of the water, the velocity calculated for the particles is a measure of the water current velocity.

ADCPs are also capable of emitting sound pulses in many directions; hence, they can measure current velocity in three-dimensional space and at different depths. For instance, in a vertical profile, they can measure the current velocity at different intervals from the surface to the seabed to give a complete picture of the current structure.

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

The instruments to be used for measuring the currents of the Rạch Giá coast should be made of reliable materials. Since it will be facing a salty, corrosive marine environment, damage from waves and floating debris may occur, so its casing must be really durable.

The equipment size should be small enough to be easily deployed, especially in areas with limited access, like in small boats or in shallow coastal waters. A lightweight design is also advantageous for it allows easy handling and installation.

Low power consumption is necessary in view of long - term measurements. It is also important that the solution is cost-effective since large-scale measurements are needed. Herein, the casing of ADCPs is made preferably from titanium alloy. Titanium alloy owns an excellent resistance to corrosion, important in the marine environment of Rạch Giá. It can resist the corrosive effects of salt water over quite some time. It is also strong and has a high strength-to-weight ratio, providing a durable yet lightweight structure that can protect the internal components of the ADCP profiler.

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

In choosing the right equipment for current measurement in Rạch Giá, the first consideration is the usage.

For Measurements from a Moving Vessel

A ship-borne ADCP meter is appropriate. It provides current data in real time while the vessel is moving around, thus offering a more three-dimensional understanding of water flow in different directions.

For Measurements at a Fixed Location near the Seabed

A bottom-mounted ADCP profiler has been quite a good choice. Continuous measurement of the current at a certain point enables good, consistent data to be available for the amount of time it has monitored the currents.

For Measurements that Must Cover a Wider Area and is not Restricted to any Specific Depth A buoy - type ADCP current profiler would be applicable. It can cover a larger area and it is not restricted to a particular depth.

For water depths up to 70m, a 600kHz ADCP is a good choice regarding the choice of frequency. For water with deeper ranges, between 70 and 110m, a 300kHz ADCP should be considered. For very deep waters up to 1000m, a 75kHz ADCP is quite recommended.

There are well - known ADCP current meter 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.