How do we measure the coastal currents of Phan Thiet?

1. Where is Phan Thiet?

Phan Thiet is an attractive coastal city situated in Binh Thuan province, Vietnam. Its location is beautifully aligned along the southeastern coast, facing the expansive area of the South China Sea. It is one of the cities in the region that boasts an appeal combining nature and culture.

The city itself is a haven for locals and tourists alike, with its landscape dotted with beautiful beaches, such as the famous Mui Ne Beach. This stretch of sand is not only a beauty to behold but also a sports field for water enthusiasts, especially those attracted to the exciting sport of kite-surfing. The local economy here thrives on tourism, fishing, and agriculture in comfortable harmony. Such a unique climate, with the addition of some amazing sand dunes, makes it all the more alluring and unique compared with other places.

The people in Phan Thiet include a number of ethnic groups, all striving to enhance the local culture that has strong roots in fishing traditions. Life here is relaxed in a coastal manner; the sea is not a background, but a part of life.

The nearby South China Sea is a teeming and dynamic aquatic world. Its waters teem with marine life-from the flamboyantly colored, manifold species of fish darting around in the waves to the slow-moving yet grand sea turtles and a myriad of shellfish. The sea provides a lifeblood to the fishing industry in this country, a source of livelihood and food for many. It is here that the tides, with their ebbs and flows, continuously renew and bring activity to the very core of Phan Thiet.

2. What is the condition of the coastal currents off Phan Thiet?

The coastal currents off Phan Thiet are influenced by several factors, which makes the water condition complex and always changing.

Such currents are closely associated with the monsoon winds, which predominantly influence their pattern. The northeast monsoon, from November to March, brings strong winds that have a southerly direction of action upon the surface currents. Contrasting with this, the southwest monsoon from May to September introduces a change in the wind direction to its northwest component, driving the currents in a northerly course. These seasonal wind patterns are thus the invisible hands that push and pull the waters, thereby setting them in motion.

Tidal currents also have a major influence on coastal circulation. The tides in the South China Sea off Phan Thiet are controlled by the moon and sun gravitational forces, with a semi-diurnal regime. This implies two high tides and two low tides every day. These tidal currents can be very strong, especially around the mouths of rivers and narrow coastal inlets. They play a very important role in the transportation of sediments, necessary for the reformation of the coasts, and nutrients that contribute to sustaining the rich marine life.

Moreover, the local bathymetry, which encompasses the shape of the seabed and the presence of underwater features like ridges and channels, acts as a modifier of the coastal currents. These underwater landscapes can redirect the flow of water, causing it to speed up, slow down, or change direction. Additionally, the discharge of rivers into the sea has an impact on the current patterns. Freshwater intake from these rivers into the marine environment causes differences in density that give rise to a complex current system interacting with the tidal and wind-driven currents.

3. How to Observe the Coastal Water Flow of Phan Thiet?

Various methods are available for observing coastal water flow in Phan Thiet; all have advantages and disadvantages.

The Surface Drifting Buoy Method

This is straightforward and practical but well-used. In this technique, buoys are thoroughly released into the water. These buoys are equipped with GPS or other reliable tracking devices. These in the water eventually drift as they get carried along with the currents. Through repeated observation of the position and movement of these buoys over time, determining the direction and speed of the surface currents becomes feasible. However, such buoys need to be designed to sustain sometimes-inhibiting marine local conditions, which include mechanical clouts by waves and chemical activities occasioned by salinity.

The Anchored Ship Method

A ship is anchored at some point in the water. From this stationariness, current-measuring devices are deployed. These are capable of measuring the velocity and direction of the water flow at various depths. This means that although this method can give a detailed study of the water flow at a particular spot, it has several drawbacks. The ship itself is considerably affected by waves and winds, which could impair the accuracy of the measurements. Moreover, it is not effective in the case where the extent of the region that is to be covered is large concerning the coastal waters.

Acoustic Doppler Current Profiler Method

As being more superior and effective method of measurement of flow of water in coastal areas, ADCP current meter finds its place. The ADCPs are devices capable of measuring the velocity profile of currents of water at a wide range of depth. They work by sending out a series of sound waves into the water. When these sound waves reflect off particles in the water-sediment, plankton, and other small particles-and return to the ADCP current profiler, the frequency of the waves reflected has changed because of what is called the Doppler effect. This frequency shift now carries valuable information related to the movement of the particles and, by extension, the velocity of the water current.

4. How do ADCPs using the Doppler principle work?

Ingeniously, ADCPs implement the so-called Doppler principle. A working ADCP emits acoustic pulses-in other words, sound waves-into the surrounding water. These sound waves then interact with the innumerable particles suspended in the water column. Such particles can range from tiny sediment grains to minuscule plankton and other microscopic entities.

A fascinating phenomenon occurs when these particles hit the sound waves and are reflected back towards the ADCP. If the particles are moving towards the ADCP current meter, then the frequency of the reflected wave is higher than the original frequency that was emitted. In contrast, if the particles are moving away from the ADCP, the frequency of the reflected wave is lower. By precisely measuring this shift in frequency, the ADCP is able to work out the velocity of the particles. Since these particles are carried along by the water current, the velocity of the particles is effectively a measure of the velocity of the water current itself.

In addition, ADCPs are designed to emit sound pulses in multiple directions. Because of this, it enables them to measure the current velocity not only in one plane but rather in three-dimensional space. They can measure the current velocity at different intervals from the surface down to the seabed. That gives a full and comprehensive picture of the whole current structure.

5. What’s needed for high-quality measurement of Phan Thiet coastal currents?

For achieving high-quality measurement of the coastal currents in Phan Thiet, several key aspects regarding the equipment need to be considered.

The construction should be made from reliable materials. The marine environment is quite harsh, and with the ever-present danger of saltwater corrosion and the physical impacts that can result from waves and debris, a hardy casing is essential. Such casing provides the first line of defense for this equipment's internal components from the elements, therefore extending life expectancy.

Another important consideration would be the size of the equipment. The devices should be small enough to be easily deployed, especially in areas that might have limited access, such as using small boats or when operating in shallow coastal waters. Lightweight design is also considerably useful because it makes the handling and installation much easier; this can result in greater convenience for practical usage under different conditions.

Long-time measurements are only possible if there is low power consumption. Usually, the equipment often needs to work continuously over extended periods; low power demand allows the equipment to do so without the need for battery replacements or interruptions in a given power source. Cost-effectiveness is also a major factor in design, as this can accomplish large-scale measurements at no unreasonable cost.

For acoustic wave and current profiler, the casing shall be made of titanium alloy. The utilization of titanium alloy provides a myriad of advantages. It is highly resistant to corrosion, which is always important in the marine environment surrounding Phaneit. It will withstand the corrosive effects of salt water over reasonably long periods without deteriorating, which would otherwise affect the sensitive internal parts of the acoustic doppler flow meter. Besides, it is a tough material featuring high strength with a lightweight and thus would be very suitable for the harsh conditions related to the measurement of coastal currents.

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

For selecting appropriate equipment to measure currents at Phan Thiet, the purpose of its usage has to be considered as the first and foremost factor.

For Measurements from a Moving Vessel

A ship-borne ADCP is ideal. From a ship-borne ADCP, one can obtain current data in real time as the ship is underway. Thus, the ship-borne ADCP continuously acquires information related to the flow of water as the vessel progresses along different areas of the coastal waters, enabling a better understanding of how the currents vary from one location to another.

For Measurements at a Fixed Location near the Seabed

An ADCP bottom-mounted is a good choice. Once fixed and correctly installed on the seabed at the site, that can deliver complete coverage of the current at the particular site continuously. The ADCP will, moreover, give consistent and reliable results in time, and therefore, it will mean valuable information in respect to the long-term behaviour of these coastal currents at this fixed position.

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

A buoy-type ADCP shall be fitted. Such an ADCP serves over larger areas because it floats freely on the water, having no restrictions to keep it at any particular depth. Thus, the device can capture a wider perspective of the flow of the coastal waters, offering a better understanding of the general current patterns in that area.

Regarding the frequency choice, the 600kHz ADCP is recommended for water depth up to 70m, while a 300kHz is best suited between 70 to 110 meters; for very deep waters up to 1000m, a 75kHz ADCP is preferred.

Some well-known brands in the market include Teledyne RDI, Nortek, and Sontek. However, there is a Chinese brand worth mentioning-China Sonar PandaADCP. The entire instrument body of this product is made from all-titanium alloy material, which will ensure durable performance and an incredibly good 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.