How to measure coastal currents in Nha Trang?

1. Where is Nha Trang?

Nha Trang is a sea-side jewel nestled in Khanh Hoa Province, Vietnam. It faces the South China Sea, an open expanse of water in which the sun gleams back and that gives shape to the identity of the city. This is a perfect holiday spot, ingeniously combined with nature and a strong buzzing urban lifestyle.

This city is a tourism mecca because of the alluring beaches and crystal blue waters that lure both domestic and international tourists. Long expanses of sandy shores are lined up with a host of resorts, hotels, and restaurants that promise a treat for those on a mission to unwind themselves with indulgence. Fishing also holds great significance in the local economy because of the rich marine resources in the area.

The local people are a mix of various ethnicities, and the culture is an interesting blend of traditional Vietnamese values and modern inputs through the tourist inflow into the area. The architecture, food, and festivals in the city all carry this unique tint.

The sea next to it, the South China Sea, is another world of itself. It is dynamic and life-filled with a kaleidoscope of marine life. Colorful coral reefs adorn the sea floor, sheltering all types of fish species. Sea turtles also frequent these waters, further enhancing the area's ecological importance. The sea is not only a tourist attraction but also an integral part of the local ecosystem, from determining the climate down to the people's livelihood.

2. How are the coastal currents around Nha Trang?

In the vicinity of Nha Trang, coastal currents are formed by the combined action of these elements, developing a very complicated and ever-changing pattern.

Monsoon winds have a far-reaching impact on coastal currents. The northeast monsoon, which sweeps through from November to March, brings with it strong winds that drive surface currents in a southerly direction. In contrast, the southwest monsoon from May to September changes the wind direction to cause currents to flow in a northerly direction. These monsoondriven currents are an important force and influence the flow of water and marine life distribution accordingly.

Tidal currents also have a major role. The tides in the South China Sea at Nha Trang are semi - diurnal, due to gravitational pulls of the moon and sun, and there are two high tides and two low tides daily. In some areas, like river mouths and narrow coastal passages, the tidal currents can be quite strong. These currents help in carrying sediments and nutrients essential for the health of the coastal ecosystem.

The local bathymetry, including the shape of the seabed and the presence of underwater features such as ridges and channels, can modify these coastal currents. Basically, the topography of the bottom can act somewhat like a guide that helps flow over the seafloor, producing areas with faster or slower currents. Other factors contributing to the current patterns include the discharge of rivers into the sea. The freshwater input of the rivers causes density differences and can give rise to rather complex current systems.

3. How to Observe the Coastal Water Flow of Nha Trang?

There are many different ways to observe the coastal water flow in Nha Trang.

The Surface Drifting Buoy Method

This is a rather straightforward, intuitive way of measuring the surface currents. Buoys are launched into the water and fitted with GPS or other tracking devices. As the buoys are taken along by the currents, one can monitor their position and movement over time. By analyzing this data, the direction and speed of the surface currents can be determined. However, this is under the condition that these buoys must be designed and fabricated for the local marine conditions in terms of both wave impact and saltwater corrosion.

Anchored Ship Method

This is a method wherein a ship is stationed at some fixed point. Current-measuring instruments are then thrown from the moving ship to carry out the water flow corresponding to different depths. While the water flow at a particular point can be reported in great detail by this approach, there are certain shortcomings of this approach. That is, the ship is prone to be washed by waves and winds that may cause inaccurate readings. Further, it is also pretty discouraging regarding covering a large area.

The Acoustic Doppler Current Profiler (ADCP) Method

The ADCP method is a more advanced way to measure coastal water flow. Flow velocities for seawater currents can be measured by an acoustic current meter over a wide range of depth intervals. Basically, this method works by casting sound waves into the water and analyzing the frequency shift of the reflected waves due to the Doppler effect. The result is rather a detailed mapping of current structure from the surface down to the seabed.

4. How do ADCPs working on the Doppler principle function?

The ADCPs make use of the Doppler Principle. Each ADCP flow meter emits an acoustic pulse-that is, a pulse of sound waves-into the water. These sound waves scatter off particles in the water; that is, sediments, plankton, and every other small-sized particle present in the water. Eventually, when these sound waves reflect back to the ADCPs after bouncing off the particles in the water, their frequency changes because of the Doppler effect.

If the particles are moving towards the ADCP current profiler, then the frequency of the reflected wave is higher than the original emitted frequency. In other words, if the particles are moving away from the ADCP current meter, then the frequency of the reflected wave is lower. Precise measurement of this frequency shift allows the ADCP to compute the velocity of the particles. Since these particles are moving with the water current, the calculated velocity of particles should give the velocity of the water current.

They can emit sound pulses in several directions, while ADCPs measure the current velocity in three-dimensional space and at different depths. A profile obtained vertically would be able to measure the speed of the current at different intervals from the surface down to the seabed, therefore giving a full view of the current structure.

5. What's needed for high - quality measurement of Nha Trang coastal currents?

It means that for highly qualitative measurement of the currents in Nha Trang, the equipment must possess certain characteristics.

First of all, its reliability should be determined by the materials the equipment will be made from. Due to the real marine environment with saltwater corrosion and possible physical influences in the forms of wave and debris impacts, casings have to be durable. The casing should be able to protect the internal components of the equipment from damage and ensure its long-term functionality.

Small size is important for easy deployment, particularly when access is limited, as on small boats, or in shallow coastal waters. Light weight is of importance too, because lighter designs are more convenient to handle and install.

Low power consumption is a must for long-term measurements. In this respect, equipment should be able to make continuous measurements without quickly depleting the power source. Cost-effectiveness is also necessary to allow large-scale measurement. For ADCPs, the casing is preferably made from titanium alloy. Titanium alloy has very good corrosion resistance, which is essential in the Nha Trang marine environment. It will be able to resist and tolerate the corrosive nature of salt water for a long period. It is also strong and has a high strength-to-weight ratio, providing a tough yet lightweight structure that is able to protect the internal components of the ADCP profiler.

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

The selection of the right equipment for the measurement of current at Nha Trang takes into consideration the usage.

For Measurements from a Moving Vessel

A ship-borne ADCP will suffice. It has the capability of offering real-time current data when the ship is in movement. This facilitates a greater understanding of the water flow at different locations as the vessel moves.

For Measurements at a Fixed Location near the Seabed

A bottom-mounted ADCP would be a good option. It is able to continuously monitor the current at a point continuously. This type of doppler current profiler is stable and can provide consistent data over time.

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

An ADCP of buoy type is suitable for this case. It can cover a larger area and does not depend on any specific depth. The buoy-type ADCP may float freely and transmit data regarding the flow of water in a wide area.

The choice of frequency: Up to 70m, the 600kHz ADCP is a good choice. From 70-110m, the 300kHz ADCP is more suitable. For the very deep waters, with 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.