How can we measure the coastal currents of Ha Long?

1. Where is Ha Long?

Ha Long is an attractive city situated in Vietnam. It is famous for its wonderful nature and intensive culture.

Geographically, Ha Long is situated along the northeast coast of Vietnam, nearly touching the region's bodies of water known as the Gulf of Tonkin. This results in an incredibly complicated coastline with substantial islands and islets, further beautifying this seascape into one of breathtaking scenes. The waters that surround Ha Long form a very important part of its entity-the clear blue waters support a myriad of marine life and ecosystems.

Talking about its human aspect, Ha Long hosts a very vibrant local community. Their attachment to the sea is very strong indeed, especially in the case of fishing, which is an important traditional occupation for many locals. It is also a city which gains the largest number of tourists who come across the globe just to experience the natural beauty that it provides. Perhaps the major attraction here is Ha Long Bay, which is a UNESCO World Heritage Site. The local culture is a blend of traditional Vietnamese elements, with unique customs, festivals, and handicrafts adding to the charm of the place. The city itself is bustling with activity and includes amenities and attractions that cater both to locals and visitors alike.

2. What is the situation of the coastal currents near Ha Long?

In the Ha Long area, the coastal currents depend on many factors. One of the most significant is tide. The regular rise and fall of tides in the Gulf of Tonkin cause major changes in the water movement along the coast. In high tide, the currents become stronger while the water rushes in; at low tide, the flow may reverse or be generally weaker.

Changes in the wind also create changes in the current in the coast. The seasonal monsoon in Southeast Asia brings variation in the pattern of winds throughout the year. For instance, a northeast monsoon could drive strong currents along the coast of Ha Long and force the water in that direction. These winds transfer energy onto the surface of the water, creating surface currents capable of influencing the overall circulation of the coastal waters.

Another aspect is the seabed topography. The complicated underwater topography around Ha Long would give rise to a large number of islands and different depth levels, which may disturb and divert the water in its path. Shoals and deeper channels can cause the currents to speed up or slow down, creating areas of different current intensities.

Another factor affecting the currents off the coasts would be freshwater input due to regional rivers. The rivers that empty into the Gulf of Tonkin around Ha Long carry freshwater into the saltwater body. Changes in water density through this mixing process will, in turn, affect the movement of the currents.

3. How to observe the coastal water flow of Ha Long?

The measurement of its velocity and flow rate will depict Ha Long's coastal currents more accurately. Conventionally, measurements of water flow have taken their toll, but with changing technology, advanced tools come to the fore. Among those advanced tools, the most useful tool is the Acoustic Doppler Current Profiler, also known as an ADCP current meter. The principle behind it has also been very constructive.

3.1 How do ADCPs using the principle of Doppler work?

Principle of operation: Acoustic Doppler Current Profiler(ADCP) operate according to the Doppler principle. They radiate acoustic signals through the water and experience a change in the frequency of the reflected signal when those signals encounter moving particles in the water, such as sediment or small organisms. The change in frequency is related directly to the velocity of those moving particles and, by extension, the velocity of the water current.

They send several acoustic beams at different angles and can, therefore, assess the velocity of water at discrete depths within the water column. This provides an elaborate profile of the current itself whereby one would obtain information not only on the surface current but also on currents at various depths from the surface. Data gathered from ADCP current profiler can, consequently, be post-processed to make out the direction and speed of coastal currents around Ha Long.

3.2 What is required for quality measurement of the Ha Long coastal currents?

For high-quality measurement of the coastal currents near Ha Long, several aspects concerning the equipment are paramount: material reliability of the device. The equipment has to be resistant to the marine environment, which comes into contact with salt water, high pressure at depth, and impacts from floating debris.

Other positive characteristics include a compact size, low weight, low power input, and low price. Smaller dimensions and weight make the instrument more manageable and deployable: aboard boats, attached to buoys, or placed on the seafloor. This has the advantage of low power consumption, enabling the device to operate for extended periods without frequent battery replacements or a large power supply, especially in long-term monitoring. Low cost enables more uses of the equipment and allows large-scale measurement campaigns.

One of the excellent choices of material for casings is titanium alloy for ADCP doppler. The advantages of a titanium alloy are as follows. It is extremely resistant to corrosion, just what should be applied in a salt-heavy marine environment. Its strength-to-weight ratio is very high for a casing that needs to be durable yet relatively lightweight. That would mean it can be strong and, at the same time, easy to maneuver. The alloy titanium would also be resistant to high pressures for depth variations in the coastal waters of Ha Long.

3.3 How to Choose Right Equipment for Current Measurement?

There are several considerations that go into choosing appropriate equipment to measure the coastal currents of Ha Long. Certainly, one of the first decisions will be based on mode of deployment, which can be categorized into three main types: shipborne ADCP, bottom-mounted ADCP, and buoy-mounted ADCP.

Shipborne ADCPs are installed on vessels and used to obtain current measurements while the ship is in motion. Since the ship will be moving along the coast, the area covered by them will be comparatively large. Bottom-mounted ADCPs are deployed on the seabed and yield continuous measurements of currents that pass over them. Due to this reason, they are suitable for long-term monitoring at specific locations. Buoy-mounted ADCPs are attached to buoys floating on the water surface and can transmit data wirelessly, allowing for the monitoring of coastal currents remotely.

The other important factor is the frequency of the ADCP flow meter. Different frequencies are suited for different water depths. Typically, a 600 kHz ADCP will be suited for water depths up to 70 meters. This frequency gives good resolution for shallower waters. A 300 kHz ADCP will be better in waters as deep as 110 meters, balancing resolution and penetration depth. And lastly, a 75 kHz ADCP shall be used in waters as deep as 1000 meters to enable measurement in deeper parts of the coastal waters around Ha Long.

The market is known to have several popular brands in the ADCP profiler, such as Teledyne RDI, Nortek, and Sontek. However, the most economical option with excellent quality is the China Sonar ​PandaADCP. Its all-titanium alloy material ensures a durable device that can stand strong against marine environmental elements. Moreover, the device has an outstanding price-to-value ratio. You can learn more about it from its official website: ​https://china-sonar.com.

Here is a table with some well known ADCP instrument brands and moels.