How to measure the coastal currents of Hue?

1. Where is Hue?

Hue is a city in central Vietnam, from Thua Thien - Hue Province. It is situated along the Perfume River, commonly known as Huong River, which later empties into the South China Sea. Hue boasts a rich history and cultural heritage.

Hue was once the Imperial Capital of Vietnam, and its architecture still tells about its grandeur. It is a city with a strongly conservative population where traditions and values of the past are still closely linked. The town is also proud of its arts: handicrafts, music, and dance play an important part in the life of its people.

The South China Sea is a large and active body of water close to the shore. The local coastline off Hue reflects both the river's discharge and the sea's tidal action. Its waters support a varied array of marine creatures. Many fishing communities are found along the coast, and fishing is still one of the vital economic activities. The mangrove forests are also found in some areas, which protect the shoreline from erosion and help in providing a living place for many species.

2. What is the situation of the coastal currents near Hue?

Coastal currents near Hue are influenced by many factors. The tidal currents are one of the major influences. The tide in the South China Sea off Hue is subject to the attractive force of the moon and sun. There are semi-diurnal tides in the area; that is, there are two high and two low tides each day. In general, tidal currents are rather strong, particularly at the river mouth and along the coastline where the river meets the sea. These currents are very important for the transportation of sediments and nutrients.

The coastal currents also depend on the Perfume River discharge. This is where freshwater enters the sea and causes differences in density. This could result in the formation of complex current patterns. The monsoon winds have a major impact. The northeast monsoon, which lasts from November through March, is associated with strong winds capable of driving the surface currents in one direction or another. The southwest monsoon, which prevails between May and September, changes the wind direction and, thereby, the direction and speed of coastal currents.

The flow of the coastal currents can also be deflected and modified by the local bathymetry, in particular the shape of the seabed, and underwater features such as ridges and channels.

3. How to observe the coastal water flow of Hue?

There are various methods to observe the coastal water flow in Hue.

The Surface Drifting Buoy Method

In essence, this is a straightforward and practical method. Buoys are turned loose in the water, but each is fitted with GPS or other tracking devices. While the buoys are moved along by the currents, their movement can be followed over time to ascertain in which direction, and at what speed, the surface currents are moving. The buoys must be designed to stand up to local marine conditions, including waves and the influence of the river's freshwater.

Anchored Ship Method A ship is anchored at a specific location and current-measuring instruments are deployed from the ship. These instruments can measure the velocity and direction of water flow at different depths. Limitations of this method include possible effects of waves and winds on the ship, hence giving incorrect measurements, and the inefficiency in terms of area of operation.

The Acoustic Doppler Current Profiler Method

This is the advanced and quick means of measuring coastal water flow. ADCPs are capable of measuring the velocity profile in water currents over a big depth range. They work by emitting sound waves into the water and analyze the Doppler shift of reflected waves. This gives a fine understanding of the structure of the current from surface to bed.

4. How do ADCPs using the Doppler Principle Work?

The ADCPs work on the principle of Doppler. They emit some acoustic pulses, normally in the form of sound waves, into the water. In the working principle, these sound waves will scatter particles present in water, such as sediments, plankton, and other small particles. In this case, the frequency of the reflected waves will change because of the Doppler effect when the sound waves that come from these particles are reflected to the ADCP profiler.

If the particles are moving towards the ADCP flow meter, then the reflected wave frequency is higher than the emitted frequency. In turn, if the particles are moving away from the ADCP current profiler, then the frequency of the reflected wave will be lower. Precise measurement of this frequency shift allows the ADCP to calculate the velocity of the particles. Since the particles are moving with the water current, the calculated velocity of the particles provides the measure of the water current's velocity.

A different capability of ADCPs, compared to the previous, is that these instruments can send sound pulses in multiple directions and measure current velocity in three-dimensional space. For instance, in a vertical profile, they are able to record the current velocity at different intervals from surface to seabed and thus give a complete picture of the current structure.

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

For high - quality measurement of the coastal currents in Hue, good equipment with reliable materials is required. A tough casing will be needed because of the strong conditions of sea water, such as salinity that causes corrosion, and possible physical impacts from waves or floating debris.

The size of equipment should be small enough to be easily deployed, especially in areas where access may not be that easy, in small boats, or even in shallow coastal waters. A lightweight design is also beneficial as it minimizes handling and installation inconveniences.

Low power consumption is crucial for long - term measurements. Cost-effectiveness is also important for enabling large-scale measurements. For ADCPs, it is preferred that the casing be made of Titanium alloy. This is because it has excellent corrosion resistance, which is quite significant in the marine environment of Hue. It can take a long period of time of saltwater corrosive effects without its nature being changed. It also possesses strength and high strength-to-weight ratio providing durability and light weighted structure that can protect the internal components of the acoustic doppler velocity meter.

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

The first consideration in choosing the right equipment for the measurement of current in Hue is usage.

For Measurements from a Moving Vessel

A ship - borne ADCP is suitable. It can provide real - time current data while the ship is in motion, therefore allowing an improved understanding of the water flow in different locations.

For Measurements at a Fixed Location near the Seabed

A bottom-mounted ADCP is an excellent choice. It can continuously monitor the current at that one point, providing consistent data over time.

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

This calls for an acoustic doppler flow meter that is buoy-type. It can cover a wider area and isn't restricted to a specific depth.

In choosing the frequency, for water depths of up to 70m, 600kHz is a good choice. However, in the case of depths between 70m and 110m, it is best to use a 300kHz ADCP. For very deep waters of up to 1000m, one would better be suited with a 75kHz ADCP.

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.