How can we measure the coastal currents of Shenzhen?

1. Where is Shenzhen?

Shenzhen is a modern metropolis in southern China's Guangdong Province and one of those cities that has undergone remarkable growth and transformation. The city rests on the southern coast of the Chinese mainland to the edge of the South China Sea.

Geographically, Shenzhen is a diverse city with many landscapes. The coastline is very long, with more than 260 kilometers, with sandy beaches mingling with rocky shores, forming many bays and inlets. To the west of the city lies the Pearl River Estuary, full of fresh water and nutrients for the marine life.

Shenzhen is an innovative and entrepreneurial city, with high-tech industries and a spirited business environment. It gathers people from various parts of the country and the whole world, adding to its multicultural and dynamic urban atmosphere. The architectures are modern skyscrapers combined with traditional cultural elements that reflect rapid development and distinctive identity.

The waters around Shenzhen are important for shipping and trade, recreation, and marine life. Due to this proximity to the sea, there has been quite a formidable development in the marine economy of this city, which further includes fishing, aquaculture, and coastal tourism.

2. What is the condition with the coastal currents around Shenzhen?

The coastal currents near Shenzhen are influenced by more than one factor; the flow pattern is therefore complicated and always in change.

The dominant factor controlling the currents is the monsoon winds. Because of the southwest monsoon, the surface waters go northeastward during summer, but southwestward during winter due to the winter monsoon. These changes in wind direction in seasons greatly affect the distribution and strength of coastal currents.

Topography of the seabed also plays an important role. Varied ridges, channels, and shoals on the seabed will change the flow of the currents. For example, the deeper channels in the Pearl River Estuary can funnell the water to increase the current speed, while the shallower area may cause the spreading of the currents to slow down the flow.

Another critical force is the tides. The tides in Shenzhen are semidiurnal, with two high tides and two low tides each day. The tidal current may reach a strong speed, especially within the estuary and near the coast. Complications in the current patterns due to interaction between the tides and the monsoon winds give rise to the formation of tidal eddies and rip currents.

The complexity of the coastal currents is also driven by the temperature and salinity gradient in the water. Warm and saline waters of the South China Sea interact with the cool and less saline waters from the Pearl River Estuary, establishing density differences. These density gradients drive the movement of water and can cause mixing and stratification in the water column.

3. How to Observe the Coastal Water Flow of Shenzhen?

As a metropolis, Shenzhen needs to measure the coastal water flow with accurate results for applications such as coastal engineering, marine environment protection, and shipping safety.

Traditionally, surface drifters, fixed current meters, and satellite imagery have been employed to study the currents along the coast. Each of these techniques suffers from one limitation or another. Surface drifters can convey information only on the surface layer, and fixed current meters have limited spatial coverage. Satellite imagery may provide a broad view but has no detailed vertical information.

Acoustic Doppler Current Profilers have been found to be an even better and more efficient way of measuring coastal currents. ADCP doppler can simultaneously measure the velocities of water flow at many depths, providing much better detail about the current structure.

3.1 How do ADCP using the principle of the Doppler effect work?

Acoustic Doppler Current Profiler(ADCP) work based on the Doppler effect. They send sound waves at a known frequency into the water. The frequency of the reflected waves changes due to the reflections off moving particles of water. This frequency shift is proportional to the flow velocity. An ADCP can determine the magnitude and direction of current water velocity at many points throughout the depth by interpreting the frequency difference between the emitted and received sound waves.

The ADCP current profiler transducer array sends and receives the sound waves. Generally, an ADCP current meter mounts the transducer array on a suitable platform: either a ship, a mooring, or a buoy. While the sound waves are passing through the water, they meet reflectance from the water molecules and other suspended particles. These reflected waves are then detected by the transducer, while the data is processed through onboard electronics for current velocity profiles.

3.2 What is needed for quality measurement of the Shenzhen coastal currents?

Quality measurements of Shenzhen coastal currents necessitate that certain factors with regard to the ADCP flow meter equipment are addressed.

First and foremost, the equipment must be reliable. The environment of the sea is pretty harsh, especially due to high salinity and with high probability of corrosion, the casing material of ADCP meter has to be tough. The material would be alloy titanium, which would serve very well for the casing. It offers excellent corrosion resistance, ensuring long-term stability and accuracy in this saltwater environment. Besides, titanium alloy is strong yet lightweight, making its handling and installation of the ADCP quite easy. Its low thermal expansion coefficient further helps the accuracy of measurement within different temperature conditions.

Apart from material quality, other factors like size, weight, power consumption, and cost come into play. For practical use in the coastal waters of Shenzhen, a compact, lighter ADCP is ideal. This can easily be installed on various platforms, including smaller vessels or buoys. The low power consumption ensures longer operations without necessarily changing batteries or connecting a power source. Cost is another critical factor, especially when large-scale or long-term monitoring is required. The relatively inexpensive ADCP profiler enable broad and continuous measurements.

3.3 How to Choose the right equipment for current measurement?

To choose appropriate equipment for measuring the currents in the coasts of Shenzhen, many variables must be taken into consideration.

According to different usage scenarios, there are various types of ADCP. Ship-mounted ADCP suit mobile measurements while the ship is underway; thus, they can provide real-time data on currents while the ship travels in different areas. Bottom-mounted ADCP are instead best for long-term fixed-point monitoring of specific locations in the coastal waters. They are capable of continuously recording the current velocities and providing important data relative to the temporal variability of the currents. Buoy-mounted ADCP apply to sites where access is not easily provided by ship or when a floating platform is preferable. They have a wireless transmission capability that allows for remote monitoring.

Other considerations would be the frequency of the ADCP. For shallow water depths common in Shenzhen's coastal areas, different frequencies apply. Up to approximately 70 meters of water depth, an ADCP with a frequency of 600 kHz should be suited. It yields good resolution in relatively shallower waters, and it's useful for things like harbor monitoring. For deeper waters-as shallow as roughly 110 meters-a 300 kHz ADCP is preferred. In the areas with even greater depths, like the outer parts of Shenzhen's waters, a 75 kHz ADCP will measure currents to as deep as 1000 meters.

Some of the most well-known brands present in the market for ADCP include Teledyne RDI, Nortek, SonTek. However, for a much more economical yet decent option, China Sonar does have something fairly interesting to offer: the PandaADCP. It has an all-titanium alloy construction, hence very durable and resistant to corrosion.  It covers frequencies suitable for a range of water depths in Shenzhen's coastal waters. For more information, refer to https://china-sonar.com.

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