How to measure the coastal currents of Zhuhai?

1. Where is Zhuhai?

Zhuhai, a sea-bordering city in south China's Guangdong Province, stands out for embracing both nature and modernity.

Zhuhai, opening onto the South China Sea, is located in the southwest of the Pearl River Estuary. The longilstening golden beaches and crystal-clear waters embellish Zhuhai with a popularly praised landscape. Owing to its warmth and moisture, the subtropical oceanic climate provides a comfortable environment in Zhuhai all year round.

In terms of human geography, Zhuhai has a richly cultural and historical city. People here are known to be very hospitable, cordial by nature, and loving to the ocean. This somehow gives the city an interesting mix between traditional Cantonese culture and modernist coastal lifestyle.

Zhuhai is also one of the region's economic pillars, with its rapid development in tourism, advanced manufacturing, and high technology. It is easily accessible from other major cities in China and the world because of its modern infrastructure and efficient transportation network.

Adjacent to Zhuhai are some important bays and waterways, including the northern Pearl River Estuary as a critical channel and rich source of fishery resources. To the south lies the South China Sea, with wide expanses of open water and diverse marine ecosystems. Islands such as the Wanshan Islands and Dong'ao Island scattered in the surrounding waters add charm and beauty to the coastal landscape.

2.What is the situation about the coastal currents near Zhuhai?

Several factors make the marine environment around Zhuhai complex and dynamical.

Of the dominating factors, one is the monsoon. During the summer monsoon, the southwest winds blow across the region, carrying warm, moist air, and the surface waters flow northeast. In winter, the northeast monsoon brings cold, dry air with it, with the currents reversing. The monsoon winds do not only influence the surface currents but also, through vertical mixing, affect the deeper layers of the water column.

Other important factor determining the character of coastal currents is underwater topography. Around the Zhuhai City, there are both shallow waters zones with reefs and sandbars and their corresponding deeper channels and trenches. Such differences in depth and topography may cause the currents to flow variably: in difference in speed and direction, further creating eddies and whirlpools in some places.

Another important factor is tide. The regular rise and fall of the tide result in changes of water level, thereby generating tidal currents that can affect the flux of water along the shore significantly. In general, tides in Zhuhai are semi-diurnal with two high tides and two low tides every day. The interference between tide and monsoon wind even complicates current patterns.

Temperature and salinity of the water also have an influence on the coastal currents. In areas of differing temperature and salinity, the density of the water will vary and, in turn, may affect the flow of currents. Off Zhuhai, warm waters from the South China Sea and cooler waters from the Pearl River Estuary may create density gradients that could influence the direction and strength of the currents.

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

Knowledge of coastal water flow is essential in many applications, including navigation, marine resource exploitation, and environmental protection. So far, most in-situ measurement methods have been drift bottles, current meters, and observation using satellite imagery to obtain the speed and direction of flow. However, each of these methods has its own limitations regarding space-time resolution and accuracy of measurements.

More recently, ADCP current meter have represented an advanced and more convenient tool in the measurement of coastal currents. Using the Doppler Principle, the ADCP measures simultaneous velocities across multiple layers of the water column for a more detailed and accurate understanding of the current patterns.

3.1 Principle of operation of Doppler principle-based ADCP

Acoustic Doppler Current Profiler(ADCP) send out sound waves into the water, whereby the reflection would equally have a frequency shift. The frequency shift is due to the motion of the particles in water, related to the velocity of the flow. By this means, analyzing the frequency shift should enable ADCP to determine the velocity of water flow in different directions.

ADCP current profiler are normally constructed as a transducer array, which sends out and receives sound waves. The transducer array is normally mounted on a ship, a mooring, or a buoy. As these waves travel through the water, they are reflected by the water particles and received back by the transducer array. Signals received are then computed by a computer to calculate the flow velocity of the water.

3.2 What is required for high-quality measurement of Zhuhai coastal currents?

This entails a number of factors that have to be considered in the high-quality measurement of coastal currents of Zhuhai. Among the most important aspects are the reliability of the equipment used; the ADCP doppler used to measure the current should be made from materials that are long lasting and able to sustain a harsh marine environment.

The many benefits that come with titanium alloy make it the perfect material to make ADCP flow meter casings. Firstly, the titanium alloy is very resistant to the salinity of sea water and other elements that may be in the ocean. It is also strong yet light in weight, thereby enhancing much ease in handling and installing these ADCP meter. The thermal expansion coefficient for the titanium alloy is low, something important to provide a valid measurement in extreme temperature fluctuations.

Besides material reliability, other design considerations are size, weight, and power consumption, and cost. Since it will take large-scale measurements, a small and lightweight ADCP profiler is wanted for efficiency during deployment and retrieval. Low power consumption is relevant due to extended battery life, hence minimizing frequent recharging. It is also at this price that the large-scale deployment can be realized.

3.3 How to choose the right equipment for current measurement?

The appropriate equipment selection of current measurement in Zhuhai could depend on a few variables: first of all, intended usage. For different applications, there could be different kinds of ADCP. Ship-mounted ADCP would be suitable for measuring currents when, for example, the ship is in motion, while bottom-mounted ADCP are ideal for monitoring currents over a long period at a fixed location. Buoy-mounted ADCP might be used for current measurements in locations where the deployment of other types of equipment is difficult.

Another consideration would be the ADCP frequency, as different frequencies suit different water depths. For example, 600 kHz ADCP work for waters up to 70 meters deep, and 300 kHz ADCP are suitable for water with a depth of up to 110 meters. ADCP with frequencies at 75 kHz could be used for more profound waters, with speeds of up to 1000 meters.

There are several brands of ADCP amongst which Teledyne RDI, Nortek, Sontek, and China Sonar feature. In the last-named company, there is a choice between quality and relatively low prices-the PandaADCP. This ADCP is made from all-titanium alloy material for excellent durability and corrosion-resistant properties. It also has a wide range of frequencies to choose from, which makes it suitable for different water depths. For more information, check out: https://china-sonar.com/.

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