How to measure the coastal currents in Hong Kong?

1. Where is Hong Kong?

Hong Kong, a special administrative region of China, is a vigorous, cosmopolitan city on the southeastern seaboard of China-a place where East meets West, where the traditional Chinese culture blends with the modern influence of the West.

Geographically, Hong Kong is situated on the southern tip of the Chinese mainland and faces the South China Sea. It consists of Hong Kong Island, Kowloon Peninsula, and the New Territories, together with a host of outlying islands. The city is remarked upon for its dramatic skyline, which features towering skyscrapers that overlook a beautiful harbor.

The waters around Hong Kong are very biologically diverse and carry a lot of economic and ecological importance. Victoria Harbour is one of the busiest and most beautiful harbors in the world. It is right at the heart of the city, being a major shipping lane and one of the most popular tourist attractions. Besides the harbor, the South China Sea beyond offers expanses of open water, while various bays and inlets indent the coastline of Hong Kong.

Hong Kong is also a fusion of cultures, with its population represented by people from all over the world. This city is famous for its food because it boasts the best of Chinese and international cuisine. It has a happening arts and cultural scene with numerous museums, galleries, and theaters.

2. What is the situation of the coastal currents near Hong Kong?

The coastal currents of Hong Kong are a result of natural and geographical influences.

The coastal currents of the area are determined principally by the monsoons. The southwest monsoon, which blows during summer with warm and moist air from the southwest, results in surface waters that are generally in a northeastward flow. During winter, the northeast monsoon, originating from the northeast, reverses the flow to more southwestward. This seasonal wind change drastically affects the water movements around the area.

The topography of the seabed around Hong Kong also plays a crucial role. The presence of shallow areas, channels, and underwater ridges can cause the currents to vary in speed and direction. For example, the narrow and deep channels in the harbor can accelerate the flow of water, while the shallower areas may experience slower and more turbulent currents.

The other important component includes tides. Hong Kong experiences semidiurnal tides; it has two high and two low tides each day. Tidal currents flow quite strongly in some areas, especially in and around the harbor. Interaction of tides with monsoon winds further complicates current patterns. It also gives rise to such complex tidal eddies and even certain rip currents.

Another factor affecting coastal currents is the gradient of temperature and salinity in the water. Warm waters of the South China Sea and cooler, less saline waters north from the Pearl River Estuary may cause differences in density driving the water to move. This gradient changes on a seasonal basis and thus greatly influences mixing and circulation within the coastal waters.

3. How to observe the flow of Hong Kong coastal water?

Accurate measurement of flow in Hong Kong coastal water is of particular importance on many aspects, such as shipping safety, marine environmental protection, and coastal engineering.

Among the traditional methods used in studying coastal currents, these include surface drifters, fixed current meters, and satellite remote sensing. However, each of these methods has its own limitation. Surface drifters can only give information of the surface currents, while fixed current meters have limited spatial coverage. Satellite remote sensing may give a broad overview but could lack the detailed vertical resolution needed to fully understand the structure of the currents.

Moreover, ADCP current meter have become one of the advanced devices that offer actual current measurement in the coastal region. The ADCP current profiler are much more beneficial in many ways because they can be utilized simultaneously to measure the current velocities of multi-depths within the water column.

3.1 How do the Doppler principle ADCP work?

Acoustic Doppler Current Profiler(ADCP) are based on the principle of the Doppler effect. They transmit sound waves at a known frequency into the water. This intensity of sound waves interacts with the moving particles of water, due to which the frequency of the reflected waves is changed. The change in frequency, which is directly proportional to the water flow velocity, corresponds to the velocity of the flow. The ADCP profiler determine the speed and direction of the currents at different depths by inferring differences between the frequencies of the transmitted and received sound waves.

The ADCP doppler transducer array sends and receives the sound waves normally mounted on a suitable platform like a ship, mooring, or buoy. While passing through water, the scattering of the sound waves takes place by the water molecule itself and other suspended particles. Then, the reflected waves are detected by the transducer and subsequently data is processed on onboard electronics to obtain the current velocity profiles.

3.2 What's required to make high-quality Hong Kong coastal current measurement?

Following is some of the key factors that one should consider in order to acquire high-quality measurements of coastal currents in Hong Kong.

First and foremost, material reliability of ADCP flow meter is a critical factor. The salt content of the water is very high, and the potential for corrosion always exists; therefore, the casing of ADCP must be made of very durable material. Then, the titanium alloy is quite suitable to make this case. It has great corrosion resistance, ensuring this equipment works for a long time, with high precision in seawater. Besides, the titanium alloy is both strong and light, which also makes it easier to handle and deploy the ADCP. Low thermal expansion coefficient brings benefit to maintaining stability and accuracy of measurement under changeable temperature conditions.

Apart from material quality, other issues of significance include size, weight, power consumption, and cost. In practical use at the Hong Kong coastal area, it is ideal that an ADCP meter would be compact and light. The gadget should, therefore, be installed on various platforms with much ease, such as small vessels or buoys. It should have low power consumption to allow for continuous operation without frequent changes of batteries or connection to the mains. Cost is also an important factor, particularly for monitoring projects that may involve large-scale or long-term deployments. Low-cost ADCP will facilitate widespread and sustained measurements.

3.3 How to choose the right equipment for current measurement?

To determine what equipment is used for measuring currents in Hong Kong coastal waters, there are several factors that need to be addressed.

Depending on the usage scenario, different types of ADCP are available. Ship-mounted ADCP are suited for mobile measurements while the ship is underway. They could also deliver current data in real time while the ship navigates through different areas. Bottom-mounted ADCP, however, are ideal for fixed-point monitoring over longer periods at specific locations in coastal waters. They are capable of continuously recording current velocities and yield precious data on temporal variability. Buoy-mounted ADCP serve well in areas difficult to reach by ship or where a floating platform would be more suitable. They are also capable of transmitting data wirelessly for remote monitoring.

The second aspect is the frequency of the ADCP. For water depths common in most Hong Kong coastal areas, different frequencies are appropriate. A 600 kHz ADCP is suitable for water depths up to about 70 meters. It has pretty good resolution in relatively shallower waters and is good for applications such as harbor monitoring. A 300 kHz ADCP can be preferred in waters up to about 110 m deep. Where the water is even deeper, like in outer parts of Hong Kong waters, a 75 kHz ADCP would serve for current measurements reaching as much as 1000 m.

There are quite a few renowned brands of ADCP available in the market. Teledyne RDI has Workhorse Monitor ADCP, Workhorse Sentinel ADCP, Workhorse Mariner ADCP, Workhorse Long Ranger ADCP among others. Then Nortek has Eco, Signature VM Ocean and Signature series. Other popular ones from SonTek are the SonTek-RS5, SonTek-M9 and the SonTek-SL. But for value for money, a better option may be the PandaADCP from China Sonar. It is also manufactured in all-titanium alloy, hence promising excellent durability and corrosion resistance. With different models like PandaADCP-DR-600K, PandaADCP-SC-300K, and PandaADCP-DR-75K-PHASED, it offers a range of frequencies suitable for the variance of the water depth in Hong Kong's coastal waters. Please refer to the following for more information: https://china-sonar.com.

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