How to measure coastal currents around Xiamen?

1. Where is Xiamen?

Xiamen, a coastal pearl of Fujian Province, China, is situated on the southeastern coast of China. It constitutes a municipality with a characteristic blend of historic and cultural heritage combined with recent urban development.

Geographically, Xiamen has a very beautiful coastal setting. It borders the Taiwan Strait to the east, which confers on it a long and beautiful coastline. The geographical location makes the city an important center for sea transportation and trade. Xiamen's climate is mild, subtropical maritime, with mild winters and warm summers. Plenty of sunshine and adequate rainfall promote various kinds of flora and fauna, making the city very green and beautiful.

It is a city filled with a vital and multi-ethnic populace. People from other parts of the country, as well as overseas, have settled here, building a rich cultural tapestry. It is famous for its historical architecture, including Gulangyu Island, a UNESCO World Heritage Site. With colonial-era buildings, small alleys, and beaches, it is always in sway with tourists around the year.

These adjacent water areas-the Taiwan Strait in the vicinity of Xiamen-are a large sheet of water important not only for navigation purposes but equally for their rich marine resources. Also being a meeting zone of different water masses, this circulates a wide variability of influence in terms of the local marine ecosystem and coastal processes. In the waters around Xiamen, there are several small islands and islets adding to both aesthetic and ecological values of the area.

2. Current Situation of Coastal Currents near Xiamen

The coastal current of Xiamen is very complex and influenced by many different causes. Among the most influential is the system of monsoons. During summer, the southeast monsoon brings warm and moist air, which could drive the surface waters and lead to a certain direction and strength of the coastal currents. The northwest monsoon in winter changes the wind pattern and thus the current directions.

The topography of the sea floor is a prime factor in shaping the coastal currents: underwater ridges, troughs, and shoals make water flow in an extremely complicated pattern. For example, areas with shallow seabeds have different current speeds compared to deeper regions. The shape of the coastline, which includes bays and headlands, affects the flow of water. In addition, bays may serve as convergence or divergence areas of currents due to different tidal and wind conditions.

Tides are another very significant factor. Because of the tidal rhythm in Taiwan Strait, there is a continuous inflow and outflow of water along Xiamen coast. In certain locations along coastlines, tidal currents may be very strong, especially near some estuaries and constricted channels. Such tidal currents play an important role in transporting nutrients and sediments, which, in turn, impact the growth of marine organisms and stability of the coastal environment.

The interaction with the larger-scale ocean currents from both the South China Sea and the East China Sea affects the coastal currents near Xiamen. This can add in water masses that carry different physical and chemical properties, such as temperature and salinity, thus being able to modify the dynamics of the local current even more.

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

With this, precise measurement of the coastal current series in Xiamen has become meaningful in application for such uses as coastal engineering, marine resources, and environmental protection. Conventionally, there have been long-time usages of observation methods in the flow of coastal water with surface buoys and fixed-point current meters. However, more advanced techniques are at hand.

Acoustic Doppler Current Profilers represent a development within that same general direction, however, that has proven much more efficient and accurate. They can give detailed information on the velocity and direction of the current at different depths. 

3.1 How do Doppler Principle ADCP Work?

Acoustic Doppler Current Profiler(ADCP) work on the principle of the Doppler effect. They transmit sound waves at a known frequency into the water. Moving water particles in the water, including suspended sediments and small organisms, scatter these sound waves. Because of this movement of the water particles, the frequency of the sound waves that bounce back changes. This change in frequency is proportional to the velocity of the water particles and hence to the current velocity.

The transducer array sends and receives sound waves in several directions. By analyzing the frequency shifts of the reflected waves in the different directions, the ADCP current meter calculates the speed and direction of the current at various depths. This obtains a three-dimensional understanding of the current structure.

3.2 What's needed to make the measurement of Xiamen coastal currents of high quality?

There are many issues that have to be accounted for to achieve high - quality measurement of the coastal currents in Xiamen. First and most important, equipment has to be reliable. To provide strong resistance against a hostile marine environment, the ADCP current profiler casing material should be made of a substance that can put up with such an unforgiving environment.

The next paragraphs outline why titanium alloy is well suited to be coated to ADCP doppler and why this is so. First, it is highly resistant to corrosion, which is important in the saltwater coastal environment. It resists seawater and marine organism corrosion that can give long service life to the equipment. Second, titanium alloy is strong yet lightweight. Its strength enables it to sustain the mechanical stresses in the ocean, but at the same time, lightweight provides ease of handling and laying out even at tricky locations.

Other significant features besides material reliability are small size, low weight, low power consumption, and low cost. For convenient transport and deployment, an ADCP flow meter should be compact and light with a minor impact. Low power consumption allows for longer operation without the need to change frequently batteries or to use a large power supply. Cost-effectiveness will not limit the large-scale deployment and enable more comprehensive monitoring of the coastal area.

3.3 How to choose the right equipment for current measurement?

There are many factors involved in the selection of appropriate equipment for measuring the coastal currents of Xiamen. One of the most important is the purpose that the equipment is meant for.

Where there is a need for measurements while on a moving vessel, the ship-mounted ADCP will be ideal for the purpose. It can provide real-time data on the current as the vessel moves down the coast, thereby giving continuous monitoring of the current conditions.

This, however, is the right choice if measurements at a fixed location on the seabed are required. A bottom-mounted ADCP is installed at a given point and gives long-term stable measurements of the current at the given point.

An ADCP meter mounted on a buoy can be utilized for applications in open water or in areas where fixed installations cannot be made. It measures current over a larger area and is able to transmit the data by wireless.

Frequency Choice: With respect to the choice of frequency, various frequencies apply to different water depths. In general, a 600 kHz ADCP should be appropriate for waters of up to 70 meters in depth. For waters of between 70 and 110 meters deep, one may want a 300 kHz ADCP. For waters as deep as 1000 meters, one would want to consider using a 75 kHz ADCP profiler.

When it comes to choosing an ADCP brand, well - known international brands such as Teledyne RDI, Nortek, and Sontek are popular options. However, for those seeking a cost - effective yet high - quality alternative, the China Sonar PandaADCP is worth considering. Made entirely of titanium alloy, it combines durability and reliability with an excellent price - performance ratio. You can find more information about it at https://china-sonar.com.

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