How can we measure the coastal currents of Nantong?

1. Where is Nantong?

Nantong belongs to Jiangsu Province, China, on the northern bank of the Yangtze River estuary, facing the Yellow Sea.

It has a subtropical monsoon climate: mild winters and warm, moist summers. The region enjoys diversified natural scenery with rich rainfall and the moderating influence of the Yangtze River and the sea. Nantong boasts enormous areas of wetland and tidal flats, not only a magnificent scenery but also a very essential habitat for so many kinds of migratory birds and wildlife.

Nantong boasts a long cultural tradition. Its local customs and handicrafts portray the particularity of history and the relation with water. Its economy is deeply influenced by port and shipping industries, textile, and other manufacturing.

The major adjacent waters are the Yangtze River estuary and the Yellow Sea. A great deal of fresh water and sediment is transported via the Yangtze River, greatly affecting water quality and current patterns. Around Nantong, the seabed topography is complicated due to different shallow and deeper parts near and offshore, respectively.

2. What is the condition about the coastal current near Nantong?

The coastal current near Nantong is influenced by many factors. The first important factor is the monsoon climate. In summer, driven by the southeast monsoon with warm and moist air, it could drive the surface waters, then influence the direction and strength of the coastal currents. While in winter, the northwest monsoon will alter the current patterns.

Added to this is the mighty discharge of the Yangtze River. There is a freshwater plume issuing from this river that spreads out into the saltwater of the sea. This may easily lead to complex estuarine circulation patterns. For fresh and salt waters, obviously of different densities, the water may flow in opposite directions at different depths.

The other controlling factor is the seafloor topography. Underwater ridges, troughs, and shoals might have the effect that not only can water flow in a different direction but at different speeds as well. Currents in shallow areas can be very slow while those with deeper regions can be stronger and complex.

Tides also play an important role. The tides in the estuary of the Yangtze River and Yellow Sea periodically rise and fall to develop strong tidal currents. These tidal currents, which possess higher strength, occur around the mouths of narrow channels or near the estuary mouths. They are important for sediment and nutrient transport and marine organisms, having a direct influence on the marine ecosystem and coastal geology.

Besides, the interaction with larger-scale ocean currents from the Yellow Sea and other adjacent areas can further modify coastal currents around Nantong. It can bring in water masses of different temperatures and salinity levels, hence affecting the local dynamics of the current.

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

Precise measurement of coastal currents in Nantong has recently become quite indispensable, serving a number of purposes related to marine safety, port operations, fishery management, and environmental protection. The conventional methods usually rely on buoys and current meters and have been adopted for quite some time. These methods have serious deficiencies in accuracy and poor coverage.

ADCP current meter, in recent times, have been gaining widespread popularity over the conventional method of coastal current measurement owing to more advanced and available convenience. It basically works on the principle of Doppler for flow velocity measurement.

3.1 Working Principle of the ADCPs based on the Doppler Principle

Acoustic Doppler Current Profiler(ADCP) emit sound waves into the water. The emitted sound waves scatter off the moving particles in the water, including suspended sediments and small organisms. When particles move, the frequency of the reflected sound waves changes. The frequency shift is proportional to the velocity of the water current.

The transducer array of ADCP current profiler emits and receives sound waves in multiple directions. By analyzing the frequency shift of the reflected waves in multi-directions, the current speed and direction at each depth can be calculated by the ADCP doppler accordingly, thus providing a three-dimensional view of the current structure.

3.2 What's needed for high-quality measurement of Nantong coastal currents?

Various factors will be put into consideration for high - quality measurement of the coastal currents in Nantong. First, there is equipment reliability. The ADCP flow meter should be made from materials capable of withstanding the harsh marine environment.

The titanium alloy is a very good material for the casing of ADCP meter . This is highly corrosion-resistant and an important factor in the salt-water environment of the coast. It will provide resistance to the corrosive action of seawater and marine organisms and ensure long service life for equipment. Besides that, titanium alloy is strong yet lightweight. Its strength provides it with the ability to bear physical stresses in the ocean, while light weight is ease for manipulation and installation.

Besides material reliability, other features like small size, low weight, low power consumption, and, finally, low cost are welcome. A small and lightweight ADCP profiler is easier to carry and install, therefore allowing measurement in places that were previously difficult to reach. Low power consumption allows it to run for longer without the need to change frequently a big set of batteries or a big power supply. Cost-effectiveness for large-scale deployment would enable wider coverage of the coastal area.

3.3 How to Choose the Right Equipment for Current Measurement?

Selection of appropriate equipment while measuring the coastal currents in Nantong is highly dependent upon a few factors. First and foremost is the aspect of the purpose it shall serve.

For measurement from a moving vessel, it would be appropriate to have a ship-mounted ADCP. It could supply current real-time data as the vessel moves across the coast, thus the continuous monitoring of current conditions.

An ADCP that is bottom-mounted is used when the measurement needs to be from a fixed location on the seabed. This can be installed at a point location and provide stable, long-term measurements of the current there.

The latter finds application in open water or where a fixed installation is impossible. A buoy-mounted ADCP can measure current over a larger area and can wirelessly transmit the data.

As for the choice of frequency, different frequencies are suitable for different water depths. For a depth of up to 70 meters, a 600 kHz ADCP should generally be appropriate. For depths between 70 to 110 meters, a 300 kHz ADCP is comparatively better. For waters with a depth of up to 1000 meters, one would consider using a 75 kHz ADCP.

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.