How could we measure the coastal currents of Shanghai?

1. Where is Shanghai?

Shanghai is a world metropolis and one of the great coastal cities of China, situated at the mouth of the Yangtze River where it empties into the East China Sea. It is a dynamic economic and cultural center with a unique geographical position.

The city shoreline follows the estuary outside into the sea. Shanghai is classified under a subtropical monsoon climate, with generally mild winters and warm, humid summers. Sufficient rainfalls, along with the influence of the Yangtze River, support a rich variety of flora and fauna. This also includes vast wetlands and tidal flats within the city limits, which help form important stopovers for migratory birds and other wildlife.

Shanghai is a melting pot, with a history that dates back to the beginning of international trade and exchanges. Its skyline brims with modern skyscrapers, amongst which stands the historical landmark of the Bund. The local economy is diverse, with strong sectors in finance, trade, shipping, and manufacturing.

Adjacent waters, therefore, include the Yangtze River Estuary and the East China Sea. The Yangtze River discharges a huge amount of freshwater and sediment into the sea, which may alter the water properties and circulation in this region. The seafloor topography is quite complicated near Shanghai, added to that are some shallow areas just near the mouth of the estuary and others of deeper waters.

2. What are conditions of the coastal currents around Shanghai?

Several factors give rise to the coastal currents near Shanghai. The first is the monsoon climate. During summer, the southeast monsoon fetches warm and moist air that might drive the surface waters and affect the velocity and direction of the coastal currents. In winter, the northwest monsoon changes the current patterns.

Apart from this, the enormous discharge of the Yangtze River occurs with great impact. The fresh water outflow from this river creates a plume that interacts with the sea's saltwater. This can produce very intricate current patterns, including estuarine circulation. The differing densities between fresh and salty water can cause the water to move in different directions at different depths.

Another very important factor is sea-floor topography. The presence of undersea ridges, troughs, and shoals can make the water move in different directions at various speeds. Shallower areas next to an estuary can have slower-moving currents, while the deeper regions further out can have more powerful and complicated flows.

Tides also play a very important role in this connection. The normal ebb and flow of the Yangtze River Estuary and East China Sea tides are capable of producing high tidal currents. They are especially particular near the mouth of the estuary and narrow channels. These tidal currents are very essential for the transportation of sediments, nutrients, and marine organisms that have a direct impact on the marine ecosystem and coastal geology.

In addition, interactions with larger - scale ocean currents from the East China Sea and the Kuroshio Current could also change coastal currents around Shanghai. These oceanic currents may cause the invasion of water masses with different temperatures and salinities, which may further influence the local dynamics of currents.

3. How to observe the coastal water circulation of Shanghai?

The accurate measurement of coastal current in Shanghai is very important for marine safety, port operations, environmental protection, and scientific research. Traditionally, this was achieved by using buoys and current meters over a long period, but there are some limitations regarding these methods related to accuracy and coverage.

Acoustic Doppler Current Profiler (ADCP) have emerged as a more advanced and convenient way to measure coastal currents. ADCPs apply the Doppler principle to measuring water flow velocity.

3.1 How Does the Doppler Principle-Based ADCP Work?

The ADCP current meter send sound waves into the water. Suspended sediments, small organisms, and other moving particles in water intercept these emitted sound waves. When the suspended particles move, the frequency of the reflected sound waves is changed. The change in frequency is proportional to the velocity of the water current. 

The array of an ADCP current profiler transducer operates by first emitting and then receiving sound waves in several directions. From the returned echoes, the ADCP doppler computes the speed and direction of the current for each depth by analyzing the frequency shifts of the reflected waves in multiple directions. The result allows viewing the three-dimensional structure of currents.

3.2 What is required to measure Shanghai coastal currents in high quality?

There are quite a few elements that enable the high-quality measurement of the coastal currents in Shanghai. First, the reliability of the equipment is very important; that is to say, the ADCP flow meter must have hardy materials able to withstand the hostile seawater environment.

Titanium alloy is very good material for ADCP casing. It resists corrosion very well, which is very important in the seawater environment of the coast.

It would resist the corrosive action of seawater and marine organisms, ensuring the long-term performance of equipment. The other added property of titanium alloy is that it is very strong yet light. This strength allows it to bear the physical forces in the marine environment, and its light weight in turn makes handling and installation easier.

Those features make it reliable not only in material but also in small size, low weight, low power consumption, and low cost. A smaller and lighter ADCP meter will be more convenient in transportation and deployment for measurements in many varied locations. Low power consumption allows for longer operation without the need for frequent battery replacement or a large power supply. This cost-effectiveness is important in large-scale deployment, as it monitors an increased area of the coastline.

3.3 How to Choose Right Equipment for Current Measurement?

The choice of appropriate equipment for measuring the coastal currents in Shanghai depends on quite a number of factors. First is the purpose of the equipment to be used. 

For measurements from a moving vessel, a ship-mounted ADCP would be the best option. This can provide real-time data of the current while moving the vessel along the coast, hence allowing the continuously changing conditions of the current to be monitored. 

If the measurement has to be done at a fixed position on the sea floor, then the bottom-mounted ADCP would be used. It can be deployed in a particular spot and register a long-term stable measurement of currents covering that area. 

A buoy-mounted ADCP profiler is better suited for applications in open water or where fixed installation is impossible. Its measurements can cover a larger area and then wirelessly transmit the data. 

Choice of Frequency，Different frequencies suit different water depths. For example, a 600 kHz ADCP would generally be okay for water depths up to 70 meters. The 300 kHz ADCP works better for the depth range between 70 and 110 meters. For any appreciably deeper waters, up to 1000 meters, one should turn to 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.