How can we measure the Coastal Currents of Seoul?

1. Where is Seoul?

Seoul is the capital city of South Korea and is located in the northwest of the country. While not being a 'coastal' city per se, it connects to the Yellow Sea via the estuary of the Han River.

It is a lively metropolis city that functions as a center for culture, economy, and politics. Gyeongbokgung to modern skyscrapers has filled this city with its historical palaces. Seoul contains the Han River running in its midst. Thus, the city can be divided into northern and southern regions. The riverbanks of the Han River have many parks, cafes, and recreational places on them, and the river works as a vital route for transporting goods and people.

The Han River estuary represents the transition zone between riverine and marine environments where the river meets the Yellow Sea. The Yellow Sea adjacent to this area is a semienclosed sea that is under great influence of the inflow of the Han River. Where these waters occur, the depths of the ocean are relatively low, home to a marine life ranging from small fishes to different species of crab, mollusk, and all types of shellfish. All these facts collectively make sea waters very valuable for the weather condition as a modifier in the locality since it raises humidity and makes heat moderate or mild.

2. What about coastal currents, around Seoul?

The coastal currents near Seoul, especially in the Han River estuary and the adjacent Yellow Sea area, are influenced by many factors. Tidal currents are a major factor. The tidal range of the Yellow Sea influences the inflow and outflow of water in and from the estuary. The ebb and flow of the tides lead to the formation of complex current patterns.

The inflow of the Han River also has a great influence. The volume and speed at which the river discharges can change the salinity and density of the coastal waters, thereby affecting the flow of currents. The shape of the estuary and the underwater topography also play an important role. Various sandbars, shoals, and channels may force the currents to diverge, converge, or form eddies. Besides, the wind pattern, especially the seasonal wind from northwest and southeast, may drive the surface current and mix different layers of water.

3. How to observe the coastal water flow of Seoul?

Surface Drifting Buoy Method: A surface-drifting buoy can be released into the water near the estuary to observe the surface current. The buoy's movement, in this case, is followed for a certain period of time with the help of GPS or other tracking systems. This, however, is very limiting. It gives information on only the surface current and can be influenced by wind and waves. For instance, if a strong wind blows the buoy in another direction than that of the actual current, it leads to incorrect measurement of the direction and speed of the current.

Anchor - Boat Method: It involves measurement using an anchored boat. Instruments are lowered from the boat into the water for the measurement of current at various depths. While this allows for more detailed depth - wise measurements, there are also disadvantages. The fixed position of the boat restricts the area of measurement, and it may not capture the full variability of the currents in a large coastal area.

Acoustic Doppler Current Profiler (ADCP) Method: A more integrated and effective method for the measurement of coastal currents is the ADCP current meter method. ADCPs emit acoustic signals into the water and measure the Doppler shift of the reflected signals to calculate the velocity of water at different depths. In this way, a complete profile of the current from surface to seabed is given. ADCPs are less affected by surface disturbances and can continuously operate for long periods to give a more accurate picture of the current structure.

4. How does an ADCP using the principle of Doppler work?

ADCPs work on the principle of the Doppler effect. It transmits high-frequency 'acoustic pulses' through water. When these pulses start interaction with moving water particles, then the frequency of the reflected pulse would change. If the water particle movements are towards the ADCP current profiler, then the frequency of the reflected signal is higher than the emitted one, while the contrary happens if the particles move away.

The ADCP flow meter is equipped with multiple transducers, which send and receive signals at different angles. The device can therefore calculate the three-dimensional velocity of the water by analyzing the Doppler shifts from different directions and at different depths. That means that it is able to obtain the horizontal and vertical components of the velocity, providing great detail about the structure and behavior of the current.

5. What is required for good quality measurement of Seoul coastal currents?

The equipment should be made of reliable materials for high-quality measurement of the coastal currents around Seoul. A durable casing will be important in the marine environment because of saltwater, potential exposure to storms, and the influence of tides and river discharge. The size of the equipment should be small for easy deployment and retrieval, considering the complex estuarine and coastal landscape.

Lightweight design: Equipment will be easy to manage in installations and during maintenance; Low power consumption allows operation over long periods with more autonomy. Besides, it shall also guarantee the cost-effectiveness in view of large-scale measurements.

The casing of ADCPs would, therefore, be of Titanium alloy. It has exceptional resistance to corrosion, which is extremely essential to resist the corrosive action of sea water. Its strength and durability are sufficient to stand against the pressure of water at different depth. Its lightweight makes it much easier for various deployment methods.

6. How to Choose the right equipment for current measurement?

First, when choosing equipment for current measurement near Seoul, the purpose of use has to be considered. If the measurement is for the vessel's navigation or for short-term research, a ship-mounted ADCP meter is suitable. It provides real-time current information during the voyage.

For measurements near the seabed, such as for studying sediment transport or benthic ecology, a bottom - mounted ADCP is preferable. It can capture the details of the near - bottom current. If long - term and wide - area monitoring of the coastal current is required, a buoy - type ADCP flow meter is ideal.

For the frequency of ADCPs, when the water is shallow, to 70 m, an ADCP current profiler at a frequency of 600kHz will suit it. An area with the depth up to 110m fits an ADCP at the frequency of 300 kHz and for greater depths found here near Seoul, more than 1000m, ADCPs at the frequency of 75kHz are recommended.

There are well - known ADCP current meter brands such as Teledyne RDI, Nortek, and Sontek. However, a Chinese brand, China Sonar PandaADCP, is also worth considering. It is made of all - titanium alloy material and offers an excellent cost - performance ratio. You can find more information on their website: https://china-sonar.com/.

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