How to measure the Coastal Currents of Donghae?

1. Where is Donghae?

Donghae is a city in Gangwon-do, South Korea; it is on the eastern coast of the Korean Peninsula. It has been a very lively coastal city with an extensive history related to the sea. The city is enveloped by nice natural sceneries that include mountain ranges and a long coastline along the East Sea Sea of Japan.

The sandy beaches, rocky shores, and small fishing harbors fit together in a mosaic that makes up the local landscape of Donghae. People of Donghae engage in different occupations like fishing, shipping, and tourism. The East Sea near Donghae has cold water and thus nurtures all kinds of marine life. It constitutes one of the main feeding areas of fish such as mackerel, herring, and also various kinds of shellfish.

The sea also contributes much to the local climate. For this reason, cold ocean currents and sea breezes affect both temperature and humidity. Kelp forests, together with other marine vegetation along the coast, serve not only as an important part of the ecosystem but also as a nursery for many marine species.

2. What is the situation in the coastal currents near Donghae?

Various factors influence the coastal currents around Donghae. The major ocean currents in the East Sea exert a dominant influence. The interaction between the warm-water current, the Tsushima Current, and the cold-water current, the Liman Current, takes place in this region. While the Tsushima Current generally flows northward, it carries relatively warm and nutrient-rich water. In contrast, the Liman Current flows southward and is colder.

The tidal forces have a great importance. In this area, there is an appreciable tidal range and due to the rise and fall of tides, elaborate patterns of currents are formed. The shape of the coastline and underwater topography are very important. Underwater ridges, shoals, and channels can give rise to changes in the current's direction and speed. Besides, the wind pattern, in particular strong wind, which is usual over this area can force the surface current and mix the different layers of water.

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

Surface Drifting Buoy Method: A surface - drifting buoy can be used to track the surface current. The buoy is released into the water and its movement is monitored over time, which is usually done through GPS or other tracking devices. This, though, has a drawback. It's just one way of seeing the current at the surface, and it may be affected by wind and waves. For instance, if a strong onshore wind blows, this can push the buoy toward the shore, which may give an inaccurate representation of the actual direction of the current.

Anchor - Boat Method: Current could also be measured using an anchored boat. Instruments that measure currents are lowered from the boat into the water at different depths of currents to be measured. This again allows more detailed depth-wise measurements, yet has a few disadvantages. The fixed position of the boat makes its area of measurement very limited, and may not capture the full variability of the currents in a large coastal area.

ADCP Method: This is a more advanced and efficient method of measuring the currents in the coast. 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. This would provide a more comprehensive profile of the current right from the surface to the seabed. Hence, ADCPs are less affected by surface disturbances and can keep running uninterruptedly for extended periods, yielding a truer representation of the current structure.

4. How do the ADCPs based on the Doppler principle work?

The ADCPs work on the principle of the Doppler effect. They emit high-frequency acoustic pulses into the water. If these pulses interact with moving water particles, their frequency will change upon return. When the water particles are moving towards an ADCP current meter, the frequency of the reflected signal is higher than the emitted signal, and in the case of flow, it is lower.

The ADCP current profiler has several transducers, which send and receive signals at different angles. Based on analysis of the Doppler shifts in different directions, and at different depth, the device makes calculations of the three-dimensional water velocity. That is, it determines both horizontal and vertical components of the velocity, offering a fine description of the structure and behavior of the current.

5. What is required for high-quality measurements of Donghae coastal currents?

The instrument to be used should have a reliable material makeup for high-quality measurement. Considering the conditions, including cold water and saltwater, and with exposure to storms, the casing needs to be durable. It is best for the size to be smaller, given the easiness in deployment and retrieval from some coasts whose landscape is complex.

A lightweight design is preferred, as this makes the equipment more manageable at installation and maintenance. Low power consumption is required for long-term and autonomous operation. Cost-effectiveness is similarly necessary to facilitate large-scale measurements.

The casing of ADCPs is preferably made of titanium alloy. Titanium alloy has excellent corrosion resistance, vital in withstanding the corrosive effects seawater can have. It is also strong and durable, able to withstand the pressure of the water at different depths. Its lightweight nature makes it more convenient for various deployment methods.

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

The choice of equipment for current measurement near Donghae should first consider the purpose of use. For the navigation of a vessel or short-term research, a ship-mounted ADCP flow meter would be suitable. It provides current information in real time during the voyage.

For measurements near the seabed, such as for studying sediment transport or benthic ecology, a bottom-mounted ADCP is preferable, since it can capture the detail of the near-bottom current. If one wants to do long-term and wide-area monitoring of the coastal current, a buoy-type ADCP current profiler would be ideal.

The frequency of ADCPs depends on water depth: up to 70m, a 600kHz ADCP would be appropriate; up to 110m, it would be more appropriate to use a 300kHz ADCP. For further waters, such as over 1000m that might be found near Donghae, it is recommended to use a 75kHz ADCP.

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.