How to measure the Coastal Currents of Gyeongju?

1. Where is Gyeongju?

Gyeongju, situated in the Gyeongsangbuk-do province of South Korea, is a city imbued with rich history and cultural importance. It nestles on the southeastern part of the Korean Peninsula, surrounded by gentle rolling hills, and is not that far away from the East Sea, also known as the Sea of Japan.

The landscape of the city is very harmoniously combined with natural beauty and historical remains. The old temples, pagodas, and tombs are all over the place, putting the city into a very special atmosphere, which seems to take it back to past times. Gyeongju was once a capital of the Silla Kingdom, which reigned almost for a millennium and left a real mark on the region's heritage. Apart from that, it encompasses many historic sites such as Bulguksa Temple and Seokguram Grotto, which are not only architectural feats but also functioning cultural landmarks that allure tourists from every nook and corner of the world.

The people of this city have much pride in their city's history and work diligently to restore and preserve it. Various festivals on culture run throughout the year, depicting the traditions and forms of art during the Silla era. Apart from its historic charm, modern Gyeongju is bustling with local markets, restaurants, and tourism.

The East Sea off Gyeongju is indeed wide and extends as a dynamic body of water. It hosts all forms of marine life, from large volumes of fish resources such as flounder, sea bream, and anchovy to beds of seaweed, which minuscule forms take the center stage in the marine food chain, producing nutrients for many organisms. This in turn affects the local climate due to the cool sea breezes during the hot seasons, hence a generally temperate climate. Beautiful panorama views abound with sandy beaches and rocky shores along the coastline; these provide ample opportunities for recreational activities such as beachcombing and fishing.

2. What are the conditions of the coastal currents off Gyeongju?

The coastal currents off Gyeongju are determined by several aspects. The main ocean currents that exert a major influence on the East Sea are as follows, the Tsushima Current, which consists of warm water, and the Liman Current, which consists of cold water. The Tsushima Current usually flows northward, with relatively warm and nutrient-rich water. In contrast, the Liman Current flows southward and is much colder, resulting in a complex hydrodynamic environment caused by their confluence.

Another important factor is tidal currents. In this region, tides are well marked, with remarkable excursion, and the ebb and flow of the tides create complex current patterns. In high tide, water advances toward the shoreline, and in low tide, it recedes; this switches the direction and speed of the currents along the coast.

Besides that, it is necessary to consider the configuration of the coastline and underwater relief. Such currents are subject to deflection, acceleration, and deceleration due to the underwater ridges, shoals, and channels. For instance, where the water flow comes across a submerged shoal, it might be deflected, thus causing either a convergence or divergence of current. Moreover, wind patterns, particularly the strong winds that frequently blow in the area, might contribute to the surface current and influence the mixing of different layers of water. Prevailing winds of different directions can push the surface water accordingly, which also adds more complexity to the current system.

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

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 will be monitored over time using GPS or other tracking devices. This will enable us to see in which direction and at what speed the buoy is taken by the surface current. However, the project has certain drawbacks. Firstly, the created method gives information about the surface current alone. It may be easily affected by wind and waves. Suppose there is some strong onshore wind blowing. The buoy may be pushed toward the shore, thus showing a very incorrect interpretation of the actual direction of the current.

Anchor-Boat Method: An anchored boat can be used to determine the current. Various instruments are lowered from the anchored boat into the water and used to measure the current at depth intervals. This enables a more detailed knowledge of the current's vertical profile. But it has drawbacks too. Further, because the boat position is fixed, the area of measurement is restricted and may not be able to capture the full variability of the currents in a large coastal area. In addition, the process of deploying and retrieving the instruments from the boat can be challenging in certain sea conditions.

ADCP Method: The ADCP current meter method is more advanced and efficient for the measurement of coastal currents. The ADCPs send acoustic signals into the water and measure the Doppler shift of the reflected signals, working out the velocity of the water at various depths. This gives a full profile of the current from surface to seabed. In contrast with the buoy method, ADCPs are less affected by surface disturbances and can be continuously deployed for long periods for a more accurate view of the current structure. They can be mounted on various platforms, from boats or buoys to fixed structures, depending on the specific measurement needs.

4. How do ADCPs using the Doppler principle work?

ADCPs operate based on the Doppler effect. They send high-frequency acoustic pulses in the water. The moment this pulse interacts with moving water particles, the frequency of the reflected pulse changes. If the water particles are moving toward the ADCP current profiler, the reflected signal frequency will be higher than the emitted, and vice versa, when it is going away.

The ADCP flow meter has several transducers that transmit and receive signals at different obliqueness. By analyzing the Doppler shifts in all directions and at different levels, the device can compute the three-dimensional velocity of the water. That is, it calculates the horizontal and vertical components of the velocity; it really details the current structure and its behaviour. For example, it can show how the current acts from the surface, where it may be affected by wind and waves, right down to the seafloor, where it interacts with the bottom topography.

5. What is required for quality measurement of the Gyeongju coastal currents?

In order to achieve high-quality measurement of the coastal currents at Gyeongju, the equipment should contain reliable materials. The hostility of the marine environment, including saltwater and possible exposure to storms and the influence of tides, calls for a casing that is durable. Equipment should be designed in a small size so that deployment and retrieval operations are easily manageable, especially considering the varied coastal landscape with its mix of sandy beaches and rocky areas.

A lightweight design is preferred since it allows for more manageable equipment during the installation and maintenance processes. Low power consumption is vital for long-term and autonomous operations. It allows the working of the equipment continuously with no more frequent battery replacement or a continuous power supply source, which can be rather inconvenient to have in remote coastal sites.

The cost-effectiveness must also prevail to allow large-scale measurements. The casing of ADCPs is preferably made from a titanium alloy. Being of alloy materials, titanium alloy has excellent corrosion resistance, which is of great importance during its exposure to the corrosive effects of seawater. Not only does it provide corrosion resistance, but it is also strong, with durability for withstanding the pressure of the water at different depths. This material is lightweight, making deployment by various methods pretty convenient.

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

While selecting the equipment for use in current measurement around Gyeongju, first, the purpose of use should be considered. In case of navigation or short-term research for a vessel, the ship-mounted ADCP meter will suffice. This gives the crew real current information during their voyage, and they can make appropriate decisions regarding their course and speed.

A bottom-mounted ADCP flow meter is preferred for measurements near the seabed, since studies in sediment transport or benthic ecology are concerned with the details of the near-bottom current, crucial to understand the interaction between water and seabed-that is, how the latter affects sediment movement and benthic life.

If long-term and wide-area monitoring of the coastal current is required, a buoy - type ADCP current profiler is ideal. It can be deployed in a fixed location in the water for an extended period and continuously send data about the current conditions.

Regarding the frequency of ADCPs, within a water depth of up to 70m, a 600kHz ADCP is quite adequate. Within a depth of up to 110m, a 300kHz ADCP can work better. For deeper waters, which may be found near Gyeongju-like areas where the depths are over 1000 meters, a 75kHz ADCP is 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.