How can we measure Haenam Coastal Currents?

1. Where is Haenam?

Haenam is one of four counties that make up South Jeolla Province in the country of South Korea. On the map, it is situated on the southwestern corner of the Korean Peninsula essentially the southernmost front towards the Yellow Sea, surrounded by much natural beauty.

The topography of Haenam is mainly made up of open fields and, behind them, the sea. The coastline consists of a mixture of sandy beaches, mudflats, and rocky shores. The mudflat areas are one of the peculiar features that provide life to a very rich and diverse ecosystem. Haenam also consists of a number of small islands dotted around it in the sea.

The local people engage in farming, fishing, and tourism related activities. Fishing has been an important aspect of the local economy due to the rich marine resources found in the Yellow Sea. The seafood of this area, including various types of shellfish and fish such as flounder and eel, is well known. The sea near Haenam greatly affects the climate, having cool sea breezes in summer and a regulating action of temperature during the year.

2. What is the situation of the coastal currents near Haenam?

Various factors contribute to the characteristics of the coastal currents around Haenam. Tidal currents are predominant. The Yellow Sea is a sea that has quite a considerable tidal range; hence, its ebb and flood create a complex current pattern. The tides cause water to move in and out of the bays and over the mudflats, which in turn affects the overall current structure.

The shape of the coastline and underwater topography are also very important. The mudflats, along with underwater ridges and channels, can cause the currents to change direction and speed. For example, the water may slow down as it spreads over the wide mudflats during low tide and then accelerate as it funnels through channels during high tide. Besides, the wind pattern structure, mainly the seasonal winds blowing across the Yellow Sea, is able to drive surface currents and allow or inhibit the mixing of different layers. The freshwater inflow from local rivers and streams can change the salinity and density of coastal waters and hence influence current flow.

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

Surface Drifting Buoy Method: In this method, the surface current can be tracked by a surface drifting buoy. The buoy is set free in the water and its movement is followed after a certain time, usually with the help of GPS or other tracking devices. However, this method has a number of drawbacks. It gives information about the surface current only and can be influenced by wind and waves. For example, if there is a strong on shore wind, the buoy may be pushed towards the shore to give an inaccurate representation of the actual current direction.

Anchor Boat Method: In this, an anchored boat is used to determine the current. Instruments measuring current are dropped from a boat in the water. While this will allow better and more detailed depth wise measurements, this technique suffers from certain drawbacks. As the boat occupies a fixed position, only a limited area surrounding it can be measured. A small fixed area cannot depict or present the whole variation within the currents in a greater coastal area.

Acoustic Doppler Current Profiler Method: The ADCP current meter is the most advanced and efficient method to measure the coastal currents. ADCPs emit acoustic signals into the water and calculate the velocity of water at different depths from the measured Doppler shift of the reflected signals. This gives an expanded profile of the current right from the surface to the seabed. In such cases, ADCPs will have minimal surface disturbances and thus may operate continuously over a longer period, therefore yielding more realistic current structures.

4. How does an ADCP that is using the Doppler principle operate?

The principle of operation for ADCPs is based on the Doppler effect. They emit high frequency acoustic pulses into the water. As these pulses interact with moving water particles, the frequency of the reflected pulses changes. If the water particles are moving towards the ADCP profiler, then the frequency of the reflected signal is higher than that emitted, and if they are moving away, it is lower.

The ADCP meter is equipped with multiple transducers that send and receive signals at different angles. It can calculate the three dimensional velocity of the water by analyzing the Doppler shifts from different directions and at different depths. This means it can find the horizontal and vertical components of the velocity, giving a very good understanding of the current's structure and behavior.

5. What is required to perform a high quality measurement of Haenam coastal currents?

The equipment should be fabricated with reliable materials for high quality measurement of the coastal currents at Haenam. If considering the marine environment alone, the saltwater may lead to potential exposure to storms, tides, and mudflats; for that, a casing should be durable. The size of equipment should be small in order to deploy and withdraw it easily, especially keeping in mind the complex form of the coast with various beaches, mudflats, and islands.

A lightweight design is preferred since it makes the equipment more manageable at the time of installation and maintenance. Low power consumption is necessary for long term and autonomous operation. Cost effectiveness is also necessary to enable large scale measurements.

The housing of ADCPs is of titanium alloy, preferably. Due to the presence of seawater, good corrosion resistance is quite important, which the titanium alloy can provide. Besides that, it's strong, durable, and able to bear the water pressure in a different layer. Light weighted and that makes for convenience in deployment methods such as various deployments.

6. How to choose the right equipment for the measurement of current?

Equipment for current measurement, for use near Haenam, would be chosen with considerations first of the purpose of use. If the use is to be for vessel navigation or short term research, a ship mounted ADCP flow meter would be appropriate. The real time current information is given during the voyage.

For measurements near the seabed, such as in the study of sediment transport or benthic ecology, a bottom mounted ADCP is preferable. It is able to capture the details of the near bottom current. In cases where the monitoring of the coastal current is required over a long period and/or a wide area, a buoy type ADCP current profiler would be ideal.

Regarding the frequency of ADCPs, for water depths up to 70m, a 600kHz ADCP is good. For the depth up to 110m, a 300kHz ADCP is more appropriate. For deeper waters that might be found near Haenam, such as areas with a depth of over 1000m, 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.