How can we measure the Coastal Currents of Fukushima?

1. Where is Fukushima?

Fukushima is a prefecture situated on the east coast of Honshu, an island of Japan. It contains one long sea coastline with the Pacific Ocean. The prefecture stands for different industries, ranging from agriculture to fishing.

The Pacific Ocean off Fukushima is rich in marine resources, from fish resources like tuna, bonito, and mackerel to seaweed beds and shellfish. Although the local economy was seriously affected by the 2011 nuclear disaster, fishing and other coastal activities still contribute to the recovery process.

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

In the vicinity of Fukushima, coastal currents are driven by more causes. Among these, significant influence is exercised by a major current called Kuroshio Current. It is warm and north-flowing ocean water, which carries warm and rich nutrient water into the studied area. The Kuroshio Current impacts the coasts' water temperature distribution, marine life, and thus general current pattern.

Large tides are also an important factor for this area. There is a certain tidal range around Fukushima, and the tides coming in and going out create a complex current pattern. The shape of the coastline and the underwater topography are important. The presence of bays, headlands, and underwater ridges can cause the currents to change direction and speed. It may also be driven by wind, in particular seasonal winds like the trade winds, which can generate surface currents and influence mixing across various layers.

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

Surface Drifting Buoy Method: A surface - drifting buoy can be disposed in the water to observe the surface current. The position of a buoy is traced with time using GPS or another tracing system. However, this method has its limitation: it only gives information on the surface current, which can be affected by wind and waves. For example, if a strong wind blew the buoy in another direction than the actual current, this would result in erroneous measurement of the direction and speed of the current.

The anchor-boat technique is a traditional approach used to measure the current of a river. From the anchored boat, instruments are deployed into the water for measurement of currents at different depths. In so doing, the more detailed depthwise measurements are also possible. However, there are certain disadvantages associated with it. The fixed position of the boat restricts the area of measurement and might not capture the full variability in a large coastal area.

Acoustic Doppler Current Profiler (ADCP) Method: This is an advanced and quite efficient measurement of the currents in a coastline. ADCPs emit acoustics into the water and measure the Doppler shift of the reflected signals, from which it calculates the water velocity at various depths; that provides a comprehensive profile of current all the way from the surface down to the seabed. Also, ADCPs are less susceptible to surface disturbances and are thus able to operate continuously for long periods, yielding more accurate pictures of the current structure.

4. How do ADCPs that utilize the Doppler Principle Work?

The working principle of ADCPs is based on the Doppler effect. They transmit high-frequency acoustic pulses into the water. As these pulses reflect back from the moving water particles, their frequency changes. In case the water particles are flowing towards the ADCP, the reflected signal has a higher frequency than the emitted signal, while in the opposite case, it is lower.

The ADCP has a set of transducers that send and receive signals at different angles. By using the different Doppler shifts from different directions and at different depths, it can calculate the three-dimensional velocity of the water. That means it can deduce both the horizontal and vertical components of the velocity, which give a good overview of the structure and behavior of the current.

5. What is necessary for high-quality measurement of Fukushima coastal currents?

The equipment should be made from a reliable material for high-quality measurements of the coastal currents near Fukushima. A durable casing would be important in the marine environment, with saltwater, and potential exposure to storms and tides, as well as ocean currents. The size should be small for easy deployment and retrieval, especially given the complex coastal landscape with bays and beaches.

The lightweight design is needed as it makes the overall equipment easy to handle during deployment and maintenance. Low-power consumption shall be essential in allowing for long-term autonomy operation. Cost-effectiveness to enable large-scale measurements must, therefore, be established.

The casing of ADCPs is preferably made of titanium alloy. The resistance to corrosion, especially in seawater, is excellent. Besides, it is strong and durable to bear the pressure of 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?

According to the purpose of use, choose the equipment for current measurement in the waters around Fukushima. When the measurement is used for the vessel's navigation or short-term research, a ship-mounted ADCP is adequate. It gives 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 is ideal.

Regarding the frequency of ADCPs, for water depths up to 70m, a 600kHz ADCP is advisable. For depths up to 110m, a 300kHz is more appropriate, while deeper waters, which might be found near Fukushima, are above 1000m, it is recommended to use a 75kHz.

There are well - known ADCP 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.