How to measure Coastal Currents of Kumamoto?

1. Where is Kumamoto?

Kumamoto is a city on the island called Kyushu in Japan. It is not a coastline city in the strict sense, but nearby are the Ariake Sea and the Yatsushiro Sea. The city was known for its historical Kumamoto Castle and rich cultural heritage.

Ariake Sea is a large and shallow water body with unique characteristics, having a great tidal range, hence creating a diverse coastal environment. Yatsushiro Sea is another important water body, connected to other seas by various channels. The combination of agriculture, manufacturing, and fishing due to access to these productive seas supports the local economy.

Various types of marine life are found in the seas around Kumamoto. The Ariake Sea has extensive tidal flats that are home to shellfish, crabs, and other organisms. Fish species include eel, flounder, and many types of small fish. The seaweed beds are another important part of the ecosystem, providing food and shelter for many marine creatures.

2. What is the situation of coastal currents off Kumamoto?

In the coastal areas around Kumamoto, the current is influenced by several reasons. The tidal current, in particular, is great because of the large tide range of the Ariake Sea and the Yatsushiro Sea. This effect of the tide, inducing ebb and flow, results in the creation of complicated current patterns. Of course, the shape of the coastline and the underwater topography are important as well.

These may change in direction and speed due to the presence of tidal flats, channels, and shoals. Inflow from rivers and other freshwater sources can influence the salinity and density of the coastal waters, affecting the current flow. Moreover, wind patterns, especially the seasonal winds that blow across the seas, drive surface currents and influence the mixing of different layers of water.

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

Surface Drifting Buoy Method: Surface drifting buoy can be launched into the water, then taken near the coastline to observe the surface current. Movement of the buoy at different times is recorded via a Global Positioning System, or other tracking devices. This method, nevertheless, possesses several disadvantages. This technique offers data from the surface current only; wind and wave actions can directly influence this. For instance, if a strong wind blows the buoy in a different direction from the actual current, it can lead to incorrect measurements of the direction and speed of the current.

Anchor-Boat Method: In this, an anchored boat can be utilized for measuring the current. Instruments are lowered from the boat into the water at depth for measurement. Although this allows more detailed depth-wise measurements, it also has its drawbacks. 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.

The Acoustic Doppler Current Profiler method is a more advanced and efficient way of measuring the currents on coasts. ADCPs send out acoustic signals into the water and measure the Doppler shift of the reflected signals to compute the velocity of water at various depths. This provides a more comprehensive profile of the current from the surface to the seabed. Thus, ADCPs are less affected by surface disturbances and can work for longer periods continuously, which would give a much more accurate picture of the current structure.

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

ADCPs work on the principle of the Doppler effect. They transmit high-frequency acoustic pulses into the water. When these pulses encounter the moving water particles, the frequency of the reflected pulses is changed. If the water particles are moving towards the ADCP profiler, then the frequency of the reflected signal is higher than the emitted signal; if they are moving away, it is lower.

The ADCP meter is fitted with several transducers, which send and receive signals at different angles. With the ability to evaluate Doppler shifts from all different directions and at multiple depths, the device calculates three-dimensional velocity. That would be the horizontal and vertical components of the velocity and therefore give a good idea about the structure and nature of the current.

5. What is required to have high-quality measurement of Kumamoto coastal currents?

The equipment should be fabricated with reliable materials for high-quality measurement of the coastal currents around Kumamoto. There should be a durable casing considering the marine environment, saltwater, and potential exposure to storms; moreover, tides and tidal flats exert influence over it. The size should be small in order to enable easy deployment and recovery, especially in such a complex coastal landscape with its tidal flats and channels.

A lightweight design is preferred since it makes the equipment more manageable during installation and maintenance. Low power consumption for long-term and autonomous operation is also an important consideration. Cost-effectiveness is another factor so that large-scale measurements can be made.

The casing of ADCPs is preferably made of titanium alloy. Titanium alloy has excellent corrosion resistance, which is vital to withstand the corrosive effects of seawater. It is strong and durable to bear 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?

Equipment selection for current measurement near Kumamoto has to be considered based on the purpose of its use. If the measurement is for the vessel's navigation or short-term research, a ship-mounted ADCP flow meter will be suitable. It gives real-time current information during the voyage.

The measurements near the seabed are usually for purposes of sediment transport or benthic ecology studies. In such a case, a bottom-mounted ADCP will be more applicable. This can capture the details in the near-bottom current. On the other hand, if it is long-term and wide-area monitoring of the coastal current, the buoy-type ADCP current profiler would be ideal.

Frequency of ADCPs: For water depths up to 70m, the best choice is a 600kHz ADCP; for depths up to 110m, the best choice would be a 300kHz ADCP; and for deeper waters, which may be found in the vicinity of Kumamoto, with depths of more than 1000m, a 75kHz ADCP should be used.

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.