How to Measure the Coastal Currents of Fukuyama?

1. Where is Fukuyama?

Fukuyama is a city in Hiroshima Prefecture, on the main island of Honshu in Japan. It is situated on the Seto Inland Sea, which is a semi-enclosed body of water. The city is an important industrial and port city.

Manufacturing, shipping, and fishing in combination support the local economy. The Seto Inland Sea off Fukuyama is a remarkably productive marine environment. Sea bream, mackerel, and yellowtail fish are found there. Also, there are large seaweed and shellfish beds.

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

Many factors affect the coastal currents around Fukuyama. Tidal currents are one of the major contributors. The tidal range is moderate in the Seto Inland Sea; therefore, the ebbs and flows of the tides form current patterns. It is very important to mention the shape of the coastline and underwater topography. Because of the bays, channels, and shoals on the bottom, the current would change direction and speed.

This would result in changing the salinity and density of the coastal waters and influence current flow. Besides, the wind patterns, especially the seasonal winds that blow across the Seto Inland Sea, are also able to drive surface currents and affect the mixing of different layers.

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

Surface-drifting buoy method: One surface-drifting buoy may be deployed in water and observe the surface current. Further, the movement of the buoy is tracked over some time using GPS or other tracking systems. This method, too, has its drawbacks. First of all, it gives information only about the surface current. The method can also be affected by wind and waves. For instance, if there is a strong wind, the buoy might be blown into a different direction from the direction of the current, hence this could provide an inaccurate current direction and speed measurement.

Anchor-Boat Method: This involves measuring the current by an anchored boat. Instruments used for measuring the current are lowered from the boat into the water at different depths. While this provides an advantage of more detailed depth-wise measurements, it also presents some disadvantages. The position of the boat is fixed, hence limiting the area over which measurement can be taken. It may not capture the full variability of the currents in a large coastal area.

ADCP Method: The ADCP current meter method is more advanced and efficient in measuring the coastal currents. 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 provides a more comprehensive profile of the current from the surface to the seabed. ADCPs give less disturbance from the surface and are able to provide continuous operation over long periods, making them give a truer picture of the current structure.

4. How do ADCPs using the Doppler principle work?

The basic operating principle of ADCPs is the Doppler effect. It emits high-frequency acoustic pulses into the water. In cases of interaction with moving water particles, the frequency of the reflected pulses changes. When the water particles are moving toward the ADCP flow meter, then the reflected signal has a frequency higher than the emitted signal; when they are moving away, it is lower.

Each ADCP profiler is fitted with multiple transducers that send and receive signals at different angles. Through analyzing the Doppler shifts from different directions and depths, the device will calculate the three-dimensional velocity of the water. In other words, it can deduce the horizontal and vertical components of the velocity, which means detailed understanding of the current structure and behavior.

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

The equipment should be made of reliable materials for high-quality measurement of the coastal currents near Fukuyama. A durable casing is necessary, considering the marine environment with saltwater, possible exposure to storms, and the influence of tides and characteristics of the Seto Inland Sea. The size of the equipment should be small for easy deployment and retrieval, considering the complex coastal landscape with bays and channels.

The design should be lightweight, as this makes the equipment easier to handle during installation and maintenance. The power consumption should be low to enable long-term and autonomous operation. Cost-effectiveness is also necessary to make large-scale measurements feasible.

The casing of ADCPs is preferably made of titanium alloy. Titanium alloy has excellent corrosion resistance, which is essential in withstanding the corrosive effects of seawater. It is also strong and durable, capable of resisting 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 equipment for the measurement of current at Fukuyama should be decided upon according to the purpose of the use. For the cases of vessel navigation or shorter-term research, a vessel-mounted ADCP would be ideal. It can provide immediate current information in real time during the voyage itself.

A bottom-mounted ADCP would, on the other hand, become relevant in case the measurement needs to be from the seabed, say, for studying sediment transport or benthic ecology. A bottom-mounted ADCP will resolve the details of the near-bottom current. A buoy-type ADCP would again be appropriate in the cases when a coastal current needs to be monitored for long and over wide areas.

Regarding the frequency of ADCPs, a 600kHz ADCP is a good choice for water depths up to 70m, whereas the appropriate one for up to 110m depth will be a 300kHz ADCP. For more, it may go over 1000m in deep water areas near Fukuyama; in that case, it should be a 75kHz ADCP.

There are well-known ADCP 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.