How to Measure the Coastal Currents of Okazaki?

1. Where is Okazaki?

Okazaki is a city in Aichi Prefecture, in the central part of Honshu, Japan. It is relatively near the coast, close to Ise Bay, which is an inlet of the Pacific Ocean. The city is famous for its historical importance and various industries.

The local economy is diversified with major sectors such as manufacturing and some agriculture. Ise Bay, near Okazaki, is a productive marine zone. The fish resources include sea bream, mackerel, and yellowtail. Seaweed beds and shellfish are also found. The bay forms an important fishing ground and also assumes importance for transportation and trade due to its linkage to the open ocean.

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

In the case of Okazaki, a few elements combine to result in the pattern of coastal currents. One such important feature of these coastal currents is tidal currents. The tidal range is moderate in Ise Bay, and thus tidal action produces oscillating current patterns. Of most importance, however, are the shape of the coastline and underwater topography: bays, channels, and shoals can give rise to changes in the direction and speed of currents.

Freshwater input from rivers and other sources causes changes in salinity and density of the coastal waters, which can further influence the current flow. Besides, wind patterns, particularly seasonal winds that blow across Ise Bay and the Pacific Ocean, can force surface currents and further influence mixing within different layers.

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

Surface Drifting Buoy Method

A surface-drifting buoy can be deployed in the water to see the surface current. The movement of the buoy is followed over time using GPS or other tracking systems. However, the capability of this method is limited. This method only measures the surface current, and it could be interfered with by wind and wave actions. For instance, if at a given time a strong wind blew the buoy in a direction other than the current, the measurements of the current direction and speed recorded would already be incorrect.

Anchor-Boat Method

The current can be measured by an anchored boat. Instruments are lowered from the boat into the water to measure the current at different depths. While this allows for more detailed depth - wise measurements, it also has 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.

Acoustic Doppler Current Profiler (ADCP) Method

The Acoustic Doppler Current Profiler (ADCP) method is a more modern and effective way to measure the currents in coasts. The ADCPs send acoustic signals into the water and receive the reflected signal to find the Doppler shift, using which the velocity of water is calculated at various depths.

This gives a better profile of the current from the surface to the seabed. The ADCPs are less affected by surface disturbances and can run continuously for long periods, giving a more accurate picture of the current structure.

4. How do ADCPs using the Doppler principle work?

ADCPs work by utilizing the Doppler effect. They send high-frequency acoustic pulses into the water. When these pulses come in contact with moving particles of water, the frequency of the reflected pulses is altered. If the water particles are moving towards the ADCP meter, the frequency of the reflected signal is higher than that emitted, and if away, it is lower.

The ADCP current meter has multiple transducers that send and receive signals at different angles. Through the analysis of the Doppler shifts from different directions and at different depths, the device can calculate the three-dimensional velocity of the water. This means it can determine the horizontal and vertical components of the velocity, providing a detailed understanding of the current's structure and behavior.

5. What is needed for high-quality measurement of Okazaki coastal currents?

Materials in such equipment will help in measuring the coastal currents near Okazaki more qualitatively. The sea, with saltwater and probable exposure to storms and the influence of tides, calls for a casing of sturdy build; the characteristics of Ise Bay also demand this much. The size of such equipment should be small for deployment and retrieval, especially taking into consideration the complex topography of the coast featuring a bay and channels.

A lightweight design is preferable, as it makes the equipment more manageable during installation and maintenance. Low power consumption is crucial for long-term and autonomous operation. Cost-effectiveness is also an important factor to enable large-scale measurements.

The casing of ADCPs is preferably to be made of titanium alloy. Titanium alloy has very good corrosion resistance, important for withstanding the corrosive effects of seawater. It is strong, durable, and can support the pressure of water in 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 shall be made according to purpose when conducting current measurement around Okazaki. It is enough to select the ship-mounted ADCP when the measurement is used for vessel navigation or short-term research. During the voyage, it gives real-time current information.

For measurements near the seabed, as in the study of sediment transport or in benthic ecology, a bottom-mounted ADCP is preferred; it can clearly capture the details of the near-bottom current. Long-term and large-area monitoring of the coastal current requires a buoy-type ADCP as an ideal choice.

In terms of the frequency of ADCPs, for water depths up to 70m, a 600kHz ADCP is quite appropriate. For depths up to 110m, a 300kHz ADCP is more appropriate. For even deeper waters that might be found near Okazaki, such as areas with a depth over 1000m, a 75kHz ADCP is recommended.

There are well-known ADCP flow 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.