How to Measure the Coastal Currents of Tottori?

1. Where is Tottori?

Tottori is a prefecture on the Sea of Japan side of Honshu Island in Japan. The city of Tottori is its capital, and it is a coastal area. It's known for its beautiful sand dunes that go along the coast, which are a major tourist attraction.

The local economy is diverse with fishing being one of the vital aspects. The Sea of Japan off Tottori presents rich marine resources. Its bottom has a variety of fishes, including flounder, squid, and all kinds of shellfish. Extensive seaweed beds can be found in this sea for the support of the local fishing industry and the overall marine ecosystem in general.

2. What is the condition of the coastal currents near Tottori?

The coastal currents around Tottori are caused by several factors. First, there is the Tsushima Current, which is one of the major contributors. It is a warm current that branches from the Kuroshio Current and influences the temperature and nutrient distribution in the coastal waters. Its influence on the local marine life and the general pattern of currents is quite significant.

Tidal currents are another important contributor. The Sea of Japan has a wide tidal range, and the tides bring about the development of intricate current patterns. The shape of the coastline and the underwater topography play an important role. Bays, headlands, and underwater ridges may cause changes in the direction and speed of currents. For instance, the water may flow faster around headlands and be deflected into bays. Furthermore, wind, especially seasonal winds over the Sea of Japan, can promote the surface current and mix various layers of water.

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

Surface Drifting Buoy Method

The surface current can be viewed by deploying a buoy into the water. Over some time, the movement of the buoy is traced by means of a GPS or any other tracking systems. However, this technique has its drawbacks. It gives information only on the surface current and is affected by wind and waves. For instance, if a strong wind blows the buoy in a different direction from the actual current, it can result in wrong measurements of the direction and speed of the current.

Anchor-Boat Method

The current can be measured using 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.

ADCP Method

The Acoustic Doppler Current Profiler (ADCP) method is more advanced and efficient to measure 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 gives a better profile of the current from surface to seabed. ADCPs are less affected by surface disturbances and can operate continuously for long periods, giving a more accurate picture of the current structure.

4. How do ADCPs using the principle of Doppler work?

The operation of ADCPs is based on the principle of the Doppler effect. They transmit high-frequency acoustic pulses into the water. When such pulses encounter mobile water particles, they change their frequency. The frequency of the reflected signal is higher than the one emitted when the water particles are moving towards the ADCP flow meter, and vice versa.

Each ADCP current meter uses multiple transducers that transmit and receive signals at various angles from each other. Through the process of analyzing Doppler shifts from different directions and at various depths, this device calculates the three-dimensional velocity of the water-that is, horizontal and vertical components of the velocity, and thus a good understanding of the structure and behavior of the current.

5. What is needed for high-quality measurement of the currents around the Tottori coasts?

For high-quality measurement of the coastal currents near Tottori, the equipment should be made from reliable materials. With a marine environment that includes saltwater, possible exposure to storms, and the influence of tides and ocean currents, it needs to be casings that are durable. The size of the equipment should be small for easy deployment and retrieval, especially in such a complex coastal landscape with bays and headlands.

The lightweight design is preferred because it will make the equipment more manageable during installation and maintenance. Low power consumption is important for long-term and autonomous operation. Cost-effectiveness is another important factor to realize large-scale measurements.

The casing of ADCPs is preferably made of titanium alloy. Titanium alloy has excellent corrosion resistance, which is vital for withstanding the corrosive effects of seawater. 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?

The purpose of use should be considered first when choosing equipment for current measurement near Tottori. In the case of measurement for the navigation of a vessel or short-term research, a ship-mounted ADCP is adequate. It provides 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 quite suitable. In the case of depths up to 110m, a 300kHz ADCP will be more appropriate. And for deeper waters that might be found near Tottori, say areas with over 1000m of depth, a 75kHz ADCP is recommended.

There are well-known ADCP profiler 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.