How to Measure the Coastal Currents of Shizuoka?

1. Where is Shizuoka?

Shizuoka is a prefecture on the central coast of Honshu, Japan. It is a coastal area that faces the Pacific Ocean. The city of Shizuoka is known for its beautiful beaches, Mount Fuji views, and tea plantations.

The local economy is based on a combination of fishing, agriculture, and tourism. Marine resources are abundant in the Pacific Ocean off Shizuoka. All manner of fish can be found here, including tuna, bonito, and mackerel. There are also great areas of seaweed and shellfish. The warm Kuroshio Current has a major effect on the local climate and marine environment, bringing warm water and nutrient-rich substances.

2. What is the state of the coastal currents off Shizuoka?

The coastal currents around Shizuoka are caused by various factors. For one, the Kuroshio Current is the dominating one. This warm ocean current flowing northward brings in warm and nutrient-rich water that affects the local marine life and the general patterns of currents.

Tidal currents also play a very important role. In Shizuoka, a fair degree of tidal range gives rise to complex current patterns that are induced by the tides' ebb and flow. The shape of the coastline and the underwater topography are important determinants. The presence of bays, headlands, and underwater ridges may make the currents change direction and speed. For example, the water may flow faster around headlands and be deflected into bays. In addition, the wind systems, especially the seasonal wind like the trade winds, may force the surface current and affect the mixing of different layers of waters.

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

Surface Drifting Buoy Method: In order to observe the surface current, a surface-drifting buoy can be thrown into the water. By GPS or other tracking devices, the movement of the buoy is traced for some time. However, the methodology also has its shortcomings. First, it provides information only on the surface current, which is vulnerable to the effects of wind and waves. For instance, a strong wind that might blow the buoy in a different direction from that of the current could make for faulty measurement of direction and speed of flow.

Anchor-Boat Method: This approach involves the measurement of current with the use of an anchored boat. Instruments measuring current are lowered from a boat into the water at different depths. While it allows more detailed depth-wise measurement, it also has certain disadvantages. The fixed position of a boat restricts the area of measurement, and the variations in a large coastal area cannot be represented by such a system.

ADCP Method: The ADCP current meter method follows as a more advanced and quick means of measuring the coastal current. ADCPs emit an acoustic signal into the water and measure the Doppler shift of the reflected signal to calculate the velocity of the water at different depths, hence giving a better profile from surface to seabed of the current. Being less affected by surface disturbances, ADCPs can operate continuously for long periods, giving a more accurate picture of the current structure.

4. How do ADCPs using the Doppler principle work?

ADCPs work on the principle of the Doppler effect. They send high-frequency acoustic pulses into the water. When these pulses meet the moving water particles, the frequency of the reflected pulses changes. If the water particles are moving towards the ADCP meter, the reflected signal frequency is higher than the emitted one; when it is moving away, it is lower.

The ADCP current profiler has more than one transducer, which sends out and receives signals at different angles. Since the device does an analysis of the various Doppler shifts from other directions at different depths, it determines the three-dimensional velocities of water. This signifies that the horizontal and vertical components of the velocity can be determined by the device with great elaboration of its structure and behavior.

5. What is required for high quality measurements of Shizuoka coastal currents?

For high-quality measurement of the coastal currents near Shizuoka, the equipment should have reliable materials. Considering that it will be exposed to saltwater, with potential exposure to storms and the influence of tides and ocean currents, a durable casing is important. The size of the equipment should be small for easy deployment and retrieval, considering the complex landscape of coasts, bays, and beaches.

Preference should be given to lightweight design to make it manageable in the installations and their further maintenance. The low consumption of power should be meant for long and autonomous operations. Cost-effectiveness can be added for large-scale measurement.

The casing of ADCPs is preferably made of titanium alloy. This is because titanium alloy has excellent corrosion resistance, which is very important for withstanding the corrosive effects of seawater. It is also strong and durable to withstand the pressure of the water at different depths. Being lightweight makes it more feasible for various deployment methods.

6. How to Choose the right equipment for current measurement?

First, when selecting equipment for current measurement near Shizuoka, consider the purpose of use. In cases where the measurement is for the navigation of a vessel or for short-term research, a ship-mounted ADCP will suffice. It provides real-time current information during the voyage.

If measurements near the seabed, such as in the case of studying sediment transport or benthic ecology, are to be taken, then a bottom-mounted ADCP is preferred since it will capture the details of the near-bottom current. If one wants to conduct long-term and wide-area monitoring of the coastal current, then the buoy-type ADCP would be ideal.

Regarding the frequency of ADCPs, for water depths up to 70m, a 600kHz ADCP is recommended. For up to 110m, a 300kHz ADCP is suitable. And for deeper waters that might be found near Shizuoka, such as areas with a depth over 1000m, a 75kHz ADCP is suitable.

There are well-known ADCP current 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.