How can We Measure the Coastal Currents of Yokohama?

1. Where is Yokohama?

Yokohama is a major port city in Kanagawa Prefecture, on the southern coast of Honshu, Japan. It lies on Tokyo Bay, which is an inlet of the Pacific Ocean. Yokohama is known for its modern architecture, bustling ports, and vibrant international atmosphere.

Its harbor is one of the biggest in Japan and thus is very important for trade and shipping in the country. The local economy is multifunctional and involves industries like shipping, manufacturing, trade, and tourism. The coastal area of Yokohama is a favorite place for leisure, such as walking along the seafront and enjoying the sea view.

Tokyo Bay near Yokohama is a semi-enclosed body of water teeming with life. It is home to different types of fish, like sea bream, mackerel, and yellowtail. There are also considerable areas of seaweed and beds of shellfish.

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

Many factors contribute to the coastal currents around Yokohama. One is tidal currents. Tokyo Bay has a medium tidal range, and due to this, the inflow and outflow of the tides create current patterns. The shape of the coastline and underwater topography play an important role. The bay, channels, and underwater shoals can cause changes in direction and speed in the currents.

Inflow of fresh water from rivers and other sources could affect the salinity and density of the coastal waters, which then affects the flow of currents. Besides, wind patterns, especially seasonal winds that blew across Tokyo Bay and the Pacific Ocean, may drive surface currents and affect the mixing of different layers of water.

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

Surface Drifting Buoy Method: A surface-drifting buoy is generally dropped into the water to visualize the surface current. The movement of the buoy is monitored over time through GPS or other tracking systems. However, it has several disadvantages: it gives only the surface current, and it is also prone to wind and wave effects. For instance, if the buoy is blown by a strong wind in a different direction than the actual current, this can cause incorrect measurements of the direction and speed of the current.

Anchor-Boat Method: An anchored boat can be used for current measurement. Instruments are lowered into the water from the boat at various depths to measure the current. While this method is bound to give more detailed depth-wise measurements, it also has disadvantages. The fixed position of the boat restricts the area of measurement, and it might not capture the full variability of the currents in a large coastal area.

ADCP Method: This is a more advanced and efficient method of measurement of 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. It therefore gives a better profile of the current right from the surface down to the seabed. The ADCP current meter is less sensitive to surface disturbances and can be run over long periods without interruption, creating a truer picture of current structure.

4. How does an ADCP work with the principles of Doppler?

The operation of ADCPs is based on the Doppler effect. They send high-frequency acoustic pulses into the water. Whenever these pulses encounter and interact with moving water particles, their reflected frequency changes. If the movement of the water particles is in the direction towards the ADCP meter, then the frequency of the reflected signal will be higher than that of the emitted one; vice versa, if they move in a direction opposite to it.

The ADCP current profiler has several transducers that send and receive signals at different angles. The device can calculate the three-dimensional velocity of the water by analyzing the Doppler shifts from different directions and at different depths. 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 required to obtain high-quality measurements of Yokohama coastal currents?

Equipment intended for the measurement of the currents off the coast of Yokohama should be made with quality and dependability in their materials. Besides the saltwater and storm conditions, the general influences of tides and Tokyo Bay characteristics require a hard-case package. The size should also be compact for convenience during launching and recovering, especially in light of the bay and channels nature of the coastline.

A lightweight design is preferred since it makes the equipment more manageable during installation and maintenance. Low power consumption is important for long-term and autonomous operation. Cost-effectiveness also plays a major role for enabling large-scale measurements.

The casing of ADCPs is preferably made of titanium alloy. It has excellent corrosion resistance, which is vital in withstanding the corrosive effects of seawater. It is strong and durable to resist 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 purpose of use determines the choice of equipment for measuring current near Yokohama. For the navigation of a vessel or short-term research, a ship-mounted ADCP is appropriate. It provides detailed real-time current information while the voyage is in progress.

For measurements near the seabed, such as in the study of sediment transport or benthic ecology, a bottom-mounted ADCP is preferred. It captures 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 should be used. For up to 110m, one should use a 300kHz ADCP. In deeper waters, such as those that may be found near Yokohama, like in areas with more than 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.