How to Measure the Coastal Currents of Tokyo?

1. Where is Tokyo?

It is located on the eastern coast of Honshu, the largest island of Japan. It is a huge metropolis situated along Tokyo Bay, an inlet of the Pacific Ocean. The city is one of the world's major economic and cultural centers.

Tokyo Bay is a semi-enclosed body of water. The bay is a vital part of the city's infrastructure, serving as a major port for shipping and trade. The local economy related to the bay includes fishing, shipping, and various marine-based industries. The bay is home to a variety of marine life such as fish-the sea bream and mackerel-shellfish, and seaweed.

2. What is the situation of the coastal currents close to Tokyo?

There are a variety of factors that could influence coastal currents near Tokyo, including tidal currents. The tidal range in Tokyo Bay is moderate. The action of ebb and flood tides brings in currents, developing characteristic current patterns in this area. The general outline of the coast and undersea terrain comes into play as well. In this aspect, a bay, various channels, and underwater banks can significantly bend the windward flow and alter velocity.

Freshwater input from rivers, like the Sumida River, and other sources may change the salinity and density of coastal waters, which influences the current flow. Moreover, wind patterns, especially the seasonal winds that blow across Tokyo Bay and the Pacific Ocean, may force surface currents and determine the degree of mixing of different water layers.

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

Surface Drifting Buoy Method

In order to observe the surface current, a surface-drifting buoy can be cast into the water. It can be tracked over certain time via GPS or other tracking systems. This is also flawed. It gives information only about the surface current and can be affected by wind and waves. For instance, if a strong wind blows the buoy in some other direction than that of the current, then it may give erroneous readings with respect to the direction and speed of the current.

Anchor-Boat Method

It can also be measured using an anchored boat. Instruments, which are to be lowered in the water to measure the current at different depths, are temporarily taken from the boat. While this allows a greater detail in depthwise measurement, there are also disadvantages. For one thing, the restricted area of measurement is controlled by a fixed position of the boat. Large coastal areas may not be represented well with regard to the currents' variability.

Acoustic Doppler Current Profiler Method

The ADCP current profiler method is a more advanced and efficient way of making the measurements of the coastal currents. ADCPs emit an acoustic signal into the water and measure the Doppler shift of the reflected signals in calculating the velocity of the water at different depths. This provides a fuller 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 Doppler principle work?

ADCPs work on the principle of the Doppler effect. They transmit high-frequency acoustic pulses into the water. When these pulses encounter a moving water particle, the frequency of the reflected pulses changes. In the case of moving towards the ADCP meter, the frequency of the reflected signal is higher than that which was emitted, and if away, it is lower.

The ADCP current profiler has several transducers, which send and receive signals at different angles. By analyzing the Doppler shifts from different directions and at different depths, the device is able to calculate the three-dimensional velocity of the water. In other words, it can determine the horizontal and vertical components of the velocity, therefore providing a good understanding of the current's structure and behavior.

5. What needs to be done to measure Tokyo coastal currents with high resolution?

The equipment should be made of reliable materials for high-quality measurement of the coastal currents around Tokyo. A durable casing is required, since it will be in a marine environment with saltwater, possible exposure to storms, and the influence of tides and characteristics of Tokyo Bay. The size of the equipment should be small for easy deployment and retrieval, especially in a complex coastal landscape with a bay and channels.

A lightweight design is preferred to make the equipment more manageable for installation and maintenance. Low power consumption is necessary for long-term and autonomous operations. Cost-effectiveness would be important to realize measurements on a large scale.

The casing of ADCPs is preferably made of titanium alloy. It has excellent corrosion resistance, which is quite necessary against the corrosive seawater, strong, and durable enough to stand the pressure of water at different depths. Being lightweight in nature, it is more feasible for several deployment methods.

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

When choosing equipment for current measurement near Tokyo, consider the purpose of use first. In the case of navigation by a vessel or short-term research, a ship-mounted ADCP will suffice. 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.

For the frequency of ADCPs, for water depths up to 70m, a 600kHz ADCP may be a good choice; for up to 110m, a 300kHz ADCP may be more appropriate, and for deeper waters that might be found near Tokyo, a 75kHz ADCP could be appropriate in areas with more than a 1000m depth.

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.