How can we measure the Coastal Currents of Hiroshima?

1. Where is Hiroshima?

Hiroshima is a city in the Chugoku region of Honshu, Japan. It is situated on the Seto Inland Sea, which is a semi-enclosed body of water. The city is quite famous for its historical importance, especially regarding the atomic bombing during World War II.

The local economy is varied, ranging from manufacturing industries, shipbuilding, to fishing. The Seto Inland Sea near Hiroshima is quite productive in marine life. Many species of fish such as sea bream, mackerel, and yellowtail abound. Large sheets of seaweed and beds of shellfish also exist.

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

Various factors influence the coastal currents near Hiroshima. One is tidal current. The Seto Inland Sea has a medium range of tide, and the inflow and outflow of water at high and low tides respectively give rise to current patterns. The shape of the coastline and underwater topography play an important role. Bays, channels, and shoals underwater may be considered responsible for the changes in direction and speed of the currents.

Thus, depending on freshwater inflow through rivers and other sources, it determines salinity and density differences along with the flow in this water mass. The pattern of the winds, particularly those blowing periodically over the Seto Inland Sea, further enables surface currents and interferes with the various layer circulations.

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

Surface Drifting Buoy Method: A surface-drifting buoy is released into the water to observe the surface current. The movement of the buoy is followed for a certain period of time using GPS or other tracking systems. However, this method has its drawbacks. It gives information about the surface current only and may be influenced by wind and waves. For instance, if a strong wind blew the buoy in another direction from the actual current, then it would result in incorrect measurements of the current's direction and speed.

Anchor-Boat Method: In this, the current can be measured by an anchored boat. Instruments are lowered from it in the water at depth to measure the current. Although this provides more detailed depth-wise measurements, it also has some disadvantages. 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: This is a more technologically advanced and efficient measurement of coastal currents. An ADCP current meter sends acoustic signals through the water and measures the Doppler shift of the backscattered signals to infer the velocity of water at various depths. It provides a rather detailed profile of the current right from the surface down to the seabed. Being less affected by surface disturbances compared to other current meters, ADCPs can operate continuously over long periods, thus yielding an accurate picture of the current structure.

4. How Do ADCPs Using the Principle of Doppler Work?

Basically, ADCPs depend on the principle of the Doppler effect. They emanate high-frequency acoustic pulses into the water. Interacting with the water particles in motion, changes in frequency characterize the reflected pulses. That is to say, if moving towards it, the reflecting signal would be of a higher frequency from the emitted and lower in the case it is going away.

The ADCP profiler has several transducers that send and receive signals at different angles. 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 calculates the horizontal and vertical components of the velocity, therefore giving a very detailed understanding of the structure and behavior of the current.

5. What is necessary for high-quality measurement of Hiroshima coastal currents?

The equipment should be made of reliable materials for high-quality measurement of the coastal currents around Hiroshima. Since it is a marine environment with saltwater, potential exposure to storms, and the influence of tides and the characteristics of the Seto Inland Sea, it has to be in a casing that is durable. The size of the equipment should be small to facilitate deployment and retrieval, considering the complex nature of the coastal landscape with its bays and channels.

A lightweight design is preferred, as it provides ease in managing the equipment during installation and maintenance. It should have low power consumption for long-term and autonomous operation. Cost-effectiveness is very important to enable large-scale measurements.

The casing of ADCPs is preferably made of titanium alloy. The usage of a titanium alloy is due to its excellent corrosion resistance, which is vital for the seawater's corrosive nature. It is also strong and durable, capable of bearing 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?

First, consider the purpose of use when choosing equipment for current measurement near Hiroshima. If the measurement is for the vessel's navigation or short-term research, a ship-mounted ADCP meter is suitable. It provides real-time current information during the voyage.

A bottom-mounted ADCP will be proper for the measurement near the seabed, such as in studying sediment transport or benthic ecology, while 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 flow meter will be ideal.

Regarding the frequency of ADCPs, for water depths up to 70m, the ADCP current profiler frequency of 600kHz will be suitable. In cases of up to 110m, a 300kHz ADCP will be appropriate. For deeper waters, which may be found from Hiroshima, a depth of over 1000m, a 75kHz is recommended.

There are well - known ADCP current 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.