How to Measure the Coastal Currents of Osaka?

1. Where is Osaka?

Osaka is a major city in the Kansai region of Honshu, Japan. It lies on the Osaka Bay, an inlet of the Seto Inland Sea. Osaka is a bustling economic and cultural center, known for its vibrant nightlife, historical architecture, and commercial activities.

The city's port is a significant part of its infrastructure, handling a wide range of goods such as electronics, machinery, and consumer products. The local economy is diverse, with sectors including manufacturing, trade, and services. The Osaka Bay area is also a popular destination for recreational activities like fishing and boating.

The Seto Inland Sea near Osaka is a semi-enclosed sea with abundant marine life. It includes a number of fish species like sea bream, mackerel, and yellowtail. In addition, seaweed and shellfish have large beds.

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

The coastal currents around Osaka are influenced by several factors. Tidal currents are a major factor. The Seto Inland Sea has a mean tidal range of moderate value, and the inflow and outflow of water at high and low tides, respectively, result in the creation of current patterns. The shape of the coastline and underwater topography play an important role. The bay, channels, and underwater shoals can cause the currents to change direction and speed.

The inflow of freshwater from rivers and other sources can affect the salinity and density of the coastal waters, which in turn influences the current flow. In addition, wind patterns, especially the seasonal winds that blow across the Seto Inland Sea, can drive surface currents and affect the mixing of different water layers.

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

Surface Drifting Buoy Method: A buoy that is surface-drifting can be sent into the water and may be used to observe the surface current. The position of the buoy is tracked with time using GPS or other tracking systems. However, this method has several disadvantages. It only gives information on the surface current and can be interfered with by wind and waves. For example, if a strong wind blew the buoy in a different direction from the actual current, this would result in incorrect measurements of the direction and speed of the current.

Anchor - Boat Method: The current can be measured by 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: 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. This provides a more comprehensive profile of the current from the surface to the 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 the ADCPs using the Doppler principle work?

ADCPs are based on the Doppler effect. They emit high-frequency 'acoustic' pulses into the water. When these pulses hit moving water particles, the frequency of the pulses reflected back changes. If the water particles are moving towards the ADCP current meter, then the frequency of the reflected signal is higher than the emitted one; if moving away, it is lower.

In ADCP flow meter, there are multiple transducers that send and receive the signal at different angles. This equipment then calculates the three-dimensional velocity by analyzing the Doppler shift in different directions and depth, which gives the horizontal and vertical components of velocity and gives a detailed understanding of the current structure and behavior.

5. What's required for high-quality measurement of Osaka coastal currents?

The equipment should be made of appropriate, reliable materials for high-quality measurement of the coastal currents around Osaka. This is a marine environment characterized by saltwater, potential exposure to storms, and the influences of tides and specific characteristics of the Seto Inland Sea; it must have a durable casing. Equipment size should be small, making deployment and retrieval less difficult, especially considering the nature of the coastline with all its bays and channels.

The design should be lightweight, which will make the equipment easy to handle during installation and maintenance. Low power consumption is important for long-term autonomous operation. It should be cost-effective to make large-scale measurements possible.

The casing of ADCPs should be made of titanium alloy. Titanium alloy has very good corrosion resistance, which is highly needed to cope with the corrosive action of seawater. It is strong and durable to bear the pressure of water at different depths. Its lightweight nature makes it more feasible for various deployment methods.

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

First, when choosing equipment for current measurement near Osaka, the purpose of use has to be taken into consideration. If the measurement is for a vessel's navigation or short-term research, a ship-mounted ADCP would be suitable. 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.

In regards to the frequency of ADCPs, for water depths up to 70m, a 600kHz ADCP is appropriate. For up to 110m, it would be more appropriate to use a 300kHz ADCP. And for deeper waters that might be found near Osaka, such as over 1000m, 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.