How to Measure the Coastal Currents of Takamatsu?

1. Where is Takamatsu?

Takamatsu is a city on the island of Shikoku in Japan. It fronts the Seto Inland Sea, which is a semi-enclosed body of water. The city is famous for its beautiful coastal scenery, historical castles, and gardens.

Fishing, tourism, and light manufacturing are all combined to support the local economy. The bottom of the Seto Inland Sea near Takamatsu is very replete with a rich marine environment. It holds various species of fish: sea bream, mackerel, sardines, together with large beds of seaweed and shellfish.

2. What is the condition of the coastal currents around Takamatsu?

In the coastal currents around Takamatsu, a number of factors take part. First, there is a great influence from tidal currents. In the Seto Inland Sea, the tidal range is at its mean value, while the rise and fall of tides result in the pattern of current flow. Next comes the topography of the coastline and that of the ocean bottom. An area of bays and channels may shift the directions and velocities of currents.

Freshwater inflow from rivers and other sources also alters the salinity and density of the coastal waters, which can impact current flow. Moreover, the wind conditions, especially seasonal winds blowing over the Seto Inland Sea, drive surface currents that might affect mixing among the different water layers.

3. How to Observe the Flow of Takamatsu Coastal Waters?

Surface Drifting Buoy Method: The surface-drifting buoy may be launched into the water and then observe the surface current. Its movement is followed for some time by using GPS or other tracking systems. This method has its drawbacks, however. It gives information about the surface current alone and is vulnerable to wind and wave actions. For instance, if a strong wind blows the buoy in a different direction from the actual current, it can lead to incorrect measurements of the direction and speed of the current.

Anchor - Boat Method: A boat can be anchored for measuring the current. Various instruments are lowered from the boat in the water to measure the current for various depths. Although with this, more detailed depthwise measurements can be done, it has disadvantages. The fixed boat will restrict the area of measurement, and it may not capture all the variability of the currents of a large coastal area.

Acoustic Doppler Current Profiler (ADCP) Method: The Acoustic Doppler Current Profiler (ADCP) method is more advanced and much more efficient in measuring 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 gives a better profile of the current from the surface to the seabed. Being less affected by surface disturbances, ADCPs can provide a more accurate picture of the current structure while operating continuously over long periods.

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. As these pulses encounter and interact with moving water particles, the frequency of the reflected pulses is changed. If the water particles are moving towards the ADCP meter, then the frequency of the reflected signal is higher than the emitted one; if they are moving away, it is lower.

Different angle, multitransducer, signal transmission and reception take place in an ADCP flow meter. These signals, through analysis for their Doppler shifts emanating from different directions and different levels of depth, compute the three-dimensional water velocity. That is, horizontal and vertical components of the velocity provide complete insight into the structure and behavior of the current.

5. What is required in the measurement of high-quality Takamatsu coastal currents?

The equipment should be made of such materials that can ensure high-quality measurement of the coastal currents near Takamatsu. The equipment has to be deployed in a marine environment with saltwater, possible storm exposure, and the influence of tides and characteristics of the Seto Inland Sea; thus, durability in casing is required. The size of the equipment needs to be small for easy deployment and retrieval, especially at the complex coasts and their bays and channels.

A lightweight design is preferable as it makes the equipment more manageable during installation and maintenance. Low power consumption is crucial for long-term and autonomous operation. Cost-effectiveness is also important to enable large-scale measurements.

The casing of ADCPs is preferably to be made of Titanium Alloy. This is because titanium alloy has very good resistance to corrosion, which is highly important as seawater is corrosive, it can withstand pressure at various depths in the water and is lightweight for ease in multiple deployment methods.

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

Equipment for the measurement of currents around Takamatsu should first be selected considering the purpose. For vessels in navigation, or when the research involved is short-term, a ship-mounted ADCP current meter is suitable to provide real-time current information during the voyage.

According to the purpose of measurement near the seabed, like for sediment transport or benthic ecology, a bottom-mounted ADCP flow meter is preferable because it can capture the details of the near-bottom current. If long-term and wide-area monitoring of the coastal current is needed, a buoy-type ADCP is ideal.

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

There are well - known ADCP 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.