How can we measure the Coastal Currents of Kurume?

1. Where is Kurume?

Kurume is a city located in Fukuoka Prefecture, Japan. It is not a coastal city in the usual sense but is relatively close to the coast. It lies inland from the Ariake Sea, which is a large shallow bay on the western side of Kyushu Island.

It is a city of ancient and cultural importance, comprises many temples, shrines, and traditional architecture, and has a diversified economy based on manufacturing, agriculture, and service-based industries.

The Ariake Sea, lying near Kurume's vicinity, is a peculiar marine environment. It is a semi-enclosed body of water with remarkable changes in tide. The sea is comparatively shallow and hosts a wide range of marine life, including fish like eel, flounder, and shellfish. The extensive tidal flats are an important ecological feature, providing a habitat for migratory birds and a rich feeding ground for many organisms.

2. What is the status quo of coastal currents around Kurume?

The coastal current around Kurume (the Ariake Sea) includes various components. Tidal currents dominate in this particular area. The Ariake Sea belongs to one with a large tidal range in Japan, and thus strong and complex flow fields occur due to ebbs and flows.

The shape of the coastline and underwater topography also contributes. The features of tidal flats, channels, and shoals may change the directions and speeds of the currents. Inflow from rivers and other freshwater sources would change the salinity and density in the coastal waters, while the current flow depends directly on it. Additionally, the wind patterns, especially the seasonal winds that blow across the Ariake Sea, can drive surface currents and affect the mixing of different water layers.

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

Surface Drifting Buoy Method: A surface - drifting buoy can be released into the water near the coast to observe the surface current. The buoy's movement is tracked over time using GPS or other tracking systems. However, this method has limitations. It only provides information about the surface current and can be affected by wind and waves. For example, if a strong wind blows the buoy in a different direction from the actual current, it can lead to inaccurate measurements of the current's direction and speed.

Anchor - Boat Method: The current can also be measured by an anchored boat. Instruments are lowered into the water from the boat for measuring the current at different depths. While this allows more detailed depth - wise measurements, there are also disadvantages. Because the boat is in a fixed position, only a small area of measurement is covered, and it might not be representative of the total variability of currents over a large coastal area.

Acoustic Doppler Current Profiler (ADCP) Method: The ADCP method is an advanced and efficient way of measurement for 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 more comprehensive profile of the current from the surface to the seabed. Being less affected by surface disturbances, ADCPs can record continuously over quite long periods and yield far more representative current structure information.

4. How do ADCPs using the Doppler Principle Work?

The working of ADCPs is based on the Doppler effect. They emit high-frequency acoustic pulses into the water. Due to the interaction with moving water particles, the frequency of the reflected pulses changes. If the water particles are moving towards the ADCP, the frequency of the reflected signal is higher than the emitted signal, and if they are moving away, it is lower.

The ADCP has multiple transducers that send and receive signals at different angles. By analyzing the Doppler shifts from different directions and at different depths, the device can calculate the three-dimensional velocity of the water. 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 needed for high-quality measurement of Kurume coastal currents?

It is desirable to have devices that are of reliable materials so that measurement of high quality can be done. As the device will stand in a marine environment-saltwater, with possible exposition for storms, tides, and tidal flats-the need for durable casing is substantial. The equipment should not be big in size as its deployment and retrieval becomes quite easy, especially the nature of the coastal system which is a complicated landscape of tideland flats and channels.

The lightweight design makes the equipment easier to manage during installation and maintenance, which is very desirable. Low power consumption is indispensable for long-and autonomous operation. Cost-effectiveness, finally, will allow the measurements to be made on a large scale.

The casing of ADCPs is preferably made of titanium alloy. The use of titanium alloy has excellent corrosion resistance, which is very important to withstand the corrosive action of seawater. It is also strong and durable to bear 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?

For choosing the equipment for current measurement near Kurume, consider first what the purpose of the measurement is. For navigation purposes in a vessel or short - term research, a ship-mounted ADCP is suitable. The data it provides on the currents will be in real time during the voyage.

Bottom-mounted ADCP is preferred when measurements are to be made near the seabed, for instance, in the study of sediment transport or benthic ecology. It will be able to capture the details of the near-bottom current. In case long-term and wide-area monitoring of coastal currents is required, then a buoy-type ADCP would be ideal.

For the frequency of ADCPs, in water with depths up to 70m, a 600kHz ADCP will suffice. In the case of a depth up to 110m, a 300kHz ADCP would be appropriate. And for deeper waters, such as areas with more than 1000m depth that might be found near Kurume, 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.