How to measure Sendai Coastal Currents?

1. Where is Sendai?

Sendai is a major city in Miyagi Prefecture, eastern coast of Honshu Island, Japan, lying on the Pacific Ocean. The city is known for its historic sites such as Sendai Castle and is a significant economic and cultural center in the Tohoku region.

The economy of this city is very diverse, including industries such as manufacturing, trade, and services. The Pacific Ocean off Sendai is rich in marine resources. Many types of fish inhabit its waters, including tuna, bonito, and mackerel. In addition, there are extensive seaweed and shellfish beds. The area is under the influence of the Kuroshio Current, which carries warm water with rich nutrients, further increasing the productivity of the marine ecosystem.

2. What is the condition of the coastal currents off Sendai?

Many elements have been influencing the coastal currents around Sendai. This includes the Kuroshio Current. A major warm, north-flowing ocean current, it brings in water which is both warm and nutrient-rich. These further influence the temperature of the waters around the coasts, the marine life distribution, and current patterns in general.

On a much localized level, tidal currents also exist. Sendai has a fair tidal range, and with that the ebbs and flows of tides create complex current patterns. Important here are the coastline and the submarine morphology. Bays, headlands, and submarine ridges could make the currents change in both direction and speed. Wind patterns, especially seasonal winds such as the trade winds, may drive surface currents and mix different layers of water.

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

Surface Drifting Buoy Method: To observe the surface current, a surface-drifting buoy can be released into the water. The movement of the buoy is tracked by GPS or any other tracking system over some time. However, it is limited; it only gives information about the surface current, and it can be subjected to wind and waves. An example is if a strong wind blew the buoy in a different direction opposite to the current; this results in incorrect measures of the current direction and speed.

Anchor -Boat Method: Current can be measured with the use of an anchored boat. Instruments used for measuring are lowered into the water from the anchored boat to measure the current at different depths. While this allows for more depth-wise measurement details, it also introduces some drawbacks. The fixed position of a boat restricts the area of measurement that might not capture the entire variability of currents in the large coastal area.

The Acoustic Doppler Current Profiler (ADCP) Method: It is the more advanced and efficient method for the measurement of the coastal currents. The ADCPs emit the acoustic signal into the water and, based on the Doppler shift of the reflected signals, calculate the velocity of water at different depths, giving an advanced profile of the current from the surface to the seabed. This would make them less susceptible to surface disturbances and able to operate continuously over long periods, giving a more accurate picture of the current structure.

4. How does an ADCP utilizing 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 the moving water particles, the frequency of the reflected pulses is different. If the water particles are moving towards the ADCP current profiler , then the frequency of the reflected signal is higher than the emitted signal, and if they are moving away, it is lower.

Various transducers, attached to the ADCP flow meter, transmit and receive signals at different angles. From the different directions and different depth analysis of Doppler shifts, this device is able to calculate three-dimensional water velocity, that is, horizontal and vertical components of velocity. This provides a three-dimensional understanding of the structure and behavior of the current.

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

For high-quality measurement of the coastal currents near Sendai, the equipment should have reliable materials. Given the marine environment with saltwater, potential exposure to storms, and the influence of tides and ocean currents, a durable casing is called for. The size of the equipment should be small in order to enhance its deployment and retrieval with ease, given the complex nature of the coastal landscape characterized by bays and beaches.

A lightweight design is preferred since it makes the equipment more manageable during installation and maintenance. Low power consumption is essential for long-term and autonomous operation. Cost-effectiveness is also very important to realize large-scale measurements.

The casing of ADCPs is preferably made of titanium alloy. Titanium alloy has excellent corrosion resistance, which is very important for withstanding the corrosive effects of seawater. Besides, it is strong and durable to bear the pressure of 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?

In choosing the equipment for current measurement near Sendai, first consider the purpose of its use. If it is to be used for a vessel's navigation or short-term research, a ship-mounted ADCP would suffice. It provides real-time current information during the voyage.

A bottom-mounted ADCP is preferable for measurements near the seabed, such as for studying sediment transport or benthic ecology. The bottom-mounted ADCP 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.

With regards to the frequency of ADCPs, water up to 70m would need a good choice at a frequency of 600kHz. For depths from up to 110m, a good appropriate model is the 300kHz, and for deeper waters possibly reaching or found near Sendai in waters in excess of 1000 meters depth, the 75kHz works well.

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.