How can we measure the Coastal Currents of Satsumasendai?

1. Where is Satsumasendai?

Satsumasendai City is a city in Kagoshima Prefecture on Kyushu Island, Japan. It is a coastal city situated along the East China Sea. The city is basically characterized by its scenic coasts and nearby mountains.

The local economy is supported by a combination of fishing, agriculture, and small-scale manufacturing. Marine resources are abundant in the coastal waters. The East China Sea off Satsumasendai hosts a variety of fish such as tuna, mackerel, and sardines. Large areas of seaweed and beds of shellfish also exist. The warm Kuroshio Current has a big influence on the local climate and marine ecosystem, bringing in warm water and nutrients.

2. What is the condition of the coastal currents near Satsumasendai?

The coastal currents near Satsumasendai are influenced by many factors. First, there is the Kuroshio Current. This warm ocean current that moves northward carries warm and nutrient-rich water into the area, which impacts the local marine life and the overall current patterns.

Then there are tidal currents. Satsumasendai has a high tidal range, and the ebb and flow of the tides create complex current patterns. The shape of the coastline and the underwater topography are important determinants. Bays, headlands, and underwater ridges may cause the currents to change direction and speed. For example, the water may flow faster around headlands and be deflected into bays. Moreover, the wind patterns, particularly those of a seasonal nature, including the winter monsoons, may give rise to surface currents and influence the mixing of different layers of water.

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

Surface Drifting Buoy Method: A surface - drifting buoy can be released into the water to observe the surface current. The movement of the buoy is then followed for a certain period using GPS or other tracking systems. However, this method has some limitations. First, it gives information on the surface current only. Besides, it may be affected by wind and waves. For instance, when a strong wind blows the buoy sideways from the actual current, it can lead to incorrect measurements in the direction and speed of the current.

Anchor-Boat Method: The current could be measured by an anchored boat. Instruments to measure current that are attached to the boat go into the water at different depths. Although this permits more detailed depth-wise measurements, it also has disadvantages. The fixed position of the boat restricts the area of measurement and might not capture the full variability of the currents in a large coastal area.

Acoustic Doppler Current Profiler (ADCP) Method: The ADCP current meter method is more advanced in efficiency to measure the current along the coast. ADCPs emit acoustic signals into the water and measure the frequency shift due to the Doppler effect of the reflected signals to deduce the velocity of water at different depths. This produces a far more complete profile of the current from surface to 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 ADCPs using the principle of the Doppler work?

ADCPs work by the so-called Doppler effect: They send high-frequency acoustic pulses into the water, which changes their frequency back when they reflect from the moving water particles. This means that when the frequency of the reflected signal is higher than the frequency of the emitted one, the water particles are heading towards the ADCP profiler.

Several transducers are installed on an ADCP meter to transmit and receive signals through different angles. Thus, the device is able to obtain the three-dimensional velocity by analyzing the Doppler shift from various directions at every depth. This enables the horizontal and vertical components of the velocity to be gained, providing a close understanding of the current structure and its behavior.

5. What is necessary for good quality measurement of Satsumasendai coastal currents?

For high - quality measurement of the coastal currents near Satsumasendai, 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 essential. The equipment's size should be small for easy deployment and retrieval, especially considering the complex coastal landscape with its bays and beaches.

A lightweight design is preferred 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 important to be able to have large-scale measurements.

The casing of ADCPs is preferably made of titanium alloy. This alloy provides the required corrosion resistance, essential in fighting off the corrosive seawater, is very strong, and will easily survive under different depth pressure, but still lightweight to provide better comfort with several deployment techniques.

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

The purpose of use shall be considered first when choosing the equipment for current measurement at Satsumasendai. In cases where measurement is for the navigation of a vessel or short-term research, a ship-mounted ADCP flow meter is adequate. It provides real-time current information during the voyage.

If the measurements near the seabed are needed for such study as sediment transport or benthic ecology, the bottom-mounted ADCP current profiler shall be selected, and thus it may capture the detail of the near-bottom current. Long-term and wide-area monitoring of coastal current is allowed to take advantage of buoy-type ADCP.

As far as the frequency of ADCPs is concerned, for the water depths up to 70m, a 600kHz ADCP will suffice. For depths up to 110m, one would have to go for a 300kHz ADCP. And for deeper waters that might be found near Satsumasendai, such as areas having more than 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.