How can we measure the Coastal Currents of Himeji?

1. Where is Himeji?

Himeji is a city in Hyogo Prefecture, the main island of Honshu, Japan. In fact, it is not a seafront city, but lies pretty near to the Seto Inland Sea. It is known widely for its Himeji Caslte, one of the Unesco World Heritage Sites.

The Seto Inland Sea off Himeji is a partially enclosed sea. It is an essential water route for shipping and at the same time an important fishing area. The local economy linked with the sea covers industries related to fishing and shipping. Sea bream, mackerel, shellfish, and a great deal of seaweed can be found in the waters here.

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

Various factors influence the coastal currents in the vicinity of Himeji, Tidal currents are an essential factor. The average tide in the Seto Inland Sea is at medium range, and due to that, the ebbs and flows of the tides create current patterns. Generally speaking, the shape of a coastline and underwater topography influence the characteristics of these kinds of currents. Bays and channels, along with bottom and underwater shoals, deflect the currents in various ways in both direction and speed.

Some amounts of freshwater inflow from the rivers and other sources greatly impact changes in salinity and density that further affect the current flow in the coastal waters. Secondly, the wind patterns-a major one being the seasonal winds that blow across the Seto Inland Sea-drive the surface currents that have a mixing consequence for different layers of water.

3. How to Observe the Himeji Coastal Water Flow?

Surface Drifting Buoy Method: This involves releasing a surface-drifting buoy into the water, which is then observed with regard to the surface current. The buoy's travel is tracked over time through GPS or other tracking systems. However, this method is limited in some ways because it can only provide information about the surface current, and it is affected by wind and waves. For instance, 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: An anchored boat can be used in measuring current. Instruments deployed from it are lowered to the water for the current measurement at various depths. While this method has its benefits, including giving more specific depth - wise measurements, it is not without its issues. Its major drawback relates to its fixed position, further limiting the area of its measurement; hence, it may miss the total variation in large coastal areas.

Acoustic Doppler Current Profiler (ADCP) Method: The ADCP current meter method is one of the advanced and proficient methods 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 gives full profiling of the current right from the surface to the seabed. ADGPSs are less affected by surface disturbances and can work for a long period of time uninterruptedly, which makes their picture of the current structure more accurate.

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. These pulses are shifted in frequency when they reflect off the moving water particles. If the water particles are moving toward the ADCP profiler, the frequency of the reflected signal is higher than the emitted one; if they are moving away, it is lower.

Each ADCP meter possesses a number of transducers capable of sending and receiving multiple signals at various angles from one another. With that, the device will thus be able to compute, through the different Doppler shifts from different directions and various depths, the three-dimensional velocity of the water; that is, the determination of horizontal and vertical components of the velocity will be more in-depth to provide adequate knowledge of the current structure and behavior.

5. What is necessary to achieve high-quality measurement of Himeji coastal currents?

For high - quality measurement of the coastal currents near Himeji, the equipment should have reliable materials. Given the marine environment, with saltwater, potential exposure to storms, and the influence of tides and the Seto Inland Sea's characteristics, 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 channels.

A lightweight design is preferable because 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 made of titanium alloy. The resistance of the alloy to corrosion is excellent, hence it is very crucial to resist the corrosive nature of seawater. In addition, it is solid and durable enough to hold under water pressure at variable depth. Its lightweight makes the deployment easier with various deployment options.

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

The purpose of use should be considered first in choosing the equipment for the current measurement at Himeji. If the measurement is for navigation of the vessel or short-term research, a ship-mounted ADCP flow meter is suitable. It gives 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. The bottom-mounted ADCP may capture the details of the near-bottom current. If it is necessary to conduct the long-term and wide-area monitoring of the coastal current, a buoy-type ADCP current profiler should be used.

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 Himeji, such as areas with a depth exceeding 1000m, a 75kHz ADCP 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.