How can we measure the Coastal Currents of Otaru?

Explore Otaru's location, coastal currents, and measurement methods. Understand how ADCPs function, their requirements for accurate measurement, and how to select the right equipment. Discover ADCP's significance in studying Otaru's coastal currents.

1. Where is Otaru?

Otaru is a city in Hokkaido, Japan. It is situated on the western coast of Hokkaido, facing the Sea of Japan. The city is famous for its well-preserved historical canal area, beautiful glassware, and its importance as a former major port.

Traditionally, the local economy was based on fishing and trade. The Sea of Japan off Otaru is rich in marine resources. Many types of fish, such as herring, cod, and squid, are found here. There are also large seaweed beds.

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

A few factors influence the coastal currents in the vicinity of Otaru. First, the influence of the Tsushima Current predominates. This warm branch of the Kuroshio Current brings warm water, nutrients, and affects temperatures as well as the distribution of marine life.

Tidal current comes second. The Sea of Japan features a substantial tidal range. As a result of that, the tides change somewhat with the ebbs and create quite complicated current patterns. Each coastline shape and underwater topography play an important role in currents. Bays, headlands, and underwater ridges can make the currents flow in different directions with some speed. Besides, the wind patterns, especially the seasonal winds across the Sea of Japan, can drive surface currents and mix different layers of water.

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

Surface Drifting Buoy Method: A surface - drifting buoy can be released into the water to observe the surface current. The buoy's movement is tracked over time using GPS or other tracking systems. However, the approach has its limitations, it only gives information on the surface current and can easily be affected by wind and waves. For instance, when strong winds blow the buoy in another direction from the actual current, that can lead to the mismeasurement of the direction and speed of the current.

Anchor - Boat Method: An anchored boat can be used to measure the current. Instruments are lowered from the boat into the water to measure the current at different depths. While this allows for more detailed depth - wise measurements, it also has drawbacks. The boat's fixed position restricts the area of measurement, and it may not capture the full variability of the currents in a large coastal area.

ADCP Method: The ADCP current meter method is a sophisticated, highly efficient way of conducting coastal current measurements. The basic operating principle of ADCPs includes the emission of acoustic signals in water and the measurement of Doppler shift in reflected signals as a means of determining velocity at various water depths. With this, a much improved profile of the current - right from the surface down to the seabed - becomes very feasible. Because ADCPs are less affected by surface disturbances, they can operate continuously for long periods, giving a more accurate picture of the current structure.

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 the pulses encounter and interact with the moving water particles, their frequency changes. If the particles are moving towards the ADCP profiler, the frequency of the reflected signal is higher than the emitted one; if they are moving away, it's lower.

Multiple transducers fitted onto the ADCP meter transmit and receive signals at various obliquities. Through this device, one is able to analyze the different Doppler shifts received from several directions and depths by calculating three-dimensional velocities of the water, that is, horizontal and vertical components of velocity, so that one might get detailed knowledge of current structure and behavior.

5. What's needed for high - quality measurement of Otaru coastal currents?

The equipment shall contain reliable materials for the high-quality measurement of coastal currents around Otaru. It means a durable casing, because it is marine with saltwater, possible storm conditions, and influences of tides and ocean currents. The size of equipment should be small to facilitate its deployment and retrieval, especially in these complex coasts with so many bays and headlands.

Preferably lightweight design makes the equipment manageable while installing and performing maintenance tasks. Low power consumption, to make it work for long periods and even in conditions that might be considered autonomous, requires this aspect. Cost-effectiveness for large-scale measurements should be enabled.

The casing of ADCPs should be made from Titanium alloy. Titanium alloy has great corrosion resistance, which is essential in overcoming the corrosive effect from seawater. It is strong, durable, and able to withstand water pressure at various levels. Its lightweight nature makes it more convenient for various deployment methods.

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

First, consider the purpose of use in selecting the equipment for current measurement at Otaru. In cases of a vessel navigating or for short-term research, it would be appropriate to consider a ship-mounted ADCP flow meter since it would provide real current information throughout the voyage.

For the near-bottom measurements, for which the purposes are usually on sediment transport or benthic ecology, a bottom-mounted ADCP is preferred. It should be able to capture the fine details of the near-bottom flow. A buoy-type ADCP current profiler would be appropriate if coastal current monitoring is needed with long-term and wide areas.

Now regarding the frequency of ADCPs, for water depths of up to 70 m., a 600 kHz is acceptable. For depths up to 110 m., 300 kHz is more appropriate. The following are recommended for deeper water that may be found close by Otaru, exceeding 1000 m.-a 75 kHz works.

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.

Brandmodel
Teledyne RDIOcean Surveyor ADCP , Pinnacle ADCP , Sentinel V ADCP , Workhorse II Monitor ADCP, Workhorse II Sentinel ADCP, Workhorse II Mariner ADCP, Workhorse Long Ranger ADCP, RiverPro , RiverRay , StreamPro  , ChannelMaster  etc.
NORTEKEco, Signature VM Ocean, Signature ADCP, AWAC ADCP, Aquadopp Profiler etc.
SonTek SonTek-RS5, SonTek-M9, SonTek-SL, SonTek-IQ, etc.
China SonarPandaADCP-DR-600KPandaADCP-SC-300K, PandaADCP-DR-75K-PHASED, PandaADCP-DR-300K, PandaADCP-SC-600K etc.
Jack Law October 22, 2024
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