How to Measure the Coastal Currents near Sapporo

1. Location and Background of Sapporo

Sapporo is the largest city in Hokkaido, Japan. It's not a coastal city in a direct sense but is in close proximity to the Sea of Japan and the Pacific Ocean. The city is renowned for its beer, winter sports, and the Sapporo Snow Festival.

The local economy is diverse, with commerce, manufacturing, and services playing significant roles. Although it is not on the immediate coastline, the coastal currents in the surrounding seas can influence the climate and marine-related activities in the broader Sapporo region. The seas near Hokkaido are rich in fishery resources such as salmon, cod, and squid, and these resources are affected by the movement of coastal currents.

2. The Situation of Coastal Currents near Sapporo

Influence of Ocean Currents

In this area, several major ocean currents influence the coastal currents near Sapporo. The Tsushima Current is a branch of the Kuroshio Current. This warm current carries warm water and nutrients, which may influence the temperature and nutrient distribution in the Sea of Japan side near Hokkaido. On the Pacific Ocean side, the cold Oyashio Current meets the warm Kuroshio Extension. These currents interact with one another in a complex and dynamic system of currents that affects the area surrounding the city of Sapporo.

Tidal Currents

Tidal currents, however, play an equally important role. The tidal range is pretty high between the Sea of Japan and the Pacific Ocean. Because of the ebb and flow of the tide, various patterns of current are formed accordingly. These, however, might change in response to the phase of the tide as well as in accordance with the local topography.

Influence of Coastline and Underwater Topography

The shape of the coastline and underwater topography. Large bays, headlands, and underwater ridges also act to alter current's direction and speed. Near headlands, for example, the currents are usually stronger, more turbulent; whereas the current patterns can be highly complicated in bays as it also interacts with the tides which are incoming and outgoing respectively.

Wind-Driven Currents

It includes the wind patterns, particularly the seasonal winds that blow across the Sea of Japan and the Pacific Ocean, which may drive such surface currents. These winds can affect not just the mixing of different layers of water but also, in general, the direction and speed of the coastal currents.

3. Methods to Observe the Coastal Water Flow near Sapporo

Surface Drifting Buoy Method

Principle: A buoy that drifts on the surface is thrown into the water to see the current on the surface. The movements of the buoy are followed over a certain period of time using a GPS or any other location-tracking system. The merits of this are that it is comparatively simpler and inexpensive to understand the general superficial current patterns.

Limitations: This can only give information about the surface current, and wind and waves can affect it. For instance, if a strong wind blows the buoy in a direction different from that of the current, it could result in incorrect measurement of direction and speed of the current.

Anchor-Boat Method

Principle: Current is measured using an anchored boat. Instruments that are used to measure current are lowered from the boat into the water at different depths.

Advantage: It allows more detail in depth-wise measurements of the current. Limitations: The fixed position of the boat restricts the area of measurement, and it is quite unlikely to capture the full variability of the currents in a large coastal area.

Acoustic Doppler Current Profiler (ADCP) Method

Principle: 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.

Merits: This gives more detail and a better profile of the current from surface to seabed. ADCPs are relatively insensitive to surface perturbances and can be run in continuous series for long durations, resulting in a true representation of the current morphology.

How ADCPs Using the Principle of Doppler Work

ADCPs work on the principle of the Doppler effect. They transmit high-frequency acoustic pulses into the water. As these pulses encounter and 'bounce' off moving water particles, the frequency of the reflected pulses changes. If the water particles are moving towards the ADCP flow meter, then the frequency of the reflected signal is higher than the emitted signal, and if they are moving away, it is lower.

The ADCP current profiler has multiple transducers operating at different angles to send and receive signals. Through an analysis of the Doppler shifts from different directions and different depths, the device can calculate three-dimensional velocity. In this way, it will also be able to calculate both the horizontal and vertical components of velocity, thus giving detailed insights into the structure and behavior of currents.

4. Requirements for High-Quality Measurement of Coastal Currents near Sapporo

Equipment Materials

For high-quality measurement of the coastal currents in Sapporo, equipment should have reliable materials. Because it would be exposed to saltwater, possibly even storms, and influenced by tides and ocean currents, casing should be hard-wearing. Its size should not be large, in order to ease deployment and retrieval, considering how complex the coastline is due to the presence of bays and headlands.

Design Considerations

It will be preferably lightweight as that makes handling the equipment light and easier to install or maneuver during its maintenance. Power consumption needs to be small for a possible operation over longer periods autonomously. Also, for large-scale measurement cost-effectiveness is another criteria

Casing Material for ADCPs

The casing of ADCPs is preferably made of titanium alloy. Titanium alloy has excellent corrosion resistance, which is very important for resisting the corrosive action of seawater. It is also strong and durable to resist water pressure at different depths. Its lightweight nature makes it more convenient for various deployment methods.

5. How to Choose the Right Equipment for Current Measurement near Sapporo

Purpose-Based Selection

The equipment for the current measurement near Sapparo shall be selected based on the purpose of use. In case of measurement for the vessel's navigation or short-term research, a ship-mounted ADCP will be adequate. During the voyage, it offers real-time current information.

Seabed-Related Measurements

For measurements near the seabed, such as those in studies of sediment transport or benthic ecology, a bottom-mounted ADCP is better. It can resolve the detail of the near-bottom current.

Long-Term and Wide-Area Monitoring

In cases where there is a need for long-term, wide-area monitoring of coastal current, a buoy-type ADCP should be utilized.

Frequency Consideration for ADCPs

Regarding the frequency of ADCPs, a 600kHz ADCP is a good choice for up to 70m, while a 300kHz ADCP is more appropriate for water depths up to 110m. For deeper waters that might be found near Sapporo, such as an area with a depth over 1000m, a 75kHz ADCP would be 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.