How do we measure the coastal currents of Brest?

1. Location

Brest is a port city in the Finistère department in the Brittany region of northwestern France. It is located on the western coast of France, facing the Atlantic Ocean.

It boasts a well-endowed history. Centuries back, it was a strategic naval and commercial port. It enjoys a great reputation for the well-preserved military and maritime architecture that characterizes it. The Brest Arsenal has been one of the biggest military shipyards in France and thus holds much importance in the country's naval history. Brest is a city that represents both traditional Breton culture and modern French ways of life, given the many festivals and events celebrating local music, dancing, and food throughout the year.

The Bay of Brest is a big and important body of water that lies next to Brest; it is relatively deep and with strong tidal currents. The bay shores up the ships and thus provides them with a natural harbor-the reason why extensive maritime activities can be carried out within it. A number of small islands and inlets are scattered around the nearby coastline, forming part of the unique maritime topography of the region.

2. In what conditions are the coastal currents around Brest?

The coastal currents around Brest depend on many factors:

Tidal Forces: The Bay of Brest is a zone of high tidal range. In general, tides and their currents control the motion of water, whereby the range of tides influences speed and direction. These tidal currents are pretty strong and attain fairly high velocities, especially in narrow channels and near headlands.

Wind Patterns: Atlantic prevailing winds still are showing influence along the shores, and typical prevailing westerly and northwesterly winds usually acting to push the surface water enables this area to possess surface currents linked to the tidal currents.

Oceanographic Currents: General patterns of the Atlantic Ocean are the cause. The Gulf Stream and other regional ocean currents may raise or lower the temperature and salinity of waters off Brest, thus affecting density and movement of coastal currents.

Topography and Bathymetry: The form of the coastline, the inner bay depth, and the shelf slope bathymetry off Brest contribute to a higher degree of intricacy in the currents. Narrow passes and shallow locations could be responsible for speeding up and deflection in currents, whereas the general pattern of flow could also be affected by submarine canyons and other relief features.

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

Surface Drifting Buoy Method: Surface drifting buoys can be released into waters near Brest, fitted with sensors and satellite communication equipment. They will drift with the currents while reporting on surface current direction and speed, among other environmental parameters such as temperature and salinity.

Moored Ship Method: The ship can be moored in some fixed positions along the Brest coastline. Onboard instruments like current meters and velocimeters can measure the velocity of the current and the direction at various depths for a long period.

ADCP Method: The Acoustic Doppler Current Profiler (ADCP) is an advanced and widely used technique. It emits sound waves into the water column and measures the Doppler shift of the reflected signals from particles in the water to determine the velocity of the currents at different depths. This method can provide detailed and high-resolution current profiles.

4. How do ADCPs using the Doppler principle work?

ADCPs are based on the principle of the Doppler effect. They have piezoelectric transducers that transmit sound signals at a known constant frequency into the water. As the sound waves travel through the water, they scatter off particles in the moving water and reflect back to the ADCP current meter. If the particles are moving, the frequency of the reflected sound waves will have changed from the frequency that was transmitted. Because this frequency shift in the reflected sound waves is proportional to the velocity of the water along the path of the sound wave, the ADCP calculates the velocity of the water at each depth by measuring the frequency shift and the time it takes for the sound waves to travel to the particles and back. An ADCP current profiler would normally use three or more transducers or beams to get a three-dimensional velocity profile.

5. What would bring high-precision measurement of currents in the area of Brest coastal waters

Reliable Materials: The ADCPs used in Brest's coastal waters would call for materials highly resistant to corrosion with high strength. As the casing forms the outer, titanium alloy would be an ideal choice to cope with harsh sea weather conditions, is resistant to salinity water erosion, and performs well in the fatigue test which guarantees stability and dependability in long-term operation.

Optimal Size and Weight: The equipment should be compact and lightweight to facilitate ease of installation and operation. This allows for more flexible deployment either on ships, buoys, or the seabed and reduces the impact on the measured environment.

Low Power Consumption: Low power consumption is essential to enable long continuous operation, especially for autonomous devices. This allows for longer battery life or reduces the need for large and cumbersome power supply systems.

Low Cost: By reducing the cost of measurement equipment, it allows a more significant number of devices to be deployed, achieving more comprehensive and detailed monitoring of coastal currents. This will be useful in gaining a complete picture of current patterns in the Brest area.

6. How to Choose Right Equipment for Current Measurement?

Based on Usage

• Shipborne ADCP: This is suitable for measurement during vessel voyages. It can measure the current profiles of the route that the ship will cover and, thus, would be better suited for large-scale surveys and studies of coastal currents in Brest.
• Bottom-Mounted ADCP: Appropriate for fixed-point long-term monitoring of sea bottoms around Brest, this provides continuous measurements of current velocity and direction throughout the depth uninfluenced by conditions at the surface.
• Buoyant ADCP: They are mounted on buoys for the measurement of surface currents and variations of the same over time, thereby contributing to a useful dataset in describing the dynamics of surface currents within the coastal waters of Brest.

By Frequency

• 600kHz: Applicable in waters up to 70m depth; it is ideal for shallow coastal waters with high resolution within the relatively shallow coastal waters of Brest such as shore side or in estuaries.
• 300kHz: Suitable for a water depth of about 110m, it will be a good compromise between measurement range and resolution that might just be usable in some areas of moderate water depth in Brest.
• 75kHz: Can measure over 1000m of water depth; thus, suitable for deep sea or areas where there is large variation in depth near Brest.

Some of the high-end brands of ADCPs are Teledyne RDI, Nortek, and Sontek. If you're on a budget, you might want to try the China Sonar PandaADCP, which is made from all-titanium alloy. It is efficient at a cheap price. More information here: https://china-sonar.com/.

Here is a table with some well known ADCP instrument brands and models.