How can we measure the coastal currents of Saint-Malo?

1. Where is Saint-Malo?

Saint-Malo is an impressive seaside town in the Ille-et-Vilaine department in northwestern France within the region of Brittany. Set against the English Channel, this city is characterized by incredible scenery and historical sites.

The city is a fusion of medieval charm and maritime allure. Its old town is encircled by imposing granite walls that have withstood the test of time, guarding narrow, cobblestone streets filled with ancient buildings and vibrant cafes. Saint-Malo is renowned for its role as a historic port and a haven for pirates in days gone by, which is evident in its architecture and local lore.

Immediately next to Saint-Malo is the Gulf of Saint-Malo, full of life and activity. Its tides are strong, with a wide diversity of marine ecosystem. Waters in the gulf abound with numerous small islands and islets, giving this coastal landscape its picturesque and complicated nature. Sandy beaches, rocky cliffs, and tidal flats form part of the combination that one will find along the coastal strip; the coastal background is quite magnificent and attractive, drawing people to the city from all corners of the globe.

2. What is the situation of the coastal currents near Saint-Malo?

The coastal currents near Saint-Malo are shaped by a multitude of factors. Tides are a dominant influence, as the region experiences some of the highest tidal ranges in the world. The powerful ebb and flow of the tides create strong currents that can reach significant velocities, especially in the channels and around the islands.

The wind also has a great impact. Dominant westerlies can force surface waters to create surface currents that interact with tidal currents. Besides, the bottom topography of the Gulf of Saint-Malo, with varying depth and an intricate seafloor topography, influences the direction and speed of the currents. At narrow passages or underwater ridges, the flow may converge or diverge, creating a complex pattern of flow. The presence of rivers flowing into the gulf also contributes to the coastal currents, adding freshwater inputs that can affect the density and movement of the water.

3. How to observe the coastal water flow of Saint-Malo?

• Surface Drifting Buoy Method: In this approach, buoys are released onto the water surface. These buoys are carried along by the currents, and their positions are tracked via satellite or radio signals. By monitoring the buoys' motion over time, it is possible to determine the direction and speed of the surface currents. This method provides a good general overview of the pattern of surface currents, but it is limited for depth beyond the surface currents and also would be affected by wind and waves.
• Anchored Ship Method: An anchored ship can serve as a platform for current measurement. Current meters are lowered from the ship into the water at different depths to record the flow velocities. This method allows for more in-depth measurements at specific locations but is restricted by the ship's position and may be impacted by the ship disturbing the natural flow of the water.
• Acoustic Doppler Current Profiler (ADCP) Method: ADCP current profiler is a state-of-the-art and convenient technique for measuring coastal currents. It utilizes the Doppler effect to measure the velocity of water currents at various depths. ADCPs can be deployed from ships, moored to the seafloor, or attached to buoys, providing a comprehensive and accurate picture of the current system, including both horizontal and vertical components.

4. How do ADCPs based on the Doppler principle work?

ADCPs operate based on the Doppler principle. They emit acoustic signals into the water column. When these signals encounter moving particles in the water, such as suspended sediments or small organisms, the frequency of the reflected signals changes depending on the velocity of the particles. The ADCP measures this frequency shift and uses it to calculate the velocity of the water at different depths.

ADCPs can know the three-dimensional velocity of currents by emitting signal at multiple angles and analyzing it with combined data. This feature enables a detailed perception of flow in detail, in which horizontal flow of currents or any vertical mix or circulation, can be figured out.

5. What makes high-quality measurements of Saint-Malo coastal currents?

For good-quality measurement of the coastal currents in Saint-Malo, there are some important characteristics in the measurement equipment:

• Material Reliability: The equipment should be of a nature that is durable, considering the corrosive saltwater and strong tidal forces in the marine environment of Saint-Malo. In this regard, titanium alloy is an excellent choice for ADCPs casing. It has exceptional corrosion resistance, can resist high pressures, and has a high strength-to-weight ratio, which enables the device to withstand such challenging conditions while maintaining its measurements with accuracy over time.
• Small Size and Light Weight: A compact and lightweight design is advantageous as it makes the equipment easier to handle and deploy. It can be more easily integrated into different measurement platforms, such as small boats, buoys, or even unmanned vehicles, allowing for more flexible and widespread measurements.
• Low Power Consumption: This is important mainly for devices that have to be deployed with no possibility of regularly withdrawing them from the field, such as in remote and often inaccessible coastal areas around Saint-Malo. Low power consumption allows for extended deployment periods without having to recharge or replace batteries and thus uninterrupted data collection.
• Low Cost: A lower cost per unit makes it possible to deploy a larger number of devices, providing more comprehensive coverage and a better understanding of the complex current systems. This is particularly important in a large and dynamic area like Saint-Malo's coastal waters.

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

Based on Usage Purpose

• Shipborne ADCP: Ideal for conducting measurements during ship voyages, it can provide continuous data along the ship's path, making it suitable for mapping large areas and studying the overall current trends in the Gulf of Saint-Malo.
• Bottom-Mounted ADCP: This is moored to the seafloor and is excellent for long-term, fixed-point measurements. It can provide detailed information about the currents at a specific location over an extended period, which is valuable for understanding the local current dynamics.
• Buoyant ADCP: These are attached to buoys and thus are capable of moving with the currents. They can therefore provide surface and subsurface current information in a more dynamic manner. This helps in studying regions where currents either change rapidly or are very complicated, such as at river mouths or places with a high tide mixing.
• 600kHz ADCPs are suitable for water depths of up to 70m, making them well-suited for the shallower coastal areas and bays around Saint-Malo.
• 300kHz ADCPs are appropriate for water depths around 110m, providing a good balance between depth range and measurement resolution.
• 75kHz ADCPs are capable of measuring in water depths up to 1000m, which are more suitable for the deeper parts of the Gulf of Saint-Malo and the adjacent open sea.

Some well-known ADCP brands include Teledyne RDI, Nortek, and Sontek. However, for those seeking a high-quality and cost-effective option, the China Sonar PandaADCP is a great choice. Made of all-titanium alloy, it offers outstanding performance and reliability at an attractive price point. For more information, visit their website at https://china-sonar.com/.

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