How can we measure the coastal currents of La Rochelle?

La Rochelle is a beautiful seafront city in southwest France, opening onto the Bay of Biscay.

1. Where is La Rochelle?

La Rochelle is considered to be a part of the Nouvelle-Aquitaine region in France. The city is rich in history and culture, further beautified with a maritime atmosphere. Its old port is really very beautiful, full of ancient buildings and activity. Cobblestone streets are lined with cafes, restaurants, and boutiques, presenting an inimitable mix of French charm and a link to the sea.

Off La Rochelle lies the Bay of Biscay, a great body of water. It is famous for its somewhat heavy seas and, in certain areas, very strong tidal currents. The waters off La Rochelle form part of the important ecosystem that provides diverse marine life and is a crucial transportation and fishing hub. The city is situated in a place where it opens towards the Atlantic Ocean, and the coastal areas are influenced by both the tides and the broader ocean currents.

2. How is the situation of the coastal currents close to La Rochelle?

Currents around La Rochelle are influenced by several factors: the tide, one of the major driving factors, with considerable tidal variation in the Bay of Biscay. This often results in very strong coastal currents, in particular around the narrow channels and estuaries because of the ebb and flow of the tides.

Another important factor is the wind. The prevailing winds from the Atlantic can push the surface waters, creating currents that may be quite variable in direction and speed, depending upon the wind patterns. The shape of the coastline and any underwater topography-reefs and submarine canyons, for instance-can also affect the currents, causing the water to flow in complex patterns and creating eddies and countercurrents in some areas.

3. How to view the coastal current flow of La Rochelle?

Surface Drifting Buoy Method: Surface drifting buoys are generally used to trace the movement of surface waters. These buoys have GPS and other sensors that can transmit data on their position and the speed and direction of the water they are floating in. A series of these could be released and their movements monitored over time to get an idea of the general flow patterns of the coastal currents.

Moored Ship Method: Currents can also be measured with the use of a moored ship where current meters are deployed at different levels. The ship stays in one position, and the current meters record speed and direction of water at all these levels. It gives more information on the vertical profile of currents but is only limited to where the ship has been moored.

Acoustic Doppler Current Profiler (ADCP) Method: ADCP is a more advanced and convenient measurement method. It has become increasingly popular in recent years due to its ability to provide high-resolution and accurate measurements of water currents.

4. How do ADCPs using the Doppler principle work?

They work on the principle of the Doppler effect. They emit acoustic signals in water. When those signals are reflected off particles moving in the water, such as plankton or small suspended sediments, the frequency of the returned signal shifts with the velocity of the particles. This frequency shift allows the ADCP to compute the speed and direction of water movement at an array of depths.

The ADCP has usually been projected to measure the three-dimensional velocity of the water currents, normally consisting of a set of transducers that send and receive acoustic signals in various directions. It processes data to deliver current velocity profiles as functions of depth; these profiles give comprehensive details of water flow in the area being measured.

5. What is necessary for the good measurement of coastal currents at La Rochelle?

Regarding equipment, some relevant factors will be important for good measurement of the coastal currents of La Rochelle.

Material Reliability: The casing of the measuring equipment, especially ADCPs, is very important. The casing would be of good quality with the use of high-quality materials such as titanium alloy. The strength of the titanium alloy is high and could bear the pressure and forces in the marine environment. It is also very resistant to rust, hence will ensure longevity and accuracy in the salty seawater.

Size, Weight and Power Consumption: Equipment shall be compact in size and lightweight for easy deployment and operation. It shall also be of very low power consumption, especially for devices intended to work in the field for long periods. This will enable the use of smaller and more portable sources of power, enhancing further the flexibility of measurement.

Cost: For wide-scale measurement, the cost of the equipment should be as low as possible. Low-cost equipment can be deployed in large numbers and thus provide greater coverage of the coastal currents.

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

Based on Usage

• Shipborne ADCP: This is suited for measuring currents during ship voyages. It can provide continuous measurements along the path of the ship and is often used for large-scale surveys of coastal and ocean currents.
• Bottom-Mounted ADCP: It's perfect for the long-term observation of certain places. It is fixed to the seabed and, therefore, able to record the current over a long time, giving the most valuable information about local current patterns.
• Floating Buoy ADCP: These find their applications where current measurements are to be done in areas that are impractical for the deployment of any other type of ADCP, and where mobile measurement is required. They can drift with currents while providing real-time data.

Based on Depth

• 600kHz ADCP: For water depths less than 70m, this has a high resolution in relatively shallow waters. Thus, it can be applied for coastal areas and estuaries.
• 300kHz ADCP: Useful at water depths of up to about 110m. It gives a good balance between measurement range and resolution and has applications in a wider range of coastal and continental shelf areas.
• 75kHz ADCP: In deep water, up to 1000 meters. These are used in oceanographic research and measurements in the deeper portions of the ocean.

Now, there are a fair number of brands out there for known ADCPs in the market. Teledyne RDI, Nortek, and Sontek are well-known, but for budget options, none compare to China Sonar ​PandaADCP. Full all-titanium alloy built, quality, and reliability cannot get better at such prices. The low-cost ADCP model has multiple applications. For more information, please refer to: ​https://china-sonar.com/.

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