How can we measure the coastal currents of La Rochelle?

1. Where is La Rochelle?

La Rochelle is a beautiful coastal city in southwestern France, which fronts the Bay of Biscay within the Charente-Maritime department. Its geographical location endows it with a unique blend of natural beauty and historical significance.

The city is wrapped by a shoreline that alternates between stretches of sandy beaches, inviting visitors to bask in the sun, and rocky outcrops that give it a rugged charm. The waters of the Bay of Biscay are bright blue, their surface often dotted with sailboats and fishing vessels. The climate here is temperate, with mild winters and warm summers, making it an attractive destination year-round.

La Rochelle has a rich and glorious history. Major port city, it has always been a crossing point of trade and culture for several centuries. The Old Town of La Rochelle is a living museum, classified by UNESCO. Its cobblestone streets wind through medieval buildings, including the iconic双塔 (The Two Towers) and the Lantern Tower. These structures are not only architectural marvels but also stand as testaments to the city's former maritime glory. The local culture is deeply intertwined with the sea, and fishing remains a significant industry; markets around the city burst with fresh, locally caught seafood. The culinary scene reflects dishes like bouillabaisse, oysters on a leaf of lemon, and grilled fish with that certain je ne sais quoi from France.

2. What is the situation regarding coastal currents around La Rochelle?

The general setup of coastal currents in and around La Rochelle has resulted from various components acting upon this area. Of the most relevant contributors, tidal forces are the most pronounced. The Bay of Biscay has considerable variation in tides-they move the water with an apparent rhythmic oscillation-and may give rise to strong currents, particularly off shoal areas and in the approaches to the port. The speed of these tidal currents at times can be from a gentle drift up to several knots during peak tidal changes.

Wind patterns also have a strong influence. The prevailing winds in the area, such as the south-westerlies, push the surface waters to create currents that run along the coast. During periods of stormy weather, strong winds could increase these currents, resulting in heavy seas. The North Atlantic Current is a warm and powerful ocean current that has a far-reaching impact on local coastal currents. Although its immediate influence might be somewhat reduced closer to shore, it still affects the general temperature and salinity of the water, which may in turn affect density-driven currents.

The bottom topography also has a role: The irregular seabed topography, submarine canyons, and shallows will result in changes in current direction and speed. These will, in turn, produce eddies and upwelling zones to further complicate the pattern of currents.

3. How to observe the coastal water flow of La Rochelle?

Surface Drifting Buoy Method

The surface drifting buoy is a relatively simple method of observation of currents. Special buoys fitted with means for tracking, such as GPS receivers or radio transmitters, are set to float on the water surface. When these buoys are conveyed by the currents, the variations in position are recorded. After mapping the positions at regular intervals over time, one can interpret from the buoys' trajectory a set of vectors describing direction and speed of surface currents.

The good thing about this is that this is a reasonably cheaper method; it can mass-produce these buoys, and can provide information from even very vast ocean surface areas. However, the limitation includes the fact that wind and wave action may push them off course and therefore become incorrect. This system would thus present information related to the top layer of the water column, approximately over a depth of a few meters.

Anchored Ship Method

The ship is moored at the site. The current meters are suspended from it at different depths by cables. These current meters are used to measure the speed and direction of the water flow at each depth.

This technique has a lot of advantages. Very accurate measurements on the velocity of the current as well as on the direction of it can be gathered at a fixed point. The measurements in a number of depths, a profile of the currents can be obtained vertically. Contrarily, this takes a lot more time and a great quantity is required by processing. The natural flow is somehow disturbed by the ship present and the measurement is only possible from the very surroundings of the ship itself.

Acoustic Doppler Current Profiler (ADCP) Technique

Since it has become preferred, the ADCP current meter measurement method has taken over for all coastal current measurements near La Rochelle. In a working principle, an ADCP makes use of the variation in frequency or pitch of sound waves to calculate the velocity at which water moves through the metering section and at what depth. The instrument can be installed in various configurations, such as installed on a moving vessel-which provides both bottom-track velocity and water track velocity-or bottom-mounted and buoy-mounted.

They have several advantages over the other methods. They can give high-resolution data over a large volume of water, both horizontally and vertically. They do not disturb the natural flow of the currents very much, thus giving more accurate measurements. They can also operate continuously for long periods, providing a continuous record of the current conditions.

4. How do ADCPs using the Doppler principle work?

ADCPs are based on the Doppler principle. The instrument sends out acoustic pulses into the water. When these sound waves contact small particles suspended in the water, like sediment, plankton, or air bubbles, some of the sound energy is scattered back to the ADCP profiler.

If the particles are moving with the water current, then the frequency of the scattered sound waves will be offset from the frequency of the emitted waves. This so-called Doppler shift is proportional to the velocity of the particles, and hence of the water current, along the line of sight of the acoustic beam from the ADCP meter.

Most ADCPs are multibeam instruments, usually with four or more acoustic beams that are normally oriented in a conical fashion. The ADCP flow meter is able to determine the three-dimensional velocity of the water by measuring the Doppler shift in each beam. Thereafter, using some complex algorithms, this device processes the data from the different beams and finds out the current velocity at different depths within the water column.

5. What is required to make high-quality measurements of the currents around La Rochelle?

To make high-quality measurements of the currents around La Rochelle, several features are expected from the measuring equipment.

Reliability of the Material: The equipment has to be resistant to the aggressiveness of the marine environment. The saltwater can cause corrosion, while strong currents together with variable temperatures also have an impact on the device. Materials used to construct the ADCP should be those that highly resist corrosion. Titanium alloy is the best material that should be used for the casing of the ADCP. Corrosion resistance provided by a Titanium alloy is great, which means this will let the device serve longer in the salt-laden waters near La Rochelle. It is capable of withstanding years of exposure in seawater without considerable deterioration.

Size, Weight, and Power Consumption: The size of the equipment should be compact, and the weight should be light. This makes it easier to deploy in different settings. A smaller and lighter ADCP current profiler can be mounted on a buoy, a small research vessel, or moored to the seabed. Low power consumption is also crucial, especially for long-term deployments. Many ADCPs are powered by batteries or solar panels, and a device with low power requirements can operate continuously for extended periods without frequent battery replacements or recharging.

Cost-effectiveness: Especially for large-scale measurement purposes, the cost is an important factor. A cost-effective ADCP means more devices can be deployed to enhance spatial and temporal resolution for the current measurements, thus providing better understanding of the complicated coastal current pattern around La Rochelle.

6. How to Choose the Right Equipment for Current Measurement?

Selection Based on Usage

Ship-borne ADCP: this type will be adequate to map the vast areas of the coastal waters of La Rochelle and the Bay of Biscay around the city since it's done in a wider scope for great grounds, and with wide view in coast-to-coast currents survey, a ship is able to continuously measure currents while its movements along the coast allow views of large current patterns.

Sit-bottom ADCP: It is best suited for long-term fixed-point measurements. Scientists would be able to study the long-term trends and variations in current velocity and direction at a certain point, for example, near some particular port entrance or any known upwelling zone by mooring a sit - bottom ADCP on the seabed. This would further yield a continuity of data over long periods that can be really important for observing the long-term behavior of coastal currents.

Buoy-mounted ADCP: For monitoring currents in areas difficult to reach by ships, or studying the interaction between the surface currents and the atmosphere. The buoy-mounted ADCPs can be placed in remote areas or in areas with rough sea conditions. They can also be used to measure the short-term fluctuation in the surface current.

Selection Based on Frequency

600kHz ADCP: This frequency is particularly suited for measuring currents in water depths of up to approximately 70m. In relatively shallow coastal waters around La Rochelle, such as in the bays and near-shore areas, a 600kHz ADCP can give high-resolution data. The higher frequency allows for more detailed measurements of the current velocity and direction in the upper layers of the water column.

300kHz ADCP: Working on a relatively lower frequency, 300kHz ADCP offers better penetration through the water column. In this case, it should be suitable for waters of about 110m in depth. This can, therefore, be applicable for areas slightly deeper but still within the coastal zone close to La Rochelle. It balances depth penetration with resolution of data pretty well.

75kHz ADCP: For deeper waters up to 1000m, the 75kHz ADCP is used. In those outer parts of Bay of Biscay, near La Rochelle, when the water gets really deep, this low frequency allows the sound waves to travel further and therefore current measurements at greater depth are possible.

Some of the better-known brands for ADCPs are Teledyne RDI, Nortek, and Sontek. In this list of companies manufacturing the most advanced and high-quality ADCPs is the Chinese firm of China Sonar PandaADCP. Wholly made of Titanium alloy, this brand offers durability and corrosion resistance at its finest. Its price-performance ratio is highly competitive, making it an attractive option for both small-scale research projects and large-scale oceanographic surveys near La Rochelle. You can visit their website at https://china-sonar.com/ for more information.

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