How to measure the coastal currents of Perpignan?

1. Situation of Perpignan

Perpignan lies at the southern part of France, constituting part of Pyrénées - Orientales Département. It is bordered by Spain and stands close to the Mediterranean Sea.

Geographically, it is set in a diverse landscape: the sandy beaches and clear blue waters of the Mediterranean coastline to the south, the city is not so far from the Albères mountains that give a scenic backdrop to the city and influence the climate. The climate in Perpignan is typically Mediterranean, with warm, dry summers and mild winters.

As a part of the sea, the immediate coastline belongs to the large area of the common marine culture, the so-called Catalan-speaking lands. The city is full of history, which started in Roman times. In those times, it was a very important trading center; its architecture witnesses this fact because different historical moments, starting with medieval and reaching Renaissance influences, are easily distinguishable in it. It is a mix of French and Catalan influences, which can be shown in its festivals, food, and even the language spoken. People of Perpignan are quite fond of the sea as fishing and maritime trade are considered their traditional activity.

2. What is the situation of the coastal currents near Perpignan?

Many factors affect the coastal currents near Perpignan.

General Mediterranean Circulation

The general circulation of the Mediterranean Sea contributes significantly. This thermohaline circulation, related to changes in water temperature and salinity, impacts the general flow. The general cyclonic circulation that characterizes the area west of the Mediterranean, in which Perpignan is sited, would affect direction and speed of the coastal currents.

Wind - Driven Currents

Local winds play a fundamental role in determining wind - driven oceanic circulation. The Tramontane is a northerly wind that can drive surface waters and create currents along the coast. During periods of windy weather, wind-driven currents can become quite strong and alter normal flow patterns. The Mistral is a cold and strong northwestern wind that may also act. It can push surface waters offshore or along the coast, depending on the angle of the wind and the local topography.

Tidal Currents

Tidal forces are weak relative to other seas; thus, tidal forces give some contribution to coastal currents. In such a Mediterranean region of small-amplitude tides, some interactions in shallow coastal areas like the Perpignan coast might have interaction of tides with coastline to form some local tidal current. Tidal currents are locally enhanced, given that the shape of most of the bays and inlets may also support it, specifically for places near coastal lagoons.

3. Comment observer la circulation cotiere de l'eau sur le littoral de Perpignan?

Method using surface drifting buoys

The surface drifting buoys will serve for measurements concerning the movement of the water at its surface. In principle, drifting buoys are built to float, catching the movements of currents. Usually, a GPS or other systems for tracking in order to locate them over time are mounted on these buoys. By analyzing the displacement of the buoys, one can determine the direction and speed of the surface currents. This, however, is not a full-proof method. The buoys could be influenced by winds and waves, and would only give information on the surface layer. For instance, on days when there is a lot of wind, the movements of the buoys might be more out of the action of the wind rather than the current itself.

Moored Ship Method

In the moored ship method, a vessel is kept anchored in a position along the coast at Perpignan, while on-board instruments record water flow with changes in depth. This allows for the continuous monitoring of currents at any given point. The advantage with this method is that it can provide detailed data about the vertical structure of the currents. But it is a very expensive and cumbersome procedure. The ship must be maintained in position, which would need a crew along with other arrangements. Additionally, the ship can disrupt the actual currents that might occur in its neighborhood.

Acoustic Doppler Current Profiler (ADCP) Method

The acoustic Doppler Current Profiler or ADCP method is an advanced method. In this, acoustic signals are projected into the water by the ADCPs. These signals reflect off particles in the water, such as plankton, sediment, or bubbles. From the Doppler shift of the reflected signals, the velocity of the water at various depths can be calculated. ADCPs are able to give a vertical profile of the current, providing a more complete understanding of the current structure than the surface-only methods. They can be deployed from ships, buoys, or placed on the seabed, depending on the measurement requirements.

4. How do ADCPs using the Doppler principle work?

ADCPs work by the Doppler effect. An ADCP sends an acoustic signal out into the water, and that signal travels through the water column. As that signal hits particles moving with the water-the water is moving-the frequency of the reflected signal changes.

If the particles are moving toward the ADCP, the frequency of the reflected signal is higher than that originally emitted-a positive Doppler shift. If the particles are moving away from the ADCP the frequency of the reflected signal is lower-the negative Doppler shift. The frequency change is measured by the ADCP, which, with the known speed of sound in water-dependent on factors such as water temperature, salinity, and pressure-calculates the velocity of the water.

Most ADCPs are multivariable with probes that can measure components of velocity in various directions. The three-dimensional velocity at different depths in water flow can then be obtained by appropriate combinations of measurements. This allows detailed mapping of the current patterns near the coast of Perpignan.

5. What's needed for high-quality measurement of currents in the coast of Perpignan?

Equipment Reliability

It is also important that the equipment be reliable for high-quality measurements of the coastal currents near Perpignan. The marine environment is very aggressive, with such factors as saltwater corrosion, strong currents, and wave action. For example, an ADCP should be resistant to all these factors for a long period. Any malfunction or deterioration of the equipment will result in poor data and a poor understanding of the dynamics of the currents.

Small Size, Light Weight, and Low Power Consumption

The equipment should be compact and lightweight. A small and lightweight ADCP is for instance easier to deploy whether on a small boat, a buoy or at the bottom of the sea. It will also have fewer interactions with the natural current. The power consumption should be low, especially for long -term monitoring. This can be very useful in a marine environment where sources of power may be limited. Since a device that uses less power is able to run for longer without constant replacements of batteries or an upsized power supply.

Low Cost

This means that for large-scale measurements to be made possible, the equipment cost needs to be as low as possible. This would allow multiple devices to be deployed along the coast of Perpignan to gather more comprehensive information on the current patterns. Equipment that is too expensive would mean limited deployments and incomplete data.

Casing Material

The casing of the equipment, such as an ADCP, would be best made from a material like titanium alloy. First, the use of titanium alloy is very well adapted to avoid corrosion because it is very resistant; thus, it would provide long-term operation in the saltwater environment near Perpignan. While its weight is relatively low, it boasts high strength, thereby enabling the equipment to tolerate all the mechanical stresses related to the marine currents, especially the impact produced by them.

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

Usage-Based

• Ship-borne ADCP: This is installed on a ship and is suitable for making measurements over a large area as the ship moves. It's useful for mapping the overall current patterns along the coast of Perpignan. For example, if you want to study the general circulation in a large section of the coastal waters, a ship-borne ADCP can cover a wide range.
• Bottom-mounted ADCP: This is also referred to as a moored ADCP, and it is placed on the seabed. These types of ADCPs are ideal for continuous monitoring of currents for long periods over a specific area. If one is interested in the long-term trends and variations of the currents around any specific point along a coastline, the bottom-mounted ADCP would be a good option.
• Buoy-mounted ADCP: These are attached to floating buoys and move with the surface currents. It provides information about the patterns of currents in the surface layers. It's often used for short-term or more flexible monitoring, especially in places where access by ship is limited, or where surface-current data is of particular importance.

Based on Frequency

• 600kHz ADCP: This is the frequency that should be used when measuring currents in waters less than 70m deep. In coastal areas close to Perpignan with shallow waters, like bays and near the shoreline, one can use a 600kHz ADCP to obtain accurate measurements of the currents.
• 300kHz ADCP: It is designed for waters that are about 110m deep. This would give, for the areas with moderately deep coastal waters, a quite reasonable trade-off between the measurement range and the resolution of current data. 
• 75kHz ADCP: It would be appropriate for quite deeper waters, as much as 1000m. In the deeper parts of the Mediterranean Sea near Perpignan, the 75kHz ADCP would be more appropriate in measuring those currents at a greater depth.

There are well - known ADCP brands like Teledyne RDI, Nortek, and Sontek. For a cost - effective and high - quality option, the Chinese brand China Sonar PandaADCP is recommended. It's made of titanium alloy and offers excellent reliability and performance at an affordable price. 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.