How would one measure the coastal currents of Béziers?

1. Where is Béziers?

Béziers is a beautiful city in the Hérault department in southern France. It is cozily nestled along the banks of the Orb River and enjoys a strategic geographical position. It is famous for its rich history, which started in ancient Roman times, and is full of all kinds of historical monuments testifying to its great past. The old town of Béziers is a candidate for UNESCO World Heritage status, with its well-preserved medieval architecture, including the imposing Saint-Nazaire Cathedral.

As far as surrounding waters go, Béziers is not so far from the Mediterranean Sea. The immediate coasts around are those of the Languedoc - Roussillon coastline, where many bays and inlets abound. These Mediterranean waters here have crystal clear blue colors, small tides, and rich biodiversity in marine ecosystems. The region also attracts a large number of tourists every year, drawn by its warm climate, beautiful beaches, and the unique blend of French culture and coastal charm. The local population is engaged in different activities, from traditional fishing to modern-day tourism-related services, all related to the coastal environment and the behavior of the coastal currents.

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

Taken altogether, the coastal currents off Béziers are determined by several factors. First, it is the large-scale circulation features of the Mediterranean Sea. The general circulation in the western Mediterranean, driven by wind-stress, thermohaline processes, and exchange of waters through the Strait of Gibraltar, constitutes the background flow conditions for the coastal regions off Béziers.

Local winds are another important factor. The Mistral is a strong and cold north-westerly wind that can blow across southern France and has a direct impact on the surface currents. When the Mistral blows, it pushes away the surface waters from the coast and affects the direction and speed in which the water travels in relation to the coastline. On the other hand, the Sirocco is a warm and humid south-easterly wind that may also change the current patterns, sometimes bringing warmer waters from the south and changing the local circulation.

Other important processes that should be considered with respect to coastal current dynamics in Béziers are tidal forces. Although the tide ranges are rather small in comparison with certain other seas, tides interact with coastal topography and current patterns already developed in the Mediterranean. The shape of the bays and the shallowness of the coastal waters around Béziers may further contribute to an intensified effect of the tides on the local current, especially in nearshore areas.

3. Observation of Coastal Water Flow in Béziers

Surface Drifting Buoy Method

One of the traditional methods for observing coastal water flow is by using surface drifting buoys. Those buoys float on the surface, drifting with the flows. They are equipped with sensors in order to track their position, like GPS receivers. By monitoring the drift of these buoys, researchers can only be able to assume in which direction and at what speed the surface currents are moving. This system, however, is not perfect. Buoys can easily be acted upon by winds, waves, and other surface-acting forces that may or may not depict the true subsurface current conditions.

Moored Ship Method

The moored ship method involves stationing a ship at a fixed location near the coast of Béziers. These instruments, aboard a ship, measure water flow with current meters at different depths. The advantages of this technique include its capability to monitor currents continuously at one point. However, this technique is somewhat expensive and requires time, a ship, and people aboard it. In addition, the presence of a ship could alter the flow of currents naturally occurring in that very vicinity.

Acoustic Doppler Current Profiler (ADCP) Method

The ADCP current meter has now become a more modern and convenient technique in the measurement of coastal currents. ADCPs are capable of simultaneously measuring the velocity of water at several depths and provide a better view of the vertical structure of the currents. This is contrary to the previous methods, which may give information about only the surface or a single-point measurement. ADCPs detect the movement of particles in water by using sound waves and, from the Doppler shift of these sound waves, determine the velocity of the water.

4. How do Doppler Principle ADCPs work?

ADCPs are based on the principle of Doppler. The instrument emits acoustic signals into the water. These sound waves encounter small particles suspended in the water, such as plankton, sediment, or even bubbles. When the water is in movement, the particles also move. In that respect, while the sound waves reflect back through these moving particles, there is a change in the frequency of the reflected waves-a sort of frequency shift, known as the Doppler shift.

This Doppler shift is the measurement by the ADCP current profiler. In case the particles are moving towards the ADCP, then the frequency of the reflected wave is higher than the emitted frequency; if they are moving away, the frequency is lower. By accurately measuring this change in frequency, the ADCP calculates the velocity of the water in which the particles are suspended.

Most ADCPs are equipped with multiple transducers. These various transducers are utilized in sensing velocity components in different directions. By integrating measurements from these multiple transducers, an ADCP is capable of identifying the three-dimensional velocity of the water flow for different depth parts. This can help understand, in great detail, the complexity of currents in the coast near Béziers.

5. What is necessary for a quality measurement of currents on Béziers coasts?

Equipment Reliability

For high-quality measurement of the coastal currents near Béziers, the equipment used must be highly reliable. Since the marine environment can be harsh, with factors such as saltwater corrosion, strong currents, and wave action, the ADCP needs to be able to withstand these conditions over long periods.

Small Size, Light Weight, and Low Power Consumption

The ADCP flow meter should be small in size and light in weight. This makes it more deployable whether on a small boat, a buoy, or at the bottom of the sea. A smaller and lighter device minimizes the disturbance to the natural flow of the currents as well. In addition, low power consumption is highly desirable, especially for applications involving long - term monitoring. This will enable the ADCP to operate for extended lengths of time without the need to change batteries constantly or even the need for a large-scale power supply.

Low Cost

The ADCP meter needs to be relatively low in cost to facilitate its deployment on a large scale. This is due to the fact that it would be feasible to deploy multiple such units in different areas along Béziers's coast to gather further information about current trends.

Titanium Alloy Casing

The casing of the ADCP profiler shall be made preferably of titanium alloy. There is a reason for using a titanium alloy: it offers excellent resistance to corrosion, which is very important during long-term operation in the saltwater environment near Béziers. It is also rather lightweight, helping to meet the requirement of easy deployment. Besides, the high strength of the titanium alloy will protect the ADCP from the mechanical stresses generated by the impact of waves and currents in the marine environment.

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

Based on Usage

• Ship-borne ADCP: A general ADCP fixed on a moving ship. It can work over larger areas since the ship will be able to travel across various routes. Ship-borne ADCPs are usually applied for mapping large-scale current patterns along the coast of Béziers.
• Bottom-mounted ADCP: Commonly referred to as a moored ADCP, this is placed on the seabed. This is very efficient in the long term for monitoring the currents continuously at that one point. This would be appropriate in determining the trends and variation over time of the coastal currents near parts of the Béziers coast.
• Buoy-mounted ADCP: These are mounted onto floating buoys. They can thereby be advected by the surface currents and deliver information on the current patterns in the surface layer. Buoy-mounted ADCPs are often used in shorter-term or more flexible monitoring, particularly in situations where access by ship is very difficult or simply undesirable.

Based on Frequency

• 600kHz ADCP: This frequency is suitable for measuring currents in waters with a depth of less than 70m. In the shallow bays and inlets frequent in the coastal areas around Béziers, this will be an appropriate choice of current measurement by 600kHz ADCP in relatively shallow waters.
• 300kHz ADCP: It is designed for waters with a depth of about 110m. This frequency can be used in areas where the coastal waters have a moderate depth, providing a good balance between the measurement range and the resolution of the current data.
• 75kHz ADCP: This can be used for much deeper waters, up to 1000m. A 75kHz ADCP would thus be more suitable for the measurement of currents in greater depths at deeper parts of the Mediterranean Sea close to Béziers.

There are several well-known ADCP brands in the market, such as Teledyne RDI, Nortek, and Sontek. However, for those looking for a cost-effective yet high-quality option, the Chinese brand China Sonar PandaADCP is highly recommended. Made entirely of titanium alloy, it offers excellent reliability and performance at an incredibly affordable price. For more information, you can visit their website at https://china-sonar.com/.

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