How to measure the coastal currents of Bordeaux?

1. Where is Bordeaux?

Bordeaux is a major city in southwest France, the capital of Nouvelle - Aquitaine, lying on the course of the Garonne River, which reaches the Gironde Estuary. The Gironde Estuary is a vast area of water and an important waterway that links Bordeaux with the Atlantic Ocean.

The city of Bordeaux is known for its famous vineyards and wine production. The landscape is varied with vine-covered hills, rich plains, and charming small villages. The Garonne River runs wide and easy-flowing; the river has played an important part in the development of the history and economy of the city. It was once a primary transportation route for the exportation of wine, with historical buildings and warehouses lining its banks.

Where the rivers Garonne and Dordogne enter it, the Gironde Estuary can easily be imagined to be some complex body of turbulent water. Mainly being subject to tidal activity out of the Atlantic Ocean, the estuary features a distinct marine life pattern. The mix of sandy beaches and muddy and marshy coasts allows several types of birdlife and different varieties of aquatic organisms to lead healthy lives here. The entrance of the estuary is a strategic point for maritime traffic, since it constitutes the gateway to the port of Bordeaux.

2. What is the situation of the coastal currents close to Bordeaux?

The coastal currents close to Bordeaux are influenced by several factors. Tidal currents are one of the dominant forces in the Gironde Estuary. Large semi-diurnal tides from the Atlantic Ocean cause water levels to rise and fall, creating powerful currents that regularly flow in and out of the estuary. The flood tide rushes upstream, bringing with it ocean water and nutrients, while during ebb tide, it moves out towards the sea, carrying sediments and freshwater from the rivers.

The outflow of the Garonne and Dordogne rivers also influences coastal currents. The volume and speed of the river water vary the local pattern of the current. When the rivers are in spate, more so during the rainy season, the freshwater discharge has a higher impact on the density-driven current in the estuary.

A factor very coupled in action is wind: prevailing winds can actually drive surface water currents from directions such as from the west or southwest. These generally enhance or can oppose the tide and riverdriven currents; for instance, a strong onshore current during a strong westerly wind is able to advance its water level farther inland and redistribute sediments of different sizes onshore.

Shape of coastline and bathymetry of the seabed is another important factor. The wide mouth and gradually narrowing channel of the Gironde Estuary, as well as the presence of sandbanks and shoals, can cause acceleration, deceleration, or change of direction of currents. The topography of the seabed can influence the vertical structure of the currents; deeper channels may present a stronger and more regular flow.

3. How to Observe the Coastal Water Flow of Bordeaux?

Surface Drift Buoy Method

Surface drift buoys represent the low-cost means to conduct surface current observations. In principle, such buoys are designed to float on the water surface and are prepared with a positioning device like GPS. Once discharged into the water, they will be transported by the surface currents. By monitoring the position of buoys in time, some estimate of speed and direction of surface currents can be obtained. This technique also has its drawback. The wind-driven motion may influence the buoy position, and thus data obtained may be not representative of the actual current. It is also sensitive to only a surface layer of water and sometimes may not yield a complete pattern of the current.

Anchored Ship Method

The anchored ship method consists of mooring a ship in a given position, not far from the coast. Current meters and other instruments installed on board are used to measure the speed and direction of the currents at different levels. This method gives detailed measurements for a particular point, but is very limited for the spatial coverage of the area being studied. The presence of the ship itself can also affect the local currents, and the area that can be measured is limited to the vicinity of the anchored ship.

Acoustic Doppler Current Profiler (ADCP) Method

The method of Acoustic Doppler Current Profiler is a more advanced and efficient way of measurement of coastal currents. ADCPs can be deployed from ships, moored on the seabed, or attached to floating buoys. In principle, they operate on the Doppler effect principle. ADCPs emit acoustic signals into the water column. The acoustic signals reflected back by particles in the water column, such as sediment, plankton, or bubbles, are received by the instrument. When these particles move with the current, the frequency of the reflected signal changes; this is used by the ADCP profiler to calculate the velocity of the water at different depths. ADCPs can give a profile of the current structure at multiple depths over a wide area and are less affected by surface-wind-driven motion than the surface drift buoy method.

4. How do ADCPs using the Doppler principle work?

ADCPs are based on the principle of the Doppler effect. They shoot acoustic pulses into the water and measure the shift in frequency of the reflected signals. In this case, when an acoustic signal encounters a particle in the water which is moving with the current, the frequency of the reflected signal is different from the frequency of the signal originally emitted.

If the particle is moving towards the ADCP meter, then the frequency of the reflected signal is higher, a blue-shift; if it's moving away, the frequency is lower, a red-shift. This frequency shift is measured by the ADCP and is the basis for determining the velocity of the water at the location of the reflection. The ADCPs are thus able to measure the three-dimensional components of the water velocity (east-west, north-south, and vertical) by transmitting the signals in different angles and with multiple transducers. They divide the water column into multiple depth bins and measure the current velocity at each bin, creating a detailed profile of how the currents vary with depth.

5. What's needed for high-quality measurement of Bordeaux coastal currents?

Equipment Reliability

It means that the equipment should be reliable for high-quality measurement of the coastal currents near Bordeaux. Saltwater, strong currents, and variable weather conditions can be very harsh on instruments in the marine environment. ADCPs must be able to withstand such challenges. High-quality materials and robust construction are quite necessary to make the equipment work properly for a pretty long period of time. Besides, regular maintenance and calibration will also be required to maintain accuracy.

Size, Weight, and Power Consumption

Another important factor is the size and weight of the ADCP: smaller and lighter are more practical to deploy from small boats or buoys, or on the seabed, and disturb less the water around it-a feature that again can reduce errors in measurement. The power consumption is another important feature, especially when longer-term deployments are envisaged. These would enable a continuous operation of the instrument, with less frequent battery replacements or recharges of the powering source.

Cost-effectiveness

Cost is an important factor, especially when large-scale measurements need to be effected. In the case of monitoring coastal currents around Bordeaux, for example, several ADCPs may need to be deployed. The equipment should thus be cost-effective. This relates not only to the initial purchase cost but also to the costs associated with maintenance, calibration, and data acquisition.

The Advantage of Titanium Alloy Casing

A casing made of titanium alloy is very serviceable for ADCPs in the Bordeaux coastal area. Generally, titanium alloy has excellent corrosion resistance, which is very vital in a salt-water environment. It is capable of resisting seawater corrosion over a long period without significant deterioration. Besides this, titanium alloy also has a high strength-to-weight ratio. That would mean that the casing is tough, long-lasting, yet considerably lightweight; therefore, handling and deployment of the ADCP flow meter become a lot easier.

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

Based on Usage

• Ship-borne ADCP: It can be used to perform large-scale surveys of coastal currents. It can measure currents from place to place while the ship is cruising along the coast. It's useful in mapping overall current patterns and understanding the spatial variability of currents.
• Bottom-mounted ADCP: These are mounted on the sea floor and represent a practical means for acquiring long-term measurements from fixed-point locations. With the capability of bottom-mounted instruments, there is quite detailed information concerning current conditions that might exist at that one place: the near-bottom current velocity, its direction. It is interesting to note here that all this information could be of use in sediment transport and the interaction of the water with the seabed.
• Buoy-mounted ADCP: Buoy-mounted ADCPs are attached to floating buoys and are used for the measurement of currents in areas where it is difficult to measure with ships. They can provide real-time data on the surface and near-surface currents and are useful for monitoring short-term current variability.

Based on Frequency

• A 600kHz ADCP may be appropriate to water depths of about 70m. High-resolution measurements will provide detailed current structure for shallower waters near Bordeaux.
• A 300kHz ADCP may allow operation in depths up to approximately 110 m. It gives a good balance between depth penetration and resolution, which can be suitable for mid-depth applications.
• In deeper waters, say up to 1000m, a 75kHz ADCP is more suitable. Though providing lower resolution than higher-frequency ADCPs, it can go deeper into the water column.

Up till now, most well-known brands in ADCPs include Teledyne RDI, Nor-tech, and Sontek. China is producing cost-effective, high-quality ADCPs named Sonar Panda from the China Sonar brand PandaADCP, made of complete all Titanium alloy material ensuring its durability, hence reliability for any mission to produce excellent quality in cost-to-performance ratio, and their web could be looked through the following: https://china-sonar.com/.

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