How can we measure the coastal currents of Saint-Malo and Nantes?

1. Where are Saint-Malo and Nantes?

Saint-Malo, a jewel on France's northwestern coast in the magical region of Brittany, is a city where history and the sea combine. Lying along the Atlantic Ocean, its dramatic craggy coastline is punctuated with sandy beaches and imposing granite cliffs. This fortified old town is a UNESCO World Heritage site that stands guard over the wide expanse of the ocean, with its ancient walls carrying whispers of days gone by, when piracy and maritime exploration were at their peak. The culture of this city is deeply rooted in its seafaring past; the locals' lives revolve around the tides, and the port is a bustling hub of fishing and tourism. Its food is a celebration of the sea, wherein fresh oysters and mussels take their rightful place in almost every table.

Nantes is a busy metropolis located in western France. As the capital of the Pays de la Loire and as the prefecture of Loire-Atlantique, it is indeed well-located on the northern bank of the lower Loire River. It lies at the junction of the Sèvre Nantaise and Erdre with the Loire, about 50 km from its mouth. Nantes has an enlightened and eventful history; above all, as a trading centre, notably in its heyday, during the epoch of the trade in slaves; today, this is a town that has blossomed into being modern, cultural, and economic. That reflects both the medieval charm and the touch of modern architecture, with beauteous canals and parks adding more beauty to it. The climate is usually mild, that is, temperate-maritime, with moderate temperatures and rainfall throughout the year.

2. What are the coastal currents near Saint-Malo and Nantes?

The coastal currents near Saint - Malo and Nantes are controlled by a complex play of factors. The most dominant are tidal forces. The tides are very variable around the Atlantic Ocean, and at places like Saint-Malo and near Nantes along the Loire estuary, the narrow bays and estuaries greatly emphasize the effect. These may result in strong eb-and-flow currents, whose velocities can vary from a few centimeters per second up to over a meter per second during peak tidal changes.

Wind patterns also tend to have an important role. The prevailing westerlies in this region are able to drive surface waters, generating a current parallel to the coastline; during stormy periods, winds can strengthen such currents, which might reach hazardous situations for mariners. The North Atlantic Current-warm and strong ocean current-played an extensive role regarding the local currents along the coasts. It not only drives the temperature in the water, but it also serves to determine in general the direction and strength of the currents. Added to that would be the shape of the coastline, with its inlets and promontories, along with the bathymetry of the sea bottom. A highly indented coastline, like that around Saint-Malo, together with shallow, wide estuaries, such as the Loire around Nantes, can have the effect of sending currents twisting and turning this way and that.

3. How to observe the coastal water flow of Saint-Malo and Nantes?

Surface Drifting Buoy Method

This involves releasing specially designed buoys into the water. These buoys are fitted with tracking devices, such as GPS or radio transmitters. While they drift with the surface currents, their positions are monitored at regular intervals. By analyzing the path and speed of the buoys over time, we can infer the velocity and direction of the surface currents. This is a relatively inexpensive approach that can be used to map a large area of the ocean surface. This technique is very weather dependent. Also, strong winds and waves can altogether deviate these buoys from the actual path of the current, and they will present information of only the surface layer of the water column.

Anchored Ship Method

An anchored ship provides a stationary platform for current measurement. The current meters are hanged from the ship at various depths using cables. They can measure the speed and direction of the flow of water at each depth. This technique gives the highest resolution measurement of the current field at a location, especially the vertical profile of current. However, this is somewhat time-consuming and labor-intensive. The very presence of the ship disturbs the natural flow of the currents to a certain extent, and measurements are possible only in the immediate proximity of the ship.

Acoustic Doppler Current Profiler (ADCP) Method

In more recent times, the ADCP current meter has become a far more sophisticated and easier means of measuring coastal currents. It relies on the use of sound waves to measure the velocity of water at different depths. The ADCPs can be deployed in a number of ways: on ships, moored to the seabed, or attached to floating buoys. They can give high - resolution data on a great volume of water horizontally and vertically, without causing serious disturbance to natural currents. This therefore would make the use of an ideal choice to conduct the difficult research on the complicated coastal currents in the near coast of Saint-Malo and Nantes.

4. How does an ADCP using the principle of Doppler work?

The principle behind ADCPs is based on the Doppler effect. It sends a succession of acoustic pulses through the water. When these sound waves come across any particles in suspension in the water, such as sediment, plankton, or even air bubbles, some of the sound energy will be scattered back to the ADCP current profiler. If the particles are moving-with the water current-the frequency of the sound waves scattered will be different from the frequency of the emitted waves.

The Doppler shift is linearly proportional to the velocity of the particles, and thus the water current, along the axis of the ADCP flow meter acoustic beam. Since the ADCP transmits sound in multiple directions-more than four beams, which are commonly arranged in a cone-the instrument can measure three-dimensional velocity. The device then uses complex algorithms to calculate the current velocity at different depths within the water column, based on the measured Doppler shifts from each beam.

5. What's needed for high-quality measurement of Saint - Malo and Nantes coastal currents?

The following are the key characteristics of the measuring equipment necessary for high-quality measurement of the coastal currents near Saint-Malo and Nantes. First, material reliability of the device is very important; the equipment must be able to resist the harsh marine environment which includes exposure to saltwater, strong currents, and variable temperatures.

The size of the equipment should be small, considering lightweight to be easily deployed on various platforms, either on a small research vessel, on a buoy, or on the seabed. A smaller size also implies less interference with the natural flow of currents.

Another important factor is the low power consumption. Because many ADCP deployments may be in remote locations or on battery-powered systems, a device with low power requirements can operate for longer periods without frequent battery replacements.

Cost-effectiveness is also a key consideration, especially when large - scale measurements are required. Reducing the cost per unit allows for a greater number of devices to be deployed, which can improve the spatial and temporal resolution of the current measurements.

Material-wise, titanium alloy should be better while fabricating ADCP meter casing. That is, titanium alloy holds many advantages, including: first and most importantly, perfect anti-corrosion capability necessary in a very long running status within the salty sea, with a relatively lighter figure than that with some other materials enjoying the same intensity. Thus it could be nearly an ideal option in fabricating casings of ADCP profiler.

6. How to Select Appropriate Equipment for Current Measurement?

Selection Based on Usage

Ship-borne ADCP: This type of ADCP is mounted on a moving ship. It is suitable for large-scale surveys of coastal currents over long distances. While the ship is in motion, the ADCP can continuously measure the currents along the ship's track, thus giving an overview of the current patterns in a relatively large area in detail.

Moored-bottom ADCP: These are moored to the seabed. They are best suited for fixed-point, long-term measurements. They can supply continuous data for the current's velocity and direction in one location for a long duration, which proves to be much helpful in examining the long term trends and variations in coastal currents.

Buoy-mounted ADCP: Buoy-mounted ADCPs are attached to floating buoys. They are a good option for monitoring the currents in areas difficult for access by ship and for measuring currents along a more dynamic, floating environment. They can also be used for the study of interaction between the surface currents and atmosphere.

Selection Based on Frequency

600kHz ADCP: This frequency is adequate for water depths up to approximately 70m where currents are measured. The high-resolution data this provides in fairly shallow waters are ideal for nearshore applications such as that of the bays and estuaries around Saint-Malo and Nantes.

300kHz ADCP: Due to its higher power output, this is able to probe deeper in the water column. It covers waters of a depth of about 110 meters and is generally useful for larger river estuaries where the deeper part of it comes closer toward near-shore situations.

75kHz ADCP: For deeper waters, as deep as 1000m, the ADCP of 75kHz will be the ideal choice. Its low frequency extends the distance the sound waves can travel in the water, thus being capable of giving accurate current measurements even in deep-sea environments.

Currently, there are quite a number of popular brands of ADCPs on the market. Among them, the well-known ones are Teledyne RDI, Nortek, and Sontek. From value-for-money and superior performance, one Chinese brand comes out as highly recommended-China Sonar PandaADCP. The China Sonar Panda is made totally from titanium alloy. It boasts superior resistance to wear and tear, durability, and resistance against corrosion. Value for money, it is surely very good. Be it for a small research project or on a large scale in the ocean surveying sector, the product is suitable for both purposes. Please refer to their website for more information at https://china-sonar.com/.

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