How can we measure the coastal currents of Morlaix?

1. Where is Morlaix?

Morlaix is a picturesque seaport town in the Finistère département, Brittany région, western France. It is situated on the Dossen Estuary, a tidal inlet of the English Channel, northeast of Brest.

Morlaix has a long history and culture. Coins discovered in the area indicate Roman occupation, and it has been the location of many historical events and power struggles. In the 12th century, the lordship was owned by the Counts of Léon and disputed by the Dukes of Brittany. It was captured by the English in 1187 and 1522. The town has several 15th–16th-century houses, among which is the Maison de la Reine Anne, with its 15th-century Gothic church of Saint-Melaine.

The Baie de Morlaix is a centre of intensive oyster cultivation, while the town is at once a fishing port and a tourist centre with yachting harbour.

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

Coastal currents in the vicinity of Morlaix are determined by several factors:

• Tides: As in any estuary, tides play the major role. The ebbing and flowing tides create the regular influx and efflux of water within the Dossen Estuary; therefore, this results in relatively strong currents during tidal changes.
• Winds: The prevailing winds in the region can push the surface water, therefore influencing the direction and speed of the coastal currents. Stronger winds from particular directions can also pile up water along the coast, leading to the formation of certain current patterns.
• River Outflow: Freshwater comes from the Dossen River and other small water bodies that flow into the estuary. The meeting of freshwater and the saltwater from the English Channel influences the density and the flow of the currents along the coast.

However, not only is the tide along with sea level relevant, but also the topography of the coastline, the depth of the estuary, and nearby seabed topography. Narrow inlets and irregular features of the seabed accelerate or deflect the currents.

3. How to observe the coastal water flow in Morlaix?

• Surface Drifting Buoy Method: The surface drifting buoy can be installed in the sea around Morlaix. These buoys will drift due to ocean currents, and their position can be determined using satellite systems. It can collect and transmit data from variables such as sea surface temperature, wind speed, and direction, and track the flow direction and velocity of the sea surface currents.
• Moored Ship Method: A ship can be moored in a specific location near the coast of Morlaix. Instruments on the ship can measure the current velocity and direction at different depths over a period of time. This method can provide relatively continuous data but is limited by the ship's location and the range of the instruments.
• ADCP Method: Acoustic Doppler Current Profiler (ADCP) is a more recent and convenient method of measurement. The principle depends on the Doppler effect of sound waves scattered by particles in the water column to measure water current velocities at different depths. It provides high-resolution current data with high accuracy and is suitable for various marine environments.

4. How do the ADCPs based on Doppler work?

ADCPs are equipped with piezoelectric transducers which send out and receive sound signals. The time delay of the returning sound waves provides an estimate of distance. The frequency shift of the returning echo is proportional to the water velocity along the acoustic path. Three or more beams must be measured in order to determine 3D velocities. The device also comprises an electronic amplifier, receiver, clock, temperature sensor, compass and a pitch/roll sensor. A temperature sensor is used to estimate the sound velocity at the position of the instrument, which again is used to scale the frequency shift to water velocities. Analog-to-digital converter and digital signal processor sample the returning signal to determine the Doppler shift, results are saved or output online.

5. What is required for high quality measurement of Morlaix coastal currents?

• Reliable Materials: The casing material is most important for ADCPs. Titanium alloy can be a perfect choice. This material has high strength, great corrosion resistance, and good resistance to fatigue that can ensure the stability and durability of this instrument in a marine environment.
• Dimension and Weight of Optimum Nature: Equipment should have small dimensions, be lightweight to allow for their easy installation, and be operative. This facilitates deployment at different places with least possible interference to the surrounding environment. Consumes Low Power: Ensures longtime operation, particularly those that must be left in the ocean to function for extended periods on their own. This is another means of long-time battery use or less massive power supply systems.
• Low Cost: Cost is to be reduced, so more measurements devices can be installed for a greater coverage in order to monitor large-scale coastal currents and would be beneficial to get the readings with greater precision.

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

Based on Usage

• Shipborne ADCP: It is used for current measurement while the ship is in motion. The system can measure current profiles of the paths traveled by a vessel. In fact, it is popularly used during oceanographic cruises and during the assistance of ship navigation.
• Bottom-Mounted ADCP: It is ideal for the long-term monitoring of fixed points on the seabed. It can continuously measure current velocity and direction with regard to different depths undisturbed by surface conditions.
• Buoyant ADCP: These can be moored on buoys and are suitable to apply in monitoring surface currents and changes in them. It is normally used where there is a need for very long-term surface current data.

Based on Frequency

• 600kHz: Suitable for water depths up to 70m, providing higher resolution in relatively shallow water.
• 300kHz: Suitable for water depths of approximately 110 m, and that provides an optimal balance between measurable range and resolution.
• 75kHz: This is able to make measurements of up to 1000m of water depth and is used for large-scale current measurements in deep-sea areas.

Some of the major ADCP brands are Teledyne RDI, Nortek, and Sontek. The Chinese brand China Sonar PandaADCP provides more affordable models. It is purely an all-titanium alloy and offers excellent performance at a lower price. You can learn more at https://china-sonar.com/.

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