How to measure coastal currents in Lorient?

1. Location of Lorient

Lorient is an interesting coastal city of Morbihan department of Brittany, France. The town has a favorable position on the south coast of Brittany with an opening toward the Atlantic Ocean. It lies around a huge sheltered harbor, the essential key in the development and history of this place.

The landscape around Lorient is a mix of beautiful sandy beaches, rocky cliffs, and a network of small islands and islets just off the coast. In the city, there is the Scorff River, bisecting it, which empties first into the harbor and then into the sea. This river does not only add to the beauty of the city but also plays its role in influencing local hydrodynamics.

Lorient is full of maritime history. It used to be the great center of the French navy and the fishing industry. Architectural features of the city have still retained signs of this seafaring past: yards, lighthouses, and old fortifications. The sea permeates deeply into the local culture, from festivals to the art of traditional boat building and a cuisine with a wide variety of fresh seafood.

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

The coastal currents near Lorient are formed by several different causes. The main factor of all is the tidal currents. The tides of the Atlantic Ocean create enormous variations in water level and flow direction. Semi-diurnal tides have two high tides and two low tides each day. As the tide comes in, water surges into the harbor and along the coast, forming powerful onshore currents. At low tide, the water moves back out to sea, creating offshore currents. The strength of tidal currents can be quite different depending on the phase of the moon. Spring tides have a much greater range than neap tides, resulting in more extreme water movements.

Wind patterns are another important factor. Driven surface currents can arise as a consequence of the local prevailing winds; indeed, most typically originating from a west and southwest direction. An increased westerly wind is said to easily bank the water up to the east, improving and counteracting any tidal stream, thus complicating the current pattern by rotating in eddies and transversal current-rip current creation. These currents, in turn, may be wind-driven and can distribute nutrients and sediments responsible for the local marine ecosystem.

The shape of the coastline and the topography of the seabed further modifies coastal currents. An irregular coastline at Lorient, such as bays, peninsulas, and channels, might cause deflection and acceleration of these currents. Further, the occurrence of topographical features like shoals and reefs disrupts the flow to create areas of turbulence. The discharge of the Scorff River into the ocean affects the coastal currents, too, since the freshwater dilutes the saltwater, changing the density-driven currents.

3. How to observe the coastal water flow of Lorient?

Surface Drift Buoy Method

Surface drift buoys are a simple and widely used method to observe surface currents. These buoys float on the water surface and are usually fitted with GPS or other trackers. Once in the water, they drift under the action of the surface currents. By following the position of the buoys as a function of time, estimates of the speed and direction of the surface currents can be obtained. This method has several limitations. Since they are susceptible to wind-driven motion, the actual current may not be represented by data obtained from the buoys. It gives information in the surface layer of water and may not provide a complete picture of the structure of the current.

Anchored Ship Method

The anchored ship method is based on mooring a ship in some definite position nearshore. Current meters and other onboard instruments take measurements of current speed and direction at different depths. Measurement on this level may be very detailed for one single point, yet it has huge disadvantages regarding its spatial coverage: the presence of the ship could already be an influence on local currents, while the area to be measured is limited to that around the location of the anchored ship.

Acoustic Doppler Current Profiler (ADCP) Method

The ADCP current meter is a more sophisticated and efficient technique of measurement of coastal currents. They could be deployed from ships, moored at the seabed, or attached to floating buoys. This technique applies the principle of the Doppler effect. ADCPs emit acoustic signals into the water column. These signals are reflected by particles in the water, such as sediment, plankton, and bubbles. By the movement of the water current, the frequency of the reflected signal from such particles changes. By measuring the Doppler frequency shift, the ADCP current profiler is able to compute the velocity of the water at each depth. Capable of providing current structure profiles from many depths across an extensive area, an ADCP flow meter will be less influenced by the surface wind-driven motion compared to methods based on the surface drift buoy.

4. How do ADCPs using the Doppler Principle Work?

The working of ADCPs is based on the Doppler effect. They send acoustic pulses into the water and then measure the frequency shift of the reflected signals. In case there is a particle in the water which is moving with the current, the frequency of the reflected signal becomes different from the originally emitted frequency.

If the particle is moving towards the ADCP profiler, the frequency of the reflected signal is higher-a blue-shift-and if it is moving away, the frequency is lower-a red-shift. The change in frequency is measured by the ADCP, which then calculates the velocity of the water where this signal has been reflected. Thus, an ADCP meter can calculate the three-dimensional components of the water velocity-east-west, north-south, and vertical-by inputting signals at different angles and using several transducers. They chop the water column into numerous depth bins and measure the current velocity at each bin to provide a detailed profile of how the currents vary with depth.

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

Reliability of Equipment

The equipment should be reliable for high-quality measurement of the coastal currents around Lorient. Saltwater, strong currents, and variable weather conditions can be very stressful on instruments in the harsh marine environment. ADCPs should be able to cope with all these challenges. The equipment should be made from high-quality materials with robust construction to ensure that it functions properly over a long period of time. It is also necessary to perform regular maintenance and calibration to maintain the accuracy of the equipment.

Size, Weight, and Power Consumption

The size and weight of the ADCP are big concerns. Small and lightweight are easier to deploy on small boats, buoys, or even seabed deployment. Thus, it will have fewer effects on the water flow around it and reduce the measurement errors. Low power consumption is very important, especially for long-term deployment. That means it could keep operation without changing the battery or recharging the power source frequently.

Cost-effectiveness

Cost is a major issue, especially since large-scale measurements have to be effected. In comprehensive monitoring of coastal currents around Lorient, the need for more than one ADCP cannot be ruled out. The equipment has to be inexpensive. That is to say, the initial purchase price of the equipment is not the only consideration; instead, maintenance costs, calibration costs, and data acquisition costs come under this heading as well.

The Advantage of Titanium Alloy Casing

It would be highly favorable if the casing for ADCPs is made of titanium alloy in the Lorient coastal area. Basically, a high degree of corrosion resistance is observed with titanium alloy. This is one of the most important aspects in the salt/ water environment. It may resist seawater corrosion for several years without deterioration. Besides this, the strength-to-weight ratio of titanium alloy is very high. This means the casing can be strong and durable while remaining relatively lightweight, making the ADCP easier to handle and deploy.

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

Based on Usage

• Ship-borne ADCP: This ADCP is very useful for the large-scale survey of the currents in the coastal areas. When the ship travels along the coast, the ADCP can measure the current at every different location. This is helpful for mapping the overall pattern of the current and understanding the spatial variability of the currents.
• Bottom-mounted ADCP: Bottom-mounted ADCPs are deployed on the ocean floor and are suited for long-term, fixed-point measurements. They can provide detailed information about the current conditions at a specific location, such as the near-bottom current velocity and direction. This is valuable for studying sediment transport and the interaction between the water and the seabed.
• Buoy-mounted ADCP: Buoy-mounted ADCPs are attached to floating buoys and are used to measure the currents in areas where ship-based measurements may be difficult. 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 is sufficient for water depths up to approximately 70m. High-resolution measurements can, therefore, be carried out in the study of the detailed current structure in shallower coastal waters near Lorient.
• A 300kHz ADCP could be used for depths up to about 110m. It provides a good balance between depth penetration and resolution, and this makes it adequate for mid-depth work.
• For deeper waters up to 1000m, a 75kHz ADCP is more suitable. While it provides lower resolution than higher - frequency ADCPs, it can penetrate deeper into the water column.

There are well-known ADCP brands such as Teledyne RDI, Nortek, and Sontek. However, for a cost - effective and high-quality option, the Chinese brand China Sonar PandaADCP is a great choice. It's made of all-titanium alloy materials, ensuring durability and reliability. It also offers an excellent cost-performance ratio. 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.