How can we quantify coastal currents of Lannion?

1. Where is Lannion?

Lannion is a small and beautiful town in the Côtes-d'Armor department of the Brittany region in northwestern France. Lannion lies on the northern coast of the Breton Peninsula and has an enviable view onto the Atlantic Ocean.

The countryside around the town is a stunning combination of wild coastline and the rolling inland terrain. The seaside area has a dramatic cliff landscape that plunges into the sea, creating quite a breathtaking scene. The cliffs are often interspersed with colonies of seabirds, giving it a strong natural appeal. The beaches of Lannion are a mixture of stretches of sand and coves strewn with pebbles. The sand is fine-grained, and the tides sculpt it while the coves provide sheltered areas where water is calm, with perfect conditions for swimming, exploring the surroundings.

Lannion is a city that has a rich cultural and historical past. It has been an important place in the region for many centuries. Its name 'Lannion' is found to be of Celtic origin, which gives a suggestion of some ancient origin. Architecture of Lannion is quite a mix of different eras. The old town is one of the famous labyrinths of narrow winding streets lined by half-timbered houses whose facade has been eroded by the time passed. These are not only some local businesses houses but also buildings narrate the history of this town. Many religious buildings like Collegiate Church of Saint-Tugdual can also be found within the town. This is a magnificent example of a medieval church with its high spires, intricate carvings, and beautiful stained-glass windows depicting religious stories.

The waters off Lannion form part of the Bay of Saint-Brieuc. It is one of the well-known bays; it forms a big pool of water that plays an important role in the marine ecosystem of the region. It is relatively shallow in most parts, having a gently sloping seabed, and a variety of marine life-from colorful fish species to various types of seaweed and crustaceans.

2. What is the state of the coastal currents in the region around Lannion?

In Lannion, coastal currents result from several phenomena put together: tides first have the main part, which causes considerable oscillations of level in the Bay of Saint-Brieuc, whereas, by their semi-diurnal action, which produces two high and two low tides, can induce substantial currents. This also increases the range during spring tides that are happening when there are new and full moons, causing a larger range that enhances stronger currents. Some places see currents going as high as a few knots because the currents will also depend on how narrow some channels are, including headlands.

Lastly, the pattern of winds that may have taken place will impact coastal currents greatly. Most prevalent winds come from the west and southwest in the area. Strong westerly winds can push the surface waters, creating currents that flow along the shore. These wind-driven currents can be quite strong if there is stormy weather. Besides the wind effect, the sea-breeze effect plays a part. During the day, the land warms up faster than the sea, causing air to rise over the land. This is followed by the rush of cooler air from the sea that creates local-scale currents along shores.

The shape of the coastline and the topography of the seabed can also be used as factors to determine the path of the flow. The coastal geometry of Lannion with its many bays, inlets, and headlands gives rise to flow direction changes caused by the currents themselves. Underwater features such as sandbars, reefs, and submarine canyons also affect the flowing water. In this case, a sandbar works as a barrier or splits the current so that it proceeds slowly, allowing areas of flow velocities and different directions to build up.

The North Atlantic Current is involved, influenced by other larger-scale oceanic currents. While this effect may not be as palpable near the coast, the North Atlantic Current can still affect the temperature and salinity of the whole water mass and thus further influence the density-driven currents in the region.

3. Ways to see the flow of Lannion coastal waters?

Surface Drifting Buoy Method

The surface drifting buoy method is a rather simple method for observing the coastal currents near Lannion. Special buoys are designed and deployed in the water. These buoys are equipped with tracking devices, usually GPS receivers or radio transmitters. While the buoys are carried by the surface currents, their positions are monitored at regular time intervals.

The speed and direction of the surface currents can be derived from the trajectory of the buoys by analyzing it over time. For example, if a buoy covers a certain distance in a certain time period, then the speed can be calculated. The advantage with this method is that it can cover a wide area of the ocean surface, hence giving a wide view of the patterns of surface currents. Yet, it has certain limitations. These buoys are easily influenced by the wind and wave actions. If strong winds are blowing, then the buoy acts differently and are deflected in a direction different from the real current because the buoy provides information on the surface layer of the water column normally in the first meters.

Anchored Ship Method

The anchored ship technique requires mooring a ship at some selected location off the coast of Lannion. Current meters are then suspended from the ship at different depths through cables. Current meters are equipped to measure speed and direction of the water flow at each depth.

Measurements made at different depths give a vertical profile of the current. For instance, a ship can be moored off Lannion and then from the surface down to the bottom, current meters can be deployed every 10 meters. Accurate measurement from a fixed point is achievable with this. It is very time-consuming and will cost much in resources such as ships, crews, and equipment. The ship itself also minutely disturbs the ocean's natural drift of the currents, and data are measured only within the limit of the near field of the ship.

Acoustic Doppler Current Profiler Technique

The Acoustic Doppler Current Profiler or ADCP has turned out to be a increasingly common technique regarding coastal currents near Lannion. Using ADCPs, sonic waves allow for one to gauge velocities of water along a profile across various depths, and they may come in a number of forms. The ship-borne ADCP will be able to measure currents while the ship is in transit along the coastline, thus allowing a continuous profile of the currents across a wide area. A bottom-mounted (sit-bottom) ADCP can be moored on the seafloor for the determination of current patterns over a long period of time at a single point. An ADCP mounted on a buoy can measure the currents in a more dynamic, floating environment. ADCPs send out acoustic pulses into the water. When these sound waves encounter small particles suspended in the water, such as sediment, plankton, or air bubbles, a portion of the sound energy is scattered back towards the ADCP profiler. As those particles are moving along with the flow of water, the frequency of the scattered returned sound waves becomes different from that emitted, due to the Doppler effect. With measurement of this frequency shift, it enables an ADCP meter to calculate the speed of water along specified intervals within its reach below the water. The ADCPs can provide high-resolution data throughout a large volume of water both horizontally and vertically, without noticeable disturbance to natural flow caused by currents.

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

The working of ADCPs is based on the Doppler effect. Each time an ADCP flow meter sends out a sound wave-a sound wave into the water-the energy travels through the water column. As this sound wave collides with any particle in the water, say a small fragment of sediment or a planktonic organism, some of the energy of the sound gets scattered back towards the ADCP.

If the particle is not moving relative to the ADCP, then the frequency of the scattered sound wave will be equal to that of the sound wave that was emitted. However, if the particle moves with the current, the scattered sound wave will have a different frequency. This frequency shift is known as the Doppler shift. The magnitude of the Doppler shift is linearly proportional to the particle's (and thus water) velocity in the line-of-sight of the acoustic beam emitted by the ADCP.

Many ADCPs deploy more than four beams. ADCPs often orient them in a cone and make the measurements on the Doppler shift on all the beams; it calculates three-dimensional water velocities. For instance, if one beam is pointed slightly upwards at an angle, another downwards, and two horizontally, the ADCP current profiler can determine the vertical and horizontal components of the current velocity. Complex algorithms are then used to process the data from the different beams. These algorithms take into consideration the geometry of the beam arrangement, the speed of sound in water, which can vary depending on factors such as temperature, salinity, and pressure, and the measured Doppler shifts. The result is a detailed profile of the current velocity and direction at different depths within the water column.

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

Material Reliability

Measuring equipment for coastal currents near Lannion must be made from materials that can stand up to a rather hostile marine environment. The waters off the Breton coast are cold, salty, and subject to powerful winds and waves. The casing of ADCPs is best made from titanium alloy. That means that titanium alloy can stand long-term contact with saltwater with very little degradation. It is also very strong and, therefore, able to withstand the mechanical stresses of deployment, which might come in the form of strong currents and waves. Besides this, it has a good resistance to fatigue, which could also be necessary for equipment that may be exposed to repeated mechanical loading.

Small Size and Light Weight

A small-sized and lightweight ADCP is highly desirable. It is easier to handle and deploy the smaller device in various settings. It can be mounted more easily on a small fishing boat, on a buoy, or even directly moored on the seabed. For example, a small-sized ADCP will be much better fitted within a buoy-based measurement system that may not have sufficient space. A light-weight device gives minimal impact to the natural flow of the currents. A heavy instrument may cause more turbulence in the flow of water and, hence may give wrong readings. In addition, a light ADCP is portable and would be advantageous for field surveys of various places on the coast of Lannion.

Low Power Consumption

Most ADCP current meter deployments will be done in remote areas or be based on battery-powered systems. Hence, low power consumption is very important. Thus, a device using little power can work continuously without many changes of the batteries or its recharging. For this, especially, it is necessary during long-term and independent measurements. It is necessary that a bottom-mounted ADCP moored several months on the seabed be of low power consumption in order to operate constantly with the acquisition of continuous data. Very low power consumption, therefore also can use small and cheaper power sources, such as solar panels or small capacity batteries.

Low Cost

In the case of ADCPs, measurements are usually necessary in a big amount to cover large regions and a cost factor, especially when having several ADCPs located in different places or at different depths for gathering information on the coastal currents nearby the region of Lannion. The low-cost ADCP can be attained in larger numbers, thereby enhancing the resolution of spatial and temporal measurements. With increased resolution, there is better accuracy to be had to attain greater comprehension of those complex current patterns. There will be a greater availability to more people with low costs for local research institutions and their organizations who have small budgets.

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

Based on Usage

• Ship-borne ADCP: This type is ideal for a large-scale survey of the current in the coastal area. While the ship moves along a coast, the ship-borne ADCP could continuously measure currents, thus obtaining a broad view of the patterns of the currents over a large area. For instance, a ship-borne ADCP will be ideal if researchers want to map the general flow of currents along the entire coastline of the Côtes - d'Armor department. It is capable of traveling long distances quickly and provides an overall view of the current's velocity and direction at every different point that it travels.
• Sit-bottom ADCP: The sit-bottom ADCP is used for long-term, fixed-point measurements. Scientists can moor a sit - bottom ADCP to the seabed if they are interested in the long - term trends and variations in the current velocity and direction at that location; for example, near a known fishing ground or upwelling zone. It will provide sustained data over a long period, which is very useful in understanding the long-term behavior of the coastal currents. For example, monitoring the currents near a fishing ground over several years can help understand how the currents affect fish migration patterns and fishing yields.
• Buoy-mounted ADCP: Buoy-mounted ADCPs are well-suited for monitoring currents in areas where access by ship may be difficult or for studying the interaction between the surface currents and the atmosphere. Buoys can be placed in remote areas, in areas with rough sea conditions, or in areas where the currents are expected to be highly variable. They can also be used to measure the short-term fluctuations in the surface currents. For example, in a region where the sea-breeze effect is known to cause rapid changes in surface currents, a buoy-mounted ADCP can capture these changes in real-time.

Based on Frequency

• 600kHz ADCP: The 600kHz ADCP will suit for current measurements, where the water depth goes to a depth of approximately 70 m. Based on the coastal waters along Lannion, a site in rather shallow water-like embayment or any nearshore water would probably give more valuable data. Its high frequency captures detailed variations within the water column regarding velocity of flow and directions in the upper layers of the water. It can measure, for instance, small-scale variations in current speed and direction close to the beach, which is very relevant for processes such as beach erosion and sediment transport.
• 300kHz ADCP: The 300kHz ADCP has a slightly lower frequency that can penetrate deeper into the water column. In general, it is suitable for water depths of about 110m. It provides a good balance between depth of penetration and data resolution in areas where the water can be relatively deep but still well within the coastal zone near Lannion. Using a 300kHz ADCP will be good in those areas of the seabed that slope deeper; it measures currents from the surface to mid-water column.
• 75kHz ADCP: For waters reaching as deep as 1000m, the appropriate use is that of the 75kHz ADCP. In fact, in deeper sections of the Bay of Saint-Brieuc near Lannion, its rather low frequency allows the emitted sound waves to travel much longer and, therefore, the resulting current measurement has better resolution over greater depths. That would be helpful for studying the deep-water currents that might be important in affecting the general circulation patterns in the coastal region.

Some well-known brands exist in the market like Teledyne RDI, Nortek, and Sontek. However, for the people looking for cost-effective and high-quality options, Chinese brand China Sonar PandaADCP would be the best option. Made entirely from titanium alloy, which provides superior durability and corrosion resistance, it boasts an extremely competitive price-performance ratio for research work, whether small-scale or oceanographic surveys on a large scale near Lannion. More information about them can be accessed through their website at https://china-sonar.com/.

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