How do we measure the currents of Vannes along the coastline?

1. Where is Vannes?

Vannes is a city full of history and culture in northwestern France in the heart of Brittany. Located on the southern shore of the Gulf of Morbihan, the city enjoys an exceptional geographical situation that has inspired its development through the centuries.

The old town is a historic gem recognized by UNESCO. The half - timbered houses seem to whisper tales of a bygone era, the narrow, winding streets lined by them. Its imposing ramparts, once the stronghold of mighty defense, today offer breathtaking views of the surroundings. The warm hospitality and the deep-rooted love for the sea make the locals a rich heritage of maritime life. Fishing and sailing are not only professions but a part of the local culture. There are various festivals and events held throughout the year to celebrate the sea.

The Gulf of Morbihan, situated next to Vannes, is a body of water that enchants all who encounter it. With more than 400 islands, some large enough to accommodate charming villages and others just rocky outcrops, it is an extremely varied and constantly changing landscape. The waters are a haven for a wide variety of marine life, from colorful fish to majestic seabirds. Another reason the gulf is a favorite among water sport enthusiasts is the calm, sheltered bays that are the paradise for sailing, kayaking, and windsurfing. Estuaries feeding into the gulf add to an important load of nutrients, making the ecosystem even more productive.

2. How are the coastal currents near Vannes?

The coastal currents around Vannes are influenced by one complicated interplay of factors. Among the most significant tidal forces, there are semi-diurnal tides in the Gulf of Morbihan. This kind of tidal pattern makes water flow into and out of the gulf in a very periodic fashion. During high tide, water rushes into the gulf, making flood currents, while ebb currents carry water back to the open sea during low tide. These tidal currents can be very strong, especially in the narrow channels between the islands.

The shape of the coastline and the bathymetry of the gulf determine the current patterns. The many inlets, bays, and the irregular shoreline cause the currents to diverge and converge. For instance, constriction of water flow in narrow passages increases the velocity of the current. The flow of water is also affected by the varying depths of the gulf, from shallow coastal areas to deeper central regions. Deeper areas tend to have more stable currents, while shallower regions are more prone to the influence of local winds and tides.

Wind is another key factor. Prevailing winds from the Atlantic Ocean can have a significant impact on the surface currents. Westerly winds, which are common in this region, can push the surface waters towards the east, interacting with the tidal currents. In some cases, strong winds can even override the normal tidal flow, creating temporary changes in the current direction and speed.

3. How to track the coastal flow of Vannes?

Surfaces Drifting Buoy System

The basic method of capturing the coastal flows is the simple surface drifting buoy system. Essentially, buoys are small floats that swim above the water but follow the tide. They do have GPS enabled tracking devices along with them whereby their positions get tracked at prescribed intervals. By analyzing the change in the buoy's position over time, scientists can determine the direction and speed of the surface currents. However, this method has limitations. It only provides information about the surface layer of the water column, and the buoys can be affected by wind and waves, which may cause them to deviate from the actual current path.

Anchored Ship Method

In the anchored ship technique, a ship is moored in a known location and measurements of the velocity of the current are taken with the help of current meters at different depths. The current meters are lowered into the water from the ship. The meters record the velocity of the passing water, thus giving the details of the vertical profile of the currents close to the ship. However, it is rather limited as far as spatial coverage is concerned. It represents the measurement of the immediate area where the ship has gone through and has to move into different places in order to provide comprehensive data collection. This often tends to be highly time-consuming and expensive.

Acoustic Doppler Current Profiler (ADCP) Method

The ADCP method has proven to be an advanced, more practical technique for coastal current measurement. ADCPs can collect the velocity measurements of water simultaneously at several layers of depth. They can either be towed from a moving ship, be moored directly to the floor of the seafloor or attached to sea buoys. In case ADCP current meter is towing from a vessel, it records a continuous section of currents throughout the path trace of the towing vessel. Whenever it is set on the ocean floor, its long -term changes in specific locations are monitored. These buoys-mounted ADCPs can easily follow the movements of the currents and provide crucial data on dynamic behavior of water flow. Because of this adaptability, it becomes the first option for many oceanographic studies.

4. How does a Doppler-principle-based ADCP work?

ADCPs operate using the Doppler principle. They transmit sound waves into the water. These sound waves are backscattered by small particles suspended within the water column, such as plankton, sediment, or bubbles. As the water is moving, the frequency of the acoustic signal backscattered from these particles changes.

If the particles are moving towards the ADCP, the frequency of the reflected signal is higher than the frequency of the emitted signal. This is known as a positive Doppler shift. Conversely, if the particles are moving away from the ADCP, the frequency of the reflected signal is lower, resulting in a negative Doppler shift. The ADCP measures these frequency shifts and uses them to calculate the velocity of the water at different depths.

The ADCP current profiler subdivides the column of water into discrete bins, where each bin has a corresponding depth interval. Using the Doppler shift of acoustic signals reflected back from each of these bins, the ADCP is able to obtain the velocity in that depth range. This detailed vertical profile of current velocity provides rich information on the three-dimensional nature of the flow of water.

5. What’s needed for high-quality measurement of Vannes coastal currents?

Equipment Reliability

Equipment reliability is of paramount importance for high-quality measurement of the coastal currents of Vannes. The marine environment is very hostile, with saltwater corrosion, strong currents, and high-energy waves. The ADCP should be constructed to withstand such conditions. Components should be made from materials that will not degrade over time. Titanium alloy is an excellent choice for the casing of the ADCP.

Size, Weight, and Power Consumption

The size of the ADCP flow meter should be compact, and the weight should be light. This is especially important when deploying the device from a buoy or a small vessel. A smaller and lighter ADCP is easier to handle and install. Additionally, low power consumption is essential. In remote coastal areas, access to a continuous power source may be limited. A low-power ADCP will stay longer because it uses much power less. The longer duration the ADCP is active, either with a battery power source or from renewable sources such as solar panels.

Cost-effectiveness

Cost is a crucial factor, especially for large - scale measurement projects. A cost-effective ADCP allows for more widespread deployment, which in turn provides a more comprehensive understanding of the coastal current system. This is important for both research institutions with limited budgets and commercial applications that require cost-efficient data collection.

Titanium Alloy Casing for ADCP

Titanium alloy offers several advantages for the ADCP meter casing. It has excellent corrosion resistance, which is vital in the saltwater environment of Vannes' coastal waters. Saltwater can cause rapid corrosion of many materials, but titanium alloy can withstand these corrosive effects for long periods. It also has a high strength-to-weight ratio. This means that it can endure the mechanical stresses exerted by the water currents and waves while remaining relatively lightweight. This combination of properties makes titanium alloy an ideal material for the ADCP profiler casing, ensuring the long - term reliability and performance of the device.

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

Based on Usage

• Ship-borne ADCP: This type of ADCP is mounted on a moving ship. It is suitable for large - scale surveys of the coastal currents. As the ship moves, the ADCP can continuously measure the currents at different locations, providing a broad - scale view of the current patterns. Ship - borne ADCPs are often used for mapping the general flow of currents in the Gulf of Morbihan and for studying the interaction between different current systems.
• Bottom-mounted ADCP: Also known as a moored or seabed - deployed ADCP, it is fixed to the seafloor. This type is useful for long-term, continuous monitoring of currents at a specific location. It can provide detailed data on the temporal variations of the currents, such as how the currents change over the course of a day, a month, or a year. Bottom-mounted ADCPs are often used to study the local current dynamics in areas where long - term data is required.
• Buoy-mounted ADCP: These are ADCPs that are attached to floating buoys. They are specially useful for surface-layer current measurement and applications requiring mobility while simultaneously being able to measure at several depths. The buoy-mounted ADCPs can move with the currents and provide important information about the dynamic behavior of the flow in areas where the current may be complex.

Based on Frequency

The ADCP frequency depends on the depth of the water to be measured.

A 600kHz ADCP is suitable for waters up to about 70m in depth.

More detail is available with the higher frequency in shallower waters. Therefore, this will be useful in the coastal areas and the shallower part of the Gulf of Morbihan. For depths up to about 110m, a 300kHz ADCP would be more suitable. It provides a good balance between depth penetration and measurement resolution. And for measuring currents in deeper waters, up to 1000m, a 75kHz ADCP is recommended. The lower frequency can penetrate deeper into the water column, although the resolution may be slightly lower compared to higher - frequency ADCPs. There are several well - known ADCP brands in the market, such as Teledyne RDI, Nortek, and Sontek. However, for those looking for a cost-effective and high-quality alternative, the Chinese brand China Sonar PandaADCP is a great choice.

Made entirely of titanium alloy, it offers excellent corrosion resistance and durability. Its price - performance ratio is truly remarkable, making it an attractive option for coastal current measurement projects. You can find more information about China Sonar PandaADCP at their website: https://china-sonar.com/.

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