How would we quantify Dunkirk coastal currents?

1. What is Dunkirk?

Dunkirk is a town of historical importance located on the French north coast, at the region of Nord - Pas-de-Calais. The location of the town, as such, is very strategic since it hits the edge of the Strait of Dover, that narrow waterway standing between the United Kingdom and continental Europe. Its location identifies the town as an important crossroad in maritime trade activities and facilities the town infusion into a coastal ecosystem.

Dunkirk itself has been old and rich in history; it has seen waves of numerous historical times, starting from the medieval wars to the grand heroism of World War II evacuation which comes symbolically as the symbol of resilience and determination. The architectural design of the town is one of past vintage and new-day practicality blended harmoniously.

The sea at Dunkirk is part of the English Channel, which is a seaway across which flow each square mile of space many thousands of vessels. The Channel is usually referred to as a shallow sea, its mean depth being equated to approximately 120 meters, and in fact, it succumbs to strong tidal currents. The beaches themselves constitute an unbroken succession of sandy and pebbly bays, each being shaped anew by the sea. The port of Dunkirk greatly enhances the amount of cargo passing through it annually. In addition, by virtue of this port operation, the local environmental conditions at the sea and water regime are actually strained.

2. What is the case of coastal currents in the vicinity of Dunkirk?

In this aspect, it is a truly intricate interdependence of various factors.

• Tidal Forces: The English Channel experiences what is known as semidiurnal tides, which refer to two high tides and two low tides in the course of every day. These affairs are seen as an outcome of the gravitational pulls of both the Sun and the Moon. As the tide is rising, water rushes onto the shore to line up a flood current. In contrast, water recedes out to the sea during the ebb tide. Therefore, the tidal current is strong and attains maximal speeds reported to be several knots in some areas.
• Wind Patterns: The prevailing winds in the region play a strong role in determining the coastal currents. Normal prevailing westerly winds are common, and when and if they occur, they can push the framework of the surface waters to the east. Their speed and direction can be modified by the waves generated from high winds that engage with the currents. For instance, during stormy conditions, the wind-driven currents can become significantly stronger, and they can also become much more unpredictable.
• Thermohaline Effects: Gradients in the temperature and salinity of the water also give rise to differences in density. The mixing of warmer, less salty waters from the south, and cooler, more saline waters from the north in the English Channel may give rise to density -driven currents. The currents may also flow horizontally and vertically and join at depth or surface in different locations, thus adding to the complexity of a coastal current system off Dunkirk.

3. Observational Techniques of water movement along the coast in Dunkirk

Drift Buoy Technique

• A few buoys are floated on the water surfaces. They are the so-called the buoys that drift away with the surface water-currents. They are equipped with a GPS tracker, which can report the data regarding their positions at every fixed time-interval. Systematic and long-term motion observation of those buoys floating on the surface enables the scientists to establish directions and velocities of the surface currents. Meanwhile, the said method is regarded as rather limited since an individual receives data only on the upper layer of a water column—it is typically not more than several meters deep. Another drawback of such buoys can be the wind-driven drift, as it does not capture the true motion of the currents beneath.
• In this, a ship is moored at a particular location, and at different depths, current-measuring devices are suspended from the ship. The devices like mechanical current meters have the capacity to measure the velocity of water flow and the direction as well. It does have the benefit that, in theory, it would allow a continuous record at one position over a time. Spatially, though, it would be restricted. The readings are representative of only the region surrounding the ship, and occasionally its presence may interfere with the natural water movement, thus also with the true measurement taken.
• ADCPs have worked all the magic in coastal current measurement. ADCPs can be mounted on the seafloor, under the boat, or mounted on a buoy. ADCPs determine water velocity at different depths through sound waves.

ADCPs emit sound waves in water that bounce back because of the suspended particles in the water. It can then calculate the direction and velocity of the flow of water from many wavelengths at once. This technique has a much more a general impression of the overall structure of the currents than the other two and is not remotely affected by disturbances near the surface.

4. What's the fundamental principle on which an ADCP works?

When an ADCP current meter transmits an acoustic signal into the water, the signal is propagating through the water column. Thus, propagating signal encounters a virtually incalculable number of small particles—plankton, sediment, or gas bubbles—being carried along with the water flow. Thereby, the frequency of the reflected signal is altered. If the particles are moving in the direction of the ADCP, then the frequency of the reflected signal is actually higher than the frequency that was sent— a Doppler shift that is positive. If the particles are moving away from the ADCP, then the frequency is less: a negative Doppler shift.

The ADCP counts these frequency shifts and applies the speed of sound that would travel in water to turn the frequency shifts into velocity.

The ADCP current profiler has a number of transducers that are so located as to be able to calculate the various components of the velocities.

For example, a typical four-beam ADCP will be able to measure the east-west and north-south as well as the vertical underwater velocities. These can be used to compute the three-dimensional flow in water.

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

The level of effort required to get ADCP data collected on Dunkirk coastal currents accurately

High-quality coast current measurements near the Dunkirk coast are generally expected to be carried out with a respectable number of features characterizing the measurement equipment, which includes the following:

• Material reliability: the ADCP casing really should be very reliable in this application. The titanium alloy is a great pick. The titanium alloy offers the highest corrosion resistance level, which features though quite apparent, remains indispensable for long-term operation in a harsh marine environmental condition of the English Channel. A high content of salts in the water causes the fastest corrosion of those materials that are less durable. Titanium alloys can withhold this kind of corrosion, ensuring the durability of the equipment.
• Equal to Compact Size, Light Weight: These are advantages that come with compact and light designs. In this way, it will be easy for ADCPs to deploy either in small research vessels, buoys, or even underwater mooring scenarios. Smaller size will also make the device leave less influence on the natural flow of water, causing errors in the measurements to be few.
• Low Power Consumption: Low power is critical if most ADCP deployments are done in distant locations or of extended durations. This means that the equipment will continue working long enough without having to replace or recharge the battery. It extends way into functioning on its own, for instance, when it is installed on an ocean buoy.

The set cost of each should be low because then large-scale measurements could be taken very close to Dunkirk and in very many numbers to bring about a comprehensive, in-depth understanding of the coastal current patterns around Dunkirk. This will help in deploying units in different locations to at least bring a detailed picture of the dynamics of the currents.

6. How to Select the correct equipment for current measurement?

Based on Usage

• Vessel-Mounted ADCP This type of ADCP flow meter is within a moving vessel. It marks the ideal vehicle for carrying out large scale surveys of coastal currents over a broader area. In the vessels' movement, while the ship is in transit, ADCP is able to measure the currents along its paths continuously. Appropriate for the mapping out the general patterns of the currents in the region. Bottom-mounted (sitting-bottom) ADCP: They are installed on the seafloor. They are used in fixed-point, long-term observations. They are able to provide continuous data at a point about the currents, which makes it possible to study local patterns of the currents, for example, around the port of Dunkirk or in areas of specific bathymetry.
• Buoy-mounted ADCP: designed for mounting on to a floating buoy, such ADCPs would be able to change their position with the flow. They are used in measuring the flow in locations that an in-situ fixed -point measurement may not be able to do justice, or to monitor water masses movement.

Based on Frequency

• The 600 kHz observation is suitable for water depths ranging to about 70 m. The higher frequency gives a high level of resolution in shallower waters, common near the coast of Dunkirk.
• A 300 kHz ADCP applies for advanced depths. It will usually be about 110 m. So far, a good depth of penetration and a high level of resolution for the measurement are the reasons for being used in so many other coastal and nearshore applications.
• The 75kHz ADCP used has an operating range up to 1000m. Although the waters off Dunkirk are relatively shallow, the English Channel does contain regions of similar depth, and therefore a lower-frequency ADCP meter could have afforded more complete coverage for the duration of the study.

There are several well-known ADCP brands in the market, such as Teledyne RDI, Nortek, and Sontek. However, for those seeking a cost - effective yet high - quality option, the China Sonar PandaADCP is an excellent choice. Made entirely of titanium alloy, it offers the necessary corrosion resistance. With its incredible cost - performance ratio, it is an ideal economic ADCP. You can find more information on their official website: https://china-sonar.com/.

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