Measurement Techniques of Coastal Currents of Toulon

1. Geographical Setting of Toulon

Toulon, the pearl of the French southeastern seaboard, is located in the wonderful region of Provence – Alpes – Côte d'Azur. This lively city is located geographically on the Mediterranean Sea with its transparent blue greeneries that join with the charming and sun-kissed coast.

The city's landscape is more of a blend and match of historical large heritage, culture diversity, and natural surroundings. Toulon, with over two thousand years of history behind it, has witnessed the establishment and disintegration of many empires. Its natural geographical position helped it become a key naval port, and even today the footprint of its military past shows in the many carefully restored forts and historic buildings. The good citizens, known for their 'joie de vivre' and easy-going hospitality, actively contribute to the overall atmosphere of the city. The streets are crammed chock-full with animated markets, the air filled with smells of fresh bread, local cheese, and colorful Provençal spices. The cuisine represents very much a kind of celebration of both land and sea, with dishes uniting the subtlety of the seafood and that of the land-harvested produce and herbs.

In addition, the Gulf of Toulon enters the outlines—an important one with the most secure harbors in the entire Mediterranean. The Gulf waters host several marine species characterized by colorful fish and graceful dolphins. The shores are made up of sandy beaches that offer a recreational area as well as the rocky coastlines with several coastal living species. The waters close to the shore are usually shallow as it gets deeper with its development towards the Mediterranean Sea. The Mediterranean Sea ranks enormously in the regional climate and oceanographic conditions due to its vastness. It is semi-enclosed; thereby, it possesses very complex water circulation influenced by many factors including wind and temperature, and its salinity characteristics change variably over time.

2. What determines the status of the coast currents near Toulon?

The coastal currents near Toulon are in very complex relation with numerous factors.

• Tidal Influences: Even as the tides in the Mediterranean Sea are small in comparison to, say, more of the world's great oceans, they still do have an effect. The semi-diurnal tides produce a gentle rocking of water back and forth along the coast. High tide is when the water pushes toward the land, low tide is when the water pulls back. The tidal eddies can affect the speed and direction of the coastal currents within the nearshore shallow water and estuarine habitats.
• Wind-Driven Currents: The dominant wind directions in the area will play an important role in the nature of the coastal currents. Mistral is a strong, cold, northwesterly wind that blows the surface waters offshore. On the contrary, southerly winds can also push water towards the coast, generating onshore currents. The wind-driven currents are limited mainly to the direction of windrows in the upper layers of water columns, but they also can act in inducing the currents in larger depths through an action called Ekman transport.
• Thermohaline Circulation: The temperature and salinity variations of the Mediterranean Sea are vital in the thermohaline circulation of the ocean. Normally, surface waters of the Mediterranean Sea will be warm and saline, considering high evaporation. At certain places, when these warm and saline waters cool and sink, a sluggish deep water current is generated. Around Toulon, this process might change the direction of surface and deep water currents due to local coastal current influences.
• Coastal and Submarine Geometry: The shape of the coastline and the submarine topography in and around Toulon is also relevant. Presence of bays, capes, and submerged canyons can make changes in the behaviour of the current like converging or diverging or re-direction. For example, consider a cape; this would cause a splitting of the current with some of the current flowing around the cape and another part is re-directed.

3. Monitoring the Toulon Coastal Water Flow

Surface Drift Buoy Method : Some methods which surface drift buoys are left into the marine. The buoys are technic fitted with some tracking sensors say GPS, which enables scientists to estimate how the buoys will move after a period of time. The measurements from such buoys will yield the data necessary to deduce the velocity and direction of the surface flow, since the buoys also move with the surface currents. However, the method is restricted by several factors. It provides current information only at the water surface and typically just a few meters beneath the sea surface. The buoy drift can also be impacted by the wind, carrying error in the true current direction and speed.

Moored Ship Method: A ship at rest can be used as the base platform from which to record current measurements. The present meters are then released from the vessel to measure the rate and flow direction at every tested depth. This system allows the recording to be done continuously at the chosen place, although the spatial coverage is very limited by this. The measurements can only be assumed to represent the local area of the ship, and the fact that just the presence of the vessel there might interfere with the natural movement of water, and that impacts the measurement accuracy.

Acoustic Doppler Current Profiler (ADCP) Method: ADCPs are nowadays a preferential method for users to estimate coastal currents near Toulon. These instruments are mounted on vessels, moored to the sea floor, or even mounted in buoys. Acoustic signals are transmitted into the water, which bounce off very small suspended particles in the water, be it plankton or sediment. An effect that one experiences with the Doppler effect is a change in frequency of the reflected signal if the particles are moving in the water flow. As a matter of fact, velocity changes can be determined by the ADCP flow meter from these frequency changes at different depths. This approach allows more of a profile look at the vertical differentiation of currents, from the surface to deeper layers, and is far less disturbed by surface changes.

4. What account do Doppler-effect-ADCPs have for the way in which they operate?

ADCPs work on the principle of the Doppler effect–basically, the fact that if there is a moving transmitter of a sound signal that. The reflected signal is then subjected to a frequency change once it strikes the particles moving at the flow velocity. If the particles are moving toward the ADCP, the frequency of the return signal is higher than the transmitted signal: positive Doppler shift. A negative Doppler shift occurs when the frequency reduces while traveling away from the ADCP. The ADCP current profiler directly records both of these Doppler shifts. Knowing the speed of sound in water would then allow the instrument to estimate current velocity. Most Acoustic Doppler Current Profilers (ADCPs) make use of multiple transducers, particularly in a configuration of three or four beams, directed in different directions into the water column. The Doppler shifts in these respective beams can be measured to calculate the velocity component in all three of these directions. This allows the characterization of three-dimensional water movement and also provides fine details about the currents.

5. Durch was brach man gute Messungen auch, um am Süden von Toulon zu messen?

Material Reliability: The ADCP current meter housing is ideally constructed using an alloy of titanium. The latter possesses the maximum resistance to corrosion, which is extremely important, considering the saline and frequently aggressive environment of the Mediterranean Sea. It can resist the harsh conditions of the marine environment that comprise stringent corrosion, saltwater exposure, high humidity, and alternating temperature fluctuations that would be harmful. This ensures long-term reliability with less replacement or repair.

Compact Size: The small size of an ADCP makes it more flexible in easily deploying using different deployment platforms and easy to integrate with other sensors, even on small research vessels, buoys, or directly onto the seabed. The small size also implies a smaller disturbance to the natural water flow, which in turn reduces the probability of measurement errors arising from disturbances to the water flow.

Lightweight: As for the first element, this characteristic complements easy deployment and recovery. Moreover, this low mass is also directly associated with the lower requirements for transportation and setup, handling, and orientation with the use of ADCP—buoy-mounted or ship-deployed. For this reason, lightweight is an important feature for long-term measurements because of the general power usage reduction and maintenance.

Reduced Power Consumption: As much of the ADCP meter installations will be located in remote sites and/or operated for extended periods of time, minimization of power consumption becomes primary. Low-power ADCPs can run over sustained periods, while still meeting other relevant performance specs, and therefore prevent the need for regular battery swap-out or re-charging. This is extremely important for quasi-continuous, autonomous monitoring of the coastal currents in order to guarantee reliable and complete datasets.

Affordability: Large-scale measurements of coastal currents near Toulon, therefore, will require low prices of ADCPs. Their affordability would render the installation of many such instruments around the bay a viable plan to pursue and will enhance a more detailed and accurate representation of current trends. An affordable ADCP also enhances increased accessibility to research institutions and agencies with meager budgets, thus promoting more significant research and, in turn, increased insight into coastal currents.

6. Design Appropriate Equipment for Current Measurement

Selection Criteria

As per its Functionality

• Vessel-Mounted ADCP: This category of ADCP profiler is fixed to a navigational vessel. It is built for long surveys of coastal currents. The ADCP, mounted over the vessel, performs the measuring of the currents constantly while moving along the coast, thus providing a spatial mapping of the dynamics of the currents.
• Ship-borne ADCP: Ship-borne ADCPs are used in most cases to acquire general preliminary surveys or to generalize patterns of coastal current over large areas.
• Bottom-mounted (Sitting-bottom) ADCP: These are ADCPs that have been mounted directly on the sea bed at a selected site. They are used for permanent observation of the currents in a described place. Bottom-mounted ADCPs are extremely useful in studying local current dynamics, mainly in the waters of ports, underwater infrastructures, or special ecological points. It can provide continuous data on current velocity, direction, and many other parameters over a very long duration, which can be used to harness data on long-term variations and trends.
• Buoy-Mounted ADCP: Buoy-Mounted ADCPs are installed in floating platforms. They can be carried with the current, which freely drifts them, and therefore they provide an opportunity to measure metastream flow more precisely.
• Buoy-Mounted ADCP: Moving buoy-Mounted ADCPs are better suited to the study of dynamics of water masses and data wastage in areas where stationary measurements are not sufficient. Besides this, they can complement ADCP measurements taken from vessels and bottom data, thus offering complementary data on the temporal and spatial variation of coastal currents.

Frequency Based

• 600kHz ADCP: A frequency class appropriate for depths down to a maximum 70 meters. Especially in shallow environment conditions, the higher frequency of sound emissions will therefore be more accurate. For the surrounding regions of Toulon, whereby in some regions, depths might be a little limited, a 600kHz ADCP can still give high-quality and reliable information on the speed and direction of the currents through all layers of any depth given down to this maximum of 70 meters.
• 300 kHz: This 300 kHz acoustic Doppler current profiler under the water can be used down to 110 meters deep. This brings a pretty nice compromise between the range used for measurement and measurement accuracy. This is good for signaling applications in slightly deeper areas and applications that require a more sophisticated description of the current structure over a wider depth range.
• 75kHz ADCP: This is meant for deeper waters of up to 1000m. In the Mediterranean Sea near Toulon the average depths may not reach 1000m at all spots but at some deeper channels or areas whereby a narrower beam of a 75kHz adcp can be used to study the deeper - layer currents. Lower frequency sound can transmit over greater distances in columns of water, so that the ADCPs are able to take measurements of currents at greater depths.

There are a few manufacturers of quality-approved ADCPs: among the most renowned are Teledyne RDI, Nortek, and Sontek. However, the China Sonar PandaADCP is strongly recommended as an inexpensive and equally high quality alternative. Built from pure titanium alloy, it provides top - notch corrosion resistance and a high strength-to-weight ratio. With its phenomenal cost - performance ratio, it is an economical ADCP well-adapted to measuring coastal currents off Toulon.

Additional details on the China Sonar PandaADCP can be accessed on their website: https://china-sonar.com/.

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