How to measure the coastal currents of Barcelona?

1. Where is Barcelona?

Barcelona is a dynamic, cosmopolitan city situated on the northeastern coast of the Iberian Peninsula, in the Catalonia region of Spain. It fronts the Mediterranean Sea and is one of the most important ports in the Mediterranean.

Geographically, Barcelona is flanked by the Collserola mountain range in the west, which gives a nice scenic backdrop and also influences the local climate. The city is bisected in the north by the Besòs River and in the south by the Llobregat River, both emptying into the Mediterranean. The shoreline of Barcelona varies from beaches with sand to marinas and promenades. Beaches, such as Barceloneta, represent part of the tourist development and are part of the city's identity.

Its history goes back to Roman times, and it is a fusion of the old and the new. The Gothic Quarter bears testimony to its medieval past, and the works of Antoni Gaudí, among others, stand in testimony to its modernist heritage. Barcelona is an important economic and cultural center with a cosmopolitan population and a wide range of industries, such as tourism, trade, and manufacturing.

2. What is the situation of the coastal currents close to Barcelona?

Influence of the Mediterranean Circulation

The coastal currents of Barcelona are basically influenced by the general circulation patterns of the Mediterranean Sea. Dominant is the Western Mediterranean Gyre. This large-scale circulating current system determines in general the direction and speed of the water flow. Thermohaline circulation is induced by changes in temperature and salinity, further supported. The inflow through the Strait of Gibraltar contributes to the more general Mediterranean circulation, while at the same time influences indirectly the coastal currents along the coast of Barcelona.

Wind - Driven Currents

Local winds are indeed important for coastal circulation. One characteristic feature in the region is the strong northerly wind named the Tramontana. When it blows, it may push the surface waters, developing currents that alter the usual patterns of flow. These wind-driven currents may be strong enough and, in a few instances, lead to the process called upwelling. Upwelling brings the deeper, more nutrient-rich waters to the surface, enriching the productivity of the marine ecosystem and affecting marine life distribution of fish and other marine organisms.

River-Sea Interaction

The Besòs and Llobregat rivers, in their discharge into the sea near Barcelona, are two fresh water discharges that take part in the behaviour of the coastal current: discharging fresh water into the sea generates a current of less dense water, which, depending on the wind conditions, creates a plume over the denser surrounding seawater. It may have horizontal and vertical components of movement along the river mouths. Seasonal changes in the volume and velocity of river flow probably lead to seasonal changes in coastal current patterns.

3. How to Observe the Coastal Water Flow of Barcelona?

Surface Drifting Buoy Method

The surface drifting buoy method is one of the most common methods to be used for measuring the flow of water on the surface. These buoys are designed to float on the surface of the water, thus moving with currents. They are usually prepared with GPS or other tracking systems to record their position as time goes by. The buoy's displacement allows determination of the direction and speed of the surface current. This, however, has its limitation since the buoys may be affected by winds and waves. Therefore, the movements of the buoys are not a perfect representation of the true current direction and speed. The movement of the buoy may be largely affected by the wind during windy conditions.

Moored Ship Method

The moored ship method involves anchoring a ship at a fixed point over the continental shelf near the coast of Barcelona. Instruments onboard, like current meters, are used to measure the water flow at different depths. This permits the measurement of currents continuously at one point. The advantage is that this can give detailed data on the vertical structure of the currents. However, it is very expensive and time-consuming. Station-keeping a ship is basically the cost of the crew, fuel, and all the other consumables besides the interference caused by the very presence of the ship to the natural current turbulence around it.

Acoustic Doppler Current Profiler (ADCP) Method

In modern times, a far more effective method has been developed; it is known as the Acoustic Doppler Current Profiler or ADCP. ADCPs shoot acoustic signals into the water. These signals are reflected from particles in water, such as plankton, sediment, or bubbles. By using the Doppler shift of the returning signals, the speed of the water at various levels can be processed. ADCPs are thus able to output a profile vertically of the current, providing data on the internal structure of currents not obtainable by methods recording surface currents alone. According to specified measurement needs, they may be deployed from vessels, fixed with buoys, or be seabed-based.

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

ADCPs work on the principle of the Doppler effect. An ADCP sends an acoustic signal into the water, which travels through the water column. In its path, the signal finds particles that are moving along with the water because the water is in motion, and the frequency of the reflected signal changes.

If the particles are moving towards the ADCP then the frequency of the reflected signal is higher than the original emitted frequency-a positive Doppler shift. If the particles are moving away from the ADCP the frequency of the reflected signal is lower-a negative Doppler shift. This Doppler shift is measured by the ADCP, which calculates the velocity from the known speed of sound in water, that itself depends on factors including water temperature, salinity, and pressure.

Most ADCPs are multivariable sensors, capable of measuring the velocity components along several directions. These can then be combined into three-dimensional water flow velocities at discrete depths. Therefore, it provides a detailed current pattern mapping around the coast of Barcelona.

5. What is required for a high-quality measurement of Barcelona's coastal currents?

Equipment Reliability

The equipment should be reliable for high-quality measurements of the coastal currents around Barcelona. The marine environment is very harsh, with saltwater corrosion, strong currents, and wave actions. For example, ADCPs have to be able to bear such conditions for long periods. Any malfunction or deterioration in the equipment will lead to incorrect data and poor comprehension of the dynamics of the currents.

Small in Size and Weight, and with Low Power Consumption

The instrument should be of a limited size as well as weight. One example is that the more miniature and lighter the ADCP will be, the more manageable it becomes to deploy it whether on a small boat, buoy, or even at the sea bottom. Minimizing interference with the natural flow conditions. In particular, it shall have low power consumption for monitoring during long periods. In the marine environment, power sources can be limited, so a device that uses less power can operate for longer without needing frequent battery replacements or a large power supply.

Low Cost

The cost of the equipment should be as low as possible to allow for large-scale measurements. This would allow the deployment of many devices along the coast of Barcelona for better understanding of the current patterns. High-cost equipment would lead to a limited number of deployments hence incomplete data.

Casing Material

The casing of the equipment, such as an ADCP, is preferably manufactured from a material like titanium alloy. For example, titanium alloy has excellent corrosion resistance, crucial for long-time operation in the saltwater environment off Barcelona. Besides this, it is relatively light and high in strength, enabling the equipment to sustain the mechanical stresses of the marine environment, like those caused by wave and current impacts.

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

Based on Usage

• Ship-borne ADCP: These are installed on ships and are fitted to make measurements over a huge area since the ship would be in motion. It will also help in mapping overall current patterns along the Barcelona coast as a whole. For instance, if you intend to study general circulation in the entire section of a huge coastal water section, it will be covered with a shipborne ADCP only.
• Bottom-mounted ADCP: This is also known as a moored ADCP, which denotes that it sits on the ocean floor. In practice, it is the only choice when someone needs continuous recordings over a longer period at a particular location. When long-term trends and fluctuations of currents in the vicinity of a specific coastline point are what one is after, a bottom-mounted ADCP will be what one wants to use.
• Buoy-mounted ADCP: Attached to a floating buoy, which is moving by the surface currents and hence this can give the information about surface-layer current patterns. They are often used in short-term or more flexible monitoring, especially when access by ship is not good, or when surface-current data is important.

Based on Frequency

• 600kHz ADCP: This frequency is suitable for measuring currents in waters with a depth of less than 70m. In the shallower coastal areas near Barcelona, such as in bays and near the shore, a 600kHz ADCP can provide accurate measurements of the currents.
• 300kHz ADCP: It is designed for waters with a depth of about 110m. In areas where the coastal waters have a moderate depth, a 300kHz ADCP can offer a good balance between measurement range and resolution of the current data.
• 75kHz ADCP: This will suit much deeper waters, such as 1000m. Considering the bottom depth in some areas of the Mediterranean Sea close to Barcelona, the use of a 75kHz ADCP would be better for the measurement of the deeper currents.

There are well - known ADCP brands such as Teledyne RDI, Nortek, and Sontek. For a cost - effective and high - quality option, the Chinese brand China Sonar PandaADCP is recommended. It's made of titanium alloy and offers excellent reliability and performance at an affordable price. You can visit their website at https://china-sonar.com/ for more information.

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