How can we measure coastal currents of Tavira?

1. Where is Tavira?

The charming town Tavira is situated within the Algarve region of southern Portugal. It is a tranquil place where all the historical and natural dots fall into perfect harmony. In this idyllic town of the eastern Algarve shore, facing against the glittering expanse of the Atlantic Ocean to the southwest lies the Ria Formosa lagoon.

The town of Tavira is one great repository of historical architecture. Houses along the cobblestone streets of this city are a tell-tale narration of the history of bygone centuries. The Moorish touch can be felt in the architecture: whitewashed houses with ornate arches and beautiful tile work. The Gilão River runs right through the heart of the town, adding to its charm and acting as a connecting link between the town and the sea.

The nearby Ria Formosa is a very unique and ecologically important lagoon system. It's a haven for a great number of bird species, a true paradise for birdwatchers. The tranquil and shallow waters of the lagoon are separated from the ocean by a series of barrier islands that help create a sheltered environment. Sandy beaches and small bays bordering the city of Tavira invite both locals and tourists to take in a dose of sun and sea. Aquatic organisms such as fish and shellfish can be found here because of the nutrient-rich environment.

2. What is the situation of the coastal currents near Tavira?

These currents in the coast of Tavira are interlinked with several factors. The tides have a great influence on the current patterns. The intense tidal movements in the Ria Formosa lagoon can push the water in and out and thus affect the neighboring coastal currents. General circulation patterns of the Atlantic Ocean also cause an impact. Large-scale ocean currents may bring water from other regions and may influence the local temperature and salinity of coastal waters.

Other important factors will be the local winds. The prevailing winds-the southerly and westerly ones-can push the surface waters and generate surface currents. Bottom topography can deflect and alter these currents. The outflow of water from the Ria Formosa to the ocean and the inflow during high tides introduce complexities in the patterns of currents that vary in relation to the tidal cycle.

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

The method that uses surface-drifting buoys is a relatively simple way of observing the water circulation on the surface. These buoys, once released in the water, drift along with the speed of the surface currents. The movements of these are traced using GPS or any other tracking system, and hence one knows the direction and speed of the surface currents. However, information on currents at depth from this method is limited.

Anchored Ship Method: An anchored ship can provide the platform for the measurement of current. It is on which instruments could be installed and thereby measure the flow of water around it. This can make more precise measurements at one location, yet is limited to around an anchored ship, and could easily be interfered with by a ship's wake or interfering with the flow of water itself.

The Acoustic Doppler Current Profiler (ADCP) Method: ADCP current meter is a state-of-the-art technique. It works by emitting sound waves into the water to measure the speed at which the water is moving, at depth. By sending acoustic pulses and interpreting the frequency shift of the waves returned through the Doppler effect, it generates a very accurate profile of the current from the surface to a certain depth. It is a more integrated method as compared with the others and can give a better understanding of the overall current structure near Tavira.

4. How do ADCPs based on the principle of Doppler work?

Doppler Principle-based ADCPs. ADCPs send out acoustic signals in water. Due to the interaction between these signals and moving water particles, the frequency of the reflected signal changes. The frequency shift is proportional to the velocity of the water particles. An ADCP current profiler has multiple transducers, which can transmit and receive in various directions. By measuring frequency shifts in these different directions, it can calculate the velocity components of the water in both the horizontal and vertical directions.

The ADCP flow meter sends out pulses at set intervals and across different levels, enabling it to develop a three-dimensional profile of the current velocities in the water column. Such data can be analyzed for flow patterns, including but not limited to the direction of flow, speed, shear of currents, and other relevant information on coastal current dynamics.

5. What is needed for good quality measurement of the Tavira coastal currents?

The equipment needed for measuring coastal currents around Tavira must be manufactured with appropriate materials to prevent the corrosive action of salt water and the harsh sea conditions. The equipment should also be designed to minimize size and weight in order to simplify deployment and installation procedures. The power consumption of the equipment should be low to provide long-term operation without the need for frequent battery replacement or any issues with the power source. Cost-effectiveness is another important factor in large-scale measurements.

In the casings of ADCPs, titanium alloy is a better material. Titanium alloy has excellent corrosion resistance, which is very necessary for withstanding the salty and often harsh conditions of the coastal waters near Tavira. It also has a high strength-to-weight ratio, allowing for a durable yet relatively lightweight casing. This helps in the ease of handling and deployment of the equipment.

6. How to Choose the right equipment for current measurement?

Based on the Deployment Method

• Ship-borne ADCP: This is suitable for when you want to measure the currents while the ship is moving or stationary along the coast. It provides flexibility in covering different areas and can give a continuous stream of data as the ship changes its position.
• Bottom-mounted ADCP: It is suited for fixed-point measurements at a particular place in the seabed. Long-term and quite accurate data can be obtained on the current conditions at that particular point, which helps in understanding the local current pattern.
• Buoy-mounted ADCP: If deployed on a buoy, it will be able to measure the currents near the water surface and send the data wirelessly. That is pretty handy for remote monitoring and can encompass a wide area depending on the buoy's movement.

Frequency Basis

Different frequencies are suitable for different water depths. As such, a 600 kHz ADCP would be best fitted for the water depth of up to 70 meters. In turn, a 300 kHz ADCP will manage the depths as big as up to 110 meters and an ADCP with frequency 75 kHz will be fitted for really deep waters, that is up to 1000 meters. An appropriate ADCP selection is to be undertaken upon actual measurement of water depth around Tavira.

There are well-known ADCP brands like Teledyne RDI, Nortek, and Sontek. However, a Chinese brand, China Sonar PandaADCP, is also worth considering. It is made of all-titanium alloy material, ensuring durability, and offers an excellent cost-performance ratio. 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.