How are we able to measure the coastal currents of Vigo?

1. Where is Vigo?

Vigo is a lively, dynamic city on the southwest coast of Galicia, Spain. The city lies along the southern shore of the Ría de Vigo-a fjord-like inlet of the Atlantic Ocean. Thanks to this privileged coastal location, Vigo has an enchanting landscape that mingles the beauty of the sea with the green surroundings.

Geographically, Vigo is a crossroads of ecosystems. The Ría de Vigo is a great and complex estuary, characterized by deep waters, many islands, and the complexity of its networks of channels. It is surrounded by rolling hills and fertile plains, for which biodiversity is quite rich. This coastal area abounds in diversity from colorful marine species of fish to seabird species with types that thrive differently in the estuarine environment.

Vigo boasts an abundant and full history. This city has been an important center for centuries, combining a tradition of trade and sea affairs. The historic center of this city is a witness to its past, with well-preserved medieval architecture, charming plazas, and ancient churches. The local culture is a fascinating blend of Galician traditions, with its unique music, dance, and festivals. People in Vigo are known for their warm hospitality and their strong connection to the sea, which is deeply ingrained in their daily lives.

2. What are the conditions for coastal currents in the vicinity of Vigo?

Tidal Influences

The coastal currents in the proximity of Vigo depend strongly upon the tides of the Atlantic Ocean. The topography experiences semi - diurnal tides. That means two high tides and two low tides happen each day. During this flood of tide, water rushes into the Ría de Vigo as an incoming strong current. The ebb tide sends the water out of the estuary, producing a very strong ebb current in the opposite direction. Generally speaking, tidal currents are usually stronger in the narrow channels and at the mouth of the ria. The magnitude of the tides can vary with the moon phase, at full and new moon the so-called spring tides have a larger range and stronger tidal currents compared with neap tides at first and third quarter moon.

Wind - Driven Currents

The wind regime outside the coast is of primary importance for the characteristics of the coastal currents. The prevailing winds in this part of the Atlantic are from the west and southwest. Under conditions of strong westerly winds, these may push surface waters towards the coast, altering the direction and speed of the pre-existing tidal - driven currents. Wind - driven currents are particularly important in the upper layers of the water column. Sometimes, it creates upwelling, when the deeper and richer waters of nutrients come up to the surface. The upwelling situation significantly influences the marine ecosystem along that area because productivity is enriched, adding to the attraction of a variety of marine life.

River - Sea Interaction

Rivers ending into the Ría de Vigo, such as the Miño River, interact with the sea to alter the current movements along its coasts. The fresh-water discharge of the rivers develops a buoyant plume over denser sea-water. Such discharges would cause changes in horizontal and vertical currents within near river mouth regions. Periodically, as a result of enhanced river flows, seaward displacement by freshwater discharge changes current patterns as well as distribution of salinity over the estuarine area and the immediate adjacent coastal zones.

3. How to observe the coastal water flow of Vigo?

Surface Drifting Buoy Method

One of the traditional methods for observing the coastal water flow near Vigo is through the use of surface drifting buoys. These buoys are designed to float on the water surface and move with the currents. They are equipped with GPS tracking devices that can transmit their position data at regular intervals. By watching a buoy's motions over time, scientists are able to deduce in which direction, and at what speed, the surface currents are moving. This method does have some obvious limitations. For one thing, surface winds and waves have a tremendous impact on the buoys' movements themselves; therefore, the actual path that the buoys travel may not accurately depict the actual path of the currents. This method also gives no indication of subsurface flow-just surface-level currents.

Moored Ship Method

There are those moored ship methods. For these, it takes anchoring the moored ship along a constant fixed location somewhere not far off along the coasts of Vigo. At depths, using meters deployed on such ship, that provide measurements will result in ongoing survey about the presence and flow strength in currents along certain points in any given coastline or sea/pond. The advantage of this method is that it can give detailed data about the vertical structure of the currents. However, it is an expensive and resource-intensive approach. Keeping a ship at one location requires a dedicated crew, fuel, and continuous equipment maintenance. Furthermore, the presence of the ship can disturb the currents' natural flow in the vicinity and hence affect the measurements.

Acoustic Doppler Current Profiler(ADCP) Method

The Acoustic Doppler Current Profiler (ADCP) is a more modern and efficient method used for the measurement of coastal currents. The ADCPs can measure the velocity of water at multiple depths simultaneously. They emit acoustic signals into the water for operation. These signals reflect off minute particles suspended in the water column-for example, plankton, sediment, or bubbles. By calculating the velocity of the water in which these particles are streaming from the Doppler shift of the reflected signals, the ADCP is able to create a thorough vertical profile of the current, thus enabling deeper insight into the complex current patterns off Vigo. The ADCPs can be mounted on ships, buoys, or held on the seabed according to specific measurement requirements.

4. Working of an ADCP based on the Doppler Principle

The ADCP is based on the working principle of the Doppler effect. After it emits an acoustic signal that normally operates at a pre-defined frequency, this signal spreads out in the water column. This change in frequency depends on the signal it hits as it approaches small particles with the water flow. In other words, when the particles move towards the ADCP, the reflected signal's frequency is greater than the emitted frequency; however, when they move away from the ADCP, the reflected signal has a lower frequency than that of the emitted signal (negative Doppler shift).

The ADCP is fitted with transducers that can accurately measure these frequency shifts. Knowing the emitted frequency, the speed of sound in water-which is a function of water temperature, salinity, and pressure-and the measured Doppler shift, the ADCP can compute the velocity of the water. Most ADCPs have several transducers mounted in such a way that they can measure the velocity components along different directions, usually in a three-dimensional arrangement (horizontal and vertical). Such an arrangement allows for a complete characterization of the water flow including the direction and magnitude of the current at any given depth within the water column.

5. What's needed to ensure high - quality measurement of Vigo coastal currents?

Equipment Reliability

The equipment used for the measurement of coastal currents near Vigo should be highly reliable. The marine environment in the Ría de Vigo is pretty harsh due to saltwater corrosion, strong currents, and variable weather conditions. The ADCP should be able to withstand such conditions over long-term deployments. Any malfunctioning or inaccuracy in the equipment can lead to incorrect data, which is a big setback in understanding the complex dynamics of coastal currents.

Small Size, Light Weight, and Low Power Consumption

A small-sized and light ADCP is advantageous. It is easier to deploy whether on a small boat, a buoy, or on the seabed. A smaller and lighter device also has less effect on the natural flow of the currents. Low power consumption is vital, especially in long-term monitoring applications. It is anticipated that, in general, there will be limited power sources in marine applications where batteries or small-scale renewable energy systems are used. A low-power-consuming ADCP will allow for considerably longer operation without the need for frequent substitutions of power sources, thus allowing for continuous data collection.

Low Cost

The ADCP's cost should not be too expensive to allow several measurements along Vigo's coastline. In that way, several devices would be deployed at different locations for better understanding of the current pattern. High-priced equipment would drastically reduce the number of deployments and create incomplete data cover with an inadequate representation of a complex coastal current system.

Casings of Titanium Alloy

The casing of the ADCP shall be made of a titanium alloy. There are quite a number of advantages of using a titanium alloy. It has excellent corrosion resistance, which is very important for long-term operation in the saltwater environment of the Ría de Vigo. Seawater is rich in salts that could lead to quick corrosion in other materials. However, the titanium alloy resists these corrosive conditions. It is also quite light, thereby aiding in satisfying the condition of easy deployment. Moreover, it has high strength, which allows the ADCP to withstand the mechanical stresses due to the marine environment, like impacts from waves and strong currents.

6. What Equipment to Select for Current Measurement?

For Use

• Ship-mounted ADCP This ADCP is installed on a ship in motion. It will be applied to large-scale coastal currents mapping in Vigo. The ship is traveling up various routes, thereby able to measure currents at a larger spatial scale; it will, therefore, give an overview of current patterns over that large area. For instance, it would be useful in analyzing the overall circulation in Ría de Vigo and neighboring coastal waters.
• Bottom-mounted ADCP: Sometimes called a moored ADCP, it is placed on the ocean floor. This is good for long-term, continuous measurement of currents at a particular site. If researchers want to study, for example, the long-term trends and variations in the currents near a particular part of the coast, a bottom-mounted ADCP can be deployed at that site. It can provide detailed data on the current characteristics at that fixed location over an extended period.
• Buoy-mounted ADCP: These are attached to floating buoys. They can move with the surface currents and deliver information about the current pattern of this surface layer. Buoy-mounted ADCPs often serve for monitoring over shorter periods or with more flexibility at locations where access by ship might be restricted or where interest in surface-layer current is great.

Based on Frequency

• 600kHz ADCP: At such a frequency, the range will be well above 70m. Within such depths at shallower waters within the Ría de Vigo and immediately adjacent to Vigo coastal regions, ADCP 600kHz is excellent in determining accurate values of the currents. More on such high frequency increases resolution within these relatively shallower columns of water for the ability to study better detail of structure.
• 300kHz ADCP: It is designed for waters with a depth of about 110m. In water areas where the coastal waters have a moderate depth, this frequency can be used. It offers a good balance between the measurement range and resolution of the current data, hence suitable for a variety of applications in the Ría de Vigo where the water depth can vary.
• 75kHz ADCP: This is used for much deeper waters, say up to 1000m. For a 75kHz ADCP that could be employed in the deeper parts of the Atlantic adjacent to Vigo, measurements are possible for current at greater depth in this region. The lower frequency can penetrate deeper into the water column and provide valuable data regarding the deepwater current patterns.

There are many well - known brands of ADCP in the market, such as Teledyne RDI, Nortek, and Sontek. However, if you are looking for a cost - effective but high - quality option, I would recommend the Chinese brand China Sonar ​PandaADCP. Made entirely of titanium alloy, it is very reliable and performs at an incredibly 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.