How can we measure the coastal currents of Pontevedra?

1. Where is Pontevedra?

Pontevedra is an enchanting city in the southwest part of Galicia, Spain. It lies at the head of the Ría de Pontevedra, a great ria or drowned river valley opening into the Atlantic Ocean, giving the place a peculiar combination of coastal and estuarine characteristics.

It is a city covered with greenery; the rolling hills and the alluvial plains add to the richness of its agricultural lands. The Ría de Pontevedra is not only a beauty but is also an ecological hotbed. It houses various sea animals, such as different fish species, shellfish, and types of seabirds. The quiet waters and high content of food materials in the estuary make for a very lively estuarine ecosystem.

Pontevedra has had, and still has now, a lot of culture: its historic center was put on the list of UNESCO's candidates as World Heritage. Narrow, cobblestone streets wander down the old city, full of charming plazas, very old churches, and traditional houses. Traditions like Celtic music, dance, and peculiar festivals form the deep roots of the local culture and attract visitors from the entire world. The people of Pontevedra are open, warmhearted, and attached to the land and sea.

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

Influence of Tides

In the case of Pontevedra, the coastal currents are much influenced by tidal forces. Tides in the Atlantic Ocean are semi-diurnal: there are two high tides and two low tides in one day. During high tide, water enters the Ría de Pontevedra, thus creating an incoming current. During low tide, water leaves the ria and thus an ebb current. Tidal currents could be strong at some points of this channel and especially around the mouth of the ria. Tidal amplitude may also depend on the moon phase, whereby spring tides-those that happen at full and new moon-exert a greater range and stronger currents compared to neap tides at first and third quarters of the moon.

Wind - Driven Currents

The direction and speed of the coastal currents depend partly on the local wind conditions. The prevailing winds in the area are from the west and southwest. During periods of strong westerly winds, surface waters can be pushed toward the coast, while the natural tidal - driven currents would be modified. These wind-driven currents can be more influential, particularly in the upper layers of the water column. In some cases, they are responsible for the upwelling-a process in which deeper waters, rich in nutrients, reach the surface-and this leaves a great mark on the productivity of the local marine ecosystem, enriching it with all kinds of marine life.

River-Sea Interaction

The Lérez River, which passes through Pontevedra and reaches the Ría de Pontevedra, also has its influence on coastal currents. The freshwater discharge from the river can generate a buoyant plume that spreads over the denser seawater. This can influence horizontal and vertical water movements near the river mouth. In periods of high river flow, for instance following heavy rainfall, the river's discharge may push seawater further out, changing local current patterns and salinity distribution in the estuary and adjacent coastal areas.

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

Surface Drifting Buoy Method

The classical way of monitoring the coastal water flow around Pontevedra is by deploying surface drifting buoys. These buoys are meant to float on the water surface, drift with the currents, and often are fitted with GPS trackers that transmit real-time location data. Analyzing such buoys over time will allow researchers to get an approximation of direction and speed for surface currents. A number of factors limit this approach, however: surface winds and waves can easily affect the position of a buoy, sending it off track from the true movement of currents. Besides, it would only give the surface - level currents, without giving any idea about the subsurface flow.

Moored Ship Method

In the moored ship method, a ship is anchored in a fixed position off the coast of Pontevedra. The instruments that the ship is equipped with, such as current meters, record the water flow at various depths. This allows continuous monitoring of the currents at one point. The advantage of this method is that it can provide detailed data on the vertical structure of the currents. On the other hand, this is a rather expensive and resource-consuming approach. It requires a lot of work to keep a vessel in one place: a crew, fuel, and continuous maintenance of the equipment. Besides, the ship itself may alter the natural pattern of the currents in its immediate surroundings and give a wrong measure of the same.

Acoustic Doppler Current Profiler (ADCP) Method

In recent years, the use of an Acoustic Doppler Current Profiler (ADCP) has been proved to be far more effective in measuring coastal currents. ADCPs have the capability of measuring the speed of water simultaneously at different water depths. These instruments send acoustic signals through the water. These signals are reflected off small particles suspended in the water, such as plankton, sediment, or bubbles. By measuring the Doppler shift of the reflected signals, the ADCP can determine the velocity of the water in which the particles are moving. This yields a complete vertical profile of the current and allows a more in-depth understanding of the complex current patterns near Pontevedra. ADCPs can be deployed by TOVs and vessels, buoys, or even on the seabed, depending on measurement requirements.

4. How do ADCPs using the Doppler principle work?

Their working essentially relies on the Doppler effect. When an ADCP emits an acoustic signal-usually at a specific frequency-this signal travels through the water column. As the signal encounters small particles that are moving with the water flow, the frequency of the reflected signal is changed. If the particles are moving towards the ADCP, then the frequency of the reflected signal is higher than the emitted frequency; if the particles are moving away from the ADCP, the frequency of the reflected signal is lower, in what is called a positive and negative Doppler shift, respectively.

The ADCP has transducers capable of measuring the exact frequency shifts. Knowing the emitted frequency, the speed of sound in water given as a function of water temperature, salinity, and pressure, and the measured Doppler shift, the ADCP calculates the velocity of the water. Most ADCPs have multiple transducers that are arranged such that they can measure different components of velocity, mostly in three dimensions: horizontal and vertical. This fully characterizes the water flow, including the direction and magnitude of the current at different depths within the water column.

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

Equipment Reliability

The equipment for the measurement of coastal currents around Pontevedra should be very reliable. Normally, marine conditions within Ría de Pontevedra are very aggressive because of saltwater corrosion, strong currents, and variable weather conditions. For long-term deployment, the ADCP has to put up with all these conditions. Any malfunctioning or inaccuracy in the equipment leads to wrong data, which would be a major setback in understanding the complex coastal current dynamics.

Small in Size, Light in Weight, and Low Power Consumption

A small-sized and lightweight ADCP is desirable; it is easy to deploy whether on a small boat, on a buoy, or even on the seabed. A smaller and lighter device will result in less interference with the natural flow of the currents. Low power consumption is also important, especially for applications requiring long-term monitoring. The sources of power can be limited in a marine environment; for example, it can depend on batteries or small-scale renewable energy systems. A low-power-consuming ADCP can run for longer periods of time without having to change the power frequently and can continue to collect data continuously.

Low Cost

For large-scale measurement along the coastline at Pontevedra, the cost of an ADCP must be within a reasonable range so that multiple units can be placed at various points, which is quite necessary in mapping the pattern of currents at the given spot. Very expensive equipment allows for fewer deployments and, subsequently, incomplete data coverage of an inherently complex coastal current system.

Titanium Alloy Casing

The casing of the ADCP is preferably made of titanium alloy. Titanium alloy offers a number of advantages. It has excellent corrosion resistance, which is necessary for long-term operation in the saltwater environment of the Ría de Pontevedra. The high salt content in the seawater can cause rapid corrosion of other materials; however, titanium alloy is able to withstand such corrosive conditions. It is also comparably light, considering the need for easy deployment. The strength of the titanium alloy provides high strength to the ADCP in order to bear the mechanical stresses of the marine environment, including the impact of waves and strong currents.

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

Based on Usage

• Ship-borne ADCP: It is installed on a moving ship. This would suit large-scale mapping of the coastal currents around Pontevedra. It will be able to measure over a wide area by moving the ship along different routes, giving an overview of the general current patterns. This is useful for the study of overall circulation in Ría de Pontevedra and its adjacent coastal waters.
• Bottom-mounted ADCP: This is also called a moored ADCP, and it is placed on the seabed. It works very well when long-term continuous monitoring of currents is required for any particular area. Suppose the researchers have to study the long-term trend and variation of currents near any particular part of the coast. For this purpose, they will deploy the bottom-mounted ADCP. It can provide detailed data on the prevailing characteristics of that fixed location over a long period.
• Buoy-mounted ADCP: These are attached to floating buoys. They can move with the surface currents and provide information about the surface-layer current patterns. Buoy-mounted ADCPs are often used for short-term or more flexible monitoring, especially in areas where access by ship may be limited or where surface-layer current data is of particular interest.

Based on Frequency

• 600kHz ADCP: This is the frequency to be used in waters with less than 70m of depth. In the shallower areas of the Ría de Pontevedra and the coast of Pontevedra, currents can be measured accurately with a 600kHz ADCP. This higher frequency allows better resolution in the shallower water columns, thus giving very close to an accurate analysis of the current structure.
• 300kHz ADCP: This model was fitted for waters approximately 110 meters in depth. With this frequency, the ADCP can be employed where the waters off the coasts are not very deep but neither shallower. An excellent compromise between measurement range and resolution of current data is found with this unit, making it especially fitting for a series of applications to the Ría de Pontevedra where the depth may vary significantly.
• 75kHz ADCP: Much deeper water, for instance, 1000m. From the Atlantic bordering Pontevedra, in deeper waters, the currents with higher profundity would be best suited to be measured by use of a 75kHz ADCP. Thus, lower frequency is capable of deeper travel in the column of water for deep-water data acquisition on currents.

There are several well - known ADCP brands in the market, such as Teledyne RDI, Nortek, and Sontek. However, for those looking for a cost - effective yet high - quality option, the Chinese brand China Sonar ​PandaADCP is highly recommended. Made entirely of titanium alloy, it offers excellent reliability and performance at an incredibly affordable price. For more information, you can visit their website at ​https://china-sonar.com/.

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