How to measure coastal currents in Porto?

1. Where is Porto?

The historic and culturally rich Porto city lies in northern Portugal, facing the Atlantic Ocean on the Douro River. Indeed, its location makes it one of the very important port cities from time to time.

Geographically, Porto stands at the estuary of Douro, which, in addition to providing a natural harbor for the town, enriches the surrounding areas due to its sediment-rich water. The undulation characterizes Porto, basically a city of hills with the common landscape being covered with peaks and separated by the twisted route followed by the river as it courses. The nearness of the Atlantic Ocean from the west considerably influences Porto into having a gentle and ocean-type climate. Its winters are usually mild while its summers are warm but never extreme.

This is a UNESCO World Heritage Site city, which is well noted for its well-preserved historic architecture. Take a look, for example, at Ribeira, with narrow, twisted streets that meet painted houses and old buildings. The Dom Luís I Bridge is just one of the iconic structures spanning the Douro River, joining Porto with Vila Nova de Gaia. This is indeed the hub of activities, yet it can show much of Porto's rich past, which leads back to the Roman era.

The local culture in Porto is very vibrant and full of tradition. The city is well-recognized for its Port wine, produced in the Douro Valley and aged inside Vila Nova de Gaia. This tradition has a very significant hold on the lifestyle of its inhabitants, since the entire place is surrounded by wine cellars offering visits and tasting opportunities. Locals in Porto have always been perceived as friendly, and this beautiful city is never behind in enjoying every form of festival that pertains to music, food items, and other religious reasons.

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

Influence of the Atlantic Circulation

The large-scale circulation patterns of the Atlantic Ocean determine the coastal currents around Porto to a great extent. Indirectly, but to a great extent, they are influenced by the North Atlantic Current-the main oceanic current system. From the Gulf of Mexico upwards to Europe, this current might have an influence on the general direction and speed of the water masses along the Portuguese coast. Another current, the Canary Current-a cold-water current that flows southward along the western coast of Africa-takes part in the greater oceanic circulation in the area and influences the local coastal currents in the vicinity of Porto.

Wind - Driven Currents

The local winds are a very important factor that determines the characteristics of the coastal current. The westerlies are prevailing winds in this area and play the major role. Where this would happen is if these winds actually blew and thereby drove surface waters, creating currents that moved along the coast. The strength of such wind-driven currents, along with their directions, may be different with regards to the given intensity of the winds. This would include storms or strong winds where the currents would come off stronger or veer off course from usual. These winds, which vary with seasons, also contribute to the seasonality in coastal currents. For example, during winter, the winds are stronger, hence the current movements may be more marked and unpredictable.

River - Ocean Interaction

The Douro River, flowing into the Atlantic at Porto, significantly affects the coastal currents. The discharge of freshwater from this river forms a plume, which interacts with the saltwater of the ocean. This can also, in turn, affect the density and flow of water at the river mouth. The amount of water exiting the Douro would then be dependent upon the amount of rainfall and extent of water impoundment upstream. If there is high river discharge, the freshwater plume can extend further into the ocean to push the saltwater and modify the local current pattern. This can have ramifications for the distribution of marine life, as well as for human endeavors such as fishing and shipping.

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

Surface Drifting Buoy Method

Traditionally, when measuring the surface water flow near the city of Porto, one of the methods used are surface-drifting buoys. To work, they are designed to float on the surface of the water and then move with tides and currents. They are also normally equipped with GPS tracking devices capable of transmitting location data in real time. By mapping the movement of such buoys over a period of time, researchers can record the direction and velocity of the surface currents. The method does have its shortcomings. Surface winds and waves have a great effect on the buoy's movement. In windy conditions, the buoys will drift according to the wind and might not indicate the right direction of the subsurface currents. Another drawback of this method is that only the surface -level current conditions are derived, with no further information on the deeper water flow.

Moored Ship Method

The moored-ship method involves stationing a ship in one place, usually in relatively shallow waters, offshore from Porto. Instruments that are on the ship, such as current meters, are then used to measure the water flow at various depths. Continuous monitoring of currents can therefore be undertaken for a specific point. The advantages with this method are that it could provide highly detailed data regarding the vertical structure of the currents, although it is an expensive and resource-intensive approach. There should be a group of people serving onboard, also known as a crew; it has to be powered somehow and supplied with food for the staff on board and also requires some budget for routine machinery maintenance. All these features could disturb natural flow of the currents at least around its immediate location. This affects measurement results.

Acoustic Doppler Current Profiler(ADCP) Method

Acoustic Doppler Current Profiler emerged as another recent sophisticated efficient means of conducting measurement of coastal currents. ADCPs are capable of measuring water velocity at more than one depth simultaneously. They work by sending out acoustic signals into the water. These signals are reflected back by small particles suspended in the water, such as plankton, sediment, or even bubbles. By the Doppler shift of the reflected signals, the ADCP can compute the velocity of the water in which the particles are moving. This gives a full vertical profile of the current, which allows for a far better understanding of the complicated patterns of the current around Porto. ADCPs are instruments that can be deployed in different modes, such as on vessels, buoys, or the seabed, depending on the measurement needs.

4. How do ADCPs using the principle of the Doppler work?

The ADCPs work according to the Doppler effect. When an ADCP sends out an acoustic signal-usually at a certain frequency-that travels through the water column, small particles in the flow act as reflectors of this signal, which changes its frequency. When the particles are moving towards the ADCP, the frequency of the reflected signal is higher than the transmitted frequency-a positive Doppler shift. If, however, the particles are moving away from the ADCP, then the frequency of the reflected signal is lower-a negative Doppler shift.

Accordingly, the ADCP has transducers capable of measuring these frequency shifts with a high degree of resolution. Assuming an emitted frequency, a precisely calculated speed of sound in water-that is a function of water temperature and salinity, as well as pressure-and the measured Doppler shift, the velocity of the water can be determined by the ADCP. Most ADCPs are multiple transducers arranged to measure the velocity components in various directions, normally three dimensions: horizontal and vertical. This provides complete characterization of the flow of water regarding the direction and magnitude of the current at different depths within the water column.

5. What is necessary to perform a high-quality measurement of Porto coastal currents?

Equipment Reliability

For the measurement of coastal currents near Porto, the equipment has to be highly reliable. The marine environment near Porto is harsh, with saltwater corrosion, strong currents, and wave action. For example, the ADCP has to be able to resist these conditions in long-term deployments. Any malfunction or inaccuracy in the equipment leads to wrong data, which is a big setback in understanding the complex dynamics of coastal currents.

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

A small-sized and lightweight ADCP is desirable. It is easier to deploy whether it is on a small boat, a buoy, or on the seabed. A smaller and lighter device will also have less impact on the natural flow of the currents. Low power consumption is also very important, mainly for applications requiring long-term monitoring. The marine environment may provide very limited power sources, using perhaps batteries or small-scale renewable energy systems. The low power-consuming ADCP will enable longer operating periods without frequent replacements of power and maintain continuity in data collection.

Low Cost

This would also make the ADCP affordable to carry out large-scale measurements along the coast of Porto, and thus several devices could be deployed in different locations, which can give a much better understanding of the current pattern. High-priced equipment would result in fewer deployments, incomplete data coverage, and an inability to represent the complex coastal current system.

Titanium Alloy Casing

The ADCP housing would be made from the titanium alloy. Titanium alloy has a number of advantages. First, it is very resistant to corrosion, which is an essential factor for long-term operation in the salt-water environment off Porto. The high salt content of the Atlantic Ocean causes other materials to corrode at a very fast rate; however, this can be withstood by the titanium alloy. Moreover, it is comparably lightweight; this contributes to satisfying the other pre-condition of deployment ease. Secondly, the strength in the titanium alloy is high hence enabling the ADCP to withhold from these mechanical stresses; for example mechanical impacts of sea waves and heavy currents.

6. Selection Criteria of the Suitable Equipment for Velocity Measurement?

Site Usage

• Ship-borne ADCP: This is the ADCP mounted on a moving ship. Large-scale mapping of coastal currents around Porto can be carried out with the help of this type of ADCP. During various cruises of the ship, the ADCP measures currents over large areas and thus gives an overall view about the current pattern. It shall be useful in the study of overall circulation in the area and its variation at large distances.
• Bottom-mounted ADCP: This is placed on the seabed and is also called a moored ADCP. It serves best for very effective monitoring of currents at a location for long- and continuous periods. In case researchers want to study the long-term trends and variations in the currents near any particular part of the coast, a bottom-mounted ADCP can be deployed at that site. It can provide detailed information on the current characteristics at the same fixed location over a period.
• Buoy-mounted ADCP: These are mounted on floating buoys. They are capable of drifting with the surface currents while making measurements concerning the patterns of surface-layer currents. Normally, buoy-mounted ADCPs would be deployed when monitoring has to be shorter in duration or if more flexibility is desired-for instance, in cases where it might be difficult or hard to achieve access by means of a ship and/or where surface layer current information could well be quite vital.

Based on Frequency

600kHz ADCP: This is fitted for the flow of water in waters whose depth is below 70m. In more shallow coastal waters off Porto especially in bays and nearshore, this ADCP would be able to give good accurate measurements of currents. The resolution in the more shallow water column will be achieved with a frequency that is considered high, providing further details on analyzing the current structure. 

300kHz ADCP: It is designed for waters with a depth of approximately 110m. This frequency can be used in those areas where the depth of the coastal waters is considered middle. This may give the right balance between the measurement range and the resolution of the current data, thus making it appropriate for different types of applications within the coastal waters of Porto, with variable water depths.

75kHz ADCP: This would serve for far deeper waters, up to 1000m. For the deeper regions of the Atlantic Ocean around Porto, a 75kHz ADCP will be more appropriate since it measures the currents at greater depths. The lower frequency can penetrate deeper into the water column, providing valuable data on the deep-water current patterns.

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.