How are the coastal currents of Setúbal to be measured?

1. Where is Setúbal?

Setúbal is a very lovely port city in Portugal. In the District of Setúbal, it is placed on the northern shore of the Sado River estuary, sharing borders with the Atlantic Ocean.

It is a very beautiful city of nature, full of historic charm; it has around 120,000 residents with a very active fishing industry and long-standing wine tradition. The local architecture retains a mix of traditional Portuguese styles in whitewash and red-roof tiling, giving the city a very recognizably Mediterranean appearance.

The Sado River estuary near Setúbal is an area of great ecological importance. It is a home to many species of birds and the nursery of a vast number of fish species. Tides coming in and out create complex tidal flats and marshes of the estuary. In the Atlantic, Setúbal features sandy beaches and rugged cliffs. These waters serve the purpose of complementation in this city and further extend the charm by their participation in dictating the weather conditions, climate, and various forms of water sports like fishing and sailing.

2. What is the status for the coastal currents close to the city of Setúbal?

The currents along the coast at Setúbal are influenced by several elements. Tidal currents are among the most prevalent. In the estuary of the Sado River, there is a high range of tides, which creates very strong tidal streams. The tides force the water in and out of the estuary, creating complex current patterns that may vary in speed and direction with the phase of the tide.

The wind also plays an important role. The prevailing winds from the Atlantic, for instance, the westerlies, may push the surface currents. The interaction of the wind-driven and tidal-driven currents can result in the formation of eddies and other dynamic flow structures.

The shape of the coastline and underwater topography could also influence the coastal currents: the funneling shape of the Sado River estuary accelerates the water when it gets funneled in or out, while the submerged topography forces the currents over shoals and channels to bend and accelerate or decelerate, creating areas of stronger and weaker flow.

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

Surface Drifting Buoy Method

The surface drifting buoys are an accessible method to observe the current at the water surface. A buoy equipped with a positioning device such as a GPS is placed into the sea, and, driven by the current, the path taken is traced. The time series data from the position of the buoys will give the direction and speed of surface currents. This can provide only the surface layer information and may not be sufficient to depict the picture throughout the water column.

Anchor - Boat Method

Another way is to keep the boat at one place and lower instruments that measure current. Such instruments as acoustic Doppler velocimeters or mechanical current meters might be deployed from the vessel along different depths. This will give more detailed current measurements at one point, but it is very location-limited around an anchored vessel and may be influenced both by the movement of the vessel itself and by the influence of the anchor chain.

Acoustic Doppler Current Profiler (ADCP) Method

ADCP is a sophisticated and integrated measurement method used to measure coastal currents. It sends acoustic signals into the water column and measures the Doppler shift of the reflected signals from particles in the water. With this technique, it can determine the velocity of the water at many depths simultaneously. They can be installed on boats, buoys, or placed on the seafloor in order to obtain a complete picture of the structure of currents, from the surface right down to the bottom of the coastal waters at Setúbal.

4. How do the ADCPs applying the Doppler Principle operate?

The principle of operation of ADCPs is based on the Doppler effect. An ADCP sends an acoustic pulse into the water. These sound waves reflect off particles in the water-sediment, plankton, or other particles in suspension. If these particles are moving with the water current, the frequency of the reflected sound waves will be different from the frequency of the emitted waves. The ADCP measures this frequency difference, related to the velocity of the water.

Most ADCPs have several transducers, which transmit and receive acoustic signals in different directions. This enables them to measure the components of velocity in various directions and calculate the three-dimensional velocity of the water. By emitting a series of acoustic pulses and analyzing the received signals, the ADCP can build up a profile of the water current velocities from the surface to the seabed.

5. What is needed for good measurement of the currents around the coast of Setúbal?

The equipment needs to have special characteristics for high-quality measurement of the coastal currents in Setúbal. First, the materials used in the making of the measuring devices need to be of a type that is reliable and resistant to such harsh conditions of the sea. A small size and light weight are advantageous for the equipment in terms of ease of deployment and handling. For example, a lighter ADCP could easily be mounted on a small buoy or a research vessel.

Low power consumption is also needed, especially in the case of longtime measurements. It means that equipment can work for hours without changing batteries or having a big power supply. Another aspect is cost-effectiveness. A cheaper device can be used more and be placed in every single location.

For the casing of ADCPs, titanium alloy is a very good choice. First, titanium alloy has high corrosion resistance, which is quite important for withstanding the corrosive effects of seawater. It also has a high strength-to-weight ratio, meaning that the casing can be strong enough to protect the internal components while keeping the overall weight of the device relatively low.

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

Based on Usage

Ship - borne ADCP: If you want to measure the coastal currents while the vessel is in motion, a ship - borne ADCP is the best choice. It can provide continuous data as the ship travels along the coast of Setúbal, giving a broad view of the current patterns in different areas.

Bottom-mounted ADCP: In the case of long-term, fixed-point measurement, a bottom-mounted ADCP is suitable. It can be placed on the seabed along the coast of Setúbal to continuously monitor the flow at a position over a more extended period of time.

Buoy-mounted ADCP: In that case, when the focus of study is on the surface and upper-layer currents, a buoy-mounted ADCP can be applied. It can be deployed in areas such as the Sado River estuary to monitor the currents that may affect shipping and other surface-related activities.

Based on Frequency

• A 600kHz ADCP would normally be a good choice for water depths up to 70m. Its higher frequency provides better resolution for shallower waters and can give more detailed information about the current structure in the nearshore areas of Setúbal.
• For depths between 70m and 110m, a 300kHz ADCP is more appropriate. It can effectively measure the currents at these intermediate depths.
• For deeper waters, up to 1000m, an ADCP with a frequency of 75kHz is recommended; its lower frequency will manage to penetrate deeper into the water column for accurate current data in deeper parts of the Setúbal coastal area.

The ADCPs are well - known brands manufactured by Teledyne RDI, Nortek, and Sontek. However, China Sonar PandaADCP may be a good option for customers on a tight budget. This is an all - titanium alloy material with excellent value for money. More information can be seen at https://china-sonar.com/.

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