How can we measure the coastal currents of Lisbon?

1. Where is Lisbon?

Lisbon, the capital of Portugal, is located on the west coast of the Iberian Peninsula. It stands at the mouth of the Tagus River, which empties into the Atlantic Ocean. Its strategic location has made it an important port city for many centuries.

The city is alive with a vibrant blend of old and new. It is filled with historic neighborhoods, including Alfama and its narrow, winding streets lined with ancient buildings from medieval times. Lisbon's architecture tells of its rich history and eclectic cultural influences, which encompass Moorish, Gothic, and Manueline styles. It is also famous for its yellow trams clattering up and down the hilly streets, giving the passengers a very peculiar tour of the city's many sights.

As for the surrounding waters, the Tagus River estuary constitutes an integral part of the general coastal environment of Lisbon. The estuary is a complicated ecosystem that hosts a wide array of marine life and is a vital shipping lane. Just beyond the estuary, the Atlantic Ocean stretches in all its vastness, with great influence on the city's climate and coastal processes. This coastline includes sandy beaches, rocky cliffs, and small bays, presenting varied and beautiful scenery along the coasts.

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

In Lisbon, coastal currents are induced by more than one factor. Predominantly, the tides act. Semi-diurnal tides in the Tagus River estuary and along the Atlantic coast make the water go in and out, thus developing strong tidal currents. The latter reach several knots in the narrow parts of the estuary.

The other relevant factor involves wind. Dominant westerly winds can press against the surface waters and create, in general, an east-west flow along the coastline. This current is influenced by coastline geometry and underwater relief. The estuary of the Tagus River and the generally irregular form of the coastline leads to divergence/convergence and deflection of the currents.

Besides, the North-Atlantic Current that represents the general circulation of the ocean can bring warm water inside the area. The temperature and salinity of the waters near the coastline would be thus affected and consequently the density driven currents.

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

Surface Drifting Buoy Method

The surface-drifting buoys are an in-situ method of observing the surface currents in a rather simple but effective way. All buoys are equipped with GPS receivers and other sensors. A buoy is released in the water and allowed to drift freely with the surface current. The direction and speed of the surface current are determined by tracking the position of the buoy as a function of time. However, this technique will only provide information on the surface layer and might not be representative for the currents at deeper depths.

Anchor - Boat Method

Another technique used is to anchor a boat in one position and measure the current with current-measuring instruments. A boat can be deployed with mechanical current meters or electromagnetic current meters at any required depth. It allows the taking of more detailed current measurements around a single point but is limited to the area around the anchored boat and may be affected by the movement of the boat itself and by the effect of the anchor chain.

Acoustic Doppler Current Profiler(ADCP) Method

The ADCP is a fairly complex and comprehensive method for measuring coastal currents. It uses acoustic signals for the measurement of the velocity of the water at variant depths simultaneously. The ADCP sends acoustic pulses into the column of water. When these pulses hit particles in the water-sediment or plankton-it changes the frequency of the reflected signals due to the Doppler effect. Analysis of these changes of frequency results in the computation of the variant-depth velocities. The ADCPs are capable of being installed in vessels, buoys, and even underwater on the seabed, an efficient tool when one needs to observe the coastal currents in Lisbon.

4. Principles of Operation for the ADCP Based on Doppler

ADCPs work on the principle of the Doppler effect: an ADCP sends out an acoustic pulse; the sound waves travel through the water and collide with moving particles. If these particles are moving with the water current, the frequency of the reflected sound waves is different from that of the emitted waves. This frequency difference, calculated by the ADCP, is directly related to the velocity of the water.

Most ADCPs have multiple transducers transmitting and receiving acoustic signals in various directions. These make them capable of measuring different components of velocity and therefore calculating the three-dimensional velocity of the water. This is achieved because the ADCP sends several acoustic pulses and analyzes all the received signals to build up a profile of the water current velocities right from the surface down to the seabed.

5. What is required for high-quality measurement of currents off Lisbon?

The equipment for high-quality measurement of coastal currents in Lisbon needs to have several characteristics. First of all, construction materials should be reliable and resistant against the aggressive marine environment. Small dimensions and weight are highly desirable to facilitate the deployment and manipulation of the equipment. For instance, a light ADCP can easily be installed on a small buoy or a research vessel.

Another important point is the power consumption, which should be low, especially for long-term measurements. The equipment operating on low power enables it to continue for extended periods of time without replacing the batteries often or requiring a large power supply. The device should be cost-effective; a low-cost device will ensure wider use and higher frequency of measurement.

Titanium alloy is a very good choice regarding the casing of ADCPs. First, titanium alloy has very high corrosion resistance, which is 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 the measurement of the coastal currents while the vessel is in motion. It will offer continuous data as the ship travels alongside the coast of Lisbon and provide a broad view of the current patterns in various areas. Bottom-mounted ADCP: The bottom-mounted ADCP is suited for long-term and fixed-point measurement purposes. It can be laid on the seabed near the Lisbon coast and serves to monitor the currents of a specific location over a long period of time.

Buoy-mounted ADCP: Where the interest is only in the surface and upper-layer currents, then a buoy-mounted ADCP would be enough. Such units can be installed in estuaries such as that of the Tagus River to monitor currents responsible for shipping and other surface-related activities.

Based on Frequency

• A 600kHz ADCP is generally good 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 Lisbon.
• For depths between 70m and 110m, a 300kHz ADCP is more appropriate. It can effectively measure the currents at these intermediate depths.
• For larger water depths to 1000m, the 75kHz ADCP should be selected since its small frequency allows it to probe further into the water column for accurate current data in those deeper parts of the Lisbon Coastal Area.

There are well - known ADCP brands like Teledyne RDI, Nortek, and Sontek. However, for a cost - effective option, the China Sonar ​PandaADCP is worth considering. It is made of all - titanium alloy material and offers an excellent value for money. You can find more information at ​https://china-sonar.com/.

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