How can we measure the coastal currents of Figueira da Foz?

1. Where is Figueira da Foz?

Figueira da Foz is one of the most beautiful cities in the Coimbra District, which is in the heart of Portugal. The town stands at the mouth of the Mondego River, directly by the Atlantic Ocean, creating a nice coastal landscape. The city enjoys a special geographical position, practically at the midpoint of the Atlantic coast of the Iberian Peninsula, protected by the hills of Serra da Boa Viagem.

It is a coastal city, very rich not only in natural beauty but also in cultural and historical importance. In 2011, the population was 62,125 in the municipality area of 379.05 km². Figueira da Foz has a long history, and it shows traces of human settlement since the Neolithic age. The place developed conspicuously during the 18th and 19th centuries because of its busy port activities, shipbuilding, and cod-drying industry.

Today it is a popular tourist destination, famous as the "Queen of the Beaches" in Portugal, offering numerous large sandy beaches, such as its enormous seaside promenade paved with typical Portuguese pavement. The city boasts many historical landmarks, including the Sotto Mayor Palace, the Santa Catarina Fort, and several baroque-style churches.

2. What is the situation of the coastal currents close to Figueira da Foz?

The coastal currents around Figueira da Foz have a number of influences. First, there is the general pattern of circulation in the Atlantic Ocean. Large-scale ocean currents in the North Atlantic may influence the direction and speed of the local coastal currents.

The second-order effect of the outflow of the Mondego River is on the coastal currents. The volume and velocity of river water impinging on the ocean can alter the nearshore currents, especially in rainy conditions or snowmelt states of the catchment area.

Not less important is the wind. Figueira da Foz is considered a pretty windy coastal area, and by prevailing winds which may drive the surface waters, creating or modifying the coastal currents, for instance, strong westerly winds push the water to the east along the coast, and northerly or southerly winds would have different effects on both the direction and strength of the current. Tidal movements also, on one side, contribute to the coastal currents. Due to regular rises and falls of the tides, the water may flow in and out of the estuary and down the coast. Such phenomena often interact with other current-forming factors to build up complex current patterns.

3. How to observe the coastal water flow of Figueira da Foz?

Surface Drifting Buoy Method: Surface drifting buoys are small-sized devices equipped with various sensors. These drift buoys may simply be floated on the ocean, their movement dependent on that of ocean currents. Depending on the sensors available, their speed, velocity, and current motion can, thus, give useful information of water movements. Generally inexpensive, many are used across open waters but do have low resolution and possible disturbances due to winds and wave action. 

Anchored Ship Method: It would involve the usage of a ship that would be anchored relatively close to the coast. Current meters and other instruments on the ship measure the speed and direction of currents at various depths. While this method can give very detailed and accurate data at a specific location, it is limited in terms of the area that can be covered and is also affected by the ship's motion and the influence of the ship itself on the local currents.

Acoustic Doppler Current Profiler (ADCP) Method: The ADCP is an advanced and convenient measurement tool. It utilizes the Doppler effect for its working process in measuring the velocity of water currents. These can be deployed in a number of ways: on ships, moored to the seafloor, or attached to buoys. In this regard, it may provide high-resolution real-time data on current-velocity profiles over a range of depths much larger than the methods of anchored ship and more accurate than surface-drifting buoys.

4. How Do ADCPs Applying the Doppler Principle Work?

The ADCP sends out acoustic signals into the water. These are returned, by the movement of particles in the water such as suspended sediments or plankton; the frequency of the reflected signals shifts due to the Doppler effect. This frequency shift is measured and used by the ADCP for calculation of the velocity. It sends out signals at various angles and receives the backscattered signals to know the velocity components in various directions, hence a three-dimensional profile of the current velocity. Usually, the instrument divides the water column into multiple depth bins and measures the current velocity at each bin, giving a detailed picture of how the currents vary with depth.

5. What is needed for high-quality measurement of the Figueira da Foz coastal currents?

In order to carry out high-quality measurements of coastal currents in Figueira da Foz using ADCPs, a number of features are expected from the instrument.

Reliability: The ADCP should be able to operate continuously with high accuracy in a hostile marine environment, resisting the corrosion of saltwater, temperature variations, and mechanical tension.

Compact and lightweight size: Compact and light size will make ADCP more deployable and easier to handle on either a small research vessel or buoy, and in conditions of manual deployment in shallow waters.

Low power consumption accommodates longer running times operating on a battery-powered system reduces replacement or higher frequency recharging of batteries.

Low cost: The low expense of the individual equipment allows an extended deployment as well as scaling up measurement campaigns - high spatial and temporal coverage.

One material that is highly suitable for the casing of ADCPs is titanium alloy. Titanium alloy has several advantages. It is extremely corrosion-resistant, which is crucial for long-term use in the ocean. It is also strong yet lightweight, providing the necessary durability without adding excessive weight to the instrument. Additionally, titanium alloy has good mechanical properties, allowing it to withstand the pressure and impacts in the marine environment.

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

Depending on the Purpose of Use

Shipborne ADCP: it will be applicable in large-scale surveys and when measurements continuously even when the ship is in motion. It can cover a wide area and give information along the ship's track.

Bottom-Mounted ADCP: Suitable for long-term measurement in a fixed location, it is capable of providing high-resolution information of the current pattern at the bottom and along the water column above.

Buoyant ADCP: Buoy-deployed, these can measure the surface and subsurface currents in a limited area. These are often used in monitoring areas where access by ship is not possible or when shorter-term studies are required.

Based on Frequency:

600kHz ADCP is suitable for water depths within 70m, which can provide high-resolution data in relatively shallow waters.

It could be seen that a 300kHz ADCP would be good to about 110m with a good compromise of range of measurement and resolution. For waters as deep as 1000m, one would use a 75kHz ADCP; the resolution might be somewhat poorer than for higher-frequency instruments.

There are some famous brands of ADCP in the market, like Teledyne RDI, Nortek, and Sontek. In addition, if you want to get more cost-effective and high-quality ADCPs, you can try the China Sonar ​PandaADCP series. The shell is made of all-titanium alloy material, and the performance is incredible. For more details, please check the website at [​https://china-sonar.com/.

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