How do we quantify the coastal currents of Florianópolis?

1. Where is Florianópolis?

Florianópolis, the capital of southeastern Brazil's state of Santa Catarina, is a geographically and culturally significant city. It is located on a namesake island, with access to the mainland being by means of numerous bridges, and it is the largest island state capital in Brazil and is bordered by the Atlantic Ocean and has a fascinating and varied coastline.

Florianópolis is endowed with coastal ecosystems in its geography. It has over 140 kilometers of beach with a line of picturesque beaches. From the wide and sandy Praia Mole, a surfers' dream because of the consistent waves, to the more distant and less crowded Praia da Joaquina, every beach is different. The beaches are not only a big tourist attraction but also a integral part of the local ecosystem. The sand dunes on the shore are natural barriers against coastal erosion and support an extensive range of plant and animal life.

Within the interior, the island is rugged, with an elevation at Morro da Conceição of 513 meters. The hillsides are covered by dense Atlantic Forest, a breeding ground for fauna and flora containing a very high percentage of endemic species. These forested areas are cut across by many bodies of water and rivers flowing out into the ocean, which foster the diverse life in the oceans.

Florianópolis is fertile and stormy with culture. It was first inhabited by Guarani native people. Portuguese colonizers arrived in the region during the 16th century with their culture, religion, and language. The city's historic town was born out of its colonial history. It is filled with stunningly preserved colonial structures, such as the 18th-century São José Church and Fort of São José, which attest to the history of the city. The local culture is a rich mix of indigenous, Portuguese, and other European cultures, and African cultures introduced by slaves from colonial times. The cultural melting pot is also evident in the local cuisine, music, and festivals. Festa de São João, for example, a traditional Portuguese festival, is celebrated with much fervor with traditional dance, music, and a range of local dishes.

2. What are the conditions of the coastal currents around Florianópolis?

The coastal currents around Florianópolis are dominated by the complex combination of several forces. Tidal forces predominate to a certain extent. Regular tides and drops in tides of the region produce unprecedented volumes of flow of water. At high tide, the sea water flows into the bays and estuaries surrounding the island and back to the sea at low tide. The tidal currents are strongly concentrated, especially in narrow channels such as the Canal de São José between the island and the mainland. The tidal currents are responsible for the sediment, nutrients, and marine life transport, which impacts the adjacent marine ecosystem.

The wind patterns also significantly contribute to the development of the coastal currents. Southeast trade winds prevail over this area in Brazil. The trade winds are strong enough to drive surface waters along the coast and create longshore currents. The direction and strength of the longshore currents also vary relative to the time and magnitude of the wind. More energetic longshore currents are produced by more intense southeasterly winds, which tend to transport sand along the beach and erode or accrete the beach in other locations at other locations. For the instance of hard winds, there can be enhanced erosion on the southern beach, while in weaker places, sand will be deposited, changing the form and shape of the beach.

The presence of the island itself and the sea's complex bathymetry also have a respective impact on coastal currents. The shape of the island's coast and the varied depth of the seafloor, together with the shallow reefs, deep channels, and submarine canyons, are able to deflect the currents as well as change their direction. Shallow reefs, for example, can give a wall diversion of the current by splitting the current or deflecting it in another direction. Depth in water also contributes on current strength, and the more the depth of water will be the stronger and more stable currents.

3. How to monitor the coastal water current of Florianópolis?

Surface Drift Buoy Method

One of the traditional methods applied to observe coastal water currents is by using surface drift buoys. They are floating instruments with tracking devices onboard, e.g., GPS or satellite transmitters. Once they are dropped into the ocean, they send signals at regular intervals. From the varying motion of the buoys with time, scientists can estimate the speed and direction of the surface current. Surface drift buoys have limitations, however. They are very sensitive to winds and waves. If there is strong wind, the buoys will be displaced from the direction of the current, so it becomes a less-than-precise measurement. Also, they only measure the surface layer of the water column, and the subsurface currents are not measured.

Anchored Ship Method

The anchored ship method employs a stationary ship as a platform to measure currents. Scientists release current meters from the ship at different depths to record the speed and direction of the current. This method gives a better vertical profile of the current than surface drift buoys. However, it is labor and time consuming. The presence of the vessel also may cause perturbation to the natural movement of the current, and one location can be measured. Inclement weather, through strong winds and high waves, can complicate deployment and maintenance of equipment or even render it impossible.

Acoustic Doppler Current Profiler (ADCP) Method

Acoustic Doppler Current Profiler (ADCP) is now an advanced and reliable method for coastal current monitoring in the vicinity of Florianópolis. ADCPs are able to make high-resolution current velocity measurements at various depths within the water column.

4. How do Doppler principle-based ADCPs operate?

ADCPs operate on the principle of Doppler. They project sound waves into the water. When these sound waves encounter particles in the water, such as suspended sediment or small marine organisms, some of the energy in the sound will be reflected back to the ADCP current meter. When the particles are transported with the current, the frequency of the reflected sound wave will differ from the frequency of the transmitted sound wave. This difference in frequency, or Doppler shift, is directly proportional to the velocity of the particles and, thus, the present velocity.

Most ADCPs use multiple beams, typically four, to measure the Doppler shift from different directions. By analyzing the Doppler shifts from the multiple beams, the ADCP is able to measure the three- dimensional current velocity at multiple levels. The depth at which current is taken with the ADCP depends on frequency of sound waves used by ADCP. Higher - frequency ADCPs are employed for measurement in shallow water since they provide higher resolution but shorter range. Lower-frequency ADCPs possess greater ability to penetrate water column but will have lower resolution.

5. What is required to get high-quality measurement of Florianópolis coastal currents

To achieve precise measurement of the coastal currents of Florianópolis, certain attributes are of critical importance in the measuring equipment. The reliability of the material is of greatest concern. The equipment must be able to withstand the aggressive marine environment, which entails contact with seawater, strong currents, and variable weather conditions. Corrosion - resistant materials are needed to ensure long - term operationality of the device. For ADCPs, the choice of casing material is a consideration. Titanium alloy is an excellent choice for ADCP flow meter casings used in the coastal waters of Florianópolis. It is corrosion-resistant, which is a prerequisite for its capacity to withstand the corrosive effects of saltwater over long duration. It is also strong and light, providing a useful tradeoff between deployability and durability.

Size, weight, and power are also critical considerations. Equipment that is light and compact can be more readily deployed, especially in remote or hard - to - reach areas. Low - power - utilization technologies can be powered for lengthy periods without incessant battery exchange or recharging, which also is a foremost consideration when thinking about long - term monitoring activities. Cost - effectiveness is a very important consideration. To enable large-scale measurements and overall data acquisition, the equipment should be cost-effective. Costly devices will limit the number of measurement points available, hence the accuracy and representativeness of the data.

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

By Usage

The type of equipment to use to measure current would depend on the application. For the measurement of current on a ship using an ADCP mounted on a moving ship, a ship-mounted ADCP would be utilized. This ADCP can be utilized for continuous measurement of the current as the ship moves through the water and leaves a profile of the current along the ship track.

For long - term, fixed - point observations, a bottom - mounted (or sit - on - the - bottom) ADCP is appropriate. They are laid on the seafloor and can monitor current information for an extended duration at a particular location.

If the goal is to measure the current in a more mobile and scattered manner, a buoy-mounted ADCP can be used. These are installed on floating buoys and can be permitted to drift with the current, taking measurements as they move.

Choosing the Right Frequency

The frequency of the ADCP is also something to consider. A 600kHz ADCP is suitable for measuring currents in water depths of up to 70m. It delivers comparatively high - resolution readings in shallow - to - moderate - depth seas. A 300kHz ADCP can measure to depths of about 110m, and a 75kHz ADCP to much greater depths, even as much as 1000m. Frequency depends on anticipated water depth at which the current is to be measured.

There are only a couple of popular ADCP brands that have presence in the market, viz. Teledyne RDI, Nortek, and Sontek. However, for those budget-conscious, there is the ADCP manufacturer China Sonar's PandaADCP. It is made wholly of titanium alloy and performs pretty well without much expense. For price-sensitive individuals who want reliable ADCPs still, it makes a fine alternative. You may know more about them on their website: https://china-sonar.com/.

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