How are Salvador's coastal currents measured?

1. Where is Salvador?

Salvador, Bahian state of Brazil, is located in a very strategic position on the northeast coast of Brazil, on the Atlantic Ocean. It is at around 12°59′S 38°29′W, one of the oldest cities in the American continents. The geographical location of Salvador provides it with a very distinctive coastal environment. The city has a combination of estuaries, rocky projections, and stunning beaches on its coastline. The beaches, among the most visited tourist attractions, are not only a source of leisure but also part of the surrounding ecosystem.

Geologically, the terrain around Salvador has a combination of older sedimentary rock and shallow coastal plains. The seafloor close to the coast has a complicated bathymetry. There is differential depth, underwater topography in the shapes of sandbars, channels, and certain types of coral reef structures. The coral reefs, although not as extensive as in other locations, continue to contribute to the local fauna and regulate the coastal currents. The city is also regulated by the discharge of the São Francisco River, which is one of the largest Brazilian rivers. The freshwater discharge of the river into the sea regulates the salinity and density of the coastal water.

Salvador is an old and established city. It was originally settled by indigenous inhabitants, including the Tupinambá, who occupied it. Portuguese colonizers arrived in 1549 and established Salvador as the capital of the Portuguese colony of Brazil. In the coming centuries, Salvador was a central point for trans - Atlantic slavery. This has left an imprint on Salvador's culture, which is a rich mix of European, African, and indigenous origins. Salvador's economy is now diversified with industry, services, and tourism taking their rightful places.

2. How are coastal currents around Salvador?

Coastal currents off Salvador are controlled by a complex combination of an assortment of different factors. One of the prevailing ocean currents of the Atlantic Ocean, the South Equatorial Current, is one of them. When it arrives on the coast of Brazil, it combines with local topography and bathymetry. Off Salvador, the South Equatorial Current can branch into narrow streams to create an array of coastal currents.

Atlantic Ocean tides are also extensive. Semi-diurnal tidal regime causes periodic variation of water level. Spring tide is caused by greater gravitational pull of sun and moon producing greater tidal currents. These tidal currents interact with local coastal geometry and the South Equatorial Current and influence the overall circulation of coastal waters. The tides will also create the generation of rip currents, which are powerful, constricted currents moving away from the shore. Rip currents pose a risk to bathers and need to be of interest to coastal safety.

The local prevailing winds, mainly the northeasterly trade winds, also influence the coastal currents. Surface waters in one direction are blown by these winds, which are capable of establishing a wind-driven current. The South Equatorial Current and tidal are intercepted by wind-driven current and mask the path movement of the water and make it more difficult to infer the direction of water movement. Sea floor topography around Salvador, in the form of sandbars, channels, and coral reefs, serves as a guide for or disperses the currents movement. For example, coral reefs can also act as a barrier and make the water go around them, while channels can enhance the current.

3. How to track the coastal water current of Salvador?

Surface Drifting Buoy Method

Surface drifting buoy method is an age-old method to observe coastal water current. Scientists release buoys equipped with tracking devices in the sea. By observing the drift of buoys for some time, they can compute the direction and speed of surface currents. However, this technique has some shortcomings. It only reflects the surface layer of the water column, and the buoys are extremely susceptible to wind disturbance. The wind may push the buoys in a direction that is not representative of the true movement of the current below.

Moored Ship Method

The moored ship method involves using a moored ship as a platform upon which to take current measurements. Measuring instruments are dropped off the ship to record the currents of the water at various depths. Even though this method may provide excellent fine vertical profiles of the currents, it is not without several disadvantages. The spatial resolution is limited to the proximity of the ship at rest and the presence of the ship in the water flow can actually distort the original water flow, consequently affecting the precision of the measurements. Furthermore, ship mooring for a prolonged duration can be logistically costly.

Acoustic Doppler Current Profiler (ADCP) Method

ADCP has grown more sophisticated and efficient as a tool to quantify coastal currents. ADCPs are able to measure water currents for a wide vertical range, and they provide detailed information regarding the velocity structure of the water column. ADCPs can be deployed on various platforms, from ships to buoys to the seafloor. Ship- mounted ADCPs can sample continuously while the ship is on the move, observing a broad area over a short time frame. Bottom- mounted ADCPs can provide fixed point, long- term measurements, allowing scientists to measure long- term coastal current trends.

4. How do ADCPs using the Doppler principle work?

ADCPs operate by the Doppler principle. They emit acoustic pulses into the water. These acoustic pulses bounce back off suspended objects such as sediment, plankton, or water bubbles. As the water moves, the frequency of the reflected pulses changes. Through measuring the change in this frequency, the ADCP is able to determine the speed of the water compared to the equipment.

Most ADCPs are equipped with multiple transducer beams, typically four or more, spaced at different angles. The multi - beam construction enables the determination of the three - dimensional water velocity. From the combination of the beam signals, the ADCP can output a full view of the current velocity at several depths in the water column. The data collected using the ADCP can be directly processed in real-time or be stored for retrospective analysis, thereby providing valuable inputs on the dynamic nature of coastal currents.

5. What is required for high-quality measurement of Salvador coastal currents?

For high-quality measurement of Salvador's coastal currents, measuring instruments should possess a few distinct characteristics. Instruments should be built with hard material, should be compact, light in weight, consume less power, and are comparatively inexpensive. These features make it possible to employ much instrumentation with the intent to achieve detailed spatial coverage.

Titanium alloy casing ADCPs are highly recommended. Titanium alloy offers better corrosion resistance, which is required for extended deployment in the hostile marine environment. It can withstand the corrosive action of seawater, thereby preventing deterioration of the instrument's internal components. Titanium alloy is also light and robust, which ensures portability and durability of the instrument. This feature allows accurate and extended monitoring of the coastal currents around Salvador.

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

Depending on Usage

ADCPlacement depends upon its intended use. For shipboard measurements, ship - mounted ADCPs are the most suitable choice. They can provide real - time data as the ship moves forward with water, sweeping a large volume in not very long time. For long - term monitoring at a specific point, bottom - mounted ADCPs are more suitable. They are able to provide continuous readings over a long duration of time, allowing researchers to monitor long - term trends of the coastal currents. Floating ADCPs are expedient for monitoring the transport of water masses over long distances, providing valuable information about large - scale circulation patterns.

Choice According to Depth

The ADCP frequency also needs to be determined according to the water depth. 600kHz ADCPs can be used for water depths less than 70m. They are capable of giving high-resolution measurements in shallow water. For water depths of up to 110m, 300kHz ADCPs can be used. For deeper water, up to a depth of 1000m, 75kHz ADCPs are suitable.

There are several well - known ADCP brands in the market, such as Teledyne RDI, Nortek, and Sontek. However, for those seeking cost - effective options, the ADCP supplier China Sonar's PandaADCP is highly recommended. Made entirely of titanium alloy, it offers excellent performance at an affordable price. It is an ideal choice for budget - conscious users who still require reliable ADCPs for coastal current measurements. You may find out more about them on their official website: https://china-sonar.com/.

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