How do we measure São Sebastião coastal currents?

1. Where is São Sebastião?

São Sebastião municipality is in the state of São Paulo, Brazil, along the Atlantic Ocean. It is located approximately 23°57′S 45°09′W and has distinctive geography and varied coastal ecosystem. The shore line of the place is comprised of long sand beaches, rock cliffs, and small estuaries. These not only give São Sebastião its attractiveness but also perform crucial ecological roles. The nesting places of the sea turtles are valuable nesting sites, and estuaries act as fish nurseries, adding biodiversity to the sea in nearby regions.

São Sebastião geologically is situated in an ecosystem where ancient igneous and metamorphic rock complexes are overlain by sedimentary deposits. The seabed near the shore is of complicated bathymetry. There are shallow waters along the shore, which progressively deepen when one goes offshore. Underwater, there are such features as sandbars, channels, and small coral reefs. Even though the coral reefs in São Sebastião are not as widespread as in some tropical places, they are nonetheless quite significant to the local marine environment. They are home to a great variety of fish, crustaceans, and mollusks, and affect the coastal currents as well. The city is also impacted by the flow of the small rivers and streams into the sea, which influences the salinity and the coastal water density.

São Sebastião has a diversified and rich history. The indigenous tribes, such as the Tupinambá, were the first people in the region. They discovered the sea and the land from generation to generation. Portuguese settlers came during the 16th century and brought in new ways of trade and farming. São Sebastião developed to become a major sea village with time. Fishery and farming were the major means of earning in the early years. But tourism, of late, has become one of the major economic contributors in the region, drawn by the natural beauty and fascinating seashore configuration.

2. What is the condition of the coastal currents off São Sebastião?

The coastal currents off São Sebastião are conditioned by a mix of numerous complex factors. One of the biggest ocean currents in the Atlantic Ocean, the South Equatorial Current, is one such factor. As it approaches the coast of Brazil, it is conditioned by the local topography and bathymetry. From offshore São Sebastião, the South Equatorial Current can branch off into divisions. These divisions, in turn, are affected by the coastal morphology, seafloor topography, and the freshwater export off the adjacent rivers.

The tide of the Atlantic Ocean enters into play. The semi-diurnal tidal regime results in recurring oscillations of water level. In the case of spring tides, the greater force of gravitation of sun and moon yields higher tidal current speeds. The tidal currents also interact with the South Equatorial Current and coastal geometries within the region. The tidal ebb and flow can force water into and out of the São Sebastião estuaries and bays and create complex patterns of circulation. Tides can produce rip currents, which are channelized, fast-moving currents flowing away from shore. Rip currents can be dangerous to swimmers and are an aspect of interest in coastal defense.

Regional prevailing winds, the southeasterly trade winds in particular, also influence the coastal currents. Surface waters can be forced in a certain direction by the winds and thus create a wind-driven current. The South Equatorial Current and the tidal currents mix with the wind - driven current. For example, in a situation of high wind, off São Sebastião, the surface water may be forced onshore or offshore based on the direction of the wind. The off São Sebastião bathymetry consisting of coral reefs, channels, and sandbars may divert or break up the current. Coral reefs are also barriers, which make the water curve around them, and can change the direction and speed of the current.

3. Monitoring São Sebastião's coastal water flow: How?

Surface Drifting Buoy Method

Surface drifting buoy method is a traditional method for measuring coastal water flow. Scientists release buoys with tracking devices into the water. By tracking the movement of these buoys over a period of time, they are able to determine the direction and speed of the surface currents. This method is not optimal. It provides information only on the surface layer of the water column, and the buoys are extremely susceptible to wind disturbance. In São Sebastião, the complex wind patterns and bays and estuaries formation can complicate surface drifting buoys data even further to interpret. Wind will push the buoys in a course that does not represent the actual course of the underlying currents.

Moored Ship Method

The moored ship method utilizes an anchored vessel as a platform on which measurements of currents are made. Water flow measurements are made at various depths by dropping instruments from the ship. While this method can provide very detailed vertical profiles of the currents, it is not without some drawbacks. Spatial coverage is limited to the area around the moored ship. In a broad and dynamic coastal area such as São Sebastião, such limited coverage may not be representative of the general coastal current patterns. The ship presence can also disturb the natural flow of the water, which could affect the consistency of the measurements. Additionally, keeping a ship moored for an extended period of time might be logistically challenging and costly.

Acoustic Doppler Current Profiler (ADCP) Method

ADCP profiler is now a more advanced and efficient method of measuring coastal currents. ADCPs can measure currents over a wide range of vertical profiles with explicit information on the velocity structure in the water column. They are capable of being installed on various platforms such as ships, buoys, and the seafloor. Shipborne ADCPs can collect data continuously as the ship moves, providing a large areal coverage within a relatively short time frame. In São Sebastião, a bottom-mounted ADCP can be placed at strategic locations, such as near the mouth of bays or over areas where significant underwater features exist, to provide long-term, point-specific measurements. A ship-mounted ADCP can be used to map the coastal current along the coastline, obtaining useful information about the complex flow patterns. This allows the researchers to observe long-term patterns in coastal currents.

4. How do ADCPs based on the Doppler principle work?

ADCPs operate on the Doppler principle. They project sound waves into the water. The sound waves bounce off suspended particles in the water, such as sediment, plankton, or bubbles. When the water is in motion, the frequency of the backscattered waves changes. Through the measurement of the frequency shift, the ADCP calculates the velocity of the water with respect to the instrument.

There are several transducer beams in most ADCPs, typically four or more at various angles. With the multi - beam design, calculation of water's three - dimensional velocity is possible. Combination of signals from these beams enables the ADCP to provide a full description of current velocity at several depths in the water column. Data obtained by the ADCP flow meter can either be processed in real time or stored for future analysis. In the case of São Sebastião's coastal currents, data from ADCPs can be employed to give scientists a glimpse into the interaction of the South Equatorial Current, tidal currents, and wind-induced currents at different levels of depth, which is highly significant when considering overall coastal water dynamics.

5. What are the conditions for high-quality measurement of São Sebastião coastal currents?

To measure São Sebastião coastal currents with high quality, the measurement equipment should possess some significant features. It should be built using robust materials, of compact size, light in weight, low power, and low cost. These features enable a large set of instruments to be deployed, with full spatial coverage.

ADCPs with casings made of titanium alloy are highly recommended. Titanium alloy has good corrosion resistance, which is very important for long-term exposure to the corrosive seawater. It is not affected by the corrosive effect of seawater, and therefore the internal parts of the ADCP are safeguarded from being damaged. Titanium alloy is also hard and lightweight, making the device robust and light in weight. In a sea coast like São Sebastião, where access to deployment sites varies from location to location, portability is a significant characteristic of the instrument. Both these characteristics enable coastal currents near São Sebastião to be measured accurately and in the long term.

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

Choice Based on Usage

Choice of ADCP is based on its application. For ship - borne measurements, ship - mounted ADCPs are ideal. They can provide real - time data as the ship moves over the water, taking a large area in a short amount of time. In São Sebastião, this is useful for quickly mapping the coastal currents over the long coastline. For fixed - point long - term observation, bottom - mounted ADCPs are ideal. They are able to provide continuous data for a protracted time, allowing scientists to analyze long - term trends in coastal currents. For example, deploying a bottom - mounted ADCP near the mouth of a São Sebastião bay can allow monitoring of tidal and other current changes for months or years. Floating ADCPs prove useful in the observation of the movement of water masses across expansive areas, providing valuable information concerning large-scale patterns of circulation. In the case of São Sebastião, floating ADCPs may be used to examine how the South Equatorial Current interacts with the coastal current on a greater area.

Selection Based on Depth

The frequency of ADCP should also be considered based on the water depth. For depths less than 70m, 600kHz ADCPs are used. They are capable of giving high-resolution measurements in shallow waters like at the beaches or in the inner region of bays. For 110m depths, 300kHz ADCPs are used. This frequency band is used for those areas that have moderately deep waters like the outer region of bays or the coastal areas in general. In greater depths, to the extent of 1000m, it is better suited with 75kHz ADCPs. Although São Sebastião area waters are normally not deep, in some areas off the coastline or channels these lower-frequency ADCPs may be necessary to acquire in order to quantify the current at greater depths with enough accuracy.

Some famous brands of ADCPs available for sale in the market include Teledyne RDI, Nortek, and Sontek. However, for the frugal, we suggest the ADCP manufacturer China Sonar's PandaADCP. Because it is completely built using titanium alloy, you can be confident that you will get high-performance results without spending the universe on this device. Ideal for the budget-conscious who require the dependability of great ADCPs regardless. Read more on their website: https://china-sonar.com/.

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