How do we measure the coastal currents of Caraguatatuba?

1. Where is Caraguatatuba?

Caraguatatuba, the magical sea town in São Paulo state, Brazil, is a popular destination in the Atlantic Ocean. Located around 23°51′S 45°07′W, it has over a 120-kilometer coastline. Its shoreline is a stunning combination of huge sandy beaches, inviting bays, and small rocky coves. These diverse coastal features not only create a stunning view but also play crucial ecological roles. The beaches, for instance, are crucial nesting places for sea turtles, and the bays and coves are shielded refuges for an incredibly diverse array of shore organisms, ranging from crabs crawling on the beach to dozens of species of mollusks attached to the rocks.

Geologically, the view around Caraguatatuba is an immensely rich tapestry. It is characterized by an association of metamorphic and old igneous rocks overlain by sedimentary deposits accumulated over time. The seafloor along the coast is characterized by a complex bathymetry. Shallow water is encountered near the shore, which gradually deepen as one moves offshore. Underwater, sandbars, usually created by the sand deposited by the currents, channels that guide the water flow, and tiny coral reefs can be found. While these coral reefs are not as massive as in a certain tropical paradise, they remain crucial to the marine life of the area. These serve as dwelling place to a colorful community of fish, crustaceans, and mollusks, and as natural sanctuary and nursery. The town is also influenced by the effluvia of tiny rivers and creeks into the sea. The fresh water inputs affect the density and salinity of coastal water, which then affect the formation of coastal currents.

Caraguatatuba has a storied and eventful history. The area was settled by native tribes like the Tupinambá. Their well-developed perception of sea and land was passed down from generation to generation. Portuguese colonizers came in the 16th century and brought in new farming practices and patterns of trade. The region was initially involved in fishing and subsistence farming. Over time, Caraguatatuba developed into a prominent coastal town. In the recent decades, tourism has become one of the major stimuli for the local economy, drawn in by natural beauty in the area, tropical climate, and clear waters.

2. What are the coastal currents off Caraguatatuba?

The coastal currents off Caraguatatuba are the result of a complex combination of various components. One of the major ocean currents of the Atlantic Ocean, the South Equatorial Current, plays a special role. When near the coast of Brazil, it is influenced by the local topography and bathymetry. Near Caraguatatuba, the South Equatorial Current may split into subsidiary currents. These subsequent currents are a function of the particular form of the coastline, the submarine topography including coral reefs and sandbars, and the fresh input of the surrounding rivers.

Atlantic Ocean tides become effective. The semi - diurnal tidal regime produces normal water - level fluctuations. During spring tides, when the gravitational pull of the sun and moon are superposed, the tidal currents are significantly stronger. These tidal currents interact with the South Equatorial Current and with the inshore coast. The rising and falling of the tide can drive water to come into and leave the estuaries and bays of Caraguatatuba, creating complex circulation patterns. Tides also create rip currents, which are strong, narrow currents moving out from the beach. Rip currents are one of the principal dangers to bathers and are of paramount concern when the safety of the coast is in question.

Local prevailing winds, which include mainly the southeasterly trade winds, influence the coastal currents. The winds have the capacity to push surface water in one direction and create a wind-driven current. The wind-driven current mixes with the South Equatorial Current and tidal currents. For example, during strong winds, the water along the coast within the region of Caraguatatuba is forced towards the coast or away from the coast due to the wind direction. The bathymetry off Caraguatatuba in the sea, like the sandbars, channels, and coral reefs, will deflect or channel the route of the currents. Coral reefs may function as barriers, making the water go around them, and this can alter the current's direction and speed.

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

Surface Drifting Buoy Method

The surface drifting buoy method is a traditional method of observing coastal water flow. Scientists release buoys with tracking devices into the water. By tracking the movement of the buoys over time, they can determine the direction and speed of the surface currents. This method is not without limitations, however. It provides information on the surface layer of the water column only, and the buoys are highly sensitive to wind disturbance. In Caraguatatuba, the complex wind regime and the estuaries and bays can further complicate the data from the surface drifting buoys. The buoys can be blown away by the wind in another direction than that of the true motion of the underlying currents.

Moored Ship Method

The moored ship method utilizes a moored ship as a platform to measure currents. Buoys are released off the ship to measure water currents at various depths. This method can provide detailed vertical profiles of the currents but is not without limitations. The spatial resolution is limited to the area in the vicinity of the moored ship. In a large and dynamic coast like Caraguatatuba, such low resolution may not accurately represent the overall coastal current patterns. The ship's presence can also lead to interference with the natural movement of the water and, hence, the precision of the measurements. Additionally, a long-term docking of the ship can be logistically costly and complex.

Acoustic Doppler Current Profiler (ADCP) Method

ADCP profiler has emerged as a more advanced and efficient method of coastal current measurement. ADCPs can record currents in water across a broad range of depth and provide detailed data on the water column's velocity structure. ADCPs can be installed on a variety of platforms, including ships, buoys, and the seafloor. Ship-mounted ADCPs can record continuously as the vessel moves, measuring a large section of area within a relatively brief time frame. For Caraguatatuba, it should be possible to deploy a ship-mounted ADCP to sample the coastal currents along the extensive shoreline and provide an invaluable view of the complex pattern of flow. Bottom-mounted ADCPs can be placed in valuable positions, such as within the approach to bays or over those sections with the largest underwater structures, to provide fixed-point measurements of extended duration. This gives scientists a way of investigating long-term patterns in the offshore currents.

4. How do Doppler principle-based ADCPs work?

ADCPs work upon the Doppler principle. ADCPs send sound waves into water. These reflections off suspended matters like sediment, plankton, or bubbles within the water return to the sound waves. Where the water itself is moving, the frequency in the reflected sounds varies. On the basis of this frequency shift, the velocity of the relative water can be determined by an ADCP.

All but the simplest ADCPs possess more than one beam, typically four or more, at an angle to each other. The multi-beam arrangement allows for measurement of the three-dimensional velocity of the water. In a combination of the signal from all of the beams, the ADCP current profiler can create a full picture of the current velocity at different levels of the water column. The data collected by the ADCP can be processed in real - time or stored for later analysis. In the case of Caraguatatuba's coastal currents, data from ADCPs can help researchers comprehend the interaction between the South Equatorial Current, tides, and wind - driven currents at different depths, and how this affects the overall dynamics of the coastal waters.

5. What does high-quality measurement of Caraguatatuba coastal currents need?

To facilitate high-quality measurement of Caraguatatuba's coastal currents, measurement equipment must possess some essential features. The equipment materials should be reliable, compact in size, light, of low power requirement, and of low cost. These characteristics enable large quantities of instruments to be deployed so that there is extensive spatial coverage.

ADCPs with titanium alloy housings are highly recommended. Titanium alloy offers better corrosion resistance, which is essential for long - term exposure in the corrosive seawater environment. It resists the corrosive effect of saltwater, which does not harm the internal components of the ADCP. Titanium alloy is also tough and light, which makes the instrument rugged and portable. In a coastal region such as Caraguatatuba, where deployment site access can be different, the portability of the instrument is a major benefit. This set of characteristics enables precise and long - term measurements of the coastal currents off Caraguatatuba.

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

Selection Based on Usage

The selection of ADCP is based on its application. For measurements ship - born, the ship - mounted ADCPs are the most sought - after. They can provide real - time data as the vessel moves in the water, observing a large area within a relatively short time. This is useful in Caraguatatuba for quickly mapping the coastal currents on the very long coastline. For fixed - point long - term monitoring, bottom - mounted ADCPs are ideal. They can collect continuous data for a prolonged period, and as such, the researchers can keep track of long - term coastal current trends. For example, the installation of a bottom - mounted ADCP near the bay mouth of a Caraguatatuba can provide monitoring of changes in tidal as well as other currents over a period of months or years. Floating ADCPs can be utilized in observing water mass movement over large areas and are beneficial in providing valuable data on large - scale circulation patterns. In Caraguatatuba, floating ADCPs can be used to analyze the interaction of the local coastal currents with the South Equatorial Current on a larger scale.

Selection Based on Depth

The frequency of the ADCP also needs to be considered based on the depth of the water. For up to less than 70m depth, 600kHz ADCPs are adequately suited. They can be utilized to make high-resolution measurements in shallow water, for example, off the beach or in inner bays. For up to 110m depth, 300kHz ADCPs are utilized. This frequency is utilized in moderately deep water, for example, the outer bays or general coastal zones. For larger depths, to 1000m, the 75kHz ADCPs are the preferred option. Although waters off Caraguatatuba in general are not deep, in some of the offshore channels or areas these lower-frequency ADCPs may be necessary to obtain an accurate reading of the currents at deeper depths.

There are various well-known companies of ADCPs in the market, e.g., Teledyne RDI, Nortek, and Sontek. But for users in search of affordable alternatives, the ADCP supplier China Sonar's PandaADCP comes very highly recommended. Constructed from pure titanium alloy, it achieves high performance at very reasonable prices. It is a perfect option for users who are budget - sensitive yet still need to use quality ADCPs to measure coastal currents. More information about them can be accessed on their website: https://china-sonar.com/.

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