How can we measure Santos coastal currents?

1. Where Santos?

Santos, in the state of São Paulo, Brazil, is a large coastal city with a significant presence on the Atlantic Ocean. Approximately 23°58′S 46°19′W, it is a port city that flourishes as a key economic hub. The city coast is an open space, with a combination of sandy beaches, rocky outcrops, and a large artificial harbor. The beaches, such as the touristy Santos Beach, receive not only visitors but also perform essential ecological roles. They harbor many species of beach animals, including crabs, mollusks, and some species of nesting birds. The Santos port, which is one of Latin America's major ports, profoundly affects the surrounding coastal ecosystem and coastal currents run.

Geologically, the area surrounding Santos is characterized by a complex series of sedimentary rock units, which have been shaped over centuries of geological development. The seafloor near the coast is irregular in shape. Shallow waters are found near the shoreline, and gradually increase in depth as one moves further offshore. Underwater, sandbars, channels, and several small coral reef formations are present. The coral reefs, while not so widespread as in other areas of the tropics, are a vital part of the regional marine environment. They provide a habitat for an enormous range of fish, crustaceans, and mollusks and affect coastal currents as well. The city is also influenced by drainage of small rivers and streams into the sea, affecting salinity and density of the coastal water.

Santos boasts a successful and eventful past. The Tupinambá, native inhabitants, originally settled there. Portuguese colonizers arrived during the 16th century and built a colony that later developed into the city of Santos. As time went by, Santos became an important center for trade, and its harbor started playing the role of the go-between of trade transactions of products between Brazil and the rest of the world. Today, the economy of Santos is diversified with shipping, industry, and tourism being the driving forces.

2. How are coastal currents off Santos?

Coastal currents off Santos are governed by a complex mixture of several variables. Perhaps one of the most significant ocean currents in the Atlantic Ocean, the South Equatorial Current, also has an unmistakable impact. As it approaches the Brazilian coast, it comes into contact with the nearby topography and bathymetry. In the region surrounding Santos, the South Equatorial Current breaks up into small streams. These streams are also influenced by the layout of the coast, the location of the large port, and the outflow of fresh water from the surrounding rivers.

The Atlantic Ocean tides are immense. The semi-diurnal tidal regime produces periodic water-level oscillations. The stronger gravitational pull of the sun and moon during spring tides produces more forceful tidal currents. They control the South Equatorial Current as well as the interaction with the neighboring coast. They have the capability to carry the water in and out of the port as well as out of the neighborhood waters of the coast, which creates complex circulation currents. Tides also produce rip currents, rapid, shallow flow off the shore. Rip currents are dangerous to swimmers and are a point of consideration in coastal safety.

Local prevailing winds, which are dominated by the southeasterly trade winds, influence the coastal currents as well. The surface waters can be forced by the winds in a single direction and therefore create a wind - driven current. It is influenced by tidal currents as well as by the South Equatorial Current. For example, on periods of heavy wind, water near the coast off Santos gets pushed towards or away from the coast, as the case may be, with the direction of the wind. Bathymetry of the seafloor beneath the region in and around Santos, such as sandbars, channels, and coral reefs, can guide or block the current. Coral reefs may function as a barrier and cause the water to pass around them, and in so doing, change the speed and direction of the current.

3. How to study Santos' coastal water flow?

Surface Drifting Buoy Method

The surface drifting buoy method is a traditional approach to analyzing coastal water flow. Researchers release buoys equipped with monitoring instruments into the sea. By tracking the trajectory of these buoys over a period of time, they can estimate the speed and direction of the surface current. This method is not perfect, however. It only detects the surface layer of the water column, and the buoys are extremely susceptible to wind perturbation. At Santos, the combined wind conditions and the fact that the large port hinders the surface drifting buoy data even more. Wind has the potential to push the buoys to drift in a direction not representative of the true motion of underlying currents.

Moored Ship Method

Moored ship technique is a method in which a ship moored behaves as a fixed platform from where measurements of the currents are to be taken. Instruments are allowed to fall from the ship in order to record the flow of water at varying depths. Despite the fact that the method is capable of computing detailed vertical profiles of the currents, it is beset with several disadvantages. Spatial coverage is limited to the immediate area of the moored ship. For a broad and dynamic coastal region such as Santos, sparse coverage will not be sufficient to pick up general coastal current patterns. The ship presence can also disrupt the natural flow of water, which can affect the accuracy of the measurements. Additionally, maintaining a ship in a stationary position for a long time can be logistically complicated and costly, especially in a hectic port city like Santos.

Acoustic Doppler Current Profiler (ADCP) Method

ADCP flow meter is now a sophisticated and economical method of coastal current measurement. ADCPs can measure currents in water over an extensive vertical range and provide precise information about the velocity structure of the water column. ADCPs can be mounted on various platforms like ships, buoys, and on the seafloor. Ship-mounted ADCPs can take measurements continuously as the ship moves, making observations over an extended area in a relatively brief interval. On Santos, an ADCP mounted on the bottom or ship would be used to survey the coastal currents along the extended coastline and provide valuable insight into the level of detail of flow regimes. Fixed-point long-term measurements may be carried out using bottom-mounted ADCPs placed in key areas such as off the port mouth or in areas of high-underwater feature. This enables the scientists to examine long-term trends in the coastal currents.

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

ADCPs operate on the Doppler principle. They emit acoustic pulses into the water. The pulses bounce back off suspended material such as sediment, plankton, or bubbles in the water. If the water is in motion, the frequency of the returned pulses will be altered. From the frequency alteration, the ADCP is able to calculate the water speed relative to the instrument.

Most ADCPs employ multiple beams with several, typically four or more, transducers at different angles. This multi-beam arrangement provides for the determination of the three-dimensional water velocity. By combining the signals of the beams, the ADCP is able to generate a full description of the current velocity at different depths of the water column. The data collected using the ADCP can either be processed in real-time or reserved for later analysis. In the case of use in Santos' coastal currents, ADCP's data can enable the researchers' comprehension of the interaction of South Equatorial Current, tides, and wind-driven currents as functions of different depths and will therefore provide enormous insight into the dynamics of coastal waters in general.

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

In order to measure the coastal currents of Santos at high quality, the measuring device must possess certain qualities. It should be made of hard materials, possess low dimensions, low weight, low power consumption, and low expense. All these qualities enable putting a large number of instruments in the water with full spatial coverage.

ADCPs with titanium alloy housing are highly suggested. Titanium alloy is a suitable material for resisting corrosion, which is required in the long-term exposure in the harsh marine environment. It resists the corrosive effect of saltwater, thus avoiding damage to the working parts inside the ADCP. Titanium alloy is also robust and light in weight, which maintains the ruggedness and portability of the instrument. In a port town like Santos with limited access to deployment sites, the portability of the instrument is a vital plus. The combination of synergistic attributes that results from the instrument's portability makes it feasible to provide precise and long - term measurement of the off - Santos coastal currents.

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

Selection Based on Usage

ADCPtravels use is dependent on its usage. For measurements in the ship itself, ship-borne ADCPs are the ideal answer. They can provide data in real-time as the vessel moves through the water, cutting through a great extent in a reasonable space of time. This can be applied in Santos so as to chart the coastal currents along the extensive coastline quickly. For fixed - point long - term observation, e.g., at the port entry, bottom - mounted ADCPs are more suitable. They are able to deliver continuous data for a protracted period, and from that, long - term trends of coastal currents can be monitored by scientists. Floating ADCPs are useful to follow the migration of water masses over an extended region and provide valuable information regarding large - scale circulation features. To Santos, hung-up ADCPs are usable for analyzing the interaction of the South Equatorial Current with in situ local coastal currents at larger spatial scales.

By Depth Selection

Based on selection by depth, the ADCP current meter frequency should also be considered with reference to the depth of the water. Below a water depth of 70m, use of 600kHz ADCPs can be adequate. They can provide high-resolution readings in shallow waters, for instance, around the harbor or beach of the port. For depths up to 110m, 300kHz ADCPs should be employed. This frequency range is best suited for moderately deep locations, for instance, outer harbor of the port or coastal areas in general. For deeper water up to 1000m, 75kHz ADCPs should be employed. Whereas waters around Santos are generally not deep, within some offshore zones or channel reach segments, lower-frequency ADCPs would be needed to actually determine the flow at greater levels.

There are a variety of popular brands of ADCPs found on the market, from Teledyne RDI, Nortek, to Sontek. Inexpensive option users can especially find the ADCP supplier China Sonar's PandaADCP strongly recommended. Fully constructed with titanium alloy, it offers fine performance at a moderate price. It is an ideal choice for cost - sensitive users who also require stable ADCPs for measuring coastal currents. For further information, you may check their official website: https://china-sonar.com/.

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