How are we measuring the coastal currents of Natal?

1. Where is Natal?

Natal, the capital of the Brazilian state of Rio Grande do Sul, is located on the northeast coast of Brazil on the Atlantic Ocean. Its coordinates, approximately 5°55′S 35°12′W, place it in a crucial position on the coast. The city of Natal is renowned for its beautiful beaches, which extend for miles along the coast. They are a favorite among tourists but also form an integral part of the regional environment.

Geologically, the area around Natal is characterized by a mix of low - lying coastal plains and ancient sedimentary rock structures. The ocean floor off shore along the coast is extremely varied in bathymetry, with variations in depth and a wide range of underwater features such as coral reefs, sandbars, and channels. The presence of coral reefs is particularly significant, as they play a central part in controlling the coastal currents and providing a habitat for a diverse range of marine life. The city is also influenced by the nearby Potengi River, which discharges freshwater into the sea, affecting the salinity and density of the coastal waters.

Natal has a vibrant and rich history. The native tribes, including the Potiguar, originally inhabited the region. Portuguese colonists arrived in 1599 and established a settlement which developed into the city of Natal. The name "Natal" is itself the Portuguese word for "Christmas," as the city was established on Christmas Day. Over the centuries, the city has witnessed a variety of historical events, including being under colonial rule, the slave trade, and the development of a unique Afro - Brazilian culture. Today, the economy of Natal is supported by a diversified range of sectors including tourism, manufacturing, and services.

2. What is the state of the coastal currents off Natal?

The coastal currents of Natal are characterized by the effect of the interplay of numerous factors. The South Equatorial Current is among the contributing forces. The South Equatorial Current is among the largest ocean currents in the Atlantic Ocean. The current is warm, westward flowing and flows up the coast of Brazil, then, approaching Natal, combines with local topography and bathymetry. The South Equatorial Current will branch into branchlets near the coast and form a sophisticated series of coastal currents.

Atlantic Ocean tides are a primary influence. The semi - diurnal tidal cycle generates regular water - level variations. At spring tides, the stronger gravitational pull of the moon and sun generates stronger tidal currents. These tidal currents combine with the South Equatorial Current and the local shore morphology to affect the overall circulation of coastal waters. The tides also have the ability to create rip currents. These are narrow, powerful currents that move away from the beach. Rip currents are hazardous to bathers and are a matter of concern when dealing with coastal safety.

Local prevailing winds, mainly the northeasterly trade winds, influence the coastal currents. The wind can push surface water in one direction or another, creating a wind - driven current. The current, driven by the wind, overlaps with the South Equatorial Current and the tidal currents and thus makes the pattern of the water movement even more complex. Seabed bathymetry off Natal, consisting of coral reefs, sandbars, and channels, has the potential to direct or impede the flow of the water of the currents. Coral reefs, for example, are barriers that would have the water go around them, while channels can accelerate the current.

3. How is Natal's coastal water current to be observed?

Surface Drifting Buoy Method

Surface drifting buoy method is an ancient method of measuring coastal water current. Scientists put tracking devices bound on buoys in the sea. After a time span, observing where these buoys go provides the direction and speed of surface currents. It is a restricted method. It reports to us only of the surface layer of the water column, and the buoys are highly susceptible to wind disturbance. The wind can cause the buoys to be led off in a direction that is not the actual direction of movement of the underlying currents.

Moored Ship Method

The moored ship method utilizes a moored ship as a platform upon which currents may be measured. Gear is dropped from the ship to record the water flow at different depths. The technique has the capability of generating accurate vertical profiles of currents but is subject to some disadvantages. The coverage in space is limited to the area in the immediate vicinity of the moored ship, and the presence of the ship will disturb the natural water flow and influence the precision of the measurements. Also, maintaining a ship at a mooring for an extended duration can be logistically challenging and costly.

Acoustic Doppler Current Profiler (ADCP) Method

ADCP meter has evolved into a more advanced and efficient technique of coastal current measurement. ADCPs can measure water currents over a broad vertical range, offering detailed information on the velocity structure of the water column. ADCPs can be installed on various platforms, i.e., ships, buoys, and the seafloor. Ship-deck ADCPs can measure continuously as the ship travels, sampling a broad area in a brief period. Bottom-mounted ADCPs can be employed to take fixed-point, long-term measurements so that researchers can track long-term coastal current trends.

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

ADCPs operate based on the Doppler principle. They send sound waves into the water. Sound waves reflect off suspended particles in the water such as sediment, plankton, or bubbles. In a moving water condition, the frequency of the back-scattered signal is changed. The ADCP calculates the water velocity relative to the instrument based on the change in frequency.

The majority of ADCPs employ several transducer beams, typically four or more, and all pointing at right angles to each other. On the basis of the multiple-beam mode, three-dimensional water velocity is calculated. On combining the signal of the above beams, a complete profile picture of current velocity at different levels of the water column can be built by an ADCP. The data collected with the ADCP current profiler can be analyzed in real-time or recorded and analyzed afterwards, and it provides very valuable information on the dynamics of the coastal currents.

5. What does one need to measure Natal coastal currents of high quality?

For proper measurement of Natal coastal currents, the measuring device needs to possess certain characteristics. The material must be strong, the device must be compact, be light, require low power, and be inexpensive. These are the attributes which allow deployment of a large number of devices that offer complete spatial coverage.

Titanium alloy casing ADCPs are highly recommended. Titanium alloy offers better corrosion resistance, which is important for long-term use in the corrosive sea environment. It resists the corrosive action of saltwater and hence prevents damage to the internal ADCP parts. Titanium alloy is also lightweight but strong, which gives the instrument durability and portability. These features allow the instrument to make accurate and extended measurements of the coastal currents in Natal.

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

Usage-Based Choice

ADCPS are chosen based on usage. Ship-mounted ADCPs are the most appropriate for shipborne measurements. They can provide real-time data as the ship sails through the water, covering large distances in a relatively short span of time. For long-term monitoring at one location, bottom-mounted ADCPs are more desirable. They can collect data continuously over a prolonged period and allow scientists to track long-term coastal current trends. Floating ADCPs are best for tracking the advection of water masses over wide oceanic domains, providing valuable information regarding large-scale circulation features.

Depth-Based Selection

ADCP frequency must also be specified relative to the water depth. For waters less than 70m deep, 600kHz ADCPs can be used. They can provide high-resolution measurements in shallow waters. For waters with a depth of up to 110m, 300kHz ADCPs are ideal. For more depth, up to 1000m, 75kHz ADCPs are ideal.

There are various established ADCP brands available in the market, including Teledyne RDI, Nortek, and Sontek. But for individuals looking for budget - friendly solutions, the ADCP supplier China Sonar's PandaADCP is highly recommended. Constructed of pure titanium alloy, it performs well without the high price tag. It is a good fit for frugal users who need dependable ADCPs for coastal current measurement. Additional information can be found on their website: https://china-sonar.com/.

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