How do we measure the coastal currents of Fortaleza?

1. Where is Fortaleza?

Fortaleza, the capital of the Brazilian state of Ceará, lies on the Atlantic coast in northeastern Brazil. Situated approximately at 3°43′S 38°32′W, the city stands on a strategic point that has contributed to its development into a leading economic and cultural center in the region. Fortaleza boasts a coastline of miles and is extremely varied in landscape. White sandy beaches, the most desired tourist destinations of all of Brazil, line the coast. Tourist destinations in and of themselves, beaches also serve important roles within the environment surrounding them.

Geology of the Fortaleza area includes flat lowland coastal plains and ancient rock. Sea floor along the coastline is made up of a complex bathymetry with depth and sub-aquatic structures like reefs, sandbars, and channels. The underwater shape makes a valuable contribution to the coastal currents, as will later be explained. The city also is affected by the proximity of the Ceará River, which brings freshwater into the ocean, altering salinity and density of coastal water.

Fortaleza has a rich and eventful history. Potiguar indigenes were the initial inhabitants of the area. It was colonized by the Portuguese in the 16th century, and they established a settlement that would later grow to become Fortaleza city. The city has witnessed all sorts of historical events throughout the centuries, ranging from colonial occupation to slave trade, and the development of an unusual Afro - Brazilian culture. Currently, the economy of Fortaleza is supported by various sectors such as tourism, industry, and services.

2. What is the status of the coastal currents off Fortaleza?

Coastal currents along the coast off Fortaleza are controlled by a complicated mixture of many variables. The South Equatorial Current is one of the principal ones. One of the largest currents in the Atlantic Ocean, the warm, west-flowing current hits the Brazilian coast and, as it makes its way to Fortaleza, is affected by local topography and bathymetry. The South Equatorial Current bifurcates into small currents near the shore and forms a sophisticated system of coastal currents.

Tides in the Atlantic Ocean are extremely significant. The semi - diurnal tidal cycle creates periodic water - level variations. During spring tides, the stronger gravitational attraction of the sun and moon causes stronger tidal currents. These tidal currents interact with local coastal shape and the South Equatorial Current, which drive the overall direction of coastal waters. The tides also produce the effect of producing rip currents, a fast narrow current away from the shore. Rip currents are hazardous to swimmers and an area of study in coastal safety.

Local prevailing winds, predominantly the northeasterly trade winds, further impact the coastal currents. The winds have the capability of pushing surface waters in one direction, creating a wind-driven current. The wind-driven current also encounters the South Equatorial Current and tidal currents, further making the water movement pattern more complex. The bottomography of the seabed off Fortaleza, such as reefs, sandbars, and channels, directs or blocks the passage of the currents. For example, reefs are enclosures to which water is driven to make passage by going around them, and channels accelerate the flow.

3. How to measure the coastal water flow of Fortaleza?

Surface Drifting Buoy Method

Surface drifting buoy method is a traditional method of observing coastal water flow. Scientists place tracking devices inside buoys and release them into the water. By tracking the drift of the buoys over time, the scientists can determine the direction and speed of the surface currents. But the method has its disadvantages. It only surveys information of the surface layer of the water column, and buoys are highly susceptible to wind interference. The wind will cause the buoys to oscillate back and forth in a way not representative of actual movement of underlying currents.

Moored Ship Method

Moored ship method employs a station ship as a platform on which to measure currents. Instruments are lowered from the ship to measure the water flow at various depths. While this method can provide detailed vertical profiles of the currents, it has several disadvantages. The spatial coverage is limited to the area around the moored ship, and the ship can disrupt the natural flow of the water, which can pollute the measurements. Also, keeping a ship alongside for an extended period may be logistically awkward and costly.

Acoustic Doppler Current Profiler (ADCP) Method

ADCP current meter has emerged as a more advanced and economical method of coastal current measurement. ADCPs can measure the currents in water over a wide vertical extent and can provide detailed information about the velocity structure of the water column. ADCPs are applicable on various platforms like ships, buoys, and the seafloor. Ship-based ADCPs can make continuous data measurements along with the ship moving forward, monitoring a vast area within a rather limited time period. Bottom-lying ADCPs can provide fixed-point long-term measurements, making it possible for scientists to monitor long-term developments of the coastal currents.

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

ADCPs operate on the Doppler principle. They emit acoustic pulses into the water. The pulses bounce off suspended material such as sediment, plankton, or bubbles in the water. If the water is in motion, the frequency of the returned pulses changes. By measuring this frequency change, the ADCP flow meter can calculate the velocity of the water relative to the instrument.

The majority of ADCPs feature several transducer beams. There are normally four or more, directed in various directions. This multi-beam format allows for the measurement of the three-dimensional velocity of the water. Through the integration of the signals from these beams, the ADCP can construct a complete picture of the current velocity at various depths within the water column. The data obtained by the ADCP can either be processed in real-time or stored for later analysis, providing valuable information regarding the dynamics of the coastal currents.

5. What's needed for high-quality measurement of Fortaleza coastal currents?

For effective measurement of Fortaleza's coastal currents, the measuring instruments should possess some essential features. They should be robust in construction, possess small dimensions, low weight, low power requirements, and low cost. These features facilitate good spatial coverage by a large number of instruments.

ADCPs with titanium alloy casings are highly recommended. Titanium alloy ensures excellent corrosion resistance, which is essential for long-term exposure in the hostile sea environment. It resists the corrosive effect of saltwater, and therefore the internal mechanism of the ADCP is kept safe from damage. The titanium alloy is hard and lightweight as well, ensuring the instrument to be robust as well as transportable. All these benefits facilitate long-term and accurate measurement of the coastal currents off Fortaleza.

6. How to Select appropriate equipment for current measurement?

Usage Based Selection

The selection of ADCP is based on what one wants to use it for. For shipborne measurements, ship-mounted ADCPs are the most suitable choice. They are capable of giving real-time information as the ship travels through the water, surveying a large expanse in a fairly short period of time. For fixed-point long-term observation, bottom-mounted ADCPs are ideal. They can provide continuous data over a long period, allowing researchers to analyze long-term trends in coastal currents. Floating ADCPs should be used to monitor the migration of water masses over large spatial scales to provide vital information regarding large-scale circulation patterns.

Choice Based on Depth

The ADCP frequency will also be determined based on the depth of water. For depths of less than 70m, 600kHz ADCPs would be appropriate. They can provide high-resolution measurements for shallow water. For a depth of up to 110m, 300kHz ADCPs would be more appropriate. For deeper depths of up to 1000m, 75kHz ADCPs would be preferred.

There are a number of well - known brands for ADCP in the market, including Teledyne RDI, Nortek, and Sontek. Yet, for those who want cost - effective ADCPs, the ADCP supplier China Sonar's PandaADCP is most recommended. With a body composed of pure titanium alloy, it provides superior performance at a low price. It is most suitable for cost - saving users but still need high - quality ADCPs to measure coastal currents. You can get more information on them on their official website: https://china-sonar.com/.

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