How do we measure the coastal currents of Salinópolis?

1. Where is Salinópolis?

Salinópolis is a coastal municipality of Pará state, northern Brazil. Located at the mouth of the immense estuarine system of the Amazon River into the Atlantic Ocean, Salinópolis occupies a unique geographic position where it lies at the confluence of the world's largest river system and the Atlantic Ocean, with a characteristic and dynamic coastal environment.

Geographically, the Salinópolis surroundings consist of low plains. The shore is a diversified series of habitats, with sandy shores, expansive mudflats, and the world's largest mangrove forest. Mangroves in these parts are of critical value to the environment locally. They provide nurseries to millions of aquatic animals, screen out pollutants, and protect the shore from erosion.

Salinópolis is highly historic. The indigenous inhabitants first settled in the area, and their heritage can be seen in contemporary local art, culture, and language. The area was invaded by Portuguese colonizers in the 16th century, and they introduced new farming and trading customs. Salinópolis grew to become a big production unit of salt over time, which is aptly named. Today, the town's economy consists of fishing, farming, and small tourism.

2. What are the coastal currents off Salinópolis?

The coastal currents off Salinópolis are characterized by a multifaceted interaction of numerous factors. Dominant among them is the Amazon River, which discharges a tremendous volume of freshwater into the Atlantic Ocean. The immense freshwater plume prevails over the coastal water density and salinity, creating the formation of an exceptional coastal current system. The plume extends hundreds of kilometers off the coast, and seasonally varies in shape and size as a function of river discharge rate, which is regulated by precipitation regimes over the Amazon basin.

Tides in the Atlantic Ocean also play a significant role. The semi-diurnal tidal regime produces periodic variations in water levels. These are the strengthened gravitational pull of the Earth's sun and moon that produce stronger tidal currents. The tidal currents exchange with the Amazon River plume and change the overall coastal water direction.

Regional dominant winds, the northeasterly trade winds mainly, push surface waters westward. The wind current is combined with the river-plume current and tidal currents. Mixing of the currents generates a sophisticated water pattern. Seabed bathymetry, e.g., its depth profile and bottom characteristics such as sandbars, channels, and reefs, contributes further to coastal currents' complexity. Oceanic features deflect or randomize the flow of current, generating eddies and other sophisticated flow patterns.

3. How to monitor Salinópolis coastal water movement?

Surface Drifting Buoy Technique

The surface drifting buoy technique is an ancient method of tracking coastal water movement. Scientists release buoys with tracking devices in them into the sea. When the buoys drift over a time frame, they are able to capture the direction and velocity of surface currents. There are some disadvantages of this method. It provides only the wind-surface layer data of the wind column, and the buoys are very sensitive to wind perturbation. The wind may drive the buoys in any direction which is not the actual movement of the bottom currents.

Moored Ship Method

The moored ship method utilizes a moored ship as a platform for measurement of current. Equipment is lowered from the ship down to the water to record the currents at different levels of depth. The method can produce good vertical profiles of the currents with some limitations. The geographic scale is limited to around the moored ship, and the ship may disrupt free flow in the water and impact the measurement. Furthermore, it is also logistically costly to keep a vessel in moored mode over a long duration.

Acoustic Doppler Current Profiler (ADCP) Method

ADCP profiler is becoming an even more advanced and cost-effective device for measuring coastal current. ADCPs profile currents within the water having a considerable vertical extent and collect detailed velocity data on the water column. They can be mounted on a variety of various platforms, like ships, buoys, and the sea bottom. Shipboard ADCPs are capable of sampling continuously because they travel down in a ship over a vast distance within a reasonably short period of time. Bottom- mounted ADCPs also possess the ability to provide fixed- location long- term observations and thus allow the scientists to consider the long- term coastal current patterns.

4. How does Doppler principle based ADCPs operate?

ADCPs operate on the Doppler principle. ADCPs send acoustic pulses into the water. The pulses reflect off suspended material such as sediment, plankton, or air bubbles in the water. When the water is in motion, the frequency of the reflected signal differs. Based on the frequency change, the ADCP computes the relative velocity of the water to the instrument.

All ADCPs possess multiple transducer beams, typically four or more, that are tilted at different angles. The multi - beam configuration permits the measurement of three - dimensional water velocity. Based on the signals from the beams, the ADCP can form a complete picture of the current velocity at different depths within the water column. ADCPR measured data may be analyzed on real-time or stored for future evaluation, providing pertinent information on coastal current dynamics.

5. What is high-quality measurement of Salinópolis coastal currents?

For high-quality coastal current measurements of Salinópolis, the instruments need to have very essential characteristics. It needs to be constructed with strong material and should have small size, light weight, low power requirement, and low cost. These enable the application of numerous instruments and complete spatial coverage.

Heavy-duty titanium alloy casings for ADCPs are highly advised. Titanium alloy is highly resistant to corrosion, which is required for long deployment in the tough marine environment. It withstands corrosive action by seawater and prevents corrosive action on the internal ADCP flow meter components. Titanium alloy is also light and has high hardness, with high instrument strength and ease of mobility. Such a property facilitates prolonged and precise measurement collection of coastal currents off Salinópolis.

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

Usage-Based Choice

The choice of ADCP depends upon its purpose. Ship-mounted ADCPs are ideal for ship-borne measurements. They can give data in real-time while the ship travels across the water, sweeping an extensive area in relatively less time. Bottom-mounted ADCPs are ideal for point monitoring over long durations. They can provide continuous data over time, allowing scientists to analyze long-term trends in the coastal currents. Floating ADCPs are easy to use when monitoring the movement of water masses over large areas, providing useful information on circulation patterns on a large scale.

Dependent on Depth Selection

The ADCP frequency must also be chosen based on the water depth. For depths below 70m, 600kHz ADCPs are appropriate. They can be utilized to take high-resolution measurements for shallow water. For depths up to 110m, 300kHz ADCPs are appropriate. For deeper water, to a depth of 1000m, 75kHz ADCPs are appropriate.

There are some popular ADCP brands available such as Teledyne RDI, Nortek, and Sontek. But if you are on a tight budget, then I would recommend ADCP manufacturer China Sonar’s PandaADCP to you. Built with pure titanium alloy, it works very efficiently at low prices. It is the best bet for budget-friendly users who do not want to compromise on the quality of ADCPs for measuring coastal currents. You can check more about them on their official website: https://china-sonar.com/.

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