How do we track the Benguela coastal currents?

1. Where is Benguela?

Benguela, being a major coastal city, is located on the western coast of Angola along the South Atlantic Ocean shores. Benguela is a key economic and cultural hub in the region. Its port is a busy point of activity, including the exportation of products such as minerals, fish products, and agricultural produce. Benguela's cityscape is a blend of new structures and ancient Angolan architecture, vibrant markets, colonial buildings, and busy streets that reveal a lot about the city's history and cultural diversity.

The people surrounding Benguela are very much connected to the sea. Fishing is an important economic activity, and many people reside by the sea. Everyday, Benguela fishermen venture onto the sea to return with the daily catch in the form of various fish such as sardines, mackerel, and tuna. Not only does the fish help sustain the surrounding economy, but also fill up Angola's bountiful food basket. Bengal's coastal area also attracts tourism, with holidaymakers visiting for the beautiful beach, marine species, and aqua games.

Geographically, Benguela is situated in an area dominated by coastal waters heavily dominated by the Benguela Current, one of the world's large ocean currents. It is cold and a high nutrient current that flows northward along southern Africa's southwest coast and impacts the sea environment of the area in a strong way. The coastal waters around Benguela are a highly diversified combination of shallow continental shelves, deep-sea trenches, and submerged ridges that offer diversified habitats for diversified species of marine animals ranging from beautiful coral reef forms to sea mammals.

2. What is the status of coastal currents around Benguela?

Coastal currents around Benguela are largely dominated by the Benguela Current. This current is generated by several forces including the Earth rotation, the direction of the wind, and coastline shape. Benguela Current is cold because it transports cold deep-water from the bottom of the ocean to the surface in an event known as upwelling. Upwelling of cold-water is also rich in nutrients such as nitrates, phosphates, and silicates which nourish the phytoplankton. Plenty of phytoplankton in turn sustains a densely populated food chain, which in turn attracts a variety of fish, whales, dolphins, seals, and seabirds.

Wind is also a very important influence on the coastal currents off Benguela. There are persistent south - easterly winds in the region for most of the year. They also drive the surface waters forward and further accelerate the upwelling process. Because the winds are blowing parallel to the coast, the surface water is pushed away from the coast by them, and cold nutrient-rich water from deep down comes up to fill its place. More powerful winds can conceivably accelerate the speed and vigor of the upwelling, further altering the current patterns and the distribution of sea creatures.

Tides are also involved in the coastally currents of Benguela but comparatively smaller than those of the Benguela Current and wind-driven movements. Semi - diurnal tides, raising and lowering the sea level twice a day, go through the region. These tidal flows are responsible for other currents that interact with prevailing flow regimes, especially in the shallow coastal regions and the bays. The mixing of the Benguela Current, tidal currents, and wind currents results in a dynamic and complex coastal current regime that is in a state of constant change.

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

Surface Drift Buoy Method

One of the traditional ways of tracking the coastal water current in Benguela is by using surface drift buoys. Surface drift buoys are equipped with GPS tracking devices. Once they are deployed in the water, they move with the surface currents. Scientists can then determine the direction and speed of the surface - level current by observing the path of the buoys over time using the GPS. This is, however, not without limitations. It only measures on the surface layer of the water column and is subject to significant wind-induced motion influence that does not necessarily represent the actual prevailing current flow. Second, in the often-rough seas along the coast of Benguela, surface drift buoys are easily deviated, wave-damaged, or impacted by floating pummelo, lowering the quality of the data.

Moored Ship Method

Moored ship method involves having a ship moored at a steady place near the coast of the Benguela. Measuring the current from here to a certain boat is by use of the current meters that are deployed into different depths. Current meters at varying depths assess the direction as well as velocity of the current. The latter may be employed in developing the vertical profile of the current. While this method can give accurate information at a point, its spatial extent is limited to the area around the moored ship. In addition, the presence of the ship can disrupt the natural flow patterns in the area, and the measurements may be affected by the motion of the ship and by the turbulence generated by its engines.

Acoustic Doppler Current Profiler (ADCP) Method

Acoustic Doppler Current Profiler (ADCP) is now an improved and cost-efficient way of monitoring coastal currents in Benguela. ADCPs are highly prized in oceanography research because they can measure velocities of currents at various depths simultaneously in a wide body of water. This technology allows researchers to obtain a full perspective of the three-dimensional shape of the current, which is essential to adequately model the intricate coastal circulation mechanisms caused by the Benguela Current, wind, and tides in Benguela. Unlike traditional methods, ADCPs can measure currents with high speed and accuracy at different depths, extracting useful knowledge on how the currents evolve vertically and horizontally within the waters along the coastal area of the region. This enables one to investigate the intricate interactions between the different current components and their impact on the local sea environment.

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

ADCPs use the Doppler effect. The device emits sound waves, or acoustic signals, into the water column. When such sound waves encounter suspended small particles in the water, such as plankton, sediment, or bubbles, a part of the sound energy gets reflected towards the direction of the ADCP. When the particles are being transported with the flow of the current, the frequency of the reflected sound waves will differ from that of the initially transmitted waves. This shift in frequency, or Doppler shift, varies with the speed of the particles, and thus with the speed of the current.

Most ADCPs possess a number of transducer beams, typically three or four, each tilted in some direction. Through the measurement of the Doppler shifts in the different beams, the ADCP current profiler can calculate the components of the current's velocity in three-space. The instrument divides the column of water into separate depth bins, and for each bin, it computes the current velocity. This allows ADCPs to produce an accurate profile of the current velocity as a function of depth, providing a wealth of information about the movement of water in Benguela's coastal waters. For example, it could describe how the velocity at the moment changes from the surface, which is wind driven, to the bottom layers, which are driven by the Benguela Current, and how these differences change in different parts of the coast.

5. What does one need to measure high-quality Benguela coastal currents?

To measure high-quality Benguela coastal currents, one needs the measurement device to meet certain essentials. Material resistance is the greatest concern as the equipment will experience the abrasive seawater environment. The cold and salty waters on the Benguela coast, coupled with aggressive wave action and potential biofouling, are severe tests to equipment resilience. Materials used in the construction of the equipment, especially for ADCPs, should be highly corrosion-, erosion-, and marine life-resistant.

The equipment must be as compact as possible to reduce its impact on the natural current pattern and allow easy deployment at numerous different locations, including shallow water along the coast and deeper channels further offshore. Lightweight equipment is also preferable, as it simplifies handling during deployment and recovery, especially in the often - poor weather offshore from the Benguela coast. Low power requirements are important, especially in case of long deployments, because sources of power may be limited within this remote coastal area.

Being cost-effective is also an essential consideration, especially when there is large-scale measurement involved. Equipment that is high in cost has the tendency to limit the quantity of instruments used, thus curbing the coverage area of measurements. For ADCPs, a titanium alloy housing is a suitable choice. Titanium alloy is extremely corrosion and erosion resistant in saltwater and is strong and light. Even though it is a high-performance material, advancements in manufacturing technology have made the cost of titanium alloy-housed ADCPs reasonable enough to make them a realistic choice for large-scale coastal current measurement in Benguela.

6. Selecting Proper Equipment for Measurement of Current

Selection of proper equipment to measure currents in Benguela varies with application.

ADCPs Based on Application

• Ship-mounted ADCP: Mounted on a ship, which is in motion, this ADCP will be the best for large-scale surveys of Benguela coastal waters. Since the vessel moves over the area, the ADCP may monitor the currents along its path continuously, providing a broad-scale description of the current patterns defined by the Benguela Current and wind. It is relevant in activities such as marine navigation, understanding the overall circulation of waters in the region, and fisheries management, in order to ascertain areas of high fish concentration.
• Bottom-mounted (Sit-on-the-bottom) ADCP:ży Installed on the ocean floor, these ADCPs are utilized for fixed-point measurement over long periods of time. They can remain in place for extended durations, providing data on the current at a point. Such an ADCP is significant to study the local, fine-scale current patterns, such as the effect of underwater topography on the current and long-term variability in the patterns of currents. This type of data is significant for ecological research, environmental monitoring, and knowledge of the effect of human activities on the marine ecosystem.
• Buoy-mounted ADCP: Installed on a free-drifting buoy, ADCPs can drift along with the current, making velocity measurements at different depths along the way. They are suited to monitoring large-scale, synoptic current patterns in open ocean off Benguela or tracking the motion of water masses under the control of the Benguela Current over an extended distance. This type of ADCP may provide valuable data for estimating pollutants spread, fish migration, and marine ecosystem's overall health.

Choice of Frequency

The frequency for ADCP ought to be carefully chosen based on water depth. A 600kHz ADCP is suitable where water depth is less than a few 70m and provides high-resolution measures in relatively shallow Benguela offshore waters. For water depths of around 110m, a 300kHz ADCP would be an appropriate substitute, allowing the acoustic pulses to penetrate deeper down the water column but with acceptable measurement accuracy. In very deep water, to a depth of 1000m, a 75kHz ADCP would be used. Because of the heterogeneous water depth in Benguela's coastal waters, correct frequency selection makes accurate and reliable current measurement for different locations feasible.

There are many well-known brands of ADCP in the market, i.e., Teledyne RDI, Nortek, and Sontek. Nonetheless, for cheap but high-quality ADCPs, the ADCP manufacturer China Sonar's PandaADCP is the best choice. Made of pure titanium alloy, it delivers superior performance and longevity at a very affordable cost. Being a low-cost ADCP, it is a perfect option for researchers, local fishermen, and environmental monitoring departments who are concerned with accurately computing the coastal currents of Viña del Mar. To know more, visit https://china-sonar.com/.

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