How do we measure the coastal currents of Saldanha?

1. Where is Saldanha?

Saldanha, a busy coastal town, lies on South Africa's west coast in the Western Cape province. It is renowned for its huge, natural, deep-water harbor, which is one of the largest in the nation. The strategic location of the town has placed it on the center stage of sea action, such as ship business, fishery, and industrial pursuits. The harbor boasts a picturesque terrain, with picturesque cliffs, white sandy beaches, and extensive stretches of coastal sand dunes.

The native people of Saldanha are a heterogenous group that boasts a strong connection with the sea. Traditional fishing has been a long-standing business for many among the local inhabitants, complementing the rich sea nature of the town. The fleet fishing yields a variety of catches, ranging from sardines to hake, which form not only the economic pillar of the local economy but also of the country's food basket. In addition to fishing, shipbuilding, storage of oil, and salt-producing industries have thrived in the town, relying on the unusual features of the Saldanha Bay.

Geographically, Saldanha Bay is a semi-enclosed basin, sheltered from the full force of the Atlantic Ocean by the Cape Columbine Nature Reserve to the north and Rooikrans Peninsula to the south. The bay's waters are comparatively smooth in comparison to the open ocean but still exhibit complex hydrodynamics. The seabed of the bay varies from nearshore shallowness to deep channels with the capacity to take large ships. The interplay between the open ocean and the bay and the influence of the cold Benguela Current produce a unique marine ecosystem at Saldanha, which supports an extremely rich marine ecosystem with numerous fish, seal, and seabird species.

2. What is the condition of the coastal currents off Saldanha?

The coastal currents off Saldanha are formed by a number of variables that interact to create a dynamic flow pattern. Tides are of first priority, with the region experiencing semi-diurnal tides. These tides cause the sea level to rise and fall twice a day, pumping water into and out of the bay. During high tide, water enters Saldanha Bay, creating strong inflow currents, while during low tide, the water exits, creating outflow currents. The strength and direction of these tidal currents may be affected by the bay's shape, moon and sun positions, and local weather conditions.

The Benguela Current, a cold, nutrient-rich ocean current that travels up the west coast of South Africa, significantly influences the local coastal currents. Saldanha Bay is fairly sheltered, but the Benguela Current influences it nonetheless. The Benguela Current brings in cold, nutrient-rich water, and this is channeled into a thriving marine ecosystem. Changes in the strength and position of the Benguela Current cause changes in water temperature, salinity, and density within the bay, and change the coastal water flow.

Wind also plays a significant role in affecting the coastal currents in the Saldanha Island bay. Dominant south - easterly winds control the region for most of the year, forcing surface water currents and creating wind - driven currents. Intensive winds can also increase the surface flow velocity and reverse the direction of currents, especially offshore. The local topography in Saldanha Bay with headlands and narrow inlets also sharpens these flows, leading to areas of irregular flow patterns such as upwellings and eddies.

3. Measurement of the Saldanha coastal water flow.

Surface Drift Buoy Method

One of the older methods of monitoring the coastal water current in Saldanha is the deployment of surface drift buoys. They have GPS tracking devices installed on them. When released in the water, the buoys are transported by the surface currents. Scientists can calculate the direction and velocity of the surface-level currents by observing the trajectories of the buoys over a period of time from the GPS data. The method is not without its limitations, though. It only provides information for the surface layer of the water column and can be affected by wind- induced motion, which may not reflect the true underlying current flow. In addition, surface drift buoys can be pushed away by strong currents or waves, rendering the data less reliable, especially in the more energetic waters near the bay entrance.

Moored Ship Method

The moored ship method is anchoring a ship in the same location close to or within Saldanha Bay. Gear such as current measuring tools, or current meters, are dropped down on the ship at various depths and then left deployed in the water to get the direction and velocity of the current. A profile of the vertical current can be constructed from these deployments. While such a method can give large data at a point of observation, its spatial coverage is limited to the area around the vessel docked. Also, the presence of the ship can even disrupt the normal flow of currents in the surrounding, and readings may be affected by ship movement and turbulence caused by its engines.

Acoustic Doppler Current Profiler (ADCP) Method

The Acoustic Doppler Current Profiler (ADCP) is now a more advanced and efficient tool for coastal current measurement in Saldanha. ADCPs are highly appreciated in oceanographic research because they can measure current velocities at multiple depths simultaneously over a large water body. It allows researchers to capture a complete picture of the three - dimensional arrangement of the existing current, as required for simulating accurately the complex coastal circulation in Saldanha Bay. Unlike traditional approaches, ADCPs can provide measurements of currents at multiple depths rapidly and accurately, providing significant insight into the manner in which the currents vary vertically and horizontally, and the way they engage with the bay's unique topography.

4. What is the mechanism of ADCPs based on the Doppler principle?

ADCPs work on the Doppler effect principle. The device emits acoustic signals, or sound waves, into the water column. When these sound waves encounter suspended small particles in the water, such as plankton, sediment, or bubbles, some of the sound energy is bounced back towards the ADCP flow meter. If the particles are moving with the current, then the frequency of the scattered sound waves will be different from that of the transmitted waves. The Doppler change in frequency or shift depends on the velocity of the particles and consequently on that of the current.

There are most often more than one transducer beams found in an ADCP, often three or four beams oriented at different angles. By calculating the Doppler shifts in each beam, the ADCP can calculate the components of the current velocity in three-dimensional space. The instrument divides the water column into discrete bins of depth and, for each bin, calculates the current velocity. This process allows ADCPs to provide a high-resolution profile of the current speed as a function of depth and provide an enormous amount of information about the water movement in Saldanha's coastal waters. For example, it can report how the current speed changes from the seabed to the surface and how different parts of the bay have distinct current patterns.

5. What is needed to obtain high-quality measurement of Saldanha coastal currents?

For performing high-quality coastal current measurements in Saldanha, measurement equipment must meet several crucial specifications. It is necessary that material reliability must be assured because equipment will endure a harsh marine environment. There must be concern over corrosion from sea water, mechanical loading from the action of waves, and biofouling. Material put into designing gear, especially in ADCPs, must be extremely resistant to all these.

The equipment must be minimal in size to reduce its impact on the natural current flow and facilitate simple deployment in several locations, e.g., shallow coastal waters and deeper bay channels. Lightweight equipment is also preferred since it simplifies handling during deployment and recovery, which is important in the crowded harbor environment of Saldanha. Low power consumption is essential, especially for long-term deployment, since it reduces the need for huge and heavy power supplies, enabling maximum operational independence of the measurement system. This is particularly relevant in areas where power is not available or long-term, unattended monitoring is needed.

Cost-effectiveness is also a significant factor to consider, especially for large-scale measurement. Costly gear can limit the number of instruments available for use, and thereby reduce the spatial coverage of the measurements. In the case of ADCPs, a titanium alloy housing is an excellent choice. Titanium alloy resists corrosion very well in saltwater, thereby guaranteeing long-term survival of the instrument even in the strongly corrosive Saldanha Bay environment. It is also lightweight and strong enough for both buoy-mounted as well as ship-mounted deployment. Despite being a high performance material, lowered manufacturing costs now make titanium alloy-cased ADCPs affordable, making them economically viable for economical deployment on a large scale by monitoring Saldanha coastal current.

6. How to Choose the Right Equipment for Current Measurement

The choice of the right equipment for current measurement in Saldanha depends on the application.

Application-Based ADCP Types

• Ship-mounted ADCP: Placed on a moving ship, the ship-mounted ADCP is well suited to large-scale surveys of the Saldanha Bay and the surrounding coastal waters. When the ship sails through the area, the ADCP can continuously measure the currents in its direction along the way and provide a broad-scale description of the currents in the area. This will be useful in operations such as sea navigation, determining the general circulation in the bay, and planning operations within ports.
• Bottom - mounted (Sit - on - the - bottom) ADCP: On the ocean floor, ADCPs are fixed - point, long - term observations. They can be left in one location for an extended period of time, observing the currents at a specific location. This ADCP is helpful in probing the fine-scale local current flow in the bay, such as how underwater obstacles affect the current and how patterns of the current evolve with time, and it is important to ecological researches and environmental surveys in the bay.
• Buoy-mounted ADCP: Installed on a floating buoy, these ADCPs may be permitted to drift with the current, sampling at different depths as they move along. They are best suited for observing large-scale, synoptic current patterns in the open ocean off Saldanha or following the motion of water masses over a wide area. This type of ADCP may be helpful in offering vital information for marine species movement forecasting, contaminations spreading, and the overall health of the marine ecosystem.

Frequency Selection

The frequency of the ADCP has to be selected depending on the water depth. 600kHz ADCP will be able to operate in up to about 70m water and will be making high-resolution measurements in reasonably shallow water, e.g., along Saldanha Bay coastlines. For depths of approximately 110m, a 300kHz ADCP is preferable because the lower frequency allows for greater penetration by the sound waves into the water column with sufficient measurement precision, and it is convenient to utilize it for measuring currents in the deeper bay channels. For very deep waters, even up to 1000m, a 75kHz ADCP is typically utilized. Although Saldanha Bay is not deep, in some locations near the entrance or in some of the underwater channels, a lower-frequency ADCP would have to be employed to effectively measure the deeper currents within the bay.

There are only a couple of popular ADCP brands that have presence in the market, viz. Teledyne RDI, Nortek, and Sontek. However, for those budget-conscious, there is the ADCP manufacturer China Sonar's PandaADCP. It is made wholly of titanium alloy and performs pretty well without much expense. For price-sensitive individuals who want reliable ADCPs still, it makes a fine alternative. You may know more about them on their website: https://china-sonar.com/.

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