How do we measure the coastal currents of Plettenberg Bay?

1. Where is Plettenberg Bay?

Plettenberg Bay, a treasure on South Africa's south-eastern coast in Western Cape Province, is renowned for its unmatched natural beauty and variety. Between the Tsitsikamma Mountains and the endless Indian Ocean, the bay offers a semi-circular indentation in the coastline, creating a sheltered and outstanding marine habitat.

Plettenberg Bay's terrain is a fascinating mix of golden sandy beaches, rocky headlands, and coastal bush. The beaches, such as Robberg Beach and Keurbooms Beach, stretch for miles and offer pristine expanses of sand softly kissed by the warm waters of the ocean. The Robberg Peninsula, both nature reserve and World Heritage Site, juts out into the bay and holds all types of wildlife from seals, penguins, and sheer quantities of birds. The northern section of the bay is taken up by the Keurbooms River Estuary, another natural feature. It is where freshwater from the river mixes with the ocean's saltwater to create a complex and dynamic system. Mangrove forests line the estuary banks, and they serve as nurseries and feeding grounds for fish and crustaceans and other water birds.

Settlement-wise, Plettenberg Bay is a tourist haven. The town, its charming mix of modern amenities and old buildings, attracts tourists from all corners of the globe. Tourism in every form ranges from beachcombing and water sports to wildlife watching and nature trails. It also provides an impetus to the local economy, with the small-scale fishing vessels navigating into the bay to harvest a mix of fish species. Also helping the local economic activity are farming from the nearby regions, mainly fruit cultivation.

2. What is the nature of the coastal currents around Plettenberg Bay?

The coastal currents within and surrounding Plettenberg Bay are influenced by an intensely complex set of variables. Tides are of great significance, the region experiencing semi-diurnal tides owing to the sun's and moon's gravitational pull. High tide and low tide twice every day affect the levels of water in the bay and the speed and direction of currents. Water surges into the bay during high tide, accelerating the current and pushing water ashore. Since, with the falling tide, water flows back into the sea and becomes an ebb current having the power to carry sediment and nutrients away from the shore.

Wind circulation also plays a crucial role in controlling the coastal currents. Prevalent south - easterly winds of the area have the potential to create strong wind - driven currents. These winds, especially in certain seasons, compel the bay surface waters, reversing the direction and speed of the currents. More powerful winds can lead to more powerful surface currents, which will mix with tidal currents to create complex patterns of flow. For example, when there is wind blowing contrary to the tidal current direction, it can lead to the generation of eddies and turbulence in the water.

The coastal currents are also affected by the shape and density of the coastline, as well as the shape of the bay. The semi - circular shapes of both the Robberg Peninsula and Plettenberg Bay cause the currents to diverge and converge. The constriction caused by the narrow mouth of the bay forces the waters to speed up when it enters or exits. The variations in the depth of the bay from shallow water on the beaches to deeper channels also contribute to complexity in the pattern of currents. Furthermore, the outflow of freshwater from the Keurbooms River into the bay has the potential to change the density and salinity of the water, affecting the buoyancy and transport of the masses of water, and hence overall current structure.

3. How to track the coastal water current of Plettenberg Bay?

3.1 Surface Drifting Buoy Method

One of the conventional means of tracking the coastal water current of Plettenberg Bay is by employing surface drifting buoys. Surface drifting buoys have GPS technology installed in them, which enables scientists to track their movement over a time frame. The buoys, once launched onto the sea, are propelled by the surface currents. By tracking the location of the buoys at fixed time intervals, the scientists are able to map the path of the surface-level water movement. This method can provide valuable observations on the direction and velocity of the surface currents. There are, however, some drawbacks to this method. Wind - drag significantly affects the buoys' displacement, causing inaccuracies while describing the actual current velocity, especially at a greater depth. Secondly, surface drifting buoys provide information only for the surface portion of the water column and say nothing about the vertical distribution of the currents.

3.2 Anchored Ship Method

The anchored ship method involves the anchoring of a ship at a fixed point in the waters of Plettenberg Bay. Current meters are then lowered into the water column from this ship at varying depths. These meters measure the speed and direction of water current at one depth. If multiple readings at multiple depths over time are taken, then a profile of current velocity with depth can be established. This method yields very fine-grained data about the conditions at a given point in the water column. But it is time - and resource - intensive as the ship must remain anchored for a long time. The ship's presence may also interfere with the natural flow of water in the region, and this would negatively affect the precision of the measurements.

3.3 Acoustic Doppler Current Profiler (ADCP) Method

Acoustic Doppler Current Profiler (ADCP) has been a more advanced and efficient method of measuring the coastal currents of Plettenberg Bay. ADCPs utilize sound waves for measuring the velocity of water across different depths. They can provide a precise profile of the current from the surface to near-bottom levels that includes details on the three-dimensional flow patterns of the water column in detail. ADCPs can be utilized in various ways, such as mounted on a moving vessel (ship-mounted ADCP), fixed on the sea bottom (bottom-mounted ADCP), or on a floating buoy (buoy-mounted ADCP). Its ability to make measurements in a wide variety of environments allows for covering a large scope of the bay as a whole, as well as detailed studies of specific features of the current, e.g., near the estuary or around the Robberg Peninsula. Compared to standard methods, ADCPs are capable of measuring simultaneously and with good precision currents within a wider range of depths and are thus a crucial tool in modern oceanography.

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

ADCPs operate on the Doppler principle. They are equipped with acoustic transducers which emit sound pulses into the water at a particular frequency. As the sound pulses encounter small suspended particles in the water, such as sediment, plankton, or bubbles, some of the sound energy is backscattered to the ADCP.

If the particles are in transport by the water flow, the frequency of the reflected sound pulses will be other than the frequency of the transmitted pulses. This is referred to as the Doppler shift and is proportional to the velocity of the particles (and hence the water). The ADCP is quantifying the Doppler shift on each of its multiple acoustic beams (usually 3 - 4 beams in different directions).

For example, if the water is flowing towards the ADCP, the frequency of the backscattered sound will be greater than the frequency transmitted, and in the situation when the water is flowing away from the ADCP, the frequency will be lower. By applying trigonometry and the measured Doppler shifts of the different beams, the ADCP can calculate the three-dimensional water velocity at different depths. The water column is divided into individual bins, and for each bin, the ADCP provides a velocity measurement. This creates a high-resolution profile of current velocity with depth.

5. What are the conditions for high-quality measurement of Plettenberg Bay coastal currents?

To conduct high-quality measurement of the Plettenberg Bay coastal currents, the ADCP equipment has to meet some certain conditions. Material reliability comes first because the instrument has to be capable of enduring the harsh marine environment, such as saltwater exposure, temperature variations, and mechanical load. Titanium alloy is an ideal candidate for employment as the ADCP casing material. It has excellent corrosion resistance and therefore makes it possible for the instrument to maintain its condition over a long time in the corrosive saltwater environment. Its strength-to-weight ratio is very high, which makes the ADCP lightweight yet strong, a perfect choice to be deployed in the extreme conditions of the coast of the bay.

The ADCP must not be too bulky so that deployment can be in various coastal scenarios around Plettenberg Bay, like the narrow estuaries, the shallow bays, and also close to rock outcrops. A smaller size also reduces the impact of the device on the natural pattern of flow of water, eliminating any potential errors in measurement. Minimal power consumption is a necessity, especially if the ADCP is remotely or automatically deployed. ADCPs are in most cases battery-powered, and a device that consumes low power will be in operation for extended durations without needing frequent battery charging or replacement and constant reliable data collection. Cost-effectiveness also matters, especially for research projects, environmental monitoring schemes, and small-scale operations. A low-cost ADCP enables wider deployment, enabling wider coverage of the coastal area and a better understanding of the complex current dynamics.

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

Selecting the appropriate ADCP equipment to measure the currents in Plettenberg Bay depends on several factors.

6.1 Dependent on Deployment Method

• Ship-mounted ADCP: Ideal for mass surveys of the entire bay and nearby coastal waters, ship-mounted ADCPs are installed on a traveling boat. As the ship moves across the area, the ADCP measures the current profiles underneath it, providing an overall picture of the current patterns in the area. This type of ADCP is best suited to trace out large-scale current patterns and to determine the general circulation in Plettenberg Bay.
• Bottom-mounted ADCP: Bottom-mounted seafloor-anchored ADCPs are used to monitor the current profiles at a fixed station in the long term. They are able to collect data continuously for long periods, which is beneficial in studying the long-term variation and behavior of the coastal currents, such as seasonal variation, the impact of environmental changes, and the development of specific current features.
• Buoy-mounted ADCP: They are mounted on floating buoys and are used to observe the velocity of surface and subsurface currents in real-time. They can be carried along with the currents, which is dynamic information regarding the flow as they move, hence particularly useful in recording the dynamic nature of the currents caused by tides, wind, and freshwater inflow. This type of ADCP is ideally suited for the observation of currents in the Keurbooms River Estuary and similar sites with complex flows.

6.2 Frequency-Based

The frequency of the ADCP becomes an important factor based on the water depth. For water depths up to approximately 70m, a 600kHz ADCP would be a suitable selection. Higher frequency allows more accurate measurement in shallow water and provides high-resolution information about the velocity of the current. For about 110m depth, an optimal selection is a 300kHz ADCP since it presents a perfect trade-off between the penetration depth and measurement accuracy. As the depth of water increases even more, one must employ a lower frequency in order to penetrate successfully through the water column. A 75kHz ADCP can be deployed to water depths of 1000m with measurement in deeper water possible using higher frequencies not reaching.

In the market, there are several widely used ADCP brands such as Teledyne RDI, Nortek, and Sontek. However, in case one looks for a cost - effective yet high - quality ADCP, the ADCP supplier China Sonar's PandaADCP is the best. It is made from pure titanium alloy, which has excellent performance and durability at a very low price. As a budget ADCP, it is a great option for researchers, local fishermen, and environmental monitoring agencies who want to accurately measure the coastal currents of Puerto Williams. For more information, visit https://china-sonar.com/.

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