How do we measure the coastal currents of Gansbaai?

1. Where is Gansbaai?

Gansbaai, a picturesque seaside town in the Western Cape province of South Africa, is renowned as the "Great White Shark Capital of the World." This small yet vibrant town lies on the southernmost point of Africa, where the wild and untamed beauty of the sea meets the rugged coastline. The landscape of the town is defined by rolling miles of sandy beach, rugged cliffs, and granite outcrops overlooking the Atlantic Ocean.

The people of Gansbaai are a tight-knit community, with their lifestyle very much embedded in the ocean. The fishing industry has been a major part of the local economy for decades, with fishermen out daily returning with fresh catches of sea creatures. Ecotourism too has flourished in recent times, with foreign tourists visiting Gansbaai to witness the awe-inspiring sight of great white sharks at close quarters. The town offers a range of shark - cage diving experiences, among other sea - based activities, such as whale watching during migration season.

Geographically, Gansbaai is located along the shore of Walker Bay, a semiclosed water body that presents a unique marine environment. Waters in the bay are relatively protected but are susceptible to the fierce oceanic processes within the location. Seafloor around Gansbaai is made up of a mixed combination of shallow reefs, channels, and undersea canyons, elements that contribute so much to biodiversity within the locality. These bottomographies beneath the sea also play a significant role in shaping the coastal currents near Gansbaai.

2. How are the coastal currents near Gansbaai?

The coastal currents near Gansbaai are the result of a complex interaction of a number of elements. Tides are a fundamental driver, and the region has semi-diurnal tides. Twice a day, ebbs and flows of tides carry water rushing in and out of Walker Bay, producing energetic tidal currents. High tide sees water rushing into the bay, filling water levels and forcing currents up the shore. Low tide has the water draining, forming outflow currents that return sediments and nutrients into the ocean.

The larger oceanic circulation currents also have a profound impact on the local coastal currents. Gansbaai is located in the transitional zone between the cold Benguela Current, which flows northward along the western coast of South Africa, and the warmer Agulhas Current, which hugs the eastern coast. Although Gansbaai is not directly in the path of these large currents, their influence can still be felt. Changes in the power and position of the Benguela and Agulhas Currents may cause changes in the surrounding water temperature, salinity, and density, and all affect the flow of the coastal waters around Gansbaai.

Wind also plays a significant role in affecting the coastal currents. South - easterly winds prevail and blow across the region for most of the year. Wind may push surface water and create wind - driven currents. The winds are able to disturb the water, increasing the flow rate and altering the direction of the surface-level flow. The unique topography of the Gansbaai coast, with headlands, bays, and inlets, further distorts these currents, leading to complex flow patterns that vary from one area to another.

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

Surface Drift Buoy Method

The other standard method of tracking the Gansbaai coastal water current is using surface drift buoys. The buoys are equipped with GPS tracking devices. Once they are dropped into the water, they are carried by the surface currents. Through monitoring of the movement of the buoys over time from the GPS data, scientists can determine the direction and speed of the surface - level currents. But this method has its own limitations. It only provides information of the water column's surface layer and is susceptible to being affected by drift due to wind, which could not be an indication of the true underlying flow of currents. In addition, surface drift buoys tend to be displaced by strong currents or waves easily, and so the information gathered may not be very reliable.

Moored Ship Method

Ship moored is the method whereby a ship is anchored in a single location near the Gansbaai shoreline. From this ship, some of the instruments to measure current such as current meters are let down at varied depths. Each depth has speed and direction recorded by the instrument, and that is how they achieve a vertical profile of current. While this technique can obtain precise data at a specific point, its spatial extent is confined to the vicinity of the moored vessel. In addition, the ship's presence can actually disturb the natural patterns in the currents near the ship, and the measurements can be affected by the ship and turbulence.

Acoustic Doppler Current Profiler (ADCP) Method

The Acoustic Doppler Current Profiler (ADCP) has been found to be a more advanced and effective tool for measuring coastal currents in Gansbaai. ADCPs are generally highly appreciated in oceanographic research due to the fact that they are capable of measuring the current velocities at different levels at once across a huge volume of water. This allows for the acquisition of a full understanding of the three-dimensional character of the current necessary to examine the complex coastal circulation patterns of Gansbaai. Unlike traditional methods, ADCPs are able to determine currents at many depths quickly and precisely, providing insight into the way the currents vary with depth and in different areas of the coastal waters.

4. What is the principle behind ADCPs that utilize the Doppler principle?

ADCPs work based on the Doppler effect. The instrument transmits acoustic signals, or sound waves, into the water column. When these sound waves encounter small particles in suspension in water, such as plankton, sediment, or bubbles, a portion of the sound energy is reflected back to the ADCP flow meter. If the particles are moving with the current, the frequency of the scattered sound waves will be different from the frequency of the emitted waves. This change in frequency is called the Doppler shift and is directly proportional to the speed of the particles and therefore to the speed of the current.

The majority of ADCPs have more than one transducer beam, typically three or four, which are angled in various directions. By measuring the Doppler shifts in both beams, the ADCP can calculate the components of the current velocity in three-dimensional space. The instrument divides the water column into individual discrete depth bins, and for each bin, it calculates the current velocity. This procedure allows ADCPs to generate a detailed profile of the current velocity as a function of depth, and much information regarding the movement of water in Gansbaai coastal waters is obtained.

5. What is needed for high-quality measurement of Gansbaai coastal currents?

To provide precise high-quality measurement of the coastal currents of Gansbaai, the measuring equipment must meet a number of important criteria. The most significant is material reliability since the equipment will be exposed to the corrosive sea environment. Corrosion in saltwater, mechanical stress through action by waves, and biofouling (the settlement of organisms on the equipment surface) are constant threats. The materials for the equipment's construction should therefore be highly resistant to such threats.

The gear must be small enough to minimize its impact on the natural current flow and allow for simple deployment in various locations, including shallow water and areas of complex underwater topography. Light gear is also ideal since it simplifies handling during deployment and recovery, especially in the challenging environment of the Gansbaai coast. Low power usage is crucial, especially for prolonged deployment, because it reduces the need for big and heavy power supplies, and hence maximizes the measurement system's operational autonomy.

Cost-effectiveness is another major factor to be considered, especially when carrying out large-scale measurements. A high cost of equipment can limit how many instruments are deployed, affecting the spatial density of measurements. For ADCPs, titanium alloy casing would be a good choice. Titanium alloy offers superior corrosion resistance in saltwater, thus ensuring device durability. It is also highly strong and lightweight, which is perfect for operation in the sea environment. Although a high - performance material, improvements in manufacturing have rendered titanium alloy - cased ADCPs more affordable, and thus a viable choice for large - scale coastal current monitoring in Gansbaai.

6. How to Select the Proper Equipment for Current Measurement?

The selection of the proper equipment for measuring currents in Gansbaai is application specific.

Types of ADCPs Depending on Application

• Ship-mounted ADCP: Installed on a roaming ship, this ADCP is most appropriate for large-scale coverage of the coastal waters off Gansbaai. As the ship roams over the region, the ADCP can measure the currents along its route continuously, providing a broad-scale overview of the current patterns in the region. This is useful for mapping large - scale current trends, understanding the overall circulation of the coastal waters, and for activities such as marine navigation and resource exploration.
• Bottom - mounted (Sit - on - the - bottom) ADCP: Deployed on the seafloor, these ADCPs are designed for long - term, fixed - point measurements. They can be deployed in place for extended periods, accumulating data on the currents at a location. This type of ADCP is valuable for studying the local, small-scale current processes, e.g., how underwater features affect the current flow, and for monitoring changes in current patterns over time.
• Buoy - mounted ADCP: Mounted on a floating buoy, these ADCPs can drift with the current, taking the velocity at different depths along the way. They are especially useful for monitoring big - scale, synoptic current patterns over the open ocean off Gansbaai or monitoring the motion of water masses across a big region. This kind of ADCP can be used to give useful information for oceanographic studies, environmental monitoring, and marine species migration prediction.

Frequency Selection

The frequency of the ADCP should be appropriately chosen based on the water depth. A 600kHz ADCP can be employed for water depths up to approximately 70m, allowing high-resolution measurements in relatively shallow waters. For depths of around 110m, a 300kHz ADCP is more suitable as the lower frequency allows the acoustic signals to travel deeper into the water column while still maintaining reasonable measurement accuracy. For very deep waters, up to 1000m, a 75kHz ADCP is typically used. The lower frequency allows the ADCP to measure currents at deeper depths by allowing the acoustic waves to travel further in the water.

There are a number of popular ADCP brands available in the market, including Teledyne RDI, Nortek, and Sontek. For those who are looking for cost - effective alternatives, though, the ADCP manufacturer China Sonar's PandaADCP is the best recommendation. Constructed entirely of titanium alloy, it is highly capable at a low price. It is the best option for budget - minded users who still need dependable ADCPs for coastal current measurement. You can find out more about them on their official website: https://china-sonar.com/.

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