How do we measure the coastal currents of Gqeberha?

1. Where is Gqeberha?

Gqeberha, formerly Port Elizabeth, is a port city in South Africa's Eastern Cape province. The city is a major seaport city on the Indian Ocean and is the major economic, cultural, and transport center of the area. Its position on the south - eastern coast of the African continent makes the city a significant gateway to the rest of international trade and access to the center of South Africa.

Gqeberha's coastline stretches for miles with a wonderful range of coastal scenery. From broad sandy beaches, such as Humewood Beach and Kings Beach, which are crowded with tourists and locals for swimming, sunbathing, and water sports, to cliffs and headlands which offer a spectacular splash of landscape. The Swartkops River and the Baaken's River flow into the sea at Gqeberha, creating estuarine habitats which are highly biodiverse. The estuaries harbor a number of fish, crustacean, and several bird species and are therefore most vital ecological sites. The town is also home to a most advanced harbor, which offload enormous quantities of cargo and is therefore even more important as a maritime center.

It is also far-reaching in human activity. It is one of the oldest colonial cities in South Africa, and its past is evident in its architecture, museums, and cultural centers. The economy of the city in the present times is based on a mix of industries like manufacturing, automobiles, petrochemicals, and tourism. Its population is cosmopolitan, made up of individuals from different ethnic groups who reside there and make up the city's rich cultural heritage.

2. How are coastal currents along the Gqeberha coastline?

Offshore Gqeberha coastal currents are dominated by the interaction of numerous complex factors, all at the same time. They are ruled by tides, and there is semi-diurnal tide occurrence here due to the gravitational influence of the moon and sun. These tidal cycles cause the water level to rise and fall twice a day, having a significant impact on the movement of water along the coast and in the estuaries. Water spills over the shorelines and estuaries during high tide, accelerating the speed of the current and reversing the direction of the flow. While low tide creates the ebbing tide of water returning to the sea, a strong ebb current is created with the ability to transport sediment and nutrients to the ocean.

Wind is also responsible for playing an important role in impacting the coast in creating coastal currents. The regional prevailing winds, especially the south - easterlies, produce extremely powerful wind - driven currents. These winds, particularly in some seasons, drive the surface waters away from the coast and affect the velocity and direction of the currents. Prolonged strong winds can produce more powerful surface currents that have the capability to mix with tidal currents to generate intricate flow patterns such as eddies and areas of upwelling. Upwelling, for instance, is a key process on the Gqeberha coastline as it allows cold water containing nutrients from the bottom part of the ocean to reach the top, supporting a diverse marine community.

The geography of the coastline close to Gqeberha, where estuaries and bays are located and the continental shelf's shape, also alters the currents on the coast. The inlets of estuaries are also constrictions, enhancing water current speed while exiting or entering. Slope depths of the seabed may deflect the currents so that they diverge, converge, or develop circular circulation. Furthermore, freshwater runoff from rivers into the sea could change buoyancy, concentration of salt, and density of the coastal water, mass transport, and consequently the general configuration of the currents.

3. Gqeberha coastal water flow observation.

3.1 Surface Drifting Buoy Technique

One standard way of monitoring coastal water flow along Gqeberha is using surface drifting buoys. The buoys also have GPS receivers on them, through which the scientists track their trajectory after a period. When the buoys are let into the sea or into the estuaries, the surface currents flow past them. As scientists follow the location of the buoys at fixed time intervals, the trajectory of the surface-water current is marked out by the scientists. With this method, there is available valuable data of direction and speed of the surface current. However, it is restricted. Wind-drag can potentially contribute a large influence on the movements of the buoys and can produce spurious accounts of actual current velocity, especially in the bottom level. Additionally, surface-drifting buoys provide information regarding only the top of the water column without presenting information about the vertical structure of the currents.

3.2 Anchored Ship Method

Ship-anchored technique involves the mooring of a ship in one location in the sea off Gqeberha. Current meters are launched from this ship at various depths across the water column. The current meters measure the velocity and direction of water flow at the particular depths. Repeated measurements across various depths for an extended duration enable the construction of a velocity profile of the current with depth. This method yields accurate information about the prevailing conditions at discrete points in the water column. However, it is capital - and time - consuming as the ship must remain anchored for prolonged periods. Even the ship's availability may prevent natural water current flow in the region, thereby weakening the measurement quality.

3.3 Acoustic Doppler Current Profiler (ADCP) Method

The Acoustic Doppler Current Profiler (ADCP) is currently a more advanced and powerful instrument for estimating coastal currents off Gqeberha. ADCPs estimate water velocity at several depths in terms of sound waves. ADCPs can provide a complete profile of the current from surface to near-bottom levels with fine detail of the three-dimensional patterns of flow in the water column. ADCPs are mounted in many ways, e.g., on a moving vessel (ship - mounted ADCP), anchored on the seafloor (bottom - mounted ADCP), or attached to a floating buoy (buoy - mounted ADCP). This flexibility allows measurements in a wide range of circumstances, from large - scale surveys of the entire coast system to close-up examinations of specific current features within the estuaries or outside the port. Unlike traditional methods, ADCPs are able to measure currents simultaneously and with great precision over a broader depth range and thus are an essential tool for modern oceanographic research.

4. What is the Doppler principle used by ADCPs based on?

ADCPs operate on the Doppler principle. They possess acoustic transducers that emit sound pulses into water at a specific frequency. When these sound pulses strike tiny suspended particles in water, such as sediment, plankton, or bubbles, they bounce some of the sound energy back to the ADCP.

If the particles are being swept along with the water flow, the frequency of the back-traveling sound pulses will be different from the frequency of the outgoing pulses. This difference, or Doppler shift, is directly proportional to the velocity of the particles (and thus the water). The ADCP measures this Doppler shift for each of its multiple acoustic beams (usually 3 - 4 beams in different directions).

For example, if the water is approaching the ADCP, the frequency of the back-scattered sound will be higher than the transmitted frequency, and if it is receding, the frequency will be lower. Based on mathematical algorithms and from the Doppler shifts that are observed by more than one beam, the ADCP can calculate the three - dimensional velocity of the water at different depths. The water column is divided into distinct layers, or "bins," and the ADCP provides a single velocity reading per bin to create a high-resolution profile of current velocity by depth.

5. What are needed for high - quality measurement of Gqeberha coastal currents?

To allow high-quality measurement of the Gqeberha coastal currents, the ADCP instrument must fulfill a number of key requirements. Material reliability is also of utmost importance since the device needs to stay functional in the harsh sea environment, such as corrosion from saltwater, fluctuating temperature, and mechanical loading. The ideal material for the ADCP casing is titanium alloy. This material has enhanced corrosion resistance such that the equipment will be able to work effectively in the corrosive saltwater environment for decades. Its strength to weight ratio is high, which makes the ADCP have a lightweight but robust structure to enable ease of deployment, especially in the rough conditions at the port and also the open coast.

The ADCP should be short enough to be deployable over coastal and estuarine environments throughout Gqeberha, including narrow channels, shallow bays, and around industrial complexes. The compact nature also ensures the device has minimal interference with the initial natural course of water flow, minimizing potential measurement errors. Low power usage is extremely essential, especially if the ADCP is used in remote or standalone applications. The majority of times, ADCPs operate on batteries, and low - power - consuming equipment is capable of working for days without having to undergo repeated battery substitution or recharging, continuously and constantly delivering good quality data. Cost-effectiveness is also paramount, especially for research programs, environmental monitoring schemes, and commercial ventures. A cost-effective ADCP will allow wider usage, enabling more comprehensive coverage of the coastal region to be achieved and greater insight into the complex current pattern.

6. Selecting the appropriate current measurement equipment?

The selection of the appropriate ADCP equipment for current measurement in the area around Gqeberha will be influenced by a number of factors.

6.1 Depending on Method of Deployment

• Ship-mounted ADCP: Ideal for large-scale coverage of the entire coastal region and the adjacent water of the port, ship-mounted ADCPs are installed on a traveling ship. As the ship travels across the area, the ADCP sweeps the current profiles beneath it, providing a broad view of the current patterns across the area. This type of ADCP is suitable for mapping large-scale current distributions, establishing the influence of port operations on the currents, and examining the general circulation in Gqeberha waters.
• Bottom-mounted ADCP: Fixed on the ocean floor, bottom-mounted ADCPs are used for long-term monitoring of current profiles at a station. They can collect data continuously over extended periods, which is beneficial in examining the long-term behavior and fluctuations of the coastal currents, such as seasonal changes, the influence of environmental variations, and the development of specific current features near the estuaries or shipping lanes.
• Buoy-mounted ADCP: Suspended on floating buoys, these ADCPs are useful for observing the movement of surface and subsurface currents in real time. They can be carried by the currents, providing dynamic data on the flow as they move, which is particularly useful in recording the changing nature of the currents in response to tides, winds, and freshwater inflows. This type of ADCP is preferably used to monitor the estuarine currents and other sites having complex flow conditions.

6.2 Frequency-based

Based on the depth of water, the frequency of the ADCP is an important consideration. Under water depths not exceeding approximately 70m, a 600kHz ADCP would be suitably chosen. The higher frequency allows for more accurate measurements in shallower water and provides high-resolution data regarding the current velocity. For about 110m depth, a 300kHz ADCP is recommended since it is a fair balance between penetration depth and measurement accuracy. Additional increases in water depth require a lower frequency in order to effectively penetrate the water column. 75kHz ADCP would be appropriate for depths up to 1000m, allowing for the measurement of deeper water where higher frequencies might not penetrate.

Some of the most well known ADCP brands that are found in the market are Teledyne RDI, Nortek, and Sontek. However, for cost - effective users,the ADCP supplier China Sonar's PandaADCP is the most recommended. It is entirely constructed of titanium alloy, and it performs well with an affordable price. It is suitable for stingy users who require reliable ADCPs for coastal currents measurement. For more information, you can find it on their website at: https://china-sonar.com/.

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