How do we measure the coastal currents of Port Alfred?

1. Where is Port Alfred?

Port Alfred is a small coastal town on the Eastern Cape coast of South Africa. Being situated at the mouth of the Kowie River where cold water meets warm Indian Ocean water, it offers a combination of natural beauty and history. Its location makes it ideally suited for naval operations as well as ecological studies.

Port Alfred is encircled by a wide stretch of golden sandy beaches extending over a distance, fringed by the crystal waters of the sea. The Kowie River Estuary, a critical ecological component, flows through the town and constructs a rich biocapacity of flora and fauna. The mangrove forests along the estuary banks provide a crucial nursery for fish and crustaceans and a sanctuary for various bird species. The estuary also provides a natural harbor protected from the waves of the open sea.

Port Alfred boasts a long and historic record of human usage dating back to the colonial times. The town's historical sites, such as the Royal Alfred Marina, which is named after Prince Alfred, the second son of Queen Victoria, and the numerous old buildings, attract history enthusiasts to the region. Tourism is among the most powerful economic drivers, with individuals visiting the region to swim, surf, take boat cruises, and fish along the river and the beach. The local people also fish and practice small-scale farming, depending on coastal climatic conditions and fertile land of the region.

2. How is the condition of the coastal currents off Port Alfred?

The coastal currents off Port Alfred are influenced by a combination of factors. Tides are also significant as the region has semi-diurnal tides as a result of the effect of the moon and sun's gravity. Tides cause the sea water to rise and fall twice daily and play a significant role in the run-off of the water along the coast and Kowie River Estuary. On high tide, the estuary is filled up with sea water, increasing water level and altering the direction of the current. On the ebbing tide on low tide, water runs back to the sea with a vigorous ebbing current.

Wind patterns also exert significant effects on the coastal currents. The regional prevailing winds, which seasonally change, produce wind - driven currents. Powerful southeasterly winds, which prevail at certain periods of the year, can possess the ability to drive the surface waters along the coast, raising the velocity and reversing the directions of the currents. Wind - driven currents may be mixed with the tidal currents and produce complex flow patterns.

Freshwater inflow of the Kowie River is also an important parameter. Sediment, nutrients, and freshwater are introduced by the river into the ocean and influence the salinity and density of coastal water. Less dense freshwater forms a surface layer on top of the denser ocean water and influences the flow and buoyancy of the masses of water. This mixture of the saltwater and freshwater can lead to the development of estuarine circulation patterns, in which the freshwater exits on the surface but the saltwater is introduced at the bottom, making the coastal current dynamics in Port Alfred even more complex.

3. How to observe the coastal water flow of Port Alfred?

3.1 Surface Drifting Buoy Method

One of the traditional techniques utilized in order to analyze the movement of coastal waters surrounding Port Alfred is surface drifting buoys. The buoys have GPS equipment mounted on them, thus enabling researchers to track their trajectory over a specific duration. Once released into the ocean or estuary, the buoys drift along with the surface flows. Through periodic measurement of the positions of the buoys, scientists can build a chart of the trajectory of the surface-level water movement. It provides sound information on the direction and speed of movement of the surface currents. However, it is not absolute. Wind-drag could have an influence on the buoy's movement in quite significant amounts, inducing error in relation to presenting the current speed itself at deeper levels. Besides, surface drifting buoys provide data concerning only the surface layer of the water column and have nothing to do with the vertical current structure.

3.2 Anchored Ship Method

The anchored ship method involves anchoring a ship at a single point in the Port Alfred waters. Current meters are dropped from the anchored ship at various depths in the water column. These current meters measure the water flow velocity and direction at discrete depths. By taking a number of measurements at multiple depths and over a prolonged time, a profile of the current velocity with depth can be constructed. This method gives detailed information about the current conditions at discrete points in the water column. However, it is time-consuming and costly as the ship has to remain anchored for long durations. Additionally, the presence of the ship can disrupt the natural water current in the region, which can affect the accuracy of the measurements.

3.3 Acoustic Doppler Current Profiler (ADCP) Method

The Acoustic Doppler Current Profiler (ADCP) has been an advanced and useful method of ascertaining coastal currents near Port Alfred. ADCPs employ the use of sound waves to derive the velocity of water at depths. ADCPs can provide an extensive profile of the current, from the top to near-bottom levels, offering detailed information concerning the three - dimensional flow regimes in the column of water. ADCPs can be deployed in various forms, viz., on a traveling ship (ship-mounted ADCP), bottom-mounted close to the seafloor (bottom-mounted ADCP), or mounted on a buoy (buoy-mounted ADCP). The flexibility allows for the possibility of making measurements across a broad spectrum of applications from extensive coverage of the coastal area to focused investigation of targeted current phenomena of interest in the estuary. Compared to traditional methods, ADCPs can at the same time and with excellent precision measure currents over a greater depth range and are thus a vital tool in modern oceanography.

4. What is the functioning principle of Doppler-based ADCPs?

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

If particles are swept along with the flow of water, the sound pulses bounced off them will have a frequency different from that of the pulses sent. This difference, known as the Doppler shift, is a direct proportion of the particle velocity (and therefore of the water). The ADCP measures this Doppler shift for each of its several acoustic beams (usually 3 - 4 beams in different directions).

For example, if the water is coming towards the ADCP, the frequency of the reflected sound will be higher than the frequency of the emitted sound, and if it is going away, the frequency will be lower. Using mathematical algorithms and with the aid of the measured Doppler shifts of multiple beams, the ADCP can calculate the three-dimensional velocity of the water at different depths. The water column is divided into separate layers, or "bins," and the ADCP reports velocity measurements for each bin, constructing an accurate profile of current velocity with depth.

5. What does high - quality measurement of Port Alfred coastal currents require?

In order to measure the coastal currents off Port Alfred at high quality, the ADCP gear must meet a series of important requirements. Material reliability is of utmost importance, since the device needs to withstand the corrosive marine environment that is constituted by exposure to saltwater, temperature fluctuations, and mechanical loading. Titanium alloy is an excellent choice for the ADCP casing. It offers better corrosion resistance, so that the device will be functional in the corrosive saltwater environment for a very long time. It has a high strength - to - weight ratio, and this makes the ADCP light but not weak, and it is beneficial for easy deployment, especially in the challenging situations of the estuary and the open coast.

The ADCP needs to be compact enough to ensure deployment in various coastal and estuarine conditions that are around Port Alfred, including narrow channels and shallow areas. A smaller size also reduces the device's impact on the natural flow of water, limiting the scope for error in measurements. Power usage is also crucial, especially where the ADCP is used in remote or autonomous operation. ADCPs are frequently supplied from batteries, and a low - power - consuming unit can operate over extended periods without requiring repeated battery changes or recharging, ensuring unbroken and continuous data collection. Cost - effectiveness is also a factor, especially for research studies, environmental surveillance plans, and small - scale operations. An economic ADCP will allow greater usage, so wider coverage of the coastal area may be achieved as well as improved knowledge of the complex current movements.

6. What are the procedures for selecting suitable equipment for the measurement of currents?

The appropriate ADCP equipment to measure currents around Port Alfred is a choice based on many factors.

6.1 Dependent on Deployment Means

• Ship-mounted ADCP: Large-scale studies of the coast and estuary are suited best by ship-mounted ADCPs, which are placed on a mobile ship. While the ship moves across water bodies, the ADCP makes observations of currents beneath it with a broad overview of currents within the general location. This ADCP is well-suited for sounding large sections of coastline and studying large-scale currents and how these affect each other between the estuary and the ocean.
• Bottom-mounted ADCP: Hanging suspended over the ocean floor, bottom-mounted ADCPs are used in long-term observations of profile of current at a location. They can continuously collect data over extended periods of time, which is helpful in monitoring the long-term behavior and fluctuations of the coastal currents, such as seasonal trends and the impact of environmental change. These ADCPs are normally employed in areas of ecological importance or near large shipping lanes.
• Buoy-mounted ADCP: Suspended off floating buoys, ADCPs are useful for measuring the surface and subsurface flow of currents in real time. They can be transported by the currents, providing dynamic information on the flow en route, which is useful for mapping the dynamic evolution of the currents resulting from tides, wind, and freshwater inputs.

6.2 Based on Frequency

The sampling frequency of the ADCP is a matter of consideration based on water depth. For depths up to approximately 70m, a 600kHz ADCP is a suitable choice. High frequency is utilized for taking finer measurements in shallow water, producing high-resolution data for the current velocity. For depths of approximately 110m, it is advisable to use a 300kHz ADCP since it provides a sufficient balance between depth penetration and the accuracy of the measurements. Further increase in the water depth means that a reduced frequency is used to penetrate well through the column of water. A 75kHz ADCP can be applied for water depths up to 1000m, allowing one to measure further into deeper water where higher frequency may not travel.

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.