How do we quantify the coastal currents of Bredasdorp?

1. Where is Bredasdorp?

Bredasdorp is a small town in the Western Cape province of South Africa, situated approximately 170 kilometers southeast of Cape Town. While not on the coast proper, it is near the tough southern coastline, where its shores are washed by the chilly waters of the Southern Ocean. The town itself is situated in a fertile farm land with rolling hills and great open fields with a wide variety of crops, ranging from wheat and canola to barley. There is a rich farming tradition in the area with established farming families passing down traditions and know-how from one generation to the next.

Despite its inland - type climate, Bredasdorp is an entrance to the coastal regions around it, which are characterized by steep cliffs, secluded beaches, and miles of sandy shores. Nearby Struisbaai, the largest natural harbor in the Southern Hemisphere, and Arniston with its strange limestone caves draw tourists from across the world. The seacoast waters near Bredasdorp are part of a large ocean ecosystem with many species such as seals, penguins, and fish that are born in nutrient-rich waters sustained by the impact of several oceanic currents.

2. What is the nature of the coastal currents near Bredasdorp?

The coastal currents near Bredasdorp are influenced by an array of factors. Tides play a huge role, with the region experiencing semi-diurnal tides. These tides cause repeated rising and falling of sea level, triggering high tidal streams running in and out of bays and onto the coast. Water comes pouring in along the inlets at high tide and out and back at low tide with sediment and nutrients in its wake. These tidal currents can be extremely powerful in their influence on the local marine ecosystem, altering the distribution of marine plants and animals and the pattern of coastline erosion.

The larger-scale circulation of the oceans also influences the local coastal currents. It is situated where the cold Benguela Current, which flows northwards along west South Africa, meets the warmer Agulhas Current, which is embracing eastern South Africa. Although Bredasdorp is not yet directly in the way of these huge currents, their influence still reaches there. Changes in strength and position of the above currents have the potential of introducing changes in temperature, salinity, and water density to the coastal water near Bredasdorp. Wind patterns also matter. The south - easterly dominating winds blow through the area most of the time, forcing near - surface water and creating wind - driven current. Strong winds may enhance the speed and change the direction of the surface - level flow, combining with the tidal and oceanic currents to produce complicated flow patterns.

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

Surface Drift Buoy Method

One of the traditional methods of observing the coastal water flow off Bredasdorp is through the use of surface drift buoys. Surface drift buoys are fitted with GPS tracking devices. Once deployed in the water, they are carried by the surface currents. By monitoring the movement of the buoys over time using the GPS data, scientists can determine the direction and speed of the surface - level currents. However, this method has limitations. It will only provide readings of the uppermost layer of the water column and can be subjected to wind-induced movement, which may not represent the actual underlying current. Surface drift buoys are also prone to being knocked away by strong currents or waves, thus the accuracy of the data may diminish over time.

Moored Ship Method

The moored ship method involves mooring a ship at a permanent location near the Bredasdorp coast. From this ship, current-measuring instruments such as current meters are lowered to various depths. These instruments record the velocity and direction of the current at each depth, allowing a vertical profile of the current to be built. While this method can give detailed information at a point, its spatial coverage is limited to the area around the moored vessel. Furthermore, the presence of the ship can disrupt normal direction of currents naturally flowing in the region, and the movement of the ship and whirlpool in the water can disrupt the readings.

Acoustic Doppler Current Profiler (ADCP) Method

The Acoustic Doppler Current Profiler (ADCP) is currently a more advanced and efficient method of coastal current measurement off Bredasdorp. ADCPs are particularly useful in oceanographic research as they can record current velocities at multiple depths simultaneously over a substantial body of water. This is sufficient to take a full record of the three-dimensional form of the current and to model appropriately the complex coastal circulation patterns around the area. Unlike other techniques, ADCPs can quickly and accurately survey currents at different depths, providing valuable information on how the currents vary vertically and horizontally, and hence a favorite in studying the coastal waters of Bredasdorp.

4. How do ADCPs based on the Doppler principle operate?

ADCPs operate under the Doppler effect. The device emits acoustic pulses, or sound waves, into the water column. When these sound waves hit small suspended particles in the water, such as plankton, sediment, or bubbles, part of the sound energy is sent back in the direction of the ADCP. If the particles are carried along by the current, the frequency of the sound waves reflected will be distinct from the frequency of the waves that were sent out. The Doppler shift, or frequency shift, depends directly on the particle velocity and therefore on the current velocity.

ADCPs usually have more than one transducer beam, usually three or four, but oriented in different directions. Measuring the Doppler shifts along each beam enables the ADCP to calculate the components of the current velocity in three-dimensional space. The instrument divides the water column into distinct bins of depth, and for each bin, it determines the current velocity. This effort allows ADCPs to create a true profile of the current velocity as a function of depth, providing much information about the water motion in the coastal waters off Bredasdorp.

5. What does it take for quality measurement of Bredasdorp coastal currents?

To perform quality measurement of Bredasdorp coastal currents, measurement equipment must satisfy a number of very crucial conditions. Integrity of materials is paramount since equipment will be subjected to severe marine conditions. Saltwater corrosion, mechanical stress induced by waves, and biofouling raise serious concerns. Materials used for equipment fabrication must be immune to all three to be able to endure operational conditions in the long run.

The equipment should be constructed as small as possible to minimize its impact on the free current flow of nature and ensure it is easy to deploy in various positions, including shallow water and complicating underwater geology. The equipment should also be light in weight since it will simplify handling during deployment or retrieval, especially in remote coastlines. Low power consumption is crucial, especially in the case of long - term deployments, since it reduces the need for heavy and voluminous power supplies, increasing the operational independence of the measurement system.

Cost - effectiveness is another extremely critical parameter, especially when measurements are done on a large scale. Pricy instrumentation can limit the number of instruments that can be used, reducing the spatial density of the measurements. For ADCPs, titanium alloy casing would be an excellent choice. Titanium alloy is resistant to corrosion in saltwater, and this makes the device last longer. It is hard and light, so it is durable and manageable. While a high-performance material, advances in manufacturing have decreased the cost of titanium alloy-cased ADCPs, and they are now an economical option for large-scale coastal current surveys off Bredasdorp.

6. How to Choose the Right Equipment for Current Measurement?

The choice of the right equipment for current measurement off Bredasdorp varies according to application.

Types of ADCPs According to Use

• Ship-mounted ADCP: Installed on a wandering ship, this type of ADCP is suitable for the large-scale surveying of the coastal waters off Bredasdorp. When the ship wanders over the area, the ADCP can sample the currents continuously along its track, providing an overall-scale description of the current flows in the area. This is convenient for monitoring large - scale current patterns and for understanding the overall circulation of the coastal water.
• Bottom - mounted (Sit - on - the - bottom) ADCP: These ADCPs are mounted on the ocean floor and are appropriate for long - term, fixed - point measurements. They can remain in the water for decades, observing the currents at a single point. This type of ADCP is useful for studying the local, fine-scale current dynamics, such as the effect of underwater topography on the current and the temporal evolution of current structures.
• Buoy - mounted ADCP: Installed on a floating buoy, these ADCPs are free to drift along with the current, measuring the velocity at different depths as they drift. They are particularly useful for monitoring large - scale, synoptic current patterns in the open sea off Bredasdorp or for monitoring the movement of water masses over a large area.

Frequency Selection

The frequency of the ADCP profiler should be selected according to the depth of water. Up to a depth of approximately 70m, a 600kHz ADCP would be sufficient, providing high-resolution data in quite shallow water. For a depth of approximately 110m, a 300kHz ADCP would be a more appropriate option, as the lower frequency allows the acoustics to penetrate deeper into the water column without significantly impacting the accuracy of measurements. In very deep waters, up to 1000m, the 75kHz ADCP is usually used. The lower frequency enables the ADCP to record currents at greater depths by virtue of allowing the acoustic waves to travel further within the water.

There are certain well-known well - established brands of ADCPs available in the market, e.g., Teledyne RDI, Nortek, and Sontek. But for those seeking cost - effective options, the ADCP supplier China Sonar's PandaADCP is the way to go. It's constructed from pure titanium alloy, and its performance is top - notch at an affordable price. It is the ideal choice for users seeking budget - friendly ADCPs without sacrificing coastal current measurement quality. For more details, visit their website: https://china-sonar.com/.

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