How do we measure the coastal currents of Swakopmund?

1. Where is Swakopmund?

Swakopmund, a busy coastal town, lies on the central west coast of Namibia, between the infinite horizon of the Namib Desert and the cold Atlantic Ocean waters. This unorthodox geographical position has bred a stunning landscape where the yellow sand dunes kiss the crashing waves, creating a picture. The town itself is a blend of German colonial architecture and Namibian culture, one would notice as evident in its crowded streets, old structures, and packed markets.

The local inhabitants of Swakopmund are vibrant and multicultural. One of the local economy's strongest pillars is tourism, and it attracts tourists from across the world with its adventure sport activities such as sandboarding the desert dunes and skydiving over the sea, complemented by its marine life and unique cultural interactions. Fishing is also important, and the local fishermen venture into the waters off the coast to capture some of the region's fish stocks, together with hake, horse mackerel, and octopus.

Geographically, Swakopmund is dominated by a strong cold Benguela Current as it flows up the western edge of southern Africa. This strong current forces up cold, nutrient-rich water, and this gives a productive sea environment. Seabed off Swakopmund has a mix of shallow sandy flats nearshore with small invertebrates and fish, and deeper channels farther offshore with big marine animals like seals and dolphins. Dynamic interaction of seabed topography, local climate, and the current generates a dynamic and variable coastal system.

2. How are the coastal currents off Swakopmund?

The off-shore currents of Swakopmund are the result of interaction of various factors. The Benguela Current is the dominating factor that governs these currents. Its upwelling of cold deep-water enriches the surface waters with nutrients, leading to phytoplankton growth and nourishing a food web to sustain rich marine life diversity. The intensity and direction of the Benguela Current can seasonally vary as a function of large - scale atmospheric and oceanic characteristics. For example, the wind regime variations associated with the Southern Hemisphere weather regimes can affect the current speed and direction, and consequently the coastal waters encircling it.

Wind is also another significant factor affecting the coastal currents. There are longstanding south-easterly winds blowing over the region for the majority of the year. The winds possess the capability to push surface waters, creating wind-swept currents that encounter the Benguela Current. The high winds can cause the velocity of the surface currents to be significantly increased and turned around, leading to complex flow patterns. The topography of the Swakopmund coastline, composed of headlands, bays, and inlets, also modifies these currents. Headlands are likely to speed currents by water being guided around them, while bays can create areas of eddies and comparatively calm water where the current becomes more intricate.

The tides also generate the coastal currents off Swakopmund. The region has semi - diurnal tides which lead to a rise and fall of sea level twice daily. The tidal currents create secondary currents superimposed on the existing flows, especially in the shallow areas near the coast and in the bays. A combined effect of the Benguela Current, wind currents, and tidal currents produces a dynamic and changing system of coastal currents.

3. How to monitor the coastal water flow of Swakopmund?

Surface Drift Buoy Method

Among the conventional means of tracking the coastal water current of Swakopmund is through the use of surface drift buoys. The buoys are mounted with GPS tracking devices. When put in water, the buoys get drifted by surface currents. Scientists can determine the direction and speed of surface-level currents by tracing the drift of the buoys using GPS coordinates over a duration of time. This practice also has disadvantages. It only measures the water column surface layer and can be heavily subject to wind- driven transport, which will not always accurately represent the true underlying flow of currents. Also, in the often-turbulent Swakopmund offshore waters, it is easy to displace drift buoys at the surface, have them broken by waves, or be impacted by floating trash, reducing the reliability of the data.

Moored Ship Method

Ship moored method involves the method of anchoring a ship at a fixed location near the Swakopmund coast. Current measuring instruments such as current meters are dropped from the ship to varying depths. Speed and direction of the current at each depth are recorded, which allows for plotting a vertical current profile. While this method is able to offer accurate information at a point in time, its extension is limited to the area around the anchored vessel. In addition, the presence of the vessel can have the potential to disrupt the native current flows in the area. The ship's motion, in addition to the ship's engine-created turbulence, can have the effect of biasing the validity of measurements, calling for strict calibration and adjustment of data.

Acoustic Doppler Current Profiler (ADCP) Method

The Acoustic Doppler Current Profiler (ADCP) is presently a more advanced and affordable technique for coastal current measurement in Swakopmund. ADCPs are highly appreciated by oceanographers because they are capable of measuring several depths simultaneously over a wide volume of water of current velocity. This technology allows scientists to develop a full description of the three-dimensional structure of the present, which is important to model the complex coastal circulation patterns induced by the Benguela Current, wind, and tides in Swakopmund. Unlike traditional methods, ADCPs can measure currents quickly and accurately at many depths, providing valuable information about how the currents vary vertically and horizontally in the coastal waters in the region. This allows one to examine the complex interaction among various current components and how they affect the local marine ecosystem.

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

ADCPs function on the basis of the Doppler effect. The instrument sends out acoustic signals, or sound waves, into the water column. When these sound waves encounter small particles suspended in the water, such as plankton, sediment, or bubbles, some of the sound energy scatters back towards the ADCP. If the particles are moving with the current, the frequency of the sound waves that are scattered will be different from the original frequency of the emitted waves. This change of frequency, the Doppler shift, is a function of the particles' velocity, and therefore the current velocity.

Most ADCPs are equipped with two or more transducer beams, often three or four, pointing at different angles. By measuring the Doppler shifts in all of the beams, the ADCP can calculate components of current velocity in three-space. The instrument divides the water column into separate depth bins, and for each bin, it computes the current velocity. This allows ADCPs to produce an accurate profile of the current velocity as a function of depth. For example, it can describe how the velocity of the current varies from the surface where wind plays a significant role to the bottom layers dominated by the Benguela Current, and it provides a general sense of the dynamics of the current in the coastal waters off Swakopmund.

5. What is necessary for good-quality measurement of Swakopmund coastal currents?

In order to make precise measurements of Swakopmund's coastal currents, the measuring equipment must meet some essential requirements. Most importantly, the equipment must meet material durability requirements because it will be exposed to the corrosive marine environment. The cold salty ocean along the coast of Swakopmund, as well as the erosive effects of wind-transported sand from the deserts surrounding it, pose severe challenges to equipment durability. The equipment should be constructed using extremely corrosion-resistant, erosive-resistant, and biofouling-resistant materials, especially for ADCPs.

Equipment should be compact enough so as not to hinder natural current movement and should easily be portable from one place to another, i.e., in shallow water near the shore as well as in deeper channels farther out. Lightweight equipment is also highly desirable, as handling is easier in deployment and recovery, especially under the often - adverse weather conditions off the coast of Swakopmund. Minimal power consumption is also essential, especially for long - term deployments, as the power sources could be limited in this remote coastal area.

Cost - effectiveness is also a key consideration, especially for large - scale measurements. High-cost equipment can limit the number of equipment that can be deployed, and therefore reduce the spatial density of the measurements. In the case of ADCPs, a titanium alloy housing is an excellent choice. Titanium alloy is very corrosion-resistant against seawater and abrasive erosion due to sand particles. It is also very lightweight and yet very strong, and therefore an excellent choice for deployment in the harsh environment off Swakopmund. Despite being a high-performance material, titanium alloy-cased ADCPs are now more cost-effective as a result of production improvements, and this has also made them an effective solution for large-scale coastal current monitoring within the region.

6. How to Choose Suitable Equipment for Current Measurement?

The choice of suitable equipment to measure currents in Swakopmund relies on the specific application.

ADCPs by Use

• Ship-mounted ADCP: Installed on a moving ship, this ADCP can be ideal for big-scale surveys of the coastal waters of Swakopmund. While the ship moves within the region, the ADCP is able to continuously measure the currents along the route, providing a wide-scale overview of the current patterns generated by the Benguela Current and wind. These are useful for purposes like marine navigation, research on the general circulation of the waters within the region, and management of fisheries, to locate areas of favorable fishing conditions.
• Bottom-mounted (Sit-on-the-bottom) ADCP: These ADCPs are situated on the sea bed and utilized for fixed-point, long-term measurements. They can be deployed for extended periods, collecting data on currents at a location. This type of ADCP is employed for study of local, fine-scale current patterns, such as the effect of underwater structures on the current and the development of the current patterns over time. This information is crucial for ecological research, environmental observation, and understanding the impact of human activities on the marine environment.
• Buoy-mounted ADCP: Installed on a floating buoy, these ADCPs can drift along with the current, measuring velocity at different depths as they go. They are particularly useful for monitoring large - scale, synoptic current patterns in the open oceans off Swakopmund, or for tracking the path of water masses propelled by the Benguela Current over a large region. Such an ADCP would ensure to be of enormous benefit in predicting the dispersion of contaminants, migration of marine life, and the overall status of the marine ecosystem.

Frequency Selection

The ADCP frequency should be chosen wisely with respect to water depth. An ADCP 600kHz will function in up to about 70m water depth, providing high-resolution data in relatively shallow water along the Swakopmund coast. For depths of about 110m, a 300kHz ADCP profiler will be more appropriate, allowing the acoustic pulses to penetrate deeper into the water column but still maintain sufficient measurement accuracy. In very deep water, up to 1000m, a 75kHz ADCP would be normal. But in view of typical water depths offshore Swakopmund, a 600kHz or 300kHz ADCP would most likely be adequate to meet most studies' measurements.

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.