How are we measuring the coastal currents of Lüderitz?

1. Where is Lüderitz?

Lüderitz, a picturesque seaside town, is located on Namibia's southwestern coast, stark along the Skeleton Coast. It is a place renowned for its barren, yet Utra-marous landscape, punctuated with great areas of sandy deserts which flank the frigid Atlantic Ocean. The town itself boasts a picturesque loveliness, with an integration of German colonial townscape and Namibian culture, attesting to its historical status as a German colony.

The local population of Lüderitz is small but energetic, with an existence well-entwined in the ocean. Fishing has long been a pillar of the local economy, as the port of the town is filled with fishing boats offloading hake, horse mackerel, and other sea creatures. Tourism has also grown, with visitors coming to the town's bizarre landscapes, such as the nearby ghost town of Kolmanskop, a once-thriving diamond-mining town, now buried by the sands of the desert.

Geographically, Lüderitz is situated in an area where the cold Benguela Current has a strong impact. The Benguela Current travels northwards along the west coast of southern Africa, bearing with it cold, nutrient-rich waters. The coastline of Lüderitz is rocky, with outcrops and shallow bays. The sea bed ranges from relatively shallow near the coast to deeper channels offshore, creating a diverse marine ecosystem that supports a wide variety of fish, seals, seabirds, and other marine species.

2. How are the coastal currents off Lüderitz?

The coastal currents off Lüderitz are dominated by the Benguela Current. The cold ocean current is one of the major drivers of the local marine environment. The Benguela Current's upwelling of cold, deep-water brings a constant supply of nutrients into the surface, which sustains a rich and productive marine ecosystem. The strength and direction of the current may be seasonal, with wind patterns and variations in the larger - scale ocean circulation affecting them.

Another important factor controlling the coastal currents off Lüderitz is wind. Seasonal south - easterly winds dominate the area for most of the year. These winds have the capacity to force surface waters, forming wind - driven currents that mix with the Benguela Current. Stronger winds can accelerate the surface currents and reverse their direction, developing complex flow patterns. The inlets and headlands arrangement of the Lüderitz coastline also alters these currents. Headlands will accelerate the currents as water is forced through them, and inlets will create areas of relatively calm water or eddies where the current flow is more complex.

Tides also affect coastal currents near Lüderitz, although their influence is often overshadowed by the dominant Benguela Current and wind - driven currents. Semi - diurnal tides in the region result in the sea level rising and falling twice a day. These tidal flows can produce additional currents that interact with the in - situ flow patterns, especially in the inshore bays and along the coast.

3. How can one measure Lüderitz's coastal water current?

Surface Drift Buoy Method

One of the traditional means to track Lüderitz's coastal water current is via the use of surface drift buoys. They are equipped with GPS tracking units. Upon immersion in the water, they buoy and drift according to the surface currents. By monitoring the movement of the buoys in the future with the help of GPS, scientists can quantify the velocity and direction of the surface-level currents. However, this method also has its limitations. It is restricted to the top layer of the water column and is subject to wind-induced movement, which might not necessarily be the representation of the true underlying current flow. In addition, in the sometimes - stormy waters off the coast of Lüderitz, surface drift buoys can easily become blown off course or wrecked, reducing the precision of the measurements.

Moored Ship Method

The moored ship method involves mooring a boat in one location off the coast of Lüderitz. Current - measuring equipment such as current meters are released from the boat at various depths. These instruments record the velocity and direction of the current at each depth, allowing a vertical profile of the current to be constructed. While this method can give detailed information at a point, its spatial coverage is limited to the area around the moored ship. Besides, the presence of the ship can disrupt the natural flow patterns in the region, and the measurements can be affected by the motion of the ship itself and the turbulence induced by its propellers.

Acoustic Doppler Current Profiler (ADCP) Method

The Acoustic Doppler Current Profiler (ADCP) has emerged as a superior and more efficient tool for coastal current measurements in Lüderitz. ADCPs are greatly valued in oceanographic studies because they can simultaneously measure current velocities at several depths in a large body of water. This technology enables scientists to have a complete view of the three - dimensional nature of the current, which is necessary for successfully modeling the complicated coastal circulation features driven by the Benguela Current and wind - forced forces in Lüderitz. As opposed to other techniques, ADCPs can measure currents quickly and accurately at different depths, providing valuable insight into how the currents vary vertically and horizontally in the coastal waters of the area.

4. What is the operation of ADCPs based on the Doppler principle?

ADCPs operate on the Doppler principle. The device emits acoustic pulses, or sound waves, into the water column. When these sound waves pass through small particles suspended in the water such as plankton, sediment, or bubbles, some of the sound energy will bounce back to the ADCP current profiler. If the particles themselves move with the current, the frequency of the reflected sound waves will be different from that of the transmitted sound waves. This frequency change, or Doppler shift, is proportional to the velocity of the particles, and so to that of the current.

Most ADCPs have several transducer beams, usually three or four, at different angles. By observing the Doppler shifts within each beam, the ADCP can compute the components of the current velocity in three - dimensional space. The instrument divides the water column into separate depth bins and, for each bin, computes the current velocity. This method allows ADCPs to deliver a detailed profile of the current velocity as a function of depth, providing a rich source of information regarding the movement of the water in Lüderitz's coastal waters. For example, it can show how the current speed differs from the surface, as influenced by wind, to the deeper layers as influenced by the Benguela Current.

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

In order to attain high-quality measurement of Lüderitz's coastal currents, the measuring instruments must meet several important requirements. Material reliability is of utmost importance, since the equipment will be exposed to the hostile marine environment. The freezing, brackish waters along the coast of Lüderitz, combined with the erosive action of wind - blown sand, pose significant challenges to the equipment's life. The materials used in the construction of the equipment, especially for ADCPs, need to be highly corrosion - resistant, erodible, and biofouling - resistant.

The gear must be kept as compact as possible to minimize its impact on the natural flow of current and to facilitate easy deployment in various locations, such as shallow water near the shore and deeper channels off the coast. Light gear is also preferred, as it facilitates easier handling during deployment and recovery, especially in the often - hostile weather conditions off the coast of Lüderitz. Low power consumption is essential, especially for long-term deployments, since the power sources may be scarce in this remote coastal area.

Cost-effectiveness is also a factor, especially in large-scale measurements. Costly equipment can limit the number of instruments that can be deployed, and therefore the spatial coverage of the measurements. For ADCPs, a titanium alloy housing would be a very good choice. The titanium alloy is very resistant to corrosion from seawater and erosion from sand particles. It is also hard and light and can therefore be utilized under the rough conditions off Lüderitz. Although a high-performance material, improvements in manufacture have made it possible for titanium alloy-cased ADCPs to become cost-effective, thus a reasonable choice for large-scale coastal current measurement in the region.

6. How to Choose Suitable Equipment to Measure Current?

Choice of suitable equipment for measuring currents in Lüderitz is based on the intended usage.

ADCPs Depending on Application

• Ship-mounted ADCP: Installed on a moving ship, this ADCP is most appropriate for large-scale surveys of the Lüderitz coastal waters. As the ship moves across the area, the ADCP can continuously measure the currents along its route, providing a broad-scale view of the current patterns influenced by the Benguela Current and wind. This is used for the kind of work such as navigation in the sea, the overall circulation of waters in the region, and fishery management.
• Bottom - mounted (Sit - on - the - bottom) ADCP: Installed on the bottom of the ocean, these ADCPs are designed for long - term, fixed - point observations. They are left to remain stationary for extended durations, collecting information on the currents in that place. This ADCP is useful for studying the local, fine-scale current patterns, e.g., the effects of underwater topography on the current and temporal variations of the current patterns, which is important in ecological study and environmental monitoring.
• Sailing on a buoy ADCP: Hang underneath a buoy, with ADCPs to drift with the current and measure velocity at multiple points and various depths as they pass by. They are very useful for monitoring large-scale, synoptic currents off Lüderitz in the open ocean or for monitoring water mass motion forced by the Benguela Current for large areas.

Frequency Selection

The ADCP frequency should be chosen appropriately based on the depth of water. For water depths not exceeding around 70m, a 600kHz ADCP is suitable with high-resolution measurement in quite shallow waters near the Lüderitz coastline. For about 110m depths, a 300kHz ADCP would be more suitable so that the sound waves are allowed to travel further into the water column while achieving reasonable measurements accurately. At extremely large depths, to 1000m, a 75kHz ADCP is usually utilized, which will be operable in the deeper channel offshore of Lüderitz.

There are many well-known brands of ADCP in the market, i.e., Teledyne RDI, Nortek, and Sontek. Nonetheless, for cheap but high-quality ADCPs, the ADCP manufacturer China Sonar's PandaADCP is the best choice. Made of pure titanium alloy, it delivers superior performance and longevity at a very affordable cost. Being a low-cost ADCP, it is a perfect option for researchers, local fishermen, and environmental monitoring departments who are concerned with accurately computing the coastal currents of Viña del Mar. To know more, visit https://china-sonar.com/.

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