1. Where is Nouakchott?
Nouakchott, the capital and largest city of Mauritania, is a lovely city on the shore of the Atlantic Ocean. The city lies approximately 50 kilometers north of the Tropic of Cancer and is a significant coastal town in the northwestern part of the African continent. The city has a wide sandy beach that stretches as far as one can see, to meet the endless blue Atlantic waters.
Surrounding land around Nouakchott is low-lying for the most part, and dunes of sand covering much terrain provide testament to the severe dryness in the region. Despites all the adverse setting, Nouakchott is now a vibrant city and a metropolitan presence of an eclectic demographic comprising different ethnic groups. Culture within the city is woven into being by syncretic forces of Arab - Berber community and African lineage to form an effervescent society full of texture.
The Nouakchott port is a significant economic hub for local and international trade. It serves as a point of entry for the importation of essential products and exportation of Mauritanian natural resources, such as iron ore and fish. The sea off the Nouakchott coastline is enveloped by the Mauritanian Exclusive Economic Zone, a fertile marine ecosystem of biodiversity. Although the waters are hard to sail through due to the arid climate and common strong winds, they have a variety of marine life like various fish, crustacean, and marine mammal species. Banc d'Arguin National Park, located near Nouakchott, is a renowned coastal system that shelters a large number of migrant birds along with uncommon marine species, which reflects the ecological importance of the coastal waters of the area.
2. How are the off-shore coastal currents around Nouakchott?
Offshore coastal currents off Nouakchott depend on numerous parameters generating complex and dynamic stream currents. One such critical parameter is seasonal wind regimes. The region receives strong trade winds, especially some seasons. They blow uniformly from the northeast and push the surface water against the coastline. The power of the wind has a significant impact on the direction and speed of the coastal currents, inclining to drive them southwards along the coast of Mauritania.
The second important factor is the interaction between the cold Canary Current and the inshore coastal waters. The Canary Current, a cold ocean current which flows south along the west African coast, has a striking impact on the temperature and salinity of the coastal waters off Nouakchott. This cold current carries nutrient-rich water from the deep ocean, thereby supporting an intricate marine environment. But it has an effect on the density of the water as well, which aids in determining the flow of coastal currents. It is capable of creating upwelling zones when mixing the cold Canary Current water with the relatively warm surface water off the coast since deep water rises to the surface and reverses the current direction.
Tides also play a part in the coastal currents off Nouakchott. The tides here are semi-diurnal, with two highs and two lows daily. The tidal range is variable and, at high tide, water moves towards the coast, increasing the water level and reversing the direction and speed of the currents off the coast. Whereas, at low tide, water returns, therefore exposing the intertidal areas and influencing the current flow. The interaction between these tidal oscillations and the wind - and current - induced flows results in a dynamical system of the coastal currents, which is changing constantly.
3. How to measure the coastal water flow of Nouakchott?
Surface Drift Buoy Method
The surface drift buoy method is a traditional method used to measure the coastal water current. In this method, specially designed buoys are launched into the ocean. Buoys are equipped with tracking devices, which consist of GPS receivers. When the buoys are carried by the surface current, GPS devices monitor their locations at a predetermined interval. By tracking the buoys' movement through time, scientists can estimate the direction and speed of the surface-level current. This method has its shortcomings. The buoys are susceptible to wind effects, causing drift away from the actual path of the current. It also provides only information on the surface current and not insights into the currents at different levels.
Anchor Moored Ship Method
The anchor ship moored vessel method relies on anchoring a ship in a specific location in the waters off Nouakchott coast. From this ship, the various current - measuring instruments are dropped into the water. Examples of such measuring instruments are mechanical current meters that measure the speed and direction of the current by the rotation of a propeller - like system. By taking measurements at different depths, researchers can have a profile of the current at that particular location. Even though this method gives accurate measurements at multiple depths, it only measures at the region around the anchored vessel. Moreover, the vessel can also disrupt the usual current flow at times, resulting in inaccurate measurements.
Acoustic Doppler Current Profiler (ADCP) Method
The Acoustic Doppler Current Profiler (ADCP) is nowadays a more advanced and efficient tool for monitoring coastal water current. ADCPs use sound waves to track the velocity of water current currents. ADCPs transmit acoustic pulses into the water column, and receive them back from suspended water particles, such as sediment or plankton. By analyzing the Doppler shift in the return signals, the ADCP can calculate the speed and direction of the water flow. It is feasible for this method to provide a detailed characterization of the current profile, from the surface down to near the bottom of the water body, without physically contacting the water. ADCPs are less affected by external factors like wind compared to surface drift buoys, and they are therefore a good choice for accurate current measurement.
4. What is the working principle of ADCPs that use the Doppler principle?
ADCPs operate based on the Doppler effect principle. There are transducers in the ADCP flow meter that transmit acoustic waves into the water. The sound waves travel through the water and encounter particles that are moving with the current. When the particles move towards the ADCP, the frequency of the backscattered sound waves increases, and when they move away, the frequency decreases. This shift in frequency, or Doppler shift, is directly proportional to the velocity of the motion of the water in the direction of the sound wave travel.
The majority of ADCPs monitor the three-component velocity of the water by employing several acoustic beams. Four or more beams are normally placed at different angles. Taking the Doppler shift of each beam, the ADCP meter will be able to find the horizontal and vertical components of the velocity. All the beams' information is then averaged to compute the overall current velocity and direction in different depths of the water column. ADCPs today also come equipped with additional sensors, such as temperature sensors to account for the effect of temperature on water's speed of sound, and orientation sensors so that precise measurement can be attained even if the instrument is in the process of being tilted or moved.
5. What is required for high-quality measurement of Nouakchott coastal currents?
To ensure precise measurement of the coastal currents off Nouakchott, there are several needs to be met. Firstly, the measurement gear has to be very robust in the harsh marine environment. Off Nouakchott, the sea is exposed to seawater, high winds, and potentially rough seas, and hence the materials used to construct the equipment have to be corrosion - resistant, robust, and mechanically stress - resistant.
The size of the equipment should also be considered. Instruments of smaller sizes are better suited to be placed in various sites, especially in sites with complex coastal morphology or limited accessibility. A reduced footprint will minimize the capacity to disturb the natural path of currents, offering more accurate measurements. Lightweight equipment is to be employed since they are simpler to handle when rolling out and rolling back, especially when using small vessels or offshore sites remote from power.
Low power consumption is critical, as most of the current-measurement operations would involve leaving the equipment in the field for extended periods, and where often power supply is not convenient. Low - power - consumption equipment is able to operate for a long time without the need for frequent battery replacement or recharging. Cost-effectiveness is also a concern, especially in the case of large - scale measurement exercises. Equipment that delivers good performance at a reasonable cost allows more extensive data gathering, enabling researchers to understand coastal current patterns better.
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.
How will we measure the Nouakchott coastal currents?