How do we measure the coastal currents of Libreville?

1. Where is Libreville?

Libreville, Gabon's thriving capital city, is located in Central Africa, on the west coast, along the great waters of the Gulf of Guinea. It is a city that is fascinating, blending in harmony both the beauty of nature and modern sophistication. Its coastline winds elegantly, and sandy golden stretches of land give way to the Atlantic Ocean's waves.

The city is culturally diverse, housing a cosmopolitan people whose lifestyle is closely dependent on the sea. The coastal communities have an entrenched maritime tradition, with sea fishing and seaborne commerce forming part of the economy. The ports in Libreville are bustling, with ships unloading and docking, carrying produce and connecting the city to other parts of the world. Along the coastline, colorful markets offer a wide range of fresh seafood, handicrafts, and spices, indicating the strong connection between the ocean and the city.

Together with Libreville, the surrounding waters constitute a lively marine environment. The Gulf of Guinea, bordering the city, is among the most important bodies of water in West Africa. It is characterized by its warm tropical ocean waters, which support a diverse range of marine life, from coral reefs that are teeming with tropical fish to huge whales and dolphins. The seafloor topography off Libreville is complex, with submarine ridges, canyons, and seamounts that regulate the coastal currents and distribution of marine life off Libreville.

2. What is the state of the coastal currents off Libreville?

The coastal currents to the east off Libreville are governed by a combination of large-scale oceanic and in situ processes. Regionally, the Equatorial Undercurrent and the Guinea Current are extremely crucial. The Equatorial Undercurrent is a dense subsurface stream that runs toward the east below the surface water of the Atlantic Ocean, while the Guinea Current runs toward the west along the West African shore. These global - scale currents interact with one another and may bring warm, nutrient - rich waters into the waters around Libreville.

Seasonal wind patterns also have a profound influence on the coastal currents. In the wet season, they experience strong onshore winds blowing, pushing surface waters onshore and affecting the speed and direction of the currents. On the other hand, the dry season is brought about by offshore winds that result in cold, nutrient-laden waters from the lower layers of the ocean rising towards the surface. Upwelling supports a highly developed marine life but changes the current regime near the coast.

The Libreville coastline's topography, bays, inlets, and headlands, also changes the movement of the currents. As water moves along a coast, it is forced to pass through such coastal land features, and in doing so, the currents deflect, divide, converge, and reverse. In addition, river mouths, say of the Komo River which empties into the Gulf of Guinea off Libreville, can inject fresh water into sea waters off the coast. This fresh-water inflow can influence the salinity and density of the water and hence the movement of the currents.

3. How to measure the coastal water flow of Libreville?

Various ways are available for measuring the coastal water flow of Libreville. The method of surface drifting buoy is a conventional method. Free-drifting buoys with GPS tracking units are floated on the water. These buoys are carried by the surface currents, and their trajectory is monitored with time. Scientists can estimate the direction and speed of the surface-level currents by looking at the location of the buoy. This method provides information only about surface-level currents and might not reflect the flow in lower levels.

Anchored boat method is employed where a boat is anchored at a fixed location and employs current-measuring instruments onboard. Instruments such as electromagnetic current meters are suspended from the boat to measure the water velocity at different depths. Even though this method will provide more detailed information about the current properties in the area of the boat, it is limited by the field it can cover and will be affected by the presence of the boat, which would disrupt the natural water flow.

The Acoustic Doppler Current Profiler (ADCP) method has also gained more and more prominence in coastal measurements during the past several years as a newer and better way of measuring coastal currents. ADCPs possess significant improvement over previous measurement methods. ADCPs measure water velocity at many depths simultaneously and provide a comprehensive description of water flow structure from the ocean surface down to the seafloor. They are therefore best suited for studying Libreville's complicated and changing coastal currents.

4. What are ADCPs based on the Doppler principle?

ADCPs operate on the Doppler principle. They introduce acoustic pulses into the water column. Upon reaching particles suspended within the water, such as sediment, plankton, or tiny animals, a part of the signal is scattered towards the ADCP. The backscattered frequency depends upon the velocity of the particles (and consequently of the water) relative to the ADCP.

By analyzing these frequency changes, the ADCP can calculate the water velocity at different depths. Most ADCPs use multiple acoustic beams, typically in a fan - like configuration. The data from each beam are then used to determine the three - dimensional velocity vector of the water, providing accurate information of both speed and direction of the current at a series of points along the water column. This non-invasive measurement method makes it possible to monitor the water flow continuously and precisely without sampling the water physically.

5. What's required for high-quality measurement of Libreville coastal currents?

For precise, high-quality measurement of Libreville coastal currents, there are several significant characteristics that need to be present in the measurement gear. Primarily, material reliability is essential. The aquatic environment near Libreville is harsh, with salt water, strong currents, and sun and rain exposure. The gear needs to be durable against corrosion, mechanical stress, and intense temperature changes.

Weight and size of the gear are also significant. Lighter and compact equipment is easier to transport and deploy, especially in distant or inaccessible locations on the coast of Libreville. Low power consumption is vital as it supports longer deployments without the need for frequent replacement or recharging of batteries. Also, cost-effectiveness is important, as bulk measurements may be necessary to truly understand the intricate current patterns of the region.

With respect to ADCPs, the material used for casing is very important. Titanium alloy is a better choice for the casing. It enjoys better corrosion resistance, which is necessary in the saltwater-dominated environment of Libreville. Its strength - to - weight ratio is high, so the ADCP will be strong enough to handle the forces of the strong coastal currents but light enough for easy deployment. Moreover, titanium alloy has good fatigue resistance, and the ADCP will be able to work over long durations and provide precise measurements.

6. How to Choose the right equipment for current measurement?

Choosing the right equipment for current measurement in Libreville differs with the application. In the case of in-transit measurements on board a ship, a ship-mounted ADCP would be the most suitable. It will be able to continuously measure the currents as the ship moves across different sections and provide real-time data on the flow of water. When long-term, fixed-point measurements at the bottom of the sea are required, a bottom-mounted ADCP is better. A bottom-mounted ADCP can be fixed on the sea floor to monitor currents for an extended duration. For surface - level and upper - water - column monitoring, a buoy - mounted ADCP offers a convenient solution, as it can be anchored or allowed to drift with the currents, providing valuable data on the surface and near - surface current conditions.

The frequency of the ADCP is also an important factor. A 600kHz ADCP would be suitable for depths of 70 meters or less and would be ideal for shallow coastal waters off Libreville. A 300kHz ADCP can be used for depths of 110 meters or less, and a 75kHz ADCP would be best suited for deeper water, up to 1000 meters.

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.