How do we measure Conakry's coastal currents?

1. Where is Conakry?

Conakry, the economic capital and largest city of Guinea, is a coastal gem located on the Atlantic Ocean. This vibrant city is located on Tombo Island and the adjacent Kaloum Peninsula, whose strange topography forms a natural harbour that has rendered it a necessary sea port in West Africa. The cityscape is a blend of natural scenery and urbanization, with the blue waters of the Atlantic Ocean lapping against the shores and the green of the surrounding areas forming a scenic background.

Conakry is a vibrant population, showcasing the country's rich culture. It is the economic, political, and cultural hub of the country, attracting people from all over the country as well as outside. Its economy is based on a number of sectors, including trade, fishing, and manufacturing. Its port is an ideal gateway for the export and import of commodities, facilitating commerce in Guinea as well as other nations in the globe.

Conakry borders the Gulf of Guinea, a large body of water in the Atlantic Ocean. The warm waters of the gulf and its complex oceanic systems exchange with the Conakry coastal waters, determining its marine environment. The Kaloum Estuary, found within the Conakry region, is an ecological zone of significant importance where the freshwater of the surrounding rivers meets the ocean's saltwater. This convergence constitutes a unique ecosystem, supporting high diversity of sea life and influencing the local fishery. Estuary and nearshore coastal waters are also of importance for shipping, with boats using these waters to access Conakry harbor.

2. What is the health of the coastal currents around Conakry?

The coastal currents around Conakry are impacted by a variety of factors. Tides are accountable for the movement of water along the coast. The tides in the region are governed by the moon's and sun's pull, with regular ebb and flow cycles. These tidal flows can produce extensive variations in water levels and current velocities, especially near the coast and the estuary area. During high tide, water enters the estuary and along the coast and increases the speed of the current and alters the direction of the flow in some areas.

Wind is another powerful force that affects the coastal currents. The trade winds that are the prevailing winds in the Gulf of Guinea can force surface-level currents. Strong, persistent winds over the ocean could force the ocean waters, causing long - distance currents with considerable impacts on the coastal waters off Conakry. Superimposed on this, local winds with their reliance on the city topography and surrounding landmasses, can cause short - term changes in current direction and magnitude.

The combination of the freshwater from the rivers that flow into the estuary and the saltwater from the Atlantic Ocean also changes the coastal currents. The freshwater is less dense than the saltwater, and when it flows into the ocean, it forms a surface layer. This stratification can affect water mixing and current movement. The volume of river flow, according to seasons, also plays a role. Excessive river runoff during the rainy season can introduce fresh water in huge amounts, altering the salinity level and disrupting the normal current pattern.

3. How is coastal water flow of Conakry measured?

Surface Drift Buoy Method

Surface drift buoy method is one of the traditional methods employed to estimate coastal water flow. In this method, specially designed buoys are released into the water. The buoys are equipped with tracking devices, such as GPS receivers. As the buoys are carried by the surface current, the GPS units record their locations at intervals. By analyzing the path of the buoys over time, scientists can determine the direction and speed of the surface-level current. But this method has its limitations. The buoys can be subject to wind, and therefore they could deviate from the actual course of the current. Additionally, it only provides information on the surface current and not about the patterns of the current at different depths.

Anchor Moored Ship Method

The anchor moored ship method is achieved by anchoring a ship in a specific point in the sea off the Conakry coast. From the boat, different current-measuring devices are lowered into the water. These can include mechanical current meters, which measure the velocity and direction of the current by measuring the rotation of a propeller-like instrument. By measuring at different depths, scientists can construct a profile of the current at that site. Although this technique provides precise measurements at various depths, it is confined to the area surrounding the anchored vessel. Furthermore, the ship may occasionally disrupt the natural current flow, causing spurious readings.

Acoustic Doppler Current Profiler (ADCP) Method

The Acoustic Doppler Current Profiler (ADCP) has also been a more contemporary and efficient method of coastal water flow monitoring. ADCPs take readings of the water flow speed at various depths through the use of sound waves. The ADCP fires acoustic pulses into the water column, and the pulses bounce off suspended matter in the water, such as sediment or plankton. By interpreting the Doppler shift of the returned pulses, the ADCP determines the direction and speed of the water flow. This method can provide a full description of the profile that is present, from the surface to near the bottom of the water body, without physically contacting the water. ADCPs are also less affected by external conditions like wind than surface drift buoys are, and therefore are a reliable choice for accurate current measurements.

4. How do Doppler principle ADCPs work?

ADCPs operate on the Doppler effect principle. Inside the ADCP profiler, there are transducers that emit acoustic waves into the water. The sound waves propagate in the water and strike particles flowing in the direction of the current. When the particles approach the ADCP, the frequency of the reflected sound waves is higher, and when they recede, the frequency is lower. This change of frequency, otherwise known as the Doppler shift, is in proportion to the water flow velocity in the direction of the sound wave.

Most ADCPs calculate three-dimensional water velocity from a number of acoustic beams. There are four or more beams generally spread out over different angles. By calculating the Doppler shift for each beam, the ADCP flow meter can calculate the horizontal and vertical velocities. All of the beams' data are utilized to calculate the total current velocity and direction at different depths within the water column. Modern ADCPs also have additional sensors, such as temperature sensors to account for the effect of temperature on the speed of sound in water, and orientation sensors to ensure accurate measurement even when the instrument is tilted or moving.

5. What does it take to achieve high-quality measurement of Conakry coastal currents?

For accurate measurement of the coastal currents around Conakry, there are some requirements to be fulfilled. To begin with, the equipment to be used must be very reliable in the tough marine environment. The equipment's materials for its construction must be resistant to saltwater corrosion, capable of withstanding heavy currents and waves, and robust enough to withstand long - term exposure to the weather.

The size of the equipment is also of great importance. The smaller-sized instruments are more portable and can be employed in a large number of locations of various topography of coastlines or in places with limited access. The reduced footprint also reduces the likelihood of the equipment disturbing the normal flow of currents, offering more accurate measurements.

Light-weight equipment is also preferable since it can be handled with ease while deploying and recovering the equipment. This is especially true when working from small boats or in remote locations where heavy equipment can be difficult to maneuver. Low power usage is also crucial. Because most existing - measurement activities can involve having the equipment in the field for long periods of time, and frequently in remote locations where access to power supplies is not easy, low - power - consuming devices can stay in operation for longer periods of time without frequent battery replacement or recharging.

Cost - effectiveness is also an important factor, especially for big - scale measurement projects. The equipment that delivers good performance at a reasonable price allows more extensive data collection, which in turn enables researchers to better understand the coastal current patterns.

For the case of ADCPs, the casing material is of particular importance. Titanium alloy is an ideal material to be employed in the housing of ADCPs installed in the coastal waters of Conakry. Titanium alloy is highly corrosion-resistant, which is a parameter essential to withstand long-term corrosive action of saltwater. It is extremely strong and lightweight, acting to safeguard the internal components of the ADCP but being easy to handle and install. Additionally, titanium alloy has good fatigue resistance, i.e., it can withstand repeated stress and strain without significant degradation, offering the durability and reliability of the ADCP current profiler​ in the dynamic coastal environment.

6. How to Select the appropriate equipment for current measurement?

The selection of the appropriate equipment for current measurement in Conakry depends on several factors, primarily the purpose of application and the type of the measurement site.

Types of ADCPs Based on Mounting

• Ship-mounted ADCP: This ADCP is installed on a moving ship. It is especially convenient to use in the process of taking large-scale measurements of coastal currents. As the ship moves along the coast, the ship-mounted ADCP can take data along a large distance, providing a broad view of the current trends. It comes in handy when a complete picture of the regional current system is required.
• Bottom - mounted ADCP: Bottom - mounted ADCPs are mounted on the sea floor. They are employed to measure long - term current patterns at a given point. They are capable of providing continuous information over extended periods of time, which is beneficial while studying seasonal and long - term fluctuations of coastal currents. They are normally employed in cases where one needs to monitor the stability and evolution of the current over time.
• Buoy - mounted ADCP: The floating buoys are equipped with buoy - mounted ADCPs. These are best utilized in those locations where it would be inconvenient to deploy other types of ADCPs, such as open - water or areas with strong currents that would pose danger to bottom - mounted instruments. Buoy-based ADCPs have the capability of riding the current, providing a real-time insight into the time-varying behavior of the current.

Choosing the Appropriate Frequency

The ADCP frequency is a further crucial decision in making an ADCP system choice. Slightly different frequencies are used depending on water depths. An ADCP operating on 600kHz is suitable in water to the depth of some 70m. It gives relatively high-resolution measurements in shallow water and is therefore suited to coastal areas or estuaries. 300kHz ADCP is more applicable to 110m water depth and gives a balance between resolution and penetration depth. In deeper waters, 1000m, a 75kHz ADCP must be employed. Lower - frequency ADCPs reach further into the water but degrade spatial resolution more than higher - frequency models.

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 flow meter instrument brands and models.