1. Where is Freetown?
Freetown, capital of Sierra Leone, is a coastal paradise on the estuary of the Sierra Leone River, bordering the Atlantic Ocean. This city of geography and history, where humanity converges with nature in an integral symbiosis, is an example of nature's magic and what humanity can achieve. The city was founded in 1792 as a refuge for freed slaves, a land of hope and redemption. Even its name, "Freetown," bears witness to freedom and the desire for freedom.
The cityscape itself is an interesting blend of rolling green hills, sandy beach, and densely populated urban landscape. The hills that form the periphery of the city are thickly covered with tropical flora, and serve as a breathtaking backdrop to the city, while the vast Atlantic Ocean offers one a sense of limitless space and infinite possibilities. Freetown's harbour is one of West Africa's most significant ports, and the hub of business and commercial activity not only for Sierra Leone but with nations across the globe. Freetown's population is a vibrant tapestry of people plucked from numerous ethnic groups, cultures, and lifestyles coming together and living side by side, thereby infusing the city with the dynamic and active fabric of the city.
The Freetown Harbour lies just beyond the city of Freetown and is the globe's largest natural harbour. The harbour is a significant waterway, which offers protection to a large number of ships and is a gateway to maritime trade. The Sierra Leone River, emptying into the Atlantic Ocean off Freetown, is crucial to the economy and ecosystem of the city. The confluence of the river and ocean provides a distinct ecosystem, with a rich variety of marine life, from the colorful fish to sea turtles. The river is also used as a water source for the city, and the coast is settled by individuals who rely on fishing and other river - based means of livelihood.
2. What is the condition of the coastal currents around Freetown?
The oceanic currents surrounding Freetown are influenced by a variety of factors, each with a significant role to play in the determination of water movement along the coast. Tides form part of the principal forces driving tides. The tides within the area are controlled by the gravitational attractions of the moon and sun, leading to cyclic ebb and flow patterns. At high tide, water enters the Freetown Harbour and along the coast, increasing the water level and altering the direction and velocity of current. The water is, however, pushed back by low tide, exposing the sandy beaches and opening up the intertidal flats for access. Such movements of the tides may hugely influence the volume of business of local fishermen since they have to plan their activities in relation to the tide.
Wind is a powerful force that also influences the coastal currents. The region's prevailing winds, the trade winds, have the capability of driving surface-level currents hundreds of miles. Sea-blowing strong winds have the capability to drive the water and form waves and currents influencing the Freetown coastal waters. In addition, the local wind-created patterns, which are driven by the contour and shape of the hill, can cause transient changes of speed and direction in the currents. A good example is provided by winds blowing through the hill valleys that would be capable of creating gusts, which disrupt the overall trend of the currents.
Furthermore, the mixture of the Sierra Leone River fresh and Atlantic Ocean saltwater changes the coastal currents. Freshwater, being less dense, would form a surface layer in the ocean if added. Stratification can affect mixing of waters and current transportation, forming complex regimes of flow. Volume of river flow, which varies seasonally, also complicates things. With higher river discharge during rainy times, huge amounts of freshwater will be supplied into the estuary, altering the concentration of salinity and even the normal flow patterns. It will affect the marine environment as changed salinity influences the location and activity of marine animals.
3. How can the coastal water flow of Freetown be observed?
Surface Drift Buoy Method
One of the older methods of tracking coastal water flow is the surface drift buoy method. In this method, specially prepared buoys are released into the ocean. The buoys are fitted with tracking devices, such as GPS receivers, through which scientists can monitor their movement over a period of time. As the buoys are carried by the surface current, the GPS units record their positions at intervals. By examining the path of the buoys, scientists are able to determine the direction and speed of the surface-level current. However, this method has some disadvantages. The buoys can be deflected by wind, which causes them to deviate from the actual flow of the current. It also provides data on only the surface current and not on the patterns of currents at different levels.
Anchor Moored Ship Method
The ship moored anchor technique is a method where a ship is anchored at a designated position in coastal waters off Freetown. Different current-measuring instruments are dropped into the water from the ship. These instruments can be mechanical current meters, which pick up the speed and direction of the current by the amount of rotation of a propeller-like device. By taking measurements at different depths, researchers can have a profile of the current at the point of interest. While it's feasible with this technique to measure precisely at a number of depths, it's limited to the area around the moored vessel. In addition, the presence of the vessel at times alters the original pattern of flow of current, leading to erroneous measurements.
Acoustic Doppler Current Profiler (ADCP) Method
The Acoustic Doppler Current Profiler (ADCP) has evolved as a more modern and more effective method to track coastal water movement. ADCPs are used to measure the speed of water currents at varying levels via sound waves. ADCPs transmit acoustic pulses into the water column, and the pulses bounce off suspended water particles, such as sediment or plankton. Through examining the Doppler shift of the reflected waves, the ADCP current meter is able to measure the velocity and direction of the water current. The method can provide a precise description of the current profile, from the surface to near the bottom of the water body, with zero water contact. ADCPs are less affected by the outside winds compared to the surface drift buoys and are the preferred one to implement for precise measurement.
4. How does the operation of the ADCPs' operation on the principle of the Doppler principle work?
ADCPs operate based on the principle of the Doppler effect. There are some transducers contained in the ADCP that broadcast acoustic waves to water. The sound waves propagate through the water and encounter particles transported by the flow. As the particles approach the ADCP, the frequency of the backscattered sound is higher, whereas as they are moving away from the ADCP, the frequency is lower. This frequency difference, referred to as the Doppler shift, is proportional to the velocity of the motion in the direction of the sound.
To calculate the three-dimensional velocity of the water, the majority of ADCPs utilize multiple acoustic beams. Four or more beams are commonly installed at different angles. By measuring the Doppler shift for each of them, the ADCP current profiler will find the horizontal and vertical components of the velocity. All the data from the beams are then employed to compute the combined current velocity and direction at different depths in the water column. More recent ADCPs also feature additional sensors, such as temperature sensors to compensate for the effect of temperature on the velocity of sound in water, and orientation sensors to make accurate measurements even when the device is tilted or moving.
5. What is required for high-quality measurement of Freetown coastal currents?
There are certain key requirements for high-quality measurement of the coastal currents off Freetown. First, the measuring equipment must be of highly high reliability in the adverse marine environment. Equipment needs to be built using corrosion-proof saltwater-resistant materials, being robust enough to endure currents and waves, as well as capable of enduring extended exposure to weather. This is especially important in Freetown, where it is possible to have rough water off the shore during storms.
The size of the equipment is also an essential consideration. Smaller equipment is more versatile and can be utilized in a larger number of places, including those with complex coastal topography or limited access. A smaller footprint reduces the possibility of the equipment disrupting the natural currents, offering more accurate readings. Also, lightweight equipment is better since it will be simpler to handle in recovery and deployment, especially when operating from small vessels or in remote locations.
Low power consumption is also an important parameter. Since many modern - day - measurement tasks require putting the equipment on the ground for weeks, at times in areas with no easy access to power supply, low - power - draining units will run longer without needing constant battery changes or recharge. Cost - effectiveness is also essential, especially for mass - scale measurement activities. Equipment that offers sufficient performance at a reasonable price allows for more data to be collected, which helps researchers gain a better perspective of the patterns of the coastal currents.
With respect to ADCPs, the casing material is a special case. Titanium alloy is the most suitable material to use in casing ADCPs that work in Freetown's coastal waters. Titanium alloy is very resistant to corrosion, which is critical for withstanding corrosion from saltwater over extended periods of time. It is also very strong and light, offering the protection the internal parts of the ADCP need while being light enough to handle and deploy. Also, the resistance to fatigue of the titanium alloy is good in a way that it can endure repeated stress and strain without significant degradation, ensuring long-term and reliability of the ADCP flow meter in the dynamic coastal environment.
6. How to select the appropriate equipment for current measurement?
The selection of equipment for current measurement in Freetown depends on some factors, primarily the purpose of use and characteristics of the measuring location.
- *Types of ADCPs Based on Mounting
- Ship-mounted ADCP: A ship-mounted ADCP is mounted on a floating vessel. It works best to conduct large-scale coastal current studies. As the ship moves along the coast, the ship-mounted ADCP can record data over a large area and provide a complete picture of current patterns. It is suitable for applications where a high level of the regional current regime is required, such as in navigation at sea and large - scale environmental observations.
- Bottom - mounted ADCP: Bottom - mounted ADCPs are installed on the sea bed. They are applied to measure long - term patterns of currents at a fixed point. These instruments can provide continuous data over extended periods, which is beneficial for the study of the seasonal and long - term fluctuations of coastal currents. They are often used where stability and long - term alteration of the current must be observed over time, such as near important fishing areas or marine sanctuaries.
- Buoy - mounted ADCP: Buoy - mounted ADCPs are installed on floating buoys. They are best suited for the measurement of currents in regions where it is not easy to install other forms of ADCPs, like in open-water regions or in areas with high currents that can endanger bottom-mounted instruments. Buoy-mounted ADCPs can drift with the current, providing real-time data about the dynamic fluctuations in the current, which is useful for applications like oil spill tracking and oceanographic research.
Choosing the Right Frequency
The frequency of the ADCP is another primary factor in choosing the right product. Different frequencies are suitable for different water depths. A 600kHz ADCP is suitable for water depths up to about 70m. It gives very high-resolution measurements in shallow waters and is therefore suitable for locations near the coast, e.g., around the Freetown Harbour or along the Sierra Leone River estuary. For water depths of about 110m, a 300kHz ADCP is more appropriate, striking a balance between resolution and penetration depth. For deeper waters, up to 1000m, a 75kHz ADCP would be best. Lower - frequency ADCPs probe deeper into the water but have poorer spatial resolution than higher - frequency ones.
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.
How do we measure Freetown's coastal currents?