1. Where is Banjul?
Banjul, the capital of The Gambia, is a coastal gem situated on Saint Mary's Island at the entrance of the Gambia River into the wide Atlantic Ocean. Banjul's strategic location has rendered it significant to trade, culture, and transport in West Africa. The city is characterized by its unique blend of natural landscape and urban life, with the sea and the river playing central roles in determining its character.
The island on which Banjul is located is relatively flat, with a network of waterways and mangrove swamps that harbor a great diversity of flora and fauna. The mangroves, on the other hand, are also a vital element of the nation's ecosystem as they provide a habitat to numerous fish, birds, and other wildlife. They also naturally shield the coastal region from storms and erosion. The sandy white beaches found along the coastline of Saint Mary's Island are not only a sight for the eyes but are also a prime nesting ground of sea turtles.
Banjul's population consists of a diverse collection of ethnic groups, the most dominant of which are the Mandinka, Fula, and Wolof. The city's culture too reflects this diversity in the form of a rich collection of traditional African customs blended with modern influences. The city is renowned for its bustling markets where locals and visitors alike can buy various handicrafts, spices, and fresh produce. Fishing is also a major economic activity in Banjul, with the country's domestic fishing industry relying on the abundant marine resources of the Gambia River and nearby Atlantic ocean. Banjul port is also an important point of entry for importing and exporting commodities, facilitating trade not only within The Gambia but also throughout other regional nations.
2. How is the condition of the coastal currents along Banjul?
The coastal currents along Banjul are conditioned by a complex combination of numerous factors. The Gambia River and its enormous freshwater outflow are to blame for controlling the local current regimes. The currents of the river, which seasonally vary, dump enormous amounts of freshwater into the coastal waters. This new water is more dense than Atlantic Ocean saltwater, creating a surface layer stratification. This interaction of the river water and sea water could create estuarine current patterns such that the freshwater exports out by surface flow and saltwater imports by deeper depths.
Tides also contribute an important role in shaping coastal currents along the Banjul coast. Here, the tides are semi-diurnal with two high tides and two low tides per day. The tidal range tends to be significant and greater than usual, especially during spring tides, when the sun, moon, and earth lie in a straight line with each other. During high tide, water floods into the estuary and increases the water level and reverses the direction and speed of the current. When the tide recedes at low tide, the water returns to the ocean, which reverses the current in certain places. Locals utilize it to enhance their fishing activity and sediment movement along the coast. Coastal currents also become affected by wind patterns. The regional wind pattern, or the trade winds, has a tendency to create surface-level currents. Strong winds that blow over the ocean have the potential to force the water, create waves, and shift the natural current of the coastal currents. The short - term variations in the direction and speed of the current are also created by local winds that trace the topography of the country and the shape of the coastline, making the current system even more complex.
3. How to observe the coastal water flow of Banjul?
Surface Drift Buoy Method
The surface drift buoy method is a very old method for observing coastal water currents. In this method, specially designed buoys are tossed into the water. These buoys have tracking gear, such as GPS receivers, onboard. While the buoys drift along with the surface current, the GPS units monitor their locations at regular intervals. By monitoring the path of the buoys over time, scientists can approximate the speed and direction of the surface-level current. This is not an ideal method, however. The buoys can be affected by the wind, causing them to stray from the true path of the current. It also provides only data about the surface current and nothing about the patterns of the current at different depths.
Anchor Moored Ship Technique
Anchor moored ship technique is the mooring of a ship at a specific point in the waters off Banjul coast. Several current-measuring instruments are dropped from the ship into the sea. They can be mechanical current meters, which measure the speed and direction of the current from the rotation of a propeller-like device. Through multiple measurements taken at different depths, researchers can derive a profile of the current within a point in space. As much as this method facilitates intricate multiple-depth measurement, it has limitations only around the location where the anchored vessel is positioned. The fact that the vessel even exists could cause interference to natural current flow and thus give an erroneous reading.
- *Acoustic Doppler Current Profiler (ADCP) Method
The Acoustic Doppler Current Profiler (ADCP) is now a more advanced and efficient tool to track the coastal water current. ADCPs measure the velocity of water currents at various levels by using sound waves. ADCPs emit acoustic pulses into the water column, and the pulses bounce off suspended particles in the water, such as sediment or plankton. By analyzing the Doppler shift of the backscattered pulses, the ADCP can measure the speed and direction of the water current. The method can provide a general description of the current profile, from close to the water surface to near the bottom of the water column, without touching the water. ADCPs are less sensitive to weather conditions outside, such as wind, than surface drift buoys and are therefore a good choice for accurate current measurement.
4. How does ADCPs using the Doppler principle work?
ADCPs operate on the Doppler principle. Inside the ADCP, there are transducers that emit acoustic waves into the water. The sound waves travel through the water and strike particles that are carried along with the current. When the particles are moving toward the ADCP, the backscattered sound waves have a higher frequency, and when moving away, the frequency is lower. The change in frequency, or Doppler shift, is directly proportional to the water movement speed in the direction of the sound wave.
All but the earliest ADCPs estimate the three-dimensional water velocity using more than one acoustic beam. Typically, four or more beams are deployed at different angles. By measuring the Doppler shift in each beam, the ADCP can calculate the horizontal and vertical velocity components. The data from all the beams are then combined to calculate the total current velocity and direction at different depths throughout the water column. Modern ADCPs now include other sensors, such as temperature sensors to compensate for the effect of temperature on water speed of sound, and orientation sensors so that measurements can be made even when the instrument is not level or moving.
5. High-quality measurement of Banjul coastal currents requires
In order to obtain a high-quality measurement of coastal currents surrounding Banjul, there are requirements that must be met. For starters, the equipment must be highly dependable in the harsh marine environment. Water surrounding Banjul is subject to seawater, high tides, and the effects of the river's freshwater outflow, hence the materials used to build the equipment must be resistant to corrosion, highly durable, and strong against mechanical stress.
Equipment size is also an important consideration. Small-sized equipment is more general purpose and can be placed in more varied locations, like areas with challenging coastal topography or limited access. Having a small footprint implies there is less chance of the equipment interfering with the natural current flow, and more accurate measurements will be obtained. Lightweight equipment is preferred, as it is easier to handle during deployment and recovery, particularly when operating from small boats or in remote coastal environments.
Low power usage is also essential, as most present - measurement operations will have the equipment deployed over long periods, preferably in places where power sources are not readily accessible. Low - power - consumption devices have the ability to survive longer before replacement or battery recharging is necessary. Cost - effectiveness is also a significant factor, especially for large measurement projects. Devices that offer excellent performance at an affordable cost allow researchers to obtain more comprehensive data, providing them with an overall view of the coastal current patterns.
When it comes to ADCPs, the casing material is most important. Titanium alloy is an extremely ideal material for use in the casing of ADCPs operating in Banjul's coastal waters. Titanium alloy is extremely corrosion resistant, which is most important in resisting the corrosive effect of saltwater for many years. It is also very strong and lightweight, providing covering for the ADCP's interior parts while facilitating it to be conveniently handled and deployed. Titanium alloy has one more benefit in terms of having good fatigue resistance, or its capability to resist repeated stress and strain without the failure of properties, which promises long life and reliability of the ADCP in the dynamic environment of the coast.
6. What is the process of selecting right equipment for measuring currents?
Selecting the appropriate equipment for the measurement of present in Banjul is based on many variables, primarily the uses and the character of the location on which the measurement would be made.
ADCPs Based on Mounting
- Ship-mounted ADCP: It is an ADCP mounted on a moving vessel. It is most appropriately applied while conducting massive - scale observation of coastal streams. Because the vessel is traveling along the shoreline, the ship-mounted ADCP can sample over a broad area, providing a broad overview of current regimes. It is well suited to applications such as maritime navigation, long-scale oceanographic surveys, and coastal environmental observation.
- Bottom - mounted ADCP: Bottom - mounted ADCPs are fixed on the seafloor. They are employed to measure long - term current patterns at a site. These instruments can provide continuous data for a long time, which is crucial for monitoring the seasonal and long - term variation of coastal currents and monitoring the impact of human activity on the ocean environment.
- Buoy - mounted ADCP: Buoy - mounted ADCPs are installed on floating buoys. They are most - suitable for measuring currents in areas where it is difficult to install other types of ADCPs, for instance, open - water bodies or in areas with strong currents that may endanger bottom - mounted instruments. Buoy - mounted ADCPs are able to drift with the current, providing real - time data on the current's instantaneous change, which is perfect for applications like oil spill tracking, fisheries management, and short - term oceanography.
Choosing the Right Frequency
The frequency of the ADCP is also a consideration in choosing. Different frequencies are optimal for different depths of water. A 600kHz ADCP is applicable in water depths of around 70m. It has relatively high - resolution measurements in shallow depths and would be a good choice for shore regions, such as the Gambia River estuary and Banjul beaches. A 300kHz ADCP is more appropriate for the water depth of around 110m and strikes a balance between penetration depth and resolution. For over 1000m waters, a 75kHz ADCP is the best choice. Lower-frequency ADCPs travel further into the water but possess inferior spatial resolution than higher-frequency ADCPs.
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 observe Banjul's coastal currents?