1. Where is St Louis?
St Louis, don't get this confused with the city of St Louis in America, is an interesting coastal city in Senegal. St Louis is located on the Senegal River estuary, where it leads into the Atlantic Ocean. St Louis has a very specific geographical position. This is a combination of history and nature, with the area covered by the vast expanse of the sea, the serpent-like river, and a network of small lagoons and canals.
Its landscape is quite flat, with sandy beaches along the coastline. These beaches, besides being visually appealing, add to the region's ecosystem since they provide living habitats for hundreds of marine and bird species. St Louis consists of two sections: the older island city that was the first settlement and the mainland section. The city, which is an island and has colonial-period architecture and small streets, is a UNESCO World Heritage, representing the importance of the city as a colonial trading post.
St Louis is ethnically diverse, consisting of ethnic groups including Wolof, Pulaar, and Serer. Its culture is diverse, drawing upon its coastal location and its historical connections with Europe, Africa, and the Middle East. Tourism and fishing are major St Louis economic activities. The St Louis port, albeit comparatively small compared to modern-day ports, continues to be a center of local trade, especially that of commodities brought by the Senegal River route. The adjacent waters lie on the Senegal River estuary, a richly biodiverse and energetic habitat that supports a range of fish, crustaceans, and other creatures. This estuary is also a key stopover for migratory birds, and St Louis is therefore an important place for conservation of biodiversity.
2. How are the coastal currents around St Louis?
The coastal currents around St Louis are influenced by a combination of factors that create a pattern of flow that is complex and dynamic in nature. Tides significantly influence these currents. The tides are semi-diurnal with two high waters and two low waters each day. The tidal range is not constant, and at high water, water sweeps into the estuary and along the coast, flooding the area and reversing the direction and speed of the currents. When low tide brings the tide out, the water zooms back to the ocean, this time reversing the current in some areas. The tidal cycles have a direct impact on neighborhood fishing operations since fishermen have to schedule their venture in relation to the tide for optimum yield.
The Senegal River also plays an important role in shaping coastal currents. The river's outflow, especially during the rains when the water volume is much greater, contributes a large amount of freshwater to the coastal waters. The freshwater is of different density and characteristics compared to the seawater, and it creates a surface layer that affects the mixing of water masses. The river flow may push the seawater out of the river mouth, and it affects the direction and strength of the coastal currents. Additionally, the sediment carried by the river can deposit in the estuary and on the coast, continuing to change the flow patterns.
Wind is another force that affects the coastal currents near St Louis. The dominant winds over the region, particularly the trade winds, can drive surface-level currents long distances. High-speed winds over the sea can create waves and propel the water, deflecting the natural direction of the coastal currents. Local wind circulation, depending on the topography of the land and the geometry of the shore, can also cause short-term changes in the direction and speed of the current, adding to the complexity of the current system.
3. How to observe the coastal water flow of St Louis?
Surface Drift Buoy Method
Surface drift buoy method is a traditional method by which coastal water flow is measured. In this method, specially made buoys are dropped at sea. These buoys have tracking equipment, i.e., GPS receivers with them. As the buoys pass over the surface current, the GPS equipment records their positions at periodic intervals. With the movement of the buoys over time, scientists can estimate the direction and the speed of the surface-level current. There are some disadvantages to this process. Buoys are wind-sensitive and may be redirected from the true course of the current. It provides information about the surface current only and does not give any idea of the currents at different depths.
Anchor Moored Ship Method
The anchor moored ship method involves mooring a ship at a specific location in the waters off the coast of St Louis. From the ship, a number of current - measuring instruments are lowered into the sea. These instruments can be mechanical current meters, which are used to measure the speed and direction of the current by the rotation of a propeller - type device. By taking the measurements at different depths, the scientists can maintain a profile of the current at the given position. While this method allows exact measurement at different depths, it is limited to the area of the anchored vessel. The ship itself may also hamper free current flow and lead to inaccurate measurements.
Acoustic Doppler Current Profiler (ADCP) Method
Acoustic Doppler Current Profiler (ADCP) is becoming increasingly more efficient and advanced in the sense of keeping a track on coastal water movement. ADCPs use sound wave principle in identifying the water current speed at varying depths. ADCPs introduce acoustic pulses into the column of water, and acoustic pulses echo from suspended elements of water like sediment or plankton. By analyzing the Doppler shift of the backscattered sound, the ADCP profiler can find the velocity and direction of water flow. It can provide a complete description of the current profile, from surface to near bottom of the water body, with no physical contact with the water. ADCPs are not so vulnerable to external influences like wind as drift buoys are at the surface, which is why ADCPs make for a reliable option where exact measurement of current needs to be had.
4. What are the functioning principles behind Doppler-type ADCPs?
ADCPs operate according to the principle of the Doppler effect. Transducers are provided within the ADCP for releasing acoustic waves in water. Such sound waves move into water and hit moving particles that accompany the current. As the particles approach the ADCP meter, the frequency of the return sound waves increases and decreases when receding. The resulting change in frequency, referred to as the Doppler shift, changes in a directly proportional relationship with the speed of the movement of water in the direction of the movement of the sound wave.
Most ADCPs for measurement of the three-dimensional water velocity employ multiple acoustic beams. Typically, four or more beams are separated by different angles. Taking the Doppler shift for every one of the beams, the ADCP flow meter can calculate the horizontal and vertical components of the velocity. The data from all beams is averaged to determine the total current speed and direction at multiple depths in the water column. Contemporary 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 enable accurate measurements even when the instrument is tilted or moving.
5. What's needed for high-quality measurement of St Louis coastal currents?
To obtain reliable measurement of the coastal currents near St Louis, several conditions must be met. To start with, the measurement equipment must be highly reliable within the corrosive marine environment. Near St Louis, the water is subject to saltwater, strong tides, and harsh seas, meaning that the construction materials of the equipment must be corrosion resistant, strong, and mechanically durable.
Equipment size is also an important consideration. Smaller equipment can be more versatile and applied in more varied locations, for example, in sites with complex coastal geometry or limited access. A smaller footprint also reduces the potential for the equipment to disrupt the natural flow of currents, offering more accurate measurements. Lightweight equipment is preferred as it is easier to deploy and recover, especially when working from small craft or in remote coastal locations.
Low power consumption is significant as most of today's - measurement activities will involve leaving the equipment deployed for extended periods, often in areas where there is not easy access to power sources. Low - power - consumption devices can survive for longer durations before they need to undergo many battery replacements or recharging. Cost - effectiveness is also important, especially to large - scale measurement activities. Devices with good performance at reasonable prices allow individuals to conduct more extensive data collection, thus researchers can be in a position to learn more about coastal current patterns in depth.
For ADCPs, casing material is extremely important. Titanium alloy is a suitable choice for ADCP casings installed in St Louis's coastal waters. Titanium alloy is highly resistant to corrosion and plays a vital role in withstanding corrosive action of saltwater for long periods. It is also extremely light and resilient, providing cover for the ADCP current meter components within but is very easy to manage and install. Titanium alloy is also very resistant to fatigue, which means that it is capable of enduring alternating stress and strain without any notable degradation and this essentially translates into the longevity of operation and integrity of ADCP under the dynamic coastal environment.
6. How to Select the appropriate equipment for current measurement?
Selecting the appropriate equipment for present measurement in St Louis depends on many factors, principally the objective and the environment of the area being measured.
ADCPs by Mounting
- Ship-mounted ADCP: It is mounted on a moving ship. It is ideal for conducting large-scale measurements of coastal currents. When the vessel travels along the coastal region, ADCP mounted on ships can observe in a wide sector and create an overall impression of the pattern of currents. The instrument is ideal to use for planning marine navigation routes, large oceanography, and monitoring changes in environmental conditions along the coast.
- Bottom-mounted ADCP: Bottom - mounted ADCPs are mounted on the seafloor. They are employed to measure long - term current regimes at a station. The devices can provide continuous information for extended durations, which is significant in studying the season and long - term change of coastal currents, and the observation of the influence of human activities on the marine environment.
- Buoy - mounted ADCP: Buoy - mounted ADCPs are installed on floating buoys. Buoy - mounted ADCPs are most suitable where installing other types of ADCPs is not viable, such as in open - water areas or areas with too strong currents, which can prove to be harmful for bottom - mounted instruments. Buoy-mounted ADCPs can be drifted with the current, providing real-time data on the dynamic changes in the current, helpful for such applications as tracking oil spills, fisheries management, and short-term oceanography studies.
Choosing the Appropriate Frequency
Frequency of the ADCP is also a very significant factor to be considered during selection. Different frequencies are suitable for different water depths. A 600kHz ADCP has an application in water depths of up to around 70m. It yields relatively high-resolution measurements for near-shore areas and is an appropriate choice for the inshore areas, e.g., in the estuary and along the beaches of St Louis. A 300kHz ADCP can be employed in water depths of around 110m and represents a trade-off in penetration depth vs resolution. For greater depths, to 1000m, a 75kHz ADCP is suggested. Lower - frequency ADCPs penetrate further into the water but possess lower spatial resolution than higher - frequency ones.
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 do we measure St Louis's coastal currents?