How do we quantify Accra's coastal currents?

1. Where is Accra?

Accra, the vibrant capital of Ghana, is situated on the southern coast of the country bordering the Gulf of Guinea, a large portion of the Atlantic Ocean. The city is a hub of activity, being the political, economic, and cultural center of the country, with its coastline playing a central role in defining it and its growth. The Accra coast borders the sea, creating a diversified landscape with sandy beaches, rocky outcrops, and modern port installations.

The city is inhabited by a diversified multicultural texture of ethnic communities, which constitute the city's unique cultural heritage. Fishing has been a major profession for a long time for the majority of Accra's residents. Local fishermen, who fish in wooden canoes as well as modern vessels, venture into the sea daily to catch a variety of fish, ranging from mackerel, sardines, and shrimp. These daily caught seafood are then sold in the local markets, which are vibrant centers of activity and an important part of the local economy. Besides fishing, the Accra port is a significant trade route, trading both inward and outward, and connecting Ghana with the rest of the world.

In proximity to Accra, the surrounding water is surrounded by a thriving marine ecosystem. The Gulf of Guinea tropical waters offer a diverse array of marine organisms. Coral reefs, in spite of all the dangers because of human impact and climatic shifts, continue to be homes to dozens of tropical fish species, multi-colored corals, and other invertebrates. Even larger marine mammals like whales and dolphins are also reported to swim by these waters while migrating. Offshore topography off Accra is also a complicated one with underwater ridges, canyons, and seamounts that determine local coastal currents as well as marine species distribution.

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

The coastal currents off Accra are influenced by a mixture of large - scale oceanic and local processes. The Guinea Current, a major warm - water current moving westward along the African coast, has a profound influence on the region. This flow carries with it warm, nutrient-laden water, which not only maintains a healthy marine community but also determines the direction and speed of local coastal currents. The strength and position of the Guinea Current will vary seasonally, leading to water temperature and salinity changes as well as variations in the general current pattern off Accra.

Seasonal wind patterns greatly affect the coastal currents. The onshore strong winds are blown during the rainy season, pushing surface waters onshore. This causes upwelling of some coasts because the surface water is pushed off and substituted by cold, nutrient-filled waters from deeper depths that migrate towards the surface. Upwelling events contribute significantly to the marine ecosystem regionally, as the phytoplankton production is fueled, on which the marine food chain is founded. Conversely, during the dry season, the offshore winds dominate, causing the offshore surface water flow away from the shore and altering the velocity and direction of the current.

The outstanding Accra coastline geomorphology in the form of headlands, inlets, and bays also alters the flow of currents. When the water moves along the coast, it encounters these land structures, which serve the purpose of concentrating the currents by making them converge, diverge, or take a sharp change in direction. Additionally, that there exist rivers flowing into the sea near Accra, such as the Densu River, is capable of carrying immense quantities of fresh water into the coastal water. This fresh - water flow impacts the density and salinity of the water, thereby determining the movement of the currents and generating complex mixing patterns.

3. How to observe the coastal water flow of Accra?

There are several methods for observing the movement of coastal waters of Accra. The surface drifting buoy method is a very traditional method. GPS tracking units are fixed on special buoys, and they are let out in the ocean. The buoys are carried by the surface currents, and their path is followed with time. By tracking the position of the buoys at regular intervals, researchers can compute the direction and speed of the surface-level currents. But this method only provides information about the currents at the surface and may not be representative of the flow at lower elevations.

The anchored boat method involves mooring a boat at a fixed point and using current-measuring instruments on board. Measuring instruments such as electromagnetic current meters are tossed from the boat to measure the velocity of the water at different depths. Even though this method can provide more accurate data about the current characteristics in the immediate surroundings of the boat, there are certain inadequacies associated with this method. Its operational area is limited, and the presence of the boat can affect the natural flow of the water and produce misleading data.

Acoustic Doppler Current Profiler (ADCP) method has become a more advanced and efficient tool for coastal current measurement. ADCPs are able to measure velocity in water at multiple depths simultaneously, providing an accurate description of the water flow structure from surface to ocean floor. This renders them especially effective for the study of the complex and dynamic Accra coastal currents.

4. How do ADCPs based on the Doppler principle operate?

ADCPs operate on the Doppler principle. They send acoustic pulses into the water column. As these pulses interact with suspended particles in water, e.g., sediment, plankton, or little animals, part of the pulse is echoed back to the ADCP current meter. The backscattered signal frequency shifts based on the movement of the particles (and therefore water) relative to the ADCP.

By analyzing these changes in frequency, the ADCP flow meter can calculate the water velocity at different depths. Most ADCPs employ multiple acoustic beams, typically arranged in a fan - shaped array. The information from each of the beams are then combined in order to determine the three - dimensional velocity vector of the water, providing detailed information on speed and direction of the current at many points along the water column. This non - invasive measurement method permits continuous as well as exact observation of water flow without accessing the water through sampling.

5. What is necessary for high-quality measurement of Accra coastal currents?

For accurate measurement of Accra's coastal currents, there are certain key requirements to be met in the measurement equipment. Material reliability is essential. The marine environment near Accra is aggressive, with saltwater, high currents, and exposure to sun, rain, and alternating temperatures. The equipment must be corrosion-resistant, mechanically robust, and tolerant of extreme environment conditions.

Equipment should also be light and compact. A lighter and smaller system is easier to transport, deploy, and retrieve, especially in the densely populated coastal area of Accra. Low power usage is desirable since it enables longer - term deployments with less need to replace or recharge batteries. Cost - effectiveness is also crucial since extensive measurements may be required to fully grasp the complex current patterns within the zone.

In the ADCPs' case, the casing material is a concern. A titanium alloy is an ideal material to be used in the casing. It has excellent corrosion resistance, which is vital in Accra's salty-enriched environment. It has a very high strength - to - weight ratio, ensuring that the ADCP can handle the loads from the strong coastal currents without being unreasonably hard to handle or deploy. Also, titanium alloy has a satisfactory fatigue life, allowing the ADCP current profiler to operate satisfactorily for a long period and provide accurate readings.

6. Selection of right equipment for current measurement?

The choice of suitable equipment for measuring current in Accra depends on the specific application. While measuring when moving along with a boat, a ship-mounted ADCP is the most appropriate. It is capable of measuring currents in real time as the ship traverses through different areas, providing real-time data regarding water flow, helpful in navigation, fishing, and sea operations. In case measurements on the sea floor for an extended period of time are necessary, a bottom-mounted ADCP would be preferable. This ADCP meter may be bottom-mounted to measure currents over a prolonged period, which can be utilized in underwater infrastructure planning and ecological surveys. 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 continuous data on the surface and near - surface current conditions.

The frequency of the ADCP is also of utmost concern. An ADCP of 600kHz would be appropriate for water depths of not more than 70 meters, which is appropriate for shallow coastal waters around Accra. An ADCP of 300kHz would be suitable for depths of up to 110 meters, while an ADCP of 75kHz is suitable for deep waters, 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 flow meter instrument brands and models.