How can we measure the coastal currents of Berbera?

1. Where is Berbera?

Berbera is a port city in the Somaliland region of Somalia. It fronts the Gulf of Aden, an important body of water that connects the Red Sea with the Arabian Sea and beyond.

The interesting feature about the coastal environment of this city is the meeting of sandy beaches with arid land extending further inland. The waters of the Gulf of Aden off Berbera are warm and highly saline. Marine biodiversity is so immense that a variety of fish species and other sea organisms exist in this part of the land. The presence of coral reefs along certain coasts adds to the ecological importance of the region.

From a human point of view, Berbera boasts a rich history related to trade and seafaring. Native people have traditional ties with the sea, including fishing as part of their vital economic activities. The port is an important regional and international trade hub in handling various cargoes such as livestock, minerals, and general cargo.

2. What is the situation of the coastal currents near Berbera?

A number of factors contribute to the nature of coastal currents near Berbera. First, there are the large-scale ocean circulation patterns in the Gulf of Aden and the adjacent Indian Ocean. The inflow and outflow of water masses through the straits and channels in the region affect the local current regime. Another major influence is tidal currents. The regular rise and fall of the tides produce changes in direction and speed of the coastal currents.

Another very important factor is wind patterns. Monsoon winds and other seasonal winds may force the surface waters and change the direction of the current with speed changes. The topography of the seabed at or near the Berbera coast affects the currents, too. Undersea features like ridges, canyons, and shoals can divert or disrupt the flow of the currents, creating complex, localized flow patterns close to the shore.

3. How to Observe the Berbera Coastal Water Flow?

• Surface Drift Buoy Method: This method adopted the deployment of buoys on the water surface, which are allowed to drift with currents. By detecting the movement of these buoys, generally by satellite or other positioning systems, it is possible to get information about the direction and speed of surface currents. In this method, data is mostly obtained for the surface layer, and it is not possible to obtain a full picture of currents at different depths.
• Anchor Boat Method: This involves taking a boat into the coastal waters and anchoring, while measuring the water flow at different depths around it with instruments. Though it can indeed provide detailed information about currents in the vicinity of the boat, this technique has several shortcomings as far as area coverage and disturbances are concerned due to the movements of the boat itself caused by waves and other reasons.
• ADCP technique: The technique is more progressive and efficient than that described above in current measurement. By transmitting sound waves and measuring the velocities of water concurrently at different profundity, one can estimate flow. It has been deployed, so far, on boats, on the sea bed (bottom - mounted), attached to buoys. The ADCP current meter can provide detailed and continuous data on current profiles from the surface to significant depths, which makes it preferred for studying coastal currents near Berbera.

4. How do ADCPs using the Doppler principle work?

Basically, ADCPs rely on the principle of Doppler. They send out sound pulses into the water at some frequency. The sound waves, reflecting off moving particles in water-suspended sediment or small organisms, upon return, shift in frequency due to the Doppler effect. This frequency shift is directly proportional to the velocity of the water carrying those particles.

By measuring the frequency shift of the reflected sound waves from different directions and at various depths, ADCPs can calculate multi-dimensional velocity components of the water flow, usually horizontal and vertical. At least several acoustic beams are used in an ADCP current profiler to get a comprehensive understanding of the current structure. This will, in turn, be able to construct detailed current profiles showing how the water is moving at different depths along the Berbera coast.

5. What is needed for high-quality measurements of the Berbera coastal currents?

For high-quality measurement of coastal currents around Berbera, the equipment needs to be characterised by a number of aspects. The materials of the equipment should be reliable to resist harsh marine conditions. It needs to be able to resist the chemical corrosion of saltwater, the mechanical stress from waves, and temperature variations.

It has to be compact to allow easy deployment and installation in various configurations on boats, underwater on the seabed, and attached to buoys. In that view, low weight is very advantageous for a lightweight design because of easy manipulation and transportation. Low power consumption to be able to provide long-term functioning without frequent changes of the battery or large-scale power supply will also be needed. In any case, this must be a cost-effective way so as to perform wide-range measurements.

On considering ADCP flow meter casings, the material of choice is titanium alloy: Titanium alloy has a high strength that withstands the mechanical forces in a marine environment; it is very resistant to seawater corrosion, hence assuring longevity; it is of rather low density compared to other metals, appropriate for use where weight is to be saved, hence satisfying the current measurement devices over the coasts.

6. How to Choose the right equipment for current measurement?

Based on the Use Purpose

• Ship-borne ADCP: It is applicable if measurements are required while the ship is in motion. It would update the data of currents continuously while the ship navigates along the coast. This could be helpful on research vessels and naval ships on surveys.
• Bottom-mounted ADCP: very suitable for making long-term stable measurements at a fixed location on the seabed; it provides high-resolution information over a long period on the pattern of currents within a small area and is especially useful in studies relating to the hydrodynamic conditions.
• Buoy-mounted ADCP: Good for surface and near-surface current measurement when the buoy is deployed in coastal waters. It helps study the interaction between the atmosphere and ocean surface currents.

Based on Different Frequencies

• The 600kHz ADCP can be good to go for shallow coastal water up to 70m depth since it has current measurements quite accurate in that range.
• Between 70-110 meters of water depth, a 300kHz ADCP should be better as this can get good penetration into the water column and hence get data of reliable quality.
• For deeper waters up to 1000m, as in some parts of the Gulf of Aden near Berbera, a 75kHz ADCP is most suitable for measuring currents throughout the water column.
• There are well-known ADCP brands like Teledyne RDI, Nortek, and Sontek. However, for those looking for a cost-effective option, the China Sonar PandaADCP is a good alternative. It's made of all-titanium alloy material and offers an excellent cost-performance ratio. You can visit their website at https://china-sonar.com/for more information.

Here is a table with some well known ADCP instrument brands and models.