How would we measure coastal currents of Assab?

1. Where is Assab?

Assab is a coastal city located in Eritrea at the southern tip of the Red Sea. Geographically, it lies in an area that has been crucial for regional maritime activities. It is bordered by arid landscapes with the vibrant blue of the Red Sea.

The Red Sea in this region is narrow, elongated, and part of a larger, more complicated oceanic structure. It contains warm waters that are very salty. The coasts around Assab comprise sandy and rocky shores. It is rich in sea life, consisting of different types of fish species and coral reefs.

On one hand, from the human and cultural point of view, Assab was since ancient times an essential place of trade and navigation. The local population lives in close connection with the sea through fishing and port-related activities. Just due to its historical role of a trading center, the multiplicity of cultural influences can be reflected in the architecture present within the city. The port of Assab has served as a very important link in goods transportation between Africa and the Arabian Peninsula, as well as other areas.

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

Variations in the coastal currents near Assab depend on several aspects. The large-scale circulation patterns of the Red Sea have the most significant influence. The Red Sea is interconnected with the Indian Ocean by the Strait of Bab -el-Mandeb, which already means that both inflow and outflow of water masses can influence the local current regime. Tidal movements are also influential in this regard. The regular ebb and flow of tides may cause changes both in the direction and speed of the coastal currents.

The other important factors are the wind patterns. Seasonal winds, such as monsoons, would push the surface waters and, in turn, would result in a deflection in current direction and speed. Coastal seabed topography may also affect currents. Submerged features including ridges, canyons, and shoals will result in partial deflection or channeling of the water, with resultant complicated flow conditions near shore.

3. How to Observe the Coastal Water Flow of Assab?

• Surface Drift Buoy Method: This method involves the deployment of buoys on the water surface, which are allowed to drift under the mercy of currents. The displacements of these buoys can then be monitored by satellite-based or other positioning methods to retrieve information on the direction and speed of the surface currents. This technique is good enough to provide data for the surface layer only; possibly not much information about currents at different depths.
• Anchor Boat Method: This involves anchoring the boat in any of the coastal waters while taking instruments into operation, by which it has to record and measure the water flow around a radius and also at various depths. Although this method may supply one with further minute details regarding currents around its boat, it cannot scan a vast coverage area. As the boat might start oscillations upon wave and wind interactions, that creates problems too.
• ADCP Method: More refined and efficient for current measurement is the deployment of an Acoustic Doppler Current Profiler (ADCP). It works by using sound waves for simultaneous velocity measurements of water at multiple depths. These instruments can be deployed in a variety of ways, such as on boats or on the seabed bottom-mounted, or even attached to buoys. Therefore, it remains one of the desirable tools to measure coastal currents around Assab with much detail from surface to high depth levels with high resolutions.

4. How do Doppler principle based ADCPs work?

The working principle of ADCPs is based on the Doppler effect. A known frequency sound pulse is sent into the water by them. Due to the Doppler effect, the frequency of the returning sound waves will shift as those sound waves encounter and scatter off moving particles in the water, such as suspended sediments or small organisms. This shift in frequency is proportional to the velocity of the water carrying those particles.

The measurements of frequency shift of the reflected sound waves from different directions and at different depths allow ADCPs to calculate a number of multi-dimensional velocity components-usually horizontal and vertical-of the water flow. An ADCP current meter uses several acoustic beams in order to have a comprehensive understanding of the current structure. This allows detailed current profiles to be constructed, showing how the water is moving at different depths along the coast off Assab.

5. What is needed for high-quality measurement of Assab coastal currents?

For high-quality measurement of the coastal currents around Assab, the equipment has to possess some characteristics: The materials of this equipment must be reliable enough to stand harsh marine conditions. It has to withstand seawater salinity, mechanical tension caused by waves, and temperature changes.

The equipment should be of small size for easy deployment and installation in various configurations, such as on boats, on the seabed, or on buoys. The design should also be lightweight to enable easy handling and transportation. It should consume low power to enable its operation for a long period without changing batteries or using a large-scale power supply. Cost-effectiveness is also required for large-scale measurement.

In the casings of ADCPs, titanium alloy is a perfect material. Titanium alloy has high strength, which enables it to resist mechanical forces in the marine environment. It's highly resistant to corrosion from seawater, ensuring the longevity of the equipment. Relatively low density compared to other metals makes it suitable for application in places where weight concerns are necessary, which is in line with the requirements for coastal current measurement devices.

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

Based on the Use Purpose

• Ship-borne ADCP: It is fit when the measurements are to be made while the ship is in motion. It will be able to provide continuous data about the currents while the ship is sailing along the coast. It is useful in the case of research vessels and naval ships during surveys.
• Bottom-mounted ADCP: Suitable for long-term and stable measurements at a fixed location on the seabed. It provides detailed information about the current patterns in a specific area over a long period and is useful for understanding the local hydrodynamic conditions.
• Buoy-mounted ADCP: Good for the measurement of surface and near-surface currents when deployed in the coastal waters. It will be helpful in the study of atmosphere-ocean interaction with surface current.

Based on Different Frequencies

• The 600kHz ADCP is adequate for the relatively shallow coastal waters with up to 70m depth, whereby current measurements at this range have been accurate.
• The ADCP of 300kHz shall be quite efficient for water from 70 to 110 meters in depth, while the water column would be effectively penetrated to obtain reliable data.
• For deeper waters, like the areas of the Red Sea off Assab with water depth of up to 1000m, the use of a 75kHz ADCP will be best to measure currents through the full 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.