How can we measure the coastal currents of Asmara?

1. Where is Asmara?

Asmara is the capital of Eritrea. Though not a sea-facing city as such, it is fairly proximate to the Red Sea coastline. The Red Sea coastline of Eritrea is unique in itself. The Red Sea off its coast is one of the hotspots of marine biodiversity.

The coasts along the general vicinity of Asmara are both sandy and rocky shores. The adjoining Red Sea is a semi-enclosed sea and part of the vast Indian Ocean system. It is noted for its high salinity and warm waters. The communities along the coast have an ages-long connection with the sea. For many centuries, fishing and maritime trade were very significant economic activities.

It also encompasses the regional culture, through which the various influences of a plethora of cultures and trade routes may be evaluated vis-à-vis the architecture along its coastline and also with the practice the locals perform. The history that ebbed and flowed down the coastline, from times ancient when sailors rode the oceans to the now functioning modernist ports, remains relevant in the Red Sea coastline off the shores of Asmara.

2. Current situation along the coast off Asmara?

Different elements prevail in the pattern of coastal currents around Asmara. It is greatly concerned with the large-scale ocean circulation patterns in the Red Sea. Locally, the current regime is influenced by the fact that the Red Sea exchanges water masses with the Indian Ocean. There is also an important impact exerted by tidal forces: the regular rise and fall of the tides can cause fluctuations in the coastal current patterns.

Another important determinant of these is wind patterns. Seasonal winds, for example, monsoons, drive the surface waters onward, often altering both the direction and speed of coastal currents. In addition, bottom topography around the coastline affects the flow of water: various ridges and canyons located on the seabed are able to reroute currents and form complicated flow conditions nearshore.

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

Surface Drift Buoy Method: Buoys are let into the water surface and allow drifting by the currents. Observation of these buoys-actually with satellite-based tracking systems or any other method for positioning, results in information concerning the direction and speed of surface currents. The drawback here is that the use of this particular technique is bound only to the surface layer of currents and perhaps is not completely explanatory about the characteristics at different layers of depth.

Anchor Boat Method: The boat is taken to the waters off the coastline and anchored while the instruments record the flow of water at depth intervals around the boat. Although this can give very detailed information on the currents around the immediate vicinity of the boat, it suffers a number of setbacks in area coverage and sometimes the movement of the boat due to waves and other factors.

Acoustic Doppler Current Profiler(ADCP) Method: It is a more advanced and efficient means of current measurement. ADCP depends on measuring the Doppler shift in the frequency of sound waves emitted and returned by it to calculate the water velocity at various depths simultaneously. It can be deployed in various ways, like on boats, at the seabed (bottom-mounted), or attached to buoys. The ADCP would, therefore, be the instrument of choice to study coastal currents near Asmara because of its capability to provide detailed, continuous data on current profiles right from the surface to significant depths.

4. How do Doppler Principle Using ADCPs work?

ADCPs work on the principle of the Doppler effect. They transmit sound pulses into the water at a known frequency. When these sound waves interact with moving particles in the water, such as suspended sediments or small organisms, the frequency of the reflected sound waves shifts through the Doppler effect. The shift in frequency is directly proportional to the velocity of the water carrying those particles.

Each of these is used for acquiring a three-dimensional profile (commonly horizontal and vertical components) of the current flow rate based on its measurement of a shift in the frequency of returned signals from incoming flow from multiple axes and over many levels in the water. Basically, it follows that more detailed current profiling that describes water motion at any single given point at each distinct depth may become obtainable for lengths along the coasts around Asmara with multi-beams ADCP in mind for getting current structuring information in depth.

5. What shall be required in order to attain a good-quality measurement of currents in the coastal area of Asmara?

The characteristics of equipment necessary for the measurement of high-quality coastal currents at Asmara include: The materials from which the equipment is made should be strong enough to withstand quite a considerable length of time in the unfriendly marine environment. It also should be resistant to corrosion by saltwater, the mechanical action by waves, and temperature fluctuations.

The equipment should be compact in size, which can be easily deployed in different environments, such as on boats, the seabed, or buoys. It is light, and this again is advantageous when it comes to handling and transport. It will have low power consumption, making its operation last longer without any need for frequent replacement of batteries or power supplies on a large scale. It must be cost-effective for large-scale measurement.

The perfect material for ADCPs casing is titanium alloy. It has high strength, which allows it to bear the mechanical forces in the sea. It resists seawater corrosion at a very high level, ensuring longevity of the equipment. Its relatively low density compared to other metals makes it suitable for applications where weight is of concern, in line with the requirements for coastal current measurement devices.

6. Selection of Apparatus for Measuring Current and How?

Based on the Usage Purpose

• Ship -borne ADCP: ADCPS are particularly applicable when currents have to be recorded whilst a ship is under motion. From it records of the constant data of current will be reflected as movement of the ships alongside the shore. It also aids various research vessels, including naval ones, during a survey.
• Bottom - mounted ADCP: Suitable for long-term, stable measurements on the seabed at a fixed location. Provides a lot of detail on flow patterns in the same area for a long duration and is useful to understand the hydrodynamic conditions of the locale.
• Buoy-mounted ADCP: This is good for measuring surface and near-surface currents if the buoy is deployed in the coastal waters. It helps in the study of interaction between atmosphere and surface currents of the ocean.

Based on Operating Frequency:

• The 600kHz ADCP would be of much use in fairly shallow coastal waters, say up to 70m in depth, since this gives good current measurements in this range.
• In waters between 70 and 110m deep, the appropriate one would be the 300kHz ADCP because it can penetrate the water column with much greater efficiency and get more reliable data.
• In waters as deep as 1000m, such as some parts of the Red Sea near Asmara, the most appropriate will be the measurement of currents throughout the water column using a 75kHz ADCP.

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.