How to quantify Djibouti's coastal currents?

1. Situation of Djibouti

The country of Djibouti is small and thus placed in strategy in the Horn of Africa, bordered by Eritrea on the north; on the west and south, Djibouti is bordered by Ethiopia, whereas on the southeastern side, there is Somalia; thus it has a coastline along the Gulf of Aden/Red Sea.

The coasts of Djibouti are an interesting combination of arid deserts meeting the open sea. The waters of the Gulf of Aden and the Red Sea are warm and saline, with a high level of biodiversity. The coastline features a mix of sandy beaches, rocky cliffs, and small bays. The area is home to a variety of marine life such as colorful coral reefs, numerous fish species, and other sea creatures.

Djibouti is mixed ethnically from a human and cultural point of view, with the people undertaking most activities including fishing, forming part of the coastal economy. It has a strategic port of Djibouti, which remains a vital connection in international shipping and trade between Africa, Asia, and the Middle East. The strategic location of the country has also made it a hub for military activities, with many navies using its ports for refueling and other purposes.

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

The coastal currents near Djibouti are influenced by many factors. First, the large-scale oceanic circulation patterns play an important role. The Red Sea and the Gulf of Aden form part of the general circulation system of the Indian Ocean. Exchange through the narrow straits, such as the Bab-el-Mandeb Strait, influences the local current regime. Tidal forces are strong as well. The regular rise and fall of tides can change direction and speed in coastal currents.

Of important determination is the pattern of the winds. The periodic shift in wind such as monsoon winds may provide impetus to the surface waters for their flow with various directions and speed. Then comes topography into account, wherein relief features lying below water near shores break the normal direction of tides. Several seamounts, channels, and ocean banks may altogether make the structure and flow behavior locally complex closer to coasts.

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

• Surface Drift Buoy Method: The buoys are sent to the surface and left free to drift along with the currents. In this case, buoys would be followed using satellite-based tracking systems or other positioning methods for information on direction and speed of currents in surface layers. Nevertheless, this method will be restricted to the surface layer and may lack detailed information on currents at different levels.
• Anchor Boat Method: A boat is anchored in the water along the coast and instruments record the flow of water at different depths around the boat. This may yield detailed information about currents in the vicinity of the boat but, in general, presents limitations concerning the area that can be covered and also the movement that the boat can have due to waves and other factors.
• Acoustic Doppler Current Profiler (ADCP) Method: Being advanced and efficient, ADCP current meter is another means of measuring current by way of velocity with the use of sound waves. It can give simultaneous values for the velocity of water at a number of depths, and its deployment can be made in several forms: on boats, on the seabed (bottom-mounted), or attached to buoys. The ADCP current profiler technique is preferred in studying the currents of Djibouti for its ability to give continuous and detailed data of current profiles from the surface down to significant depths.

4. How do ADCPs using the Doppler principle work?

The working principle of ADCPs is based on the Doppler principle. They transmit sound pulses at a certain frequency into the water. Provided that the reflected sound waves in the water are interacting with moving particles-as a rule, this would be suspended sediment or small organisms-the frequency of these reflected sound waves will shift in accordance with the Doppler effect. This frequency shift is directly proportional to the velocity of the water carrying the particles.

Measuring the frequency shift of the reflected sound waves coming from different directions and at different depths, ADCPs are capable of calculating the velocity components of the water flow in multiple dimensions-usually horizontal and vertical. An ADCP flow meter uses multiple acoustic beams to obtain a comprehensive understanding of the current structure. In this case, it enables detailed current profiles of how the water is moving at different depths along the Djibouti coast.

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

The equipment to be used for the measurement of coastal currents around Djibouti needs to have particular characteristics. Their materials should be reliable to bear the harsh marine environment. It needs to resist the corrosive action of salt water, the mechanical stresses due to waves, and temperature variations.

The size of the equipment has to be as small as possible for easy deployment and installation either on boats, on the seabed, or on buoys. The lightweight will also be able to ensure that its handling and transport are easily possible. Low power consumption is required in order to run for a long time without the need to change batteries often or a high amount of electric power supply. Cost-effectiveness will enable large-scale measurements.

The very good material for ADCPs casing is titanium alloy. The strength of the titanium alloy is very high, hence making it able to bear the mechanical forces in the sea environment. It's highly resistant to corrosion from seawater, hence ensuring the longevity of the equipment. Its relatively low density compared to other metals makes it suitable for applications where weight is a concern, 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: This is suitable when measurements are needed while the ship is moving. It can provide continuous data on the currents as the ship sails along the coast. It's useful for research vessels and naval ships during surveys.
• Bottom-mounted ADCP: Suited for long-term and steady measurements from a fixed position at the seabed. This would give comprehensive information about current patterns for the same location and over a long duration, helping evaluate the hydrodynamic local conditions in general.
• Buoy-mounted ADCP: Good for the measurement of surface and near-surface currents when the buoy is deployed in the coastal waters. It helps in studying the interaction between the atmosphere and the ocean surface currents.

Based on Different Frequencies

• The 600kHz ADCP would work very well in water to 70m since the depth and current can be quite accurate.
• In waters where the depth lies between 70 and 110m, the use of a 300kHz ADCP would be ideal since it has shown more efficient water column penetration and generally gives very good results.
• In these deeper waters to 1000m, as portions of the Red Sea or Gulf of Aden offshore of Djibouti, it would be desirable to use a 75kHz ADCP that can measure the 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.