How do we measure Qandala coastal currents?

1. Where is Qandala?

Qandala is a coastal town in Somalia and is situated on the Gulf of Aden, hence an important point along the Somali coastline.

The town is bounded by the warm saline waters of the Gulf of Aden. The coastal area comprises a mix of sandy beaches and rocky outcrops. The sea is filled with all forms of fish species, shellfish, and in some areas coral reefs. There has been a very long connection with the sea within the local community. For them, fishing was one of the most essential aspects of earning their livelihoods, and thus, the town derives much of its history from maritime-related activities such as trade and fishing expeditions.

The surroundings are arid, and the sea is the main feature in this part of the region that influences local culture and economy. While the port area may perhaps not be on the same large scale as many of the major international ports, it nonetheless remains very important to the local and regional commerce and allows the transportation of agricultural produce and livestock across the waters.

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

In general, the coastal currents of the Qandala area are predetermined by several factors. First of all, it is necessary to mention the large-scale ocean circulation pattern in the Gulf of Aden and its connection with the open Indian Ocean. The exchange of water masses across the straits and channels in this area affects the local current regime.

The most influencing factor is that of tidal currents. Due to the regular rise and fall of the tides, the direction and speed of the coastal currents vary. While during high tide the water may move more energetically over larger parts of the shore area, during low tide flow patterns may alter and some parts may be left dry.

Again, the wind patterns have a great influence. The monsoon winds, as well as other seasonal winds, are capable of driving the surface waters and bringing changes in the direction and speed of the current. In addition, bottom topography off the Qandala coast plays an important part. Features underwater, such as ridges, canyons, and shoals, may act to deflect or disrupt the flow of the currents, creating complex and localized flow patterns close to shore.

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

• Surface Drift Buoy Method: Buoys are allowed to drift freely on the water surface with the currents. The path traveled by such buoys can be monitored, often by satellite-based tracking systems or other positioning techniques, to obtain information on the surface current direction and speed. However, this technique applies only to the surface layer and is not suitable to obtain information about currents at other depths.
• Anchor Boat Method: A boat is anchored in coastal waters, and instruments measure the flow of water at higher and lower depths around the boat. Although it can deliver much more detailed information about the currents in the closer vicinity of the boat, this method has disadvantages concerning area coverage and might suffer from movements of the boat itself due to waves and other reasons.
• Acoustic Doppler Current Profiler (ADCP) Method: ADCP current meter is an advanced and efficient method of measurement of currents. It uses sound waves to measure the velocity of water at multiple depths simultaneously. It can be deployed in different ways: on boats, on the seabed (bottom-mounted), or attached to buoys. This is because the ADCP current profiler gives a rather specific and continuous record of current profiles right from the surface to significant depths, which is important in studying coastal currents at Qandala.

4. How do ADCPs using the Doppler principle work?

ADCPs work on the principle of the Doppler effect. They send out sound pulses at a known frequency into the water. As these sound waves encounter moving particles in the water-suspended sediment or small organisms, for example-the frequency of the returning sound waves shifts, in accordance with the Doppler effect. This shift in frequency is proportional to the velocity of the water carrying the particles.

By measuring a frequency shift of reflected sound waves coming from different directions and at varied depths, an ADCP flow meter is able to calculate the multiplex-dimensional-almost always horizontal and vertical-components of water flow velocity. One ADCP meter uses multiple numbers of acoustic beams in order to have a comprehensive view of the structure of the currents. This thus allows for the construction of detailed current profiles showing how the water moves at different depths along the coast of Qandala.

5. What is necessary to have high-quality measurements of Qandala coastal currents?

For high-quality measurement of the coastal currents near Qandala, the equipment has to possess specific characteristics. First of all, the materials of the equipment have to be reliable to survive the harsh conditions of the sea. It has to be resistant to saltwater corrosion, wave-generated mechanical tension, and temperature variation.

The size of equipment should be compact to allow deployment and installation processes with ease in various environments like on boats, on the seabed, or attached to buoys. A lightweight design helps in easy maneuvering and transporting. Low power consumption is necessary for long-term use without frequent substitution of batteries and a large-scale power supply. Cost effectiveness is also helpful for large-scale measurement.

As for the casings of the ADCPs, titanium alloy is a great material. Titanium alloy is very strong and able to resist mechanical forces in seawater. Also, it resists the salinity effect of seawater very well, and that would keep the equipment long. Also, it has a relatively lower density in contrast to the other metals, so it is good to use it in lighter products, and this justifies its usage in 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: It is suited for long-term and stable measurements at a fixed location on the seabed. It gives in-depth information about the current pattern in a particular area over a long period of time and helps in understanding the local hydrodynamic conditions.
• Buoy-mounted ADCP: Good for measuring surface and near-surface currents when the buoy is deployed in coastal waters. It helps in the study of the interaction between the atmosphere and the ocean surface currents.

Based on Different Frequencies

• For a water depth ranging from shallow, say up to 70m, a frequency of 600kHz gives quite accurate current measurements, while for waters within the 70-110m depth, a 300kHz ADCP provides good penetration through the water column and thus is capable of yielding reliable data.
• For waters as deep as 1000m, like some areas in the Gulf of Aden around Qandala, a 75kHz ADCP is available to measure 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.