How can we measure the coastal currents of Beirut

1. Where is Beirut?

Beirut, Lebanon's capital, lies on the eastern shore of the Mediterranean Sea. That positions it to become a crucial location for trade, culture, and politics in the Middle East since thousands of years ago.

From being steeped in a rich historical tapestry, Beirut has been witness to the rise and fall of many a civilization. The Phoenicians, who were great seafarers and traders, saw the Romans, Byzantines, and Ottomans in turn leave their mark on its architecture and cultural heritage. Some of the ancient ruins that stand as reminders of its long-standing history are the Roman Baths and the Phoenician Port.

This can be seen through the architecture that combines old with new. Behind the modern high-rise buildings often stand traditional Lebanese houses with typical stone facades and ornate balconies. The local people are warm, friendly, and hospitable. The population has a mix of different ethnic and religious groups in its makeup. Traditional occupation included fishing whereby local fishermen sail the Mediterranean waters in small boats in search of a catch. The port of Beirut is one busy hub where both commercial shipping and recreational vessels are handled. Adjacent waters are generally much calmer compared to some other coasts, though the topography of the seabed is not as simple and features underwater cliffs and canyons in places.

2. What is the status of coastal currents near Beirut?

Several factors govern the coastal currents near Beirut. First, there are circulation patterns unique to the Mediterranean Sea, and local currents near Beirut form part of this bigger system. Tidal forces in the Mediterranean may be tiny compared to those in other seas; however, they play a role in water movement. The semidiurnal tide is the dominant tidal component; that means there are two highs and two lows of water each day-the difference between them, however, is less drastic compared to that in the English Channel.

Coastal wind seems to play a significant force behind the movement of coastal current. The dominance of north-west prevailing winds makes up the summer Etesian and can be strong enough at times to bank surface waters ashore. These might produce a coastal jet, which is a narrow strip of offshore water with higher speeds of flow. In winter, though, the winds can be less stable and there might be frequent violent storms in various directions and with variable strengths. Convergence and divergence of currents occur because the coastline has several bays and headlands, as well as because of submarine topography. The underwater cliffs and canyons may also guide the flow of water to result in areas of up - welling and down - welling. Besides, the river water from the nearby Litani River may also be another factor influencing the local current pattern since the fresh river water has different density characteristics compared with seawater.

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

Surface Drifting Buoy Method

Surface drifting buoys are among the most readily available means for observing surface currents. These come fitted with either GPS or satellite-based tracking systems. Once thrown into the water, they drift with the currents at the surface. By monitoring where they drift over time, scientists can infer the speed and direction of the surface waters. However, this technique gives information about the upper few meters of the water column and may not be representative of the deeper currents.

Moored Ship Method

In the moored ship technique, a ship is anchored at a place, and current-measuring instruments are deployed. These instruments may include electromagnetic current meters that depend on the principle of the electromagnetic field created by the flow of water. This allows the detailed vertical profile of the current velocity and direction at a given depth. However, this is essentially a single-point measurement and could be influenced by the presence of the ship that might disturb the natural water movement.

Acoustic Doppler Current Profiler (ADCP) Method

The ADCP has now assumed prime importance as an instrument to measure coastal currents. They can measure the velocity profile of the entire water column from the surface to the seabed. ADCPs use sound waves to non-intrusively measure the water flow. By emitting acoustic pulses and measuring the Doppler shift of the reflected signals from suspended particles in the water, they can calculate the current velocity at different depths. This provides a full view of the current structure and thus is quite appropriate for studying the complicated coastal current patterns around Beirut.

4. How do ADCPs using the principle of the Doppler work?

ADCPs are based on the principle of the Doppler effect. They send high-frequency acoustic pulses into the water. When these pulses hit small particles in the water, like sediment, plankton, or bubbles, the particles reflect the acoustic signals back to the ADCP. If the particles are moving with the water current, the frequency of the reflected signal is shifted compared with the transmitted signal. This shift in frequency is called the Doppler shift and is proportional to the velocity of the particles relative to the ADCP. By measuring the Doppler shift at different depths in the water column, the ADCP is able to calculate the velocity of the water at those depths. The ADCP is equipped with several transducers for measuring the different components of the velocity in a variety of directions, thus providing the three-dimensional velocity vector of the water flow.

5. What's needed for high-quality measurement of Beirut coastal currents?

Equipment Material Reliability

The materials of equipment should be highly reliable to ensure high-quality measurement of the coastal currents near Beirut. Its casing should be made up of a material able to withstand the harsh conditions in the sea. A titanium alloy is ideal because it has a very strong strength that could support the impact that strong water currents and possible collisions with floating sea debris may impose. With a low elastic modulus, the flexibility resists mechanical damage, while excellent corrosion resistance means deployment in seawater on-site for a long period of time with minimal degradation.

Compact Size, Light Weight, Low Power Consumption, and Low Cost

A compact, lightweight ADCP would be easier to install and operate on a small fishing boat converted for research, attached to a buoy, or even on the seabed. It disturbs the natural water flow less, therefore providing more accurate measurement. Low power consumption is necessary for long-term autonomous monitoring, considering the battery-powered systems. A low-cost ADCP will enable large-scale deployment, necessary for comprehensively understanding the complex current patterns around the coast of Beirut.

6. Comment Choisir le Bon Equipe pour Mesurer les Courants?

Following the Usage

• Shipborne ADCP: Real current data on board a ship can be availed with its track and hence is very useful for oceanographic surveys, studies of the general circulation in the Mediterranean off Beirut, and for shipping route optimization.
• Bottom-mounted ADCP: suitable for long-term fixed-point monitoring of the currents at the seabed, which will be useful to study the long-term trends in the coastal currents, for instance, the influence of climate change on the local marine ecosystem.
• Buoy ADCP: Suitable for monitoring the surface-level currents over a large area while it drifts with the water, this will help understand the spatial variability of the surface currents and how the coastal waters interact with the open Mediterranean Sea.

Water Depth-Based

• 600kHz ADCP: Shall be used in approximately water depths up to 70m, for high-resolution measurements in the relatively shallow waters close to shore off Beirut.
• 300kHz ADCP: For water depths roughly about 110 m, useful for areas of intermediate water depths in the region.
• 75kHz ADCP: For deep water applications, it can measure currents to a depth of 1000m, which would be applicable in studying the deeper-layer currents in the Mediterranean Sea off Beirut.

There exist a lot of different ADCPs from different manufacturers on the market. Some well-known international brands provide the high-end solutions, although several new emerging manufacturers are also capable of providing cost-effective solutions without compromising the quality. While choosing an ADCP, one has to consider the demands on the measurement to be performed, including its intended use, water depth, and budget constraints.

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