How can we measure the coastal currents of Alexandria?

1. Where is Alexandria?

Alexandria is a city of history and culture, surrounded by the northern coast of Egypt along the Mediterranean Sea. Geographically, the city has stood at an important port for over thousands of years, serving as a vital connection between Africa, Europe, and Asia. It geographically lies on a narrow strip of land between the sea and Lake Mariout. The general topography is flat, and the coastline, running miles along the seafront, features on its length everything from sandy beaches and modern marinas to historic coastal landmarks.

Historically speaking, it has a city that is priceless in value. Alexander the Great found it in 331 BC. Alexandria turned out to be an enormous center for learning, trade, and culture of the Ancient World. The pride of the country is the Library of Alexandria, known worldwide in yesteryears for housing volumes of learning from the known world. It is rich in heritage due to various civilizations of Greeks, Romans, Byzantines, and Arabs among others that once stayed in the country. Centuries of fishing and sea trade have been a part of the local economy, so the cuisine of the city is closely connected with the sea, offering plenty of fresh seafood dishes.

The waters off Alexandria represent a part of the Mediterranean ecosystem. In general, the bottom topography of the sea floor varies greatly along the nearshore, with shallows, deeper channels, and reefs underwater. These features are responsible for the unique marine environment that hosts an array of marine life, from colorful fish to sea turtles and dolphins.

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

The coastal currents around Alexandria take the form of quite complicated influences. Wind patterns play a significant role. There are different wind systems in the Mediterranean Sea. The prevailing north-westerly winds will commonly cause the surface waters to move towards the coasts to form onshore currents. These onshore currents can bring in nutrients from the open sea, beneficial for the growth of phytoplankton and other marine organisms. On the one hand, while onshore winds drive water towards the shore, easterly winds push the water opposite to the shore; these result in offshore currents.

The Mediterranean tidal forces are relatively small in comparison with other seas, yet they also have their contribution to the current dynamics. Ebbs and flows of tides interact with currents caused by winds. During high tide, the level of water goes up, and the direction and speed of the current may alter. Then again, the seabed topography of the region is another vital factor. With the irregular ocean floor, consisting of underwater ridges and troughs, this may accelerate or decelerate the currents and even change directions. For example, a narrow underwater channel could constrict the flow of water, increasing the current speed, while a large reef can be a barrier, deflecting the current.

Then there is also the river runoff from the Nile River. Although the Nile itself does not discharge directly into the sea at Alexandria, the freshwater rich in nutrients spills out along the coast. This can generate density-driven currents where the fresher water, less dense than seawater, floats atop the marine waters. These density-driven currents can further interact with the wind- and tide-driven currents, further complicating the flow patterns.

3. How is the coastal water flow of Alexandria observed?

Surface Drifting Buoy Method

The surface drifting buoy method is a straightforward, simple method used to observe surface -level currents of water in the coast. Small buoys that have GPS tracking devices fitted are thrown into the water. Carried by the currents, the movements of the buoys are observed over a period of time. The path of a buoy would be followed, from which the velocity and direction of surface currents are estimated. Limitations - surface winds have a great impact on the journey of buoys, so the actual path of current may be very different. Another drawback of this method is that it gives information only about the surface water and not about the currents of deeper layers.

Anchored Ship Method

An anchored ship may be employed as a fixed platform for current measurements. Current meters are suspended at different depths from the ship. These meters record the velocity and direction of currents at each depth, thus recording a detailed vertical profile of currents at a place. However, it is restricted to the small area it represents. It represents only the vicinity near the ship itself, and even that could be affected by the presence of the ship disturbing the natural movement of the water.

ADCP Method

In recent times, ADCP profiler is preferred for the measurement of coastal currents along the coast off Alexandria. Basically, an ADCP uses sound waves in order to measure water velocity at multiple depths simultaneously. They can be deployed by ships, moored to the seabed, or attached to buoys. Such flexibility allows for greater data collection. ADCPs are less affected by surface-level disturbances like wind and hence can give more accurate measurement of the true current conditions. They can provide a three-dimensional view of the current structure in detailed presentation, having both horizontal and vertical flow components.

4. How do ADCPs using the Doppler principle work?

ADCPs work on the principle of the Doppler effect. The high-frequency sound waves emitted by an ADCP meter into the water reflect off the small particles present in the water like plankton or sediment, even tiny bubbles. The particles scatter these sound waves back towards the ADCP. The frequency of the scattered sound waves received by the ADCP flow meter is different from the frequency of the emitted waves. This frequency shift, known as the Doppler shift, is directly related to the velocity of the particles (and thus the water) relative to the ADCP.

Because most ADCPs commonly have multiple transducer beams, it can compute the velocity components of water in various directions by measuring the Doppler shift in each beam. By doing the vector addition of these velocity components, the three-dimensional velocity is obtained. It helps one understand in detail how water moves in every direction, which is very important to accurately map the coastal currents.

5. What is required for high-quality measurement of Alexandria coastal currents?

Equipment Material Reliability

The casing material of the ADCP current profiler is a factor of importance for high - quality measurements in Alexandria's coastal waters. It is desirable that the casing be made from titanium alloy. Corrosion resistance is excellent in the case of titanium alloy, which will be imperative as this equipment will be exposed to corrosive sea water throughout its life cycle. It is resistant to degradation after prolonged submergence in salinity with no marked effects, which means assurance of the life and reliability of the ADCP. The strength-to-weight ratio is such that the ADCP maintains its structural integrity during strong currents or heavy seas.

Size, Weight, and Power Consumption

The ADCP must be of compact size and lightweight. A compact design makes deployment in a number of settings more feasible. For instance, it can be installed on a small research vessel or buoy more easily. A smaller size minimizes the impact on the flow field being measured. Another important consideration is low power consumption. In this way, the ADCP can operate for extended periods without the need to replace batteries frequently or have sources of external power. This will be particularly important for the long-term deployments that are planned around the coastal areas of Alexandria.

COST-EFFECTIVENESS

The ADCP current meter should be inexpensive enough to enable extensive and wide-scale monitoring of coastal currents at Alexandria. Less expensive ADCP allows other research institutions, environmental monitoring groups, and even local initiatives to purchase equipment for a wider number of participants in data gathering that may be the key to higher understanding of the very complex coastal current dynamics.

6. How to Select Proper Equipment for the Measurement of Current?

Usage Purpose Basis

• Shipborne ADCP: This type of ADCP is mounted on a moving ship. It finds application in carrying out large-scale surveys of the coastal currents. It would go around the coast continuously measuring currents over large areas with a broad view of current patterns. Such kinds of observations become helpful during the initial phase of the research work or study of variation of the same current pattern over a vast geographical area.
• Bottom-mounted ADCP: Bottom-mounted ADCPs are placed on the seabed, while bottom-mounted ADCPs are used for long-term fixed-point monitoring. They can provide continuous data on the currents at a specific depth and location. This is valuable to understand the local hydrodynamics, such as how the currents behave in a particular bay or near a specific underwater feature.
• Buoyant ADCP: These types of ADCP's attached with a floating buoy, buoyant ADCPs could float along the surface currents, thereby suitable to monitor surface as well as current pattern of nearly at the surface. They may be utilized as one way in observing movement change from water lots throughout the process and play a role in study transports of nutrition, pollutants or even marine life.

Based on Water Depth

• For water depths within 70m, a 600kHz ADCP is a suitable option. The higher frequency of the sound waves allows for more detailed measurements in shallower waters. It can provide high - resolution data on the current structure, making it easier to detect small - scale changes in the currents.
• A 300kHz ADCP is appropriate for water depths of about 110m. This gives a balance between the measurement range and the resolution. The ADCP is able to go deeper in the water column than a 600kHz ADCP can while yielding relatively good resolution for mid-depth coastal waters.
• In deeper waters up to 1000m, a 75kHz ADCP is recommended. The lesser frequency of the sound waves can penetrate deeper, although the resolution may be finer compared to higher - frequency ADCPs. This would be fit for measurements into the deeper depths in the Mediterranean Sea near Alexandria.

There are several well - known ADCP brands on the market, including Teledyne RDI, Nortek, and Sontek. However, for those seeking a cost-effective yet high-quality option, the China Sonar PandaADCP is a great choice. Made of all-titanium alloy, it offers excellent durability and performance at an affordable price. It is an economic-class ADCP that provides great value for money. You can find more information about it on their official website: (https://china-sonar.com/).

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