How can we measure the coastal currents of Thessaloniki?

1. Where is Thessaloniki?

Thessaloniki, Greece's second-largest city, is a lively metropolis that fronts onto the Thermaic Gulf, an inlet of the Aegean Sea. It is located at the crossroads of major trade routes by both land and sea and has been, for thousands of years, a very important cultural, economic, and political center.

Geographically, the city is situated between the sea and the towering Mount Olympus to the west. The coastal aspect is provided by the Thermaic Gulf-a relatively shallow water body, with a gradual deepening towards the open Aegean Sea. The coastline of Thessaloniki is constituted of urban-developed areas that include modern ports and promenades, as well as natural sandy beaches. The architecture of the city is a perfect blend of old and modern features. Ruins from as early as ancient times coexist with modern structures, like the Rotunda of Galerius, and speak volumes of the rich historical heritage this city has.

Local culture in Thessaloniki is vibrant, basically representing the culmination of Greek, Roman, Byzantine, and Ottoman influences. It is well known for festivals: celebrating music, art, and even the sea. People from Thessaloniki have the sea in their souls, while fishing and shipping are important contributors to the local economy. All kinds of sea species, such as colorful fish and different species of shellfish, can be found in the waters of Thermaic Gulf.

2. What is the condition of the littoral currents in the neighborhood of Thessaloniki?

The coastal currents around Thessaloniki have both physical and wind-related origins. Wind conditions can be very relevant. The Aegean Sea is subject to several wind directions. A strong north-west blowing wind, the meltemi, is particularly frequent during the summer period but at times may also blow during spring and early autumn. This creates an onshore current that has a profound influence upon the local coastal ecosystem, bearing nutrient-rich water from the open sea. The nutrients support the growth of plankton, which form the base of the marine food chain.

On the contrary, southerly winds somehow succeed in driving this water offshore to form offshore currents. Seasonality in the direction and strength brings about seasonality in these coastal current patterns. The current dynamics receive extra contribution from tidal forces, even though small tides exist around the Aegean Sea as compared with quite a number of regions around the globe. Such ebb and flow of tides interact with wind-driven currents in a manner to either reinforce them or even modify the effects.

Another significant influence is the topography of the Thermaic Gulf. The bottom has a rather uneven topography, with submarine valleys, ridges, and shallows. These can accelerate, decelerate, or even alter the direction of the currents. For example, a narrow submarine channel might constrict the flow of water by increasing the current speed, while a large submarine ridge might deflect the current. In addition, the coastal currents are also influenced by river runoff from nearby rivers such as the Axios River. Freshwater input from the rivers modifies the density of the seawater and thus creates a system of density-driven currents interacting with the already existing wind- and tide-driven currents.

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

Surface Drifting Buoy Method

The surface drifting buoy is a simple approach to the observation of coastal currents at the surface level. Buoys with GPS tracking devices are tossed into the water. As the buoys are carried by the currents, their movement over time is recorded. From this analysis of the buoy's trajectory, an estimation for the speed and direction of the surface currents can be obtained. However, this method does suffer from limitations. Surface winds may completely change the path of the buoys from the actual path of the current. Also, this method provides information only about the surface layer water, not about deeper-layer currents.

Anchored Ship Method

An anchored ship can act like a fixed platform for the measurement of currents. Current meters are suspended from the ship at different depths. These meters record the velocity and direction of the currents at each depth. This gives a detailed vertical profile of the currents at that location. However, this is not very effective for the area it can cover. The data is representative only for the immediate area around the ship and the presence of the ship may affect the natural flow of water in that area.

Acoustic Doppler Current Profiler (ADCP) Method

The ADCP has become one of the preferred methods for measuring near-shore coastal currents around Thessaloniki. ADCPs work by sending out sound waves in a beam that measures the velocity of water simultaneously at several depths. They can be ship-deployed, moored to the seabed, or attached to buoys. This flexibility allows comprehensive data collection. Being less influenced by surface disturbances, such as wind, the ADCPs measure the actual current conditions more precisely. They can also provide a three-dimensional view of the structure of the current in detail for both horizontal and vertical flow components.

4. How do ADCPs using the Doppler principle work?

The ADCPs work on the principle of the Doppler effect. If an ADCP sends a high-frequency sound wave into the water, it encounters the small particles in the water-like plankton, sediment, or even bubbles. These particles scatter the sound waves back to the ADCP. The frequency of the scattered sound waves reaching the ADCP is different from the frequency of the emitted waves. This frequency shift, commonly referred to as the Doppler shift, is linearly proportional to the velocity of the particles and hence the water relative to the ADCP.

Most ADCPs are multibeam transducer systems. Measuring the Doppler shift in each beam allows the ADCP to calculate the velocity components of the water in multiple directions. The vector addition of these velocity components gives the three-dimensional velocity of the water. In this way, a very good knowledge of the current structure is obtained, which is really important for mapping coastal currents in great detail.

5. What is necessary to have high-quality measurement of coastal currents in Thessaloniki?

Material Reliability in Equipment

Taking into great consideration the high demands on the high-quality measurements within the coastal waters of Thessaloniki, special attention needs to be focused on the ADCP casing material. In these circumstances, very suitable is an implementation of titanium alloy. Such a material shows excellent resistance against corrosion because this equipment continuously contacts saline water, which is corrosive in nature. It will not degrade in the long term due to being immersed in salty conditions, and thus the longevity and reliability of the ADCP are assured. Its high strength-to-weight ratio means that the ADCP would be able to hold its own in strong currents or rough seas.

Size, Weight, and Power Consumption

The ADCP should be designed to be small in size and light in weight. A compact design makes it easier to deploy on a ship, attached to a buoy, or placed on the seabed and minimizes its impact on the flow field that is being measured. Low power consumption is highly important, mainly for long-term deployments. This enables the ADCP to operate for extended periods without frequent battery replacements or power from external sources, ensuring data collection is done uninterruptedly.

Cost-effectiveness

The ADCP shall be reasonably priced to make possible the large-scale and full coverage of Thessaloniki's coastal currents. A more affordable ADCP would make this equipment accessible for a wider audience of research institutions, environmental monitoring groups, and local initiatives to allow wider data coverage, necessary for understanding complex coastal currents' dynamics.

6. How to Select Correct Equipment for Current Measurement?

Depending upon the purpose of Usage

• Shipborne ADCP: This system is mounted aboard a moving vessel. It applies to large-scale surveys of currents along coastlines. As a ship moves in a given fashion along the coasts, while in motion it can continuously observe currents with this shipborne ADCP, offering a wide vision of current features over a general area. Its usefulness is first in preliminary surveys or in analyzing changes in flows over a given wide geographical scale.
• Bottom-Mounted ADCP: These are deployed on the sea floor. Bottom-mounted ADCPs are useful for long-term fixed-point measurements. They provide a continuous measurement of the current profile at any given depth or location. That will be of value in making the local hydrodynamics-in particular, what the currents actually do in certain areas of Thermaic Gulf-understandable.
• Buoyant ADCP: While attached to a floating buoy, buoyant ADCPs are free to move with the surface currents. They are well-suited for monitoring surface and near-surface current patterns. They also can be used to track movement of water masses over time important in the study of the transport of nutrients, pollutants, or marine organisms.

Based on Water Depth

• For waters with 70m, a 600kHz ADCP would suffice. Because of the higher frequency of the sound waves, it is possible to have more details in the shallow waters. High-resolution current structure data is presented, thereby assisting in detecting changes in minor currents.
• A 300kHz ADCP is good to about 110m of water depth. A good balance for the range of measurement and resolution deeper into the water column, than could be achieved with a 600kHz ADCP but with relatively decent measurements in mid-depth coastal waters.
• A 75kHz ADCP can be used for waters as deep as 1000m. Its lower frequency sound waves can go deeper, but with somewhat reduced resolution compared to higher-frequency ADCPs. Therefore, it would be good enough to measure currents in the deeper parts of Thermaic Gulf and Aegean Sea near Thessaloniki.

Among all, some of the well-acknowledged ADCP brands include Teledyne RDI, Nortek, and Sontek. However, those who aim for an inexpensive yet quality product should consider the China Sonar PandaADCP. Constructed from all-titanium alloy, this will ensure the product is very durable and will have very good performance at a much lower price. It is an economic class ADCP that one can get with great value. More details can be found on their official website: (https://china-sonar.com/).

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