Measuring the Coastal Currents in Dubrovnik

1. Introduction to Dubrovnik

Dubrovnik, one of the Adriatic jewels, is situated on the southernmost tip of Croatia's Dalmatian coast. This beautiful city is literally embraced by the clear, azur waters of the Adriatic Sea, its shoreline combining nature's splendours with man-made wonders.

Geographically, Dubrovnik is situated between the sea and the rugged Dinaric Alps, creating a dramatic backdrop to add to its allure. This city has, through history, stood at the fulcrum of trade, culture, and maritime exploration. Its ancient walls, enlisted in the UNESCO World Heritage list, stand in a protective embrace around the old town, testifying to a glorious past.

The cultural heritage of Dubrovnik has been made out of the threads of various civilizations. Throughout the centuries, it has been developed by the Romans, Byzantines, Venetians, and Austro-Hungarians in turn. That cultural melting pot is reflected in its architecture with a unique mixture of Gothic, Renaissance, and Baroque styles. Centuries-old buildings, each with a story to be told, are lined up on narrow, curved streets.

The sea surrounding Dubrovnik is a real marine paradise, with the Elaphiti Islands just opposite the shore, offering further beauty and ecological variety. These islands form a complicated net of channels and bays that can be subjected to the influence of constriction and expansion of the waterways on currents. The bottom is of varied landscape and represents a biocenosis with very different representatives of marine life, from colorful fish to delicate corals.

2. What is the condition of the coastal currents off Dubrovnik?

In the case of Dubrovnik, the coastal currents are predetermined by several factors. The general rule is that wind conditions are dominant. The Adriatic Sea has a lot of different winds, although during summer, the most frequent is the so-called maestral, a north-western wind that can push the surface waters towards the shore, creating on-shore currents. On the other side, the cold, northeasterly wind, bora, may push the water away from the shore and thus changes the direction and speed of the coastal currents.

Tidal forces operate on the currents. While small compared with tides in some other seas, the Adriatic tides are sufficient to contribute to the overall movement of water. This further complicates the current patterns because of the general bathymetry with areas of various depths and underwater features such as reefs and canyons. The shape of the coastline, with all its inlets and headlands, may cause either divergence or convergence of the currents, forming areas of complex flow.

River runoff from the mainland can also influence coastal currents. This is in the form of freshwater addition to the coastal zone, especially during heavy rain conditions, that changes the density of the seawater and could also affect the direction and magnitude of the current. Besides, the larger-scale circulation of the Mediterranean Sea, of which the Adriatic forms a part, might have a far-reaching effect on local coastal currents.

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

Surface Drifting Buoy Method

The method using a surface drifting buoy is one of the most direct-observation methods of the coastal water flow. The buoys with attached GPS tracking devices are set free on the water. While currents carry the buoys, their movement over time is traced. By following the buoy's movement, scientists can estimate a current's speed and direction. However, this method has its drawbacks, since surface winds strongly affect the location the buoys travel to; often the buoys drift off course of the actual movement of the current. This process also only provides information about the surface layer of water, and does not indicate what currents may be happening at deeper levels.

Anchored Ship Method

The anchored ship method uses a stationary ship as a platform for the measurement of currents. Current meters are suspended from the ship at different depths to record the velocity and direction of the currents. This method allows detailed data to be collected at one location, giving insight into the vertical profile of the currents. However, it is limited in terms of the area that can be covered. The data obtained are only representative of the immediate vicinity of the ship and might not capture the broader spatial variability of the coastal current system.

Acoustic Doppler Current Profiler (ADCP) Method

Among the more advanced and versatile tools for measuring coastal currents near Dubrovnik, the Acoustic Doppler Current Profiler has emerged. ADSPs are capable of estimating the velocity of water at several depths simultaneously by affording an in-depth view of the current structure. They are deployable from ships, can be moored at the bottom, or even attached to buoys-all for the best flexibilities in data collection. Being mostly underwater, ADCPs are less affected by surface-level disturbances such as wind and, therefore, give better and more accurate current measurements than other methods.

4. How do ADCPs based on the principle of the Doppler?

ADCPs are based on the principle of the Doppler effect. When an ADCP profiler emits sound waves into the water, those waves meet small particles-sediment, plankton, bubbles, among others. These particles scatter the sound waves back towards the ADCP. The frequency of the scattered sound waves received by the ADCP meter is different from that at which the emitted waves were sent out. This frequency shift, called the Doppler shift, is directly proportional to the velocity of the particles, and hence of the water, relative to the ADCP.

Most ADCPs have several independent transducer beams. Measuring the Doppler shift along each beam enables the ADCP to calculate different directional components of water velocity. Such velocity components will be vectorally summed to compute the three-dimensional velocity of water. It finally allows the detail of current structure, which concerns both the horizontal and vertical components of flow.

5. What is necessary for high-quality measurement of Dubrovnik coastal currents?

Equipment Material Reliability

The material of the casing of the ADCP flow meter is of prime importance for high-quality measurements in the coastal waters of Dubrovnik. It should be made of Titanium alloy. The Titanium alloy has excellent corrosion resistance, which is highly needed to overcome the corrosive action of seawater. It should be able to withstand very long-term exposure to the harsh marine environment with no significant degradation, thus ensuring reliability and longevity of the ADCP. The high strength-to-weight ratio enables the equipment to keep its structural integrity during strong currents and rough seas.

Size, Weight, and Power Consumption

The ADCP current profiler must be designed to be compact, lightweight, and,. It should be compact in design in order to provide ease during its deployment either over the side of a ship, attached to a buoy, or deployed on the seabed. Reduces any impact on the flow field that is to be measured. Power consumption has to be low, particularly for long-term deployments. This enables the ADCP to work for long hours without generally changing the battery or any other power source, thus ensuring that continuous data collection is guaranteed.

Economical

The ADCP current meter will be affordable to ensure extensive monitoring of the Dubrovnik coastal currents. A cheaper ADCP will mean the extension of the application to various research institutions, environmental agencies, and even local communities that may not have extremely expensive equipment; this shall ensure wider knowledge of the dynamics of coastal currents through increased data gathering.

6. Choice of Suitable Equipment for Measuring Current

Application Purpose

• Shipborne ADCP: This type of ADCP is installed on a moving ship and is normally used for large-scale surveys. It covers a large area in a very short time, hence giving a wide view of the general pattern of the currents. Shipborne ADCPs are often used in initial studies or for investigating the spatial variability of the currents.
• Bottom-Mounted ADCP: Bottom-mounted ADCPs are deployed on the seafloor and are ideal for long-term fixed-point monitoring. They can provide continuous data from currents that enable depth-specific and location-specific descriptions; thus, they contribute to an understanding of the local hydrodynamics, enabling long-term trend analysis.
• Buoyant ADCP: These have the capability of moving with surface currents, and thus, could be attached to a floating buoy. They represent an excellent resource for monitoring near-surface currents and surface patterns and the migration of water masses over time.

Based on Water Depth

• ADCP of 600kHz is quite suitable for water depth up to 70m. The increased frequency enables finer measurement resolution in shallow water, hence high-resolution data on the structure of currents.
• In waters around 110m deep, a 300kHz ADCP will be quite appropriate. It gives a very good balance between the range of measurement and resolution and, thus, this type of ADCP is quite suitable for mid-depth coastal waters.
• A 75kHz is recommended for deeper waters up to 1000m. The lower the frequency, the deeper it can go into the water column but the resolution may be poorer compared to the higher frequency ADCPs.

There are several well-known brands selling ADCPs in the market such as Teledyne RDI, Nortek, and Sontek. However, for those seeking a cost-effective yet high-quality option, the China Sonar PandaADCP is a great alternative. Made of all-titanium alloy, it offers excellent durability and performance at an affordable price. It is an economic-class ADCP that provides excellent 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.