Biokovo's Rain Shadow and the Bora: Acoustic Velocity Profiling in Makarska's Coastal Waters

Executive Summary

Measuring currents off Makarska is a nightmare for the uninitiated because of the extreme interaction between the Biokovo Mountain range and the Adriatic basin. We aren't dealing with simple tidal oscillations here. The real challenge is the Bora wind—a violent, cold northeasterly that slams into the coast, creating sudden, high-velocity offshore surges that can flip surface current directions in hours. Most generic models fail here because they ignore the steep bathymetric drop-off characteristic of the Dalmatian coast. To get a clean signal, you have to account for the sharp thermoclines and the erratic wind-driven shear that makes surface-mounted sensors almost useless during winter storms.

The Biokovo-Adriatic Interface

Makarska sits in a precarious geographic spot. To the east, the Biokovo Mountains rise abruptly, acting as a massive atmospheric wall. This creates a rain shadow and, more importantly, channels the Bora wind directly toward the sea. I've worked in similar Mediterranean pockets, but the acceleration of water movement here is distinct. The seabed drops off rapidly into the deep Adriatic, meaning you have a very narrow continental shelf. This steep slope means that wind-driven surface currents don't just move water—they trigger complex Ekman transport patterns that push nutrient-rich deeper waters toward the surface (upwelling) or shove surface water far offshore.

Tides in the Adriatic are negligible—usually under 20cm—so if you're seeing significant velocity shifts in your data, it's almost certainly wind-driven or a result of larger-scale Adriatic gyres. The local bathymetry, riddled with rocky reefs and limestone caves, creates localized turbulence that can mess with your acoustic backscatter if your instrument isn't positioned perfectly.

Unique Measurement Challenges at Makarska

The primary headache here is the seasonal volatility. During the summer, the Maestral (northwesterly) creates a predictable onshore flow. But come winter, the Bora turns the coastal zone into a chaotic environment. We've seen instances where surface velocities spike, but the bottom boundary layer remains stagnant. This creates massive vertical shear. If you're using a low-resolution ADCP, you'll likely miss the transition zone, leading to 'noisy data' that looks like instrument failure but is actually just extreme physical turbulence.

Another issue is the biological interference. The Adriatic is biologically productive. During plankton blooms, the water becomes 'acoustically thick.' This leads to signal attenuation. I remember a deployment where we lost the top three bins of data because the organic load was so high the pings simply didn't return. You can't just 'set it and forget it' in Makarska; you need to adjust your blanking distance and sampling intervals based on the season.

Site-Specific ADCP Configuration

For this specific environment, I strongly recommend a 300kHz ADCP. Why? Because the 600kHz units, while great for shallow estuaries, lack the range needed to capture the full water column before the steep slope takes over. You need that deeper penetration to see the interaction between the surface Bora-driven flow and the deeper, slower currents.

Bottom-mounting is the only way to go here. Vessel-mounted surveys are too fragmented to capture the Bora's onset. We typically use a heavy tripod mount with a weighted base to prevent 'tilt' during high-velocity events. If the instrument tilts even a few degrees, your horizontal velocity vectors become garbage. I always insist on a sanity check by deploying a current meter at a fixed depth to ground-truth the ADCP's vertical profile.

Our typical setup involves:

• Frequency: 300 kHz for optimal depth penetration.
• Bin size: 0.5m to 1.0m to accurately map the shear layer.
• Sampling interval: 15-minute averages to filter out wave-induced orbital motion.
• Mooring: Bottom-mounted tripod with an acoustic release.

Representative Measurement Data

Below is a typical profile we might see during a moderate Bora event. Notice how the velocity drops off precipitously as you move away from the surface.

The data reveals a classic wind-driven surface layer. The reversal at 60m is a textbook example of the compensatory flow required to maintain mass balance in the Adriatic basin. If you don't see this reversal, your signal fence is likely too tight, or you're dealing with a massive storm surge.

Operational Impact on Local Maritime Activities

These currents aren't just academic. They dictate everything for the Makarska fishing fleet. The Bora-driven offshore currents push larval fish away from the coast, affecting recruitment in local nurseries. For the sailing community, these sudden shifts can be dangerous. A boat heading into a perceived calm can suddenly be hit by a shear current that pushes the bow off course.

From an engineering perspective, the high-energy events caused by the Bora make the placement of underwater cables or sensors risky. We've seen equipment 'walk' across the seabed because the drag forces on the mooring head were underestimated. Any infrastructure project in this zone needs to account for these periodic, high-velocity surges to avoid catastrophic failure.

Internal Context and Broader Applications

Comparing Makarska to the more open waters of the Ionian Sea, the 'channeling' effect of the Dalmatian coast is far more pronounced. The interaction between the mountains and the sea creates a micro-climate that mimics some of the fjords in Norway, albeit with different temperature profiles. We often use the data from these sites to refine Doppler profiling algorithms for other high-slope coastal regions.

To get the full picture, you can't rely on ADCPs alone. Pairing this with CTD (Conductivity, Temperature, Depth) sensors allows us to see if the current is moving a salt wedge or a freshwater plume from coastal runoff. In my experience, the salinity gradient during the autumn rains significantly alters the acoustic backscatter, which can lead to 'bin contamination' if you aren't careful with your data processing.

About the Author

Dr. Alistair Vance. A senior oceanographic engineer with over 20 years of experience specializing in acoustic telemetry and deep-sea instrumentation. He has led numerous ADCP deployment campaigns across the Mediterranean and North Atlantic, focusing on high-energy coastal environments.