Mediterranean Inflow Dynamics and Port Basins
The Port of Ashdod sits at a precarious intersection of the Eastern Mediterranean's coastal currents and the anthropogenic modifications of its harbor basins. We often see current velocities shifting abruptly near the breakwaters, where the prevailing north-northwesterly winds drive surface waters that collide with the port's structural geometry. This creates complex eddies and shear layers that make standard current monitoring a nightmare. If you ignore the interaction between the Levantine Intermediate Water (LIW) and the shallow port bathymetry, your velocity profiles will be wrong.
The challenge here isn't just the flow; it's the variability. During winter storm surges, the energy flux into the port increases significantly. This pushes sediment-laden water into the navigation channels, causing sudden spikes in acoustic backscatter. I have observed that the transition from the open sea to the sheltered berths creates a sharp gradient in turbulence intensity. This isn't a steady-state environment. It is a chaotic mix of wind-driven surges and tidal oscillations that, while small in the Mediterranean, still influence the movement of silt and pollutants within the harbor.
Most operators treat the harbor as a stagnant pond. They are wrong. The actual flow dynamics involve a constant exchange of water masses. The salinity levels stay high, usually above 38 PSU, but the temperature swings between seasons affect the speed of sound. If you don't calibrate your ADCP for the specific sound velocity of Ashdod's waters, your depth bins shift. A 1% error in sound speed might seem trivial, but across a 30-meter water column, it ruins your vertical resolution.
The Ashdod Navigation Channel and Breakwater Geometry
The primary access channel, centered roughly around 34.7° N, 34.6° E, serves as the sole artery for ultra-large container vessels. The bathymetry here is a product of constant maintenance dredging. We see depth contours that drop sharply from the shallow coastal shelf into the dredged channel. This creates a 'canyon effect' where currents accelerate. When the Mediterranean current hits the northern breakwater, it doesn't just stop; it deflects, creating a jet that can push vessels off-course during docking maneuvers.
The breakwaters are designed to shield the berths, but they also trap organic matter and fine sediments. This creates a stratified layer of suspended solids near the bed. In my experience, these 'bottom-heavy' sediment loads cause significant signal attenuation. The transition from the deep channel to the shallower berth areas (often less than 15 meters) means the ADCP must handle rapidly changing blanking distances to avoid losing the bottom track.
Acoustic Propagation Challenges in This Environment
Ashdod's water is far from clear. The high concentration of suspended particulate matter—mostly fine sands and organic debris—scatters the acoustic signal. This creates 'noisy data' in the lower bins. When the signal-to-noise ratio (SNR) drops, the ADCP struggles to find a reliable backscatter peak. I've seen cases where the correlation magnitude plummets during high-turbidity events, leading to 'hole' in the data where the instrument simply cannot resolve a velocity.
Salinity gradients also complicate things. While the Mediterranean is generally homogeneous, the port's enclosed nature can lead to localized salinity pockets, especially after heavy rainfall or near industrial discharge points. These gradients bend the acoustic beams (refraction). If you rely on a constant sound speed of 1500 m/s, you are guessing. To get a clean signal, you need real-time CTD (Conductivity, Temperature, Depth) integration. Without it, you're just looking at an approximation of the current.
Frequency Selection: 300 kHz vs 600 kHz Analysis
Choosing the right frequency is a trade-off between range and resolution. For the deep navigation channel, a 300 kHz transducer is the logical choice. It penetrates deeper and handles the higher sediment load better. However, the 300 kHz unit has a larger sample volume, which means you lose the fine-scale shear data near the bed. In the shallower berths, the 600 kHz unit is superior. It provides the vertical resolution needed to see how the current interacts with the quay walls.
Honestly, the 600 kHz unit outperformed in the berths, but it struggled during the winter siltation peaks. We found the 300 kHz unit more robust for long-term deployments in the channel. For a sanity check, we often deploy both side-by-side. If the 600 kHz shows a velocity spike that the 300 kHz doesn't, it's usually a sign of localized turbulence or 'bin contamination' from fish schools rather than a true current shift.
Data Interpretation and Field Findings
Our data shows a distinct 'bottom-up' flow pattern during certain tidal phases. We measured velocities of 0.4 m/s at the surface, but near the bed, the flow reversed. This indicates a complex recirculating cell within the port basin. This is a critical finding. It explains why sediment accumulates in specific 'dead zones' despite the dredging efforts. The water isn't just flowing in and out; it's swirling.
We also noted a significant discrepancy between the ADCP's bottom-track and the GPS-fixed position of the mooring. This 'drift' suggested that the seabed was shifting—likely due to the migration of sand ripples. Ground-truthing with a fixed seabed transponder confirmed that the sediment was moving. This makes the 'bottom-track' mode unreliable for absolute velocity measurements in Ashdod's sandy channels. I always recommend using a GPS-referenced mooring for any serious hydrodynamic study here.
Operational Implications
These current profiles directly impact pilotage. When a 300-meter vessel enters the channel, a 0.5 m/s cross-current can exert massive lateral force. Understanding the exact shear profile allows pilots to anticipate 'push' more accurately. It's not just about the surface current; it's about how the deep-water current acts on the keel while the surface current acts on the superstructure.
Furthermore, the sediment transport data helps the port authority optimize dredging schedules. Instead of dredging the whole channel on a fixed calendar, they can target the 'deposition hotspots' identified by the ADCP's backscatter intensity maps. This saves money and reduces operational downtime. If you can map the current, you can map the sand.
About the author: Dr. Kenji Sato. A specialist in underwater acoustics with 20 years of experience designing instrumentation for extreme marine environments. He has led over 50 field campaigns focusing on river-sea interfaces and harbor hydrodynamics.
Analyzing Mediterranean Boundary Current Influence on Near-Bed Velocity Profiles in the Port of Ashdod