Deployment Notes: Milos Port, Cyclades, September 2023
The salt air was thick and heavy as we unloaded the gear at the Milos quay just after 05:00. Looking out over the harbor, the water had that deceptive, glass-like stillness typical of the Aegean before the Meltemi winds kick in. But the stillness is a lie. Milos is a volcanic anomaly; the bathymetry here is erratic, with steep drop-offs and hidden rocky outcrops that create chaotic local eddies. For anyone trying to get a clean velocity profile, this place is a nightmare because the currents don't follow a simple linear path—they swirl around the volcanic morphology of the coastline.
The weather held, but the water state was tricky. We noticed significant surface chop coming from the open sea, which usually suggests a strong subsurface shear. I spent an hour checking the local charts against our actual sonar pings. The depth readings were jumping around (likely due to the rugged volcanic floor), making it a challenge to find a stable, flat spot for the tripod. We eventually settled on a site near the main shipping channel, where the interaction between the incoming Aegean swells and the port's geometry creates complex circulation patterns that could easily push a docking vessel off course if the pilot isn't paying attention.
What We Found
The data hit us hard during the first retrieval. We saw a massive spike in current velocity—hitting 0.7 m/s—at a depth of only 4 meters, which completely contradicted the surface observations. It was a classic case of internal waves or a localized jet caused by the port's narrow entrance. Most surprising was the tidal asymmetry. We expected a standard semi-diurnal pulse, but the flood tide was significantly more aggressive than the ebb. This suggests the harbor geometry is trapping water or that the volcanic ridges outside the port are funneling the flow in a way that accelerates the inflow. It's a dangerous setup for smaller vessels.
I spent three hours scrubbing the data, looking for bin contamination. In several of the lower bins, we saw some noisy data that looked like sediment transport. Given Milos's volcanic nature, we're dealing with coarse sands and basaltic fragments. These particles act like mirrors for the acoustic pings, creating 'ghost' velocities. Once I filtered out the noise, the real picture emerged: a complex, rotating cell of water sitting right in the middle of the harbor. It's not just a flow-through system; it's a mixing bowl. This explains why some of the berths experience unexpected siltation despite the periodic dredging.
Equipment Performance
We deployed a 300kHz ADCP for this run. Honestly, I debated going with the 600kHz unit for better resolution in the shallow bins, but the 300kHz gave us the vertical reach we needed to see the full water column. The instrument held its position well, though the tripod shifted about 15 centimeters during a peak surge. The signal-to-noise ratio was decent for the first 72 hours, but then it dipped. I suspect some organic fouling or a sudden plume of suspended volcanic sediment. Still, the velocity vectors were consistent enough to pass a sanity check against the surface drifters. The battery life was exactly where the manual said it would be, though the internal clock drifted by about four seconds—negligible for this kind of study, but annoying for a perfectionist.
Recommendations for Future Deployments
If we go back to Milos, we can't just 'drop and hope.' The seabed is too unpredictable. We need a more rigorous pre-deployment survey to avoid placing the unit on a slope.
- Use a heavier gravity base instead of a tripod to prevent shifting during Aegean surge events.
- Deploy a secondary CTD (Conductivity, Temperature, Depth) sensor to correlate velocity spikes with salinity gradients.
- Increase the ping rate during the spring tide window to capture the peak asymmetry of the flood currents.
- Apply a more aggressive blanking distance to avoid surface noise from cruise ship wakes.
Field report by Sarah Jenkins. Sarah is a senior oceanographic engineer specializing in the interaction between coastal morphology and acoustic current profiling.
Field Deployment Report: Bottom-Mounted ADCP Profiling at Milos Port, Greece