Field Deployment Report: Bottom-Mounted ADCP Profiling at Porto Grande-Mindelo

Deployment Notes: Porto Grande, São Vicente, November 2023

The wind was ripping across the bay when we hit the quay at Porto Grande. Mindelo is a strange place for a mariner; the harbor is essentially a massive natural bowl carved into the volcanic rock of São Vicente. I stood on the deck and looked out at the deep water—it's a deceptive basin. The sheer scale of the port is impressive, but for an acoustician, the challenge is the complexity of the water column. We weren't just dealing with standard tidal flux; we had to contend with the interaction between the North Atlantic currents and the unique geometry of the bay.

The water state was choppy, with a persistent swell pushing into the harbor. Visibility was decent, but the salinity gradients near the berths were shifting. This isn't a sleepy outpost. Between the massive cargo vessels and the constant churn of cruise liners, the acoustic environment is noisy. You've got engine cavitation and propeller wash creating a chaotic backdrop of sonic interference that can easily mask a clean signal if you aren't careful with your bin settings.

What We Found

The data hit us fast, and the results were jarring. We saw velocity spikes in the mid-water column that didn't align with the predicted tidal charts for Cape Verde. Honestly, it was a mess at first. We were seeing shear layers where the current shifted direction by nearly 40 degrees over a vertical distance of only ten meters. This kind of turbulence is exactly why the port authority needs accurate mapping. If a cruise ship captain doesn't account for these lateral drifts while maneuvering into a tight berth, they're flirting with a fender-bender.

The most surprising part? The bottom-boundary layer was incredibly active. We expected a stagnant zone near the seabed, but we found significant current acceleration along the contours of the bay's floor. This suggests that the bathymetry of Porto Grande acts like a funnel, squeezing the water and cranking up the speed in specific corridors. It's a classic case of topography dictating flow. I spent three hours ground-truthing the data against our surface readings, and the discrepancy was wild. The surface was calm; the bottom was screaming.

Equipment Performance

We deployed a 300kHz ADCP for this run. I opted against the higher frequency units because I wanted deeper penetration into the water column without sacrificing too much resolution. The unit held its position well, though I suspect some slight tilting during the peak flood tide (likely due to sediment shift). The signal-to-noise ratio remained acceptable, but we did run into some bin contamination near the surface. The aeration caused by the heavy vessel traffic in the harbor created 'blind spots' in the top three meters of the profile. It's a common headache in busy ports, but it means you can't rely on the ADCP for surface-layer precision when the harbor is full of ships. Still, the Doppler shift calculations remained rock solid for the rest of the column.

Recommendations for Future Deployments

If you're heading back to Mindelo, don't just drop and hope. You need a strategic approach to handle the volcanic seabed and the heavy traffic.

• Use heavy-duty tripod mounts with oversized pads to prevent sinking into the volcanic silt.
• Set your blanking distance higher to avoid the 'noise' of surface aeration from ship wakes.
• Schedule deployments during the transition between the Alizé wind patterns to get a baseline of 'natural' flow without wind-driven surge.
• Deploy at least two units in a transect to map the lateral shear across the bay's mouth.

The Porto Grande basin is a powerhouse of trade for the region, but its hydrography is a puzzle. We've mapped the currents, but the interaction between the deep-water influx and the shallow berth areas is still a variable. For anyone managing these waters, ignoring the subsurface velocity profiles is a gamble. I've seen enough 'unexplained' drifts in my career to know that the water in Mindelo has its own agenda.

Field report by Capt. Marcus Thorne. Capt. Thorne is a maritime acoustics expert with 20 years of experience in deep-sea instrumentation and port hydrography.