Deployment Notes: Tarrafal de São Nicolau, Cape Verde
The humidity hit us the moment we stepped off the boat at Tarrafal de São Nicolau. I remember staring at the quay, watching the local fishing boats bobbing in a restless swell that didn't match the calm surface of the harbor. We were there to get a real handle on the current velocities—not just the surface drift, but the full water column. The port is a lifeline for São Nicolau, but its geography makes it a nightmare for precise current mapping. You have these steep volcanic slopes dropping off quickly, which creates erratic eddies when the Atlantic pushes into the harbor.
The water was surprisingly clear, but the wind was whipping across the berths, making the deployment of the tripod frame a gamble. We spent the first hour just fighting the surge to ensure the ADCP was seated perfectly level on the seabed. If the tilt is off by even a few degrees, your vertical velocity components get messy, and you spend three days in the lab trying to correct the data. The tide was pushing in, and the current felt stronger than the local charts suggested.
What We Found
The data gave us a wake-up call. We saw unexpected velocity spikes in the lower bins that completely contradicted the surface readings. While the top few meters were drifting sluggishly toward the harbor mouth, the deeper layers were shearing in the opposite direction. I suspect these are localized recirculation cells caused by the port's specific bathymetry. It's a classic case of surface-level observations lying to you. We recorded peak flows that would make docking a medium-sized cargo ship a lot more nerve-wracking than the harbor master admits.
I noticed a significant amount of noise in the signal during the peak flood tide. It wasn't electronic interference; it was biological. The area is teeming with small pelagic fish and organic debris that the ADCP picks up as 'backscatter.' This created some annoying bin contamination in the mid-water column. However, once I filtered the noise, the current profiles showed a distinct stratification. The salinity gradients here are tricky, and the resulting density layers seem to be steering the current paths in ways that standard hydrodynamic models usually miss.
Equipment Performance
We used a high-frequency ADCP for this run because of the shallow depths near the berths. Honestly, the unit performed well, but the acoustic ping rate had to be dialed back to avoid range ambiguity. I was worried about the bottom-track losing lock due to the rocky, volcanic substrate, but the signal remained crisp. The battery life held up, though the cold-start on the internal clock gave me a brief heart attack during the initial sanity check. We did see some signal dropout during the highest turbulence events, but that's par for the course in a high-energy coastal zone like this.
Recommendations for Future Deployments
If we go back to Tarrafal, we need to stop relying on a single point of measurement. One ADCP can't capture the complexity of these eddies.
- Deploy a multi-instrument array to map the horizontal shear across the harbor entrance.
- Use a heavier mooring weight. The Atlantic surge in Cape Verde can shift a light tripod, ruining your orientation.
- Increase the sampling frequency during the spring tide window to capture the rapid acceleration of the current.
- Run a concurrent CTD cast to correlate velocity changes with temperature and salinity shifts.
We spent too much time guessing the bottom composition before the drop. Next time, a quick side-scan sonar pass would save us an hour of guesswork. The data is solid, but the environment is volatile. You can't just drop a sensor and hope for the best in a place like São Nicolau.
Field report by Elena Rodriguez. Elena is a specialist in underwater acoustics and oceanographic instrumentation with two decades of experience mapping coastal sediment transport in volatile marine environments.
Field Deployment Report: Bottom-Mounted ADCP Profiling at Tarrafal de São Nicolau Port