Deployment Notes: Sal Rei Port, Cape Verde - October 2023
The wind was howling across the west coast of Boa Vista as we prepped the gear on the quay. Sal Rei is a gritty, functional port, and the Atlantic doesn't play nice here. We hit the water early, fighting a choppy swell that made the small deployment vessel pitch violently. The immediate challenge at Sal Rei isn't just the depth; it's the erratic nature of the currents interacting with the island's rugged volcanic coastline and the specific bathymetry of the harbor entrance.
The water was a deep, clear turquoise, but the surface turbulence was deceptive. We were dealing with a complex mix of regional trade wind currents and localized tidal oscillations. The salinity was high, typical for the Cape Verde archipelago, which helps with signal propagation but doesn't stop the physical struggle of getting a heavy instrument precisely where it needs to be on a sandy, shifting bottom.
What We Found
The most surprising data point came in the first 48 hours: we saw velocity spikes near the harbor mouth that completely contradicted the local charts. The flow wasn't just a steady drift; it was pulsing. We caught these short-lived, high-velocity jets that likely result from the way the Atlantic swell pushes water into the port's geometry. It's a classic case of localized acceleration. I suspect these pulses are tied to the specific phase of the semi-diurnal tide, creating a transient 'funnel' effect that could make docking hazardous for the smaller fishing boats and cargo ships that keep this island's economy breathing.
Most of the profile showed a strong surface current moving south-southeast, but the bottom bins were noisy. We saw a distinct shear layer about three meters up from the seabed. In my experience, this usually points to a boundary layer interaction where the current is scrubbing the bottom. Honestly, the vertical velocity gradients were far steeper than I anticipated for a port of this scale. We spent a few hours ground-truthing the data against a handheld current meter, and the ADCP held its own, though the near-bottom data required some heavy filtering to remove the 'ringing' effect from the seabed reflection.
Equipment Performance
The ADCP performed reliably, but the environment tested its limits. We used a higher-frequency unit to get better resolution in the shallow water, and it paid off. The signal-to-noise ratio stayed clean for the most part, though we did hit some bin contamination during a particularly heavy swell event. I noticed a slight drift in the compass heading during the first six hours—likely a calibration quirk—but it stabilized once the unit settled into the sediment. The battery life is the real win here; despite the cold deep-water temperatures (relatively speaking), the power draw remained linear. I've seen cheaper units fail in these high-energy environments, but this setup handled the turbulence without dropping a single data packet.
Recommendations for Future Deployments
If you're heading back to Sal Rei, don't trust the seabed stability. The sand shifts. I'd suggest a heavier tripod base to prevent tilting, which would kill your vertical alignment and ruin your profiles.
- Use a 600kHz or 1200kHz transducer to avoid the 'blanking distance' issues in these shallow berths.
- Increase the ping rate during the spring tide cycles to capture those transient velocity spikes.
- Deploy a secondary pressure sensor for an independent sanity check on the tidal height.
- Apply a more aggressive bottom-track filter to handle the sandy substrate.
The port's growth in tourism and cargo means we need better real-time data. Relying on old charts in a place as dynamic as Boa Vista is a recipe for disaster. We need a permanent array to see if these current pulses are seasonal or permanent features of the harbor's hydrography.
Field report by Sarah Jenkins. Sarah is a specialist in underwater acoustics and oceanographic instrumentation with two decades of experience mapping continental shelf currents.
Field Deployment Report: Bottom-Mounted ADCP Profiling at Sal Rei Port, Boa Vista