Field Deployment Report: Bottom-Mounted ADCP Profiling at the Port of Lomé

Deployment Notes: Lomé Littoral, October 2023

The humidity hit us the moment we stepped off the plane, but the real heat was waiting at the harbor. We arrived at the port entrance just as the South-Westerly monsoon winds were peaking, turning the Atlantic into a churning, opaque soup of suspended sediment. Standing on the deck of the survey vessel, you could practically see the Guinea Current shoving the shoreline eastward. It isn't a subtle flow; it's a relentless, high-energy conveyor belt that treats the Togolese coast like a sanding block.

The water state was aggressive. We were dealing with a chaotic mix of heavy swell and localized turbulence caused by the shallow bathymetry near the navigation channels. The visibility was nearly zero. I watched a handful of sediment settle in a sample bucket—it didn't just sink; it clumped, showing the sheer volume of solids moving through the water column. This isn't a standard port environment. It's a hydrodynamic battleground where the bed-load transport is so intense it threatens to choke the harbor entrance in a matter of weeks.

What We Found

The data came back with a shock: the vertical velocity gradient is far more extreme than the port authority's existing models suggested. We saw a massive discrepancy between the surface shear and the bottom boundary layer. While the surface currents were screaming eastward, the flow near the seabed was behaving erratically, showing localized acceleration zones that likely contribute to the rapid siltation of the channels. If you only rely on surface floats here, you're lying to yourself about the dredging reality. We caught several spikes in velocity that coincided with swell events, proving that the bed-load movement isn't just a steady drift—it's a series of violent pulses.

The most frustrating part? The noise. We saw significant bin contamination in the lower cells. The acoustic signal wasn't just hitting water; it was bouncing off dense clouds of suspended sand. In some of the early profiles, the data looked like a jagged mess. I spent three hours scrubbing the raw files just to find a clean signal. It's clear that the interaction between the deep-water Guinea Current and the rising seabed at 6.1° N creates a turbulence profile that would baffle a standard current meter. We aren't just measuring water movement; we're measuring a liquid landslide.

Equipment Performance

I insisted on the 300kHz ADCP, and thank god I did. A 600kHz unit would have been blind in this turbidity, and 1200kHz is useless for the depths we needed to monitor at the port mouth. The 300kHz gave us the penetration required to actually see the bottom boundary layer, though it still struggled with the sediment load. The real MVP was the heavy-duty tripod frame. We used oversized footpads because the seabed here is alive. In previous deployments in West Africa, I've seen frames vanish into the sand in a single tidal cycle. We did a sanity check with a vessel-mounted unit via a short tow-fish run before the final drop. The alignment was off by a few degrees, which told me the seabed was already shifting under the unit. We had to adjust the tilt correction in post-processing to make the data usable. Honestly, without that ground-truthing, the entire dataset would have been skewed.

Recommendations for Future Deployments

Lomé is a brutal environment for instrumentation. To get data that actually means something, stop treating it like a stable shelf. You need gear that can survive burial and sensors that can pierce through sediment clouds.

• Stick to 300kHz units. Anything higher will lose signal in the turbid bottom layer.
• Use oversized, wide-base footpads on all tripods to prevent the unit from sinking into the shifting sands.
• Mandate a tow-fish calibration run immediately prior to deployment to identify tilt and burial issues.
• Increase the sampling frequency during monsoon peaks to capture the pulse-like nature of the sediment transport.
• Apply aggressive signal-to-noise filtering to account for the heavy suspended solids in the lower bins.

Field report by Sarah Jenkins. Sarah is a specialist in underwater acoustics and continental shelf currents with twenty years of experience deploying instrumentation in high-energy coastal zones.