Deployment Notes: Quelimane Estuary, Mozambique Channel, November 2023
The humidity hit us the moment we stepped off the boat, a thick, clinging heat that usually signals the start of the rainy season. We arrived at the mouth of the Bonsperi River just before dawn, hoping to catch the peak of the flood tide. The water was the color of milky coffee, opaque and heavy with suspended sediment. You could practically feel the density of the water against the hull. It is a volatile place. Here, the freshwater discharge from the interior clashes violently with the salt wedge pushing in from the Mozambique Channel, creating a chaotic mixing zone that makes standard current measurements a gamble.
The bathymetry in this delta is a nightmare. We spent the first few hours scouting for a deployment site, only to find that the sandbars had shifted significantly since the last survey (likely due to the recent heavy rains upstream). The water state was erratic. Surface currents were ripping seaward, but the deeper layers were fighting back, pushing saline water inland. This vertical shear is the primary headache in Quelimane. If you rely on surface readings, you are lying to yourself about the actual energy of the water column.
What We Found
The data came back with a shock. We recorded velocity reversals that were far more aggressive than the regional tide tables suggested. At the pycnocline—that sharp boundary where the fresh river water sits atop the denser seawater—the shear was brutal. In some bins, we saw the surface water moving at 0.6 m/s toward the channel while the bottom layers were surging inland at nearly 0.9 m/s. It is a textbook salt wedge, but on steroids. The turbidity maximum zone had migrated further offshore than we anticipated for November, trapping organic debris in a dense, swirling slurry that acted like a physical barrier to the acoustic pings.
Honestly, the most surprising part was the stability of the lower salt wedge. While the surface was a mess of turbulence and wind-driven noise, the saline layer remained a disciplined, powerful conveyor belt. This creates a dangerous situation for vessel safety. A captain looking at the surface might think they have plenty of steerage, while the under-hull current is actually dragging them toward the banks. We spent hours ground-truthing the ADCP data against our CTD casts. The correlation was tight, but it confirmed that the density shifts here are violent enough to trigger significant bin contamination if your calibration is off by even a fraction.
Equipment Performance
We opted for a 600kHz ADCP, and it was the right call. I've used higher frequencies in clearer waters, but in the Bonsperi confluence, a 1200kHz unit would have been blinded by the silt. The 600kHz unit punched through the turbidity and gave us a clean signal, though we still saw some attenuation in the highest sediment peaks. We mounted the unit on a heavy-duty tripod frame—a necessity here. I've seen too many sensors vanish into the shifting sands of the Mozambique coast because the team used a simple weight. The tripod kept us off the bottom, preventing the unit from being buried during the tidal swing. The 15-minute averaging interval was a lifesaver; it stripped out the high-frequency noise from surface chop while keeping the tidal signal intact. We did experience some signal dropout during the peak flood, but nothing that compromised the overall dataset.
Recommendations for Future Deployments
If you are heading into the Quelimane delta, don't wing it. The environment is too aggressive for 'off-the-shelf' settings. You need to prioritize penetration over raw resolution.
- Stick to 600kHz: Higher frequencies attenuate too quickly in this silt.
- Use Tripod Mounts: Single-weight anchors will get buried in the shifting sandbars within 48 hours.
- Tighten Bin Size: Set bins to 0.5m or 1.0m. Any larger and you'll miss the sharp interface of the salt wedge, leading to smeared data.
- Mandatory CTD Pairing: Never deploy an ADCP here without a concurrent conductivity-temperature-depth sensor. Without a sanity check on the salinity gradient, your velocity readings are just guesses.
- Seasonal Timing: Avoid the peak of the monsoon rains unless you have a high-tolerance mooring system; the discharge from the Bonsperi can shift the seabed in a single afternoon.
The key to success in this region is accepting that the water is not a single body, but a layered cake of different densities and directions. Once you stop treating the estuary as a uniform channel, the data starts making sense.
Field report by Dr. Alistair Vance. Dr. Vance is a specialist in underwater acoustics and estuarine dynamics with twenty years of experience deploying instrumentation in high-turbidity environments.
Field Deployment Report: Bottom-Mounted ADCP Profiling in the Quelimane Delta